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Observations and temperatures of Io's Pele Patera from Cassini and Galileo spacecraft images
Abstract
Pele has been the most intense high-temperature hotspot on Io to be continuously active during the Galileo monitoring from 1996-2001. A suite of characteristics suggests that Pele is an active lava lake inside a volcanic depression. In 2000-2001, Pele was observed by two spacecraft, Cassini and Galileo. The Cassini observations revealed that Pele is variable in activity over timescales of minutes, typical of active lava lakes in Hawaii and Ethiopia. These observations also revealed that the short-wavelength thermal emission from Pele decreases with rotation of Io by a factor significantly greater than the cosine of the emission angle, and that the color temperature becomes more variable and hotter at high emission angles. This behavior suggests that a significant portion of the visible thermal emission from Pele comes from lava fountains within a topographically confined lava body. High spatial resolution, nightside images from a Galileo flyby in October 2001 revealed a large, relatively cool (
... Revised results for the Pillan eruption temperature indicated at least basaltic compositions, with the possibility of ultramafic lavas (Keszthelyi et al., 2007). Magma composition inferred from blackbody temperature fits to remotely sensed spectral data incorporate many factors (Wright et al., 2011;Wright and Flynn, 2003); however, the consensus is that temperatures of lavas on Io imply either basaltic or ultramafic lavas Radebaugh et al., 2004;Davies et al., 2018). ...
... Remote observations of lava lakes on Earth can be directly compared with remote observations of volcanoes on Io Davies et al., 2001;Radebaugh et al., 2004;Lopes et al., 2004;Marchis et al., 2005;Howell and Lopes, 2007;Davies et al., 2011). Studies of terrestrial lava lakes can help us understand how lava lakes might behave on Io and what their composition is likely to be (e.g., Davies et al., 2011;Radebaugh et al., 2016). ...
... The Pele volcano on Io has not been imaged at high resolution in visible wavelengths, and so its appearance is not well characterized (Fig. 16). Morphologically, we can say little about what type of volcano Pele is, although we do know that this landform has been a constant source of high, short-wavelength thermal output when observed from Earth or spacecraft (e.g., Rathbun et al., 2004;Radebaugh et al., 2004;Howell and Lopes, 2011;Davies et al., 2012). Eruptions from Pele appear to be highly confined as they are located in the same place every time activity is observed (e.g., Rathbun et al., 2004;Spencer et al., 2007). ...
Active lava lakes are rare on Earth, with only ten documented examples, all formed by lavas of basaltic composition and housed inside summit craters or calderas. The existence of lava lakes on other planetary bodies may imply similarities in either composition or volcano-tectonic settings, and so has important implications for understanding the link between melt production and volcanism. We review lava lakes on Earth and other planets, particularly active caldera-like features interpreted to be currently containing active lava lakes on Jupiter's moon Io, and features interpreted as remnants of lava lakes on Venus and Mars. Mercury and the Moon do not boast the major calderas or shield volcanoes of their larger rocky neighbors, partly due to a horizontally compressive tectonic regime arising from global contraction that makes it difficult to move melts through the upper crust. We discuss the evidence for active lava lakes on Io and show how modeling based on terrestrial lava lakes can reveal how these phenomena differ on both bodies; the superficial similarities do not necessarily imply that the plumbing is similar. Observations of the largest lava lake on Io, Loki Patera, provide insight into the nature of Ionian lava lakes in general, which may be more similar to eruptive episodes on the East Pacific Rise on Earth, which lead to temporary lava lakes. Although temporal data for Io's lava lake activity are scarce, studies of temporal variability of lava lakes on Earth are useful for providing ground truth for comparisons. Future studies of Earth by remote sensing and field observations, and of Io by both ground-based observations and future missions, are needed to answer many questions, including why lava lakes, rare on Earth, appear to be common on Io.
... The best Galileo camera and infrared spectrometer data can only constrain the lower limit of the likely temperature range present to be in excess of about 1400 K. The analysis of Galileo Solid State Imaging experiment (SSI) data yielded a minimum temperature of~1400 K (Keszthelyi et al., 2001;Milazzo et al., 2005;Radebaugh et al., 2004) at a number of locations on Io's surface. Near Infrared Mapping Spectrometer (NIMS) data suggested temperatures close to 1400 K at the Pele lava lake and possibly ultramafic temperatures at Pillan (Davies et al., 2001). ...
... Previously (e.g., Davies et al., 2011aDavies et al., , 2011bDavies et al., , 2016, it was noted that only certain styles of volcanic eruption are suitable for the purpose of measuring eruption temperature, viz., modes of eruption where relatively large areas close to initial eruption temperatures are isolated from cooler areas. For Io, these eruption styles include: large lava fountains (Davies, 1996;de Kleer et al., 2014;Milazzo et al., 2005); smaller fountains in overturning lava lakes (Davies et al., 2011a;Davies et al., 2001;Radebaugh et al., 2004); and lava tube skylights (Davies et al., 2016). For small pulsating lava fountains in an active, overturning basalt lava lake (Erta'Ale volcano, Ethiopia) Davies et al. (2011aDavies et al. ( , 2011b modelled the effect of an induced time delay in the acquisition of imaging data at different wavelengths on the derivation of eruption temperature. ...
... Pele is not actually located within a patera but is in a graben-like feature adjacent to Pele Patera. Some of the high spatial resolution data obtained by Galileo SSI (Radebaugh et al., 2004) (Fig. 5) and NIMS (Davies et al., 2001) were saturated, but data analysis of the unsaturated data allowed the highest temperatures present to be established. This temperature is in excess of about 1400 K. Other possible candidates for active and overturning lava lakes include Tupan Patera (Davies and Ennis, 2011) and Janus Patera . ...
The highly variable and unpredictable magnitude of thermal emission from evolving volcanic eruptions creates saturation problems for remote sensing instruments observing eruptions on Earth and on Io, the highly volcanic moon of Jupiter. For Io, it is desirable to determine the temperature of the erupting lavas as this measurement constrains lava composition. One method of determining lava eruption temperature is by measuring radiant flux at two or more wavelengths and fitting a blackbody thermal emission function. Only certain styles of volcanic activity are suitable, those where detectable thermal emission is from a restricted range of surface temperatures close to the eruption temperature. Volcanic processes where this occurs include large lava fountains; smaller lava fountains common in active lava lakes; and lava tube skylights. Problems that must be overcome to obtain usable data are: (1) the rapid cooling of the lava between data acquisitions at different wavelengths, (2) the unknown magnitude of thermal emission, which has often led to detector saturation, and (3) thermal emission changing on a shorter timescale than the observation integration time. We can overcome these problems by using the HOT-BIRD detector and a novel, advanced digital readout circuit (D-ROIC) to achieve a wide dynamic range sufficient to image lava on Io without saturating. We have created an instrument model that allows various instrument parameters (including mirror diameter, number of signal splits, exposure duration, filter band pass, and optics transmissivity) to be tested to determine the detectability of thermal sources on Io's surface. We find that a short-wavelength infrared instrument on an Io flyby mission can achieve simultaneity of observations by splitting the incoming signal for all relevant eruption processes and still obtain data fast enough to remove uncertainties in accurate determination of the highest lava surface temperatures. Observations at 1 and 1.5 μm are sufficient for this purpose. Even with a ten-way beam split, instrument throughput generates acceptable signal-to-noise values. Accurate constraints on lava eruption temperature are also possible with a visible wavelength detector so long as data at different wavelengths are obtained simultaneously and integration time is very short. Fast integration times are important for examining the thermal emission from lava tube skylights due to rapidly changing viewing geometry during close flybys. The technology described here is applicable to instruments observing terrestrial volcanism and for investigating proposed volcanic activity on Venus, where lava composition is not known.
... Remote observations of lava lakes can be directly compared with observations of volcanoes on Jupiter's moon Io (McEwen et al., 2000;Davies et al., 2001;Radebaugh et al., 2004;Marchis et al., 2005;Howell and Lopes, 2007;Davies et al., 2011) many of which appear to be lava lakes (Lopes et al., 2004). Only remote observations are possible of Io's volcanoes, from spacecraft in close proximity to Io or from space-based or ground-based Earth telescopes. ...
... The determination of Io's eruptions as being lava lakes is based on the observations that they are sources of persistent, high heat flow (Lopes et al., 2004;Davies et al., 2001), exhibit temperatures consistent with predicted magma temperatures (McEwen et al., 1998;Radebaugh et al., 2004;Milazzo et al., 2005;Allen et al., 2013), are highly confined, not moving position from one observation to another (Lopes et al., 2004) and are within topographic lows, when observation resolution permits morphologic interpretation. Although temporal coverage is poor compared to the data we have for Earth, observations ranging from 1979 (Voyager spacecraft encounters) to -2001 and 2007 (New Horizons), interspersed with observations from Earthbased telescopes show that some, if not most, of Io's volcanoes appear to be persistently active (e.g., de Kleer and de Pater, 2015). ...
... In contrast, Pele is a volcano on Io that has never been imaged at high resolution in visible wavelengths (Fig. 12) but is a consistent source of high, short-wavelength thermal output when observed from Earthbased telescopes as well as in flybys by multiple spacecraft (Pearl and Sinton, 1982;McEwen et al., 1998;Rathbun et al., 2004;Lopes et al., 2004;Radebaugh et al., 2004;Marchis et al., 2005;Spencer et al., 2007;Davies et al., 2012). The temperatures observed at Pele (1147°C + −100°C) are consistent with predicted eruption temperatures for lavas on Io, indicating the presence of exposed lavas . ...
Observations from field remote sensing of the morphology, kinematics and temperature of the Marum/Mbwelesu lava lake in the Vanuatu archipelago in 2014 reveal a highly active, vigorously erupting lava lake. Active degassing and fountaining observed at the ~ 50 m lava lake led to large areas of fully exposed lavas and rapid (~ 5 m/s) movement of lava from the centers of upwelling outwards to the lake margins. These rapid lava speeds precluded the formation of thick crust; there was never more than 30% non-translucent crust. The lava lake was observed with several portable, handheld, low-cost, near-infrared imagers, all of which measured temperatures near 1000 °C and one as high as 1022 °C, consistent with basaltic temperatures. Fine-scale structure in the lava fountains and cooled crust was visible in the near infrared at ~ 5 cm/pixel from 300 m above the lake surface. The temperature distribution across the lake surface is much broader than at more quiescent lava lakes, peaking ~ 850 °C, and is attributed to the highly exposed nature of the rapidly circulating lake. This lava lake has many characteristics in common with other active lava lakes, such as Erta Ale in Ethiopia, being confined, persistent and high-temperature; however it was much more active than is typical for Erta Ale, which often has > 90% crust. Furthermore, it is a good analogue for the persistent, high-temperature lava lakes contained within volcanic depressions on Jupiter's moon Io, such as Pele, also believed from spacecraft and ground-based observations to exhibit similar behavior of gas emission, rapid overturn and fountaining.
... Remote observations of lava lakes can be directly compared with observations of volcanoes on Jupiter's moon Io (McEwen et al., 2000;Davies et al., 2001;Radebaugh et al., 2004;Marchis et al., 2005;Howell and Lopes, 2007;Davies et al., 2011) many of which appear to be lava lakes (Lopes et al., 2004). Only remote observations are possible of Io's volcanoes, from spacecraft in close proximity to Io or from space-based or ground-based Earth telescopes. ...
... The determination of Io's eruptions as being lava lakes is based on the observations that they are sources of persistent, high heat flow (Lopes et al., 2004;Davies et al., 2001), exhibit temperatures consistent with predicted magma temperatures (McEwen et al., 1998;Radebaugh et al., 2004;Milazzo et al., 2005;Allen et al., 2013), are highly confined, not moving position from one observation to another (Lopes et al., 2004) and are within topographic lows, when observation resolution permits morphologic interpretation. Although temporal coverage is poor compared to the data we have for Earth, observations ranging from 1979 (Voyager spacecraft encounters) to -2001 and 2007 (New Horizons), interspersed with observations from Earthbased telescopes show that some, if not most, of Io's volcanoes appear to be persistently active (e.g., de Kleer and de Pater, 2015). ...
... In contrast, Pele is a volcano on Io that has never been imaged at high resolution in visible wavelengths (Fig. 12) but is a consistent source of high, short-wavelength thermal output when observed from Earthbased telescopes as well as in flybys by multiple spacecraft (Pearl and Sinton, 1982;McEwen et al., 1998;Rathbun et al., 2004;Lopes et al., 2004;Radebaugh et al., 2004;Marchis et al., 2005;Spencer et al., 2007;Davies et al., 2012). The temperatures observed at Pele (1147°C + −100°C) are consistent with predicted eruption temperatures for lavas on Io, indicating the presence of exposed lavas . ...
We documented eruption activity at three primary vents at Yasur volcano, Tanna Island, Vanuatu using portable instrumentation in the field over a period of 5 h on 21 May 2014, and acquired aerial images of the craters and vents on 22 May 2014. Although limited in duration, our observations of eruption intervals, durations, temperatures, and speeds of ejected material illustrate the characteristics of the activity at the time at each of the primary vents, providing a useful snapshot of eruption behavior and revealing continued variability at Yasur in comparison to other observation campaigns. Hand-held, high-resolution, near-infrared observations of one of the vents gave peak temperatures of 850 °C to 930 °C for ejected clasts, with a maximum temperature of 1033 °C. These temperatures are significantly higher than previous measurements because exposed lavas could be resolved at timescales less than a second. Our aerial near-infrared images allowed us to estimate the combined area of the active vents within the crater to be ~ 150 m2, and comparison to MODIS radiance measurements in the same time frame yields temperatures, averaged over the combined vent area, of 530–730 °C. In the context of previous observations at Yasur, the activity in May 2014 exhibited lower overall intensity, as well as differences in the nature of the eruptions at the various vents, providing insight regarding the temporal variability of Yasur's activity.
... In addition, Galileo data indicated regions with SO 2 seepage that was triggered by nearby volcanic activity. Eruptions of large plumes such as in Io's prominent volcano Pele are frequent and variable with short (minutes) outbursts of activity and some events ongoing over weeks to years (e.g., Radebaugh et al. 2004). These large plumes produced orange or red oval annuli that are stretched in northsouth directions with maximum radii about 500-550 km. ...
... Using direct IR temperature measurements and the temperature-dependent color of elemental sulfur modifications, temperatures of around 650K could be associated with the regions of the large plumes (McEwen and Soderblom 1983). Galileo and Cassini observations indicated that the large region near Pele is ringed with bright hotspots that are relatively cool (< 800 K) but that the strongest thermal emissions reach 1500 ± 80 K (from the Cassini Imaging Science Subsystem, ISS, data) and 1605 ± 220 and 1420 ± 100 K (from a subset of Galileo Solid State Imaging, SSI, data) which are interpreted as regions of Pele's lava lake and upwelling and fountaining of basaltic lava at the lake's margins (see Radebaugh et al. 2004). ...
We review sulfur chemistry of the gas giant planets and their moons where sulfur compounds are observed. The major S-bearing gas in the upper atmospheres of the giant planets is H2S and is removed from their observable atmospheres by condensation into cloud layers (NH4SH on all four planets and additionally H2S ice on Uranus and Neptune). Any remaining H2S at higher altitudes is destroyed photochemically. Among the moons Io is the world dominated by sulfur. We summarize the sulfur cycle on Io and how pyrovolcanism is spreading sulfur across the Jovian system. Implantation of sulfur into icy surfaces of the other Galilean moons via magnetospheric transfer and radiolysis are major processes affecting the sulfur chemistry on their icy surfaces. On the icy worlds, we are literally looking at the top of the icebergs. Subsurface liquid salty bodies reveal themselves through cryovolcanism on Europa, Ganymede, and Enceladus, where salt deposits are indicated. Subsurface oceans are suspected on several other moons. We summarize the sulfur cycle for the icy Galilean moons. The occurrence of sulfates can be explained by salt exchange reactions of radiolytically produced H2SO4 with brine salts (carbonates and halides), or from a subsurface ocean that has become acidified by uptake of H2SO4 leaked from ice. In the primordial oceans of the moons that accreted with high ice rock ratios, sulfur is expected as sulfide and bisulfide anions and H2S in aqueous solution. Cosmochemical constraints suggest that pyrrhotite, tochilinite and green rusts could be important sulfide bearing compounds found with hydrous silicates such as serpentine, and magnetite on the sea floors. In N-C-rich worlds such as Titan, sulfides such as NH4SH and possibly thiazyl compounds could be important, and sulfates are unstable. Nothing is known about the sulfur chemistry on the Uranian and Neptunian moons.
... The viewing angle into the skylight had some effect on the obtained pixel brightnesses and resultant brightness temperatures. This has been noted to be important on Earth [29,21] and on Jupiter's moon Io [30]. Observations into the same skylight from a different viewing angle (Fig. 4d) show temperatures of 1208°C for the hottest material in the open channel. ...
... Similar characteristics for an imaging system -high spatial resolution, rapid image acquisition, and the ability to obtain eruption temperatures -are necessary attributes for imaging active volcanoes on Jupiter's moon Io [44,25]. Volcanoes are constantly erupting across the body in lava flows, tubes, and fountaining lava lakes [45,46,30,47]. Lavas there are mafic to ultramafic silicate in composition, so they require imaging in the same spectral range as the Handycam -visible to short-wavelength infrared -in order to obtain accurate eruption temperatures at appropriate image resolutions (with the addition of multi-color data; [44,25]). ...
Remote thermal monitoring of active volcanoes has many important applications for terrestrial and planetary volcanic systems. In this study, we describe observations of active eruptions on Kilauea and Erta Ale volcanoes using a short-wavelength, high-resolution, consumer digital camcorder and other non-imaging thermal detectors. These systems revealed brightness temperatures close to the eruption temperatures and temperature distributions, morphologies and thermal structures of flow features, tube systems and lava fountains. Lava flows observed by the camcorder through a skylight on Kilauea had a peak in maximum brightness temperatures at 1230 °C and showed brightness temperature distributions consistent with most rapid flow at the center. Surface brightness temperatures of cooling lava flows on Kilauea were close to 850 °C. Centimeter-scale thermal features are evident around pahoehoe ropes and inflated flows and stalactites in lava tubes. Observations of the fountaining Erta Ale lava lake in February 2011 extend the baseline of observations of the eruptive episode begun in late 2010. We observed a fountain using the camcorder and found a peak in maximum brightness temperatures at 1164 °C, consistent with previous studies. Steep temperature gradients were observed across centimeter-scale distances between the highly exposed fountain and cracks and the much cooler lava lake surface and crater walls. The instrument and methods described here lead to robust pictures of the temperatures and temperature distributions at these volcanoes and reveal desired characteristics of planetary remote sensing platforms for the study of volcanically active bodies such as Io.
... At 30 km  20 km, the average diameter of Pele's patera is relatively small compared to the ionian mean patera diameter of 41 km, but its vigorous volcanic activity is evident from every observation of Pele during the Voyager and Galileo missions ( Spencer and Schneider, 1996) and the New Horizons spacecraft ( Spencer et al., 2007). Despite its small diameter, temperature estimates of Pele's caldera surface from spectral analysis are consistently above 1300 K, and its thermal output of $230 GW is one of the greatest on Io ( Radebaugh et al., 2001Radebaugh et al., , 2004Howell and Lopes, 2011). Allen et al. (2013) determined that Loki and Wayland paterae have a similar thermal output to Pele. ...
... The observed thermal output of the surface of Pele's lava lake constrains the magma temperatures and supplies insight to the probable magma compositions. Evaluations of temperature data from Cassini, Voyager, Galileo, and the Hubble Space Telescope (HST), including observations of the plume and the lava-covered patera surface, are consistently within the range of 1200-1600 K ( Radebaugh et al., 2004;Howell and Lopes, 2011;Allen et al., 2013) similar to basaltic through ultramafic magmas on Earth. Zolotov and Fegley (2000) determined a temperature of 1440 (±40) K and an oxygen fugacity À11.5 (±0.1) log 10 units for the magma in Pele's lava lake through HST observations of the sulfurous components contained within the plume and measured SO 2 , SO, and S n concentrations ( Spencer et al., 2000;McGrath et al., 2000). ...
... The styles of eruption on Io generally had to be inferred from very-low-spatial-resolution data, with high-spatial-resolution observations by the SSI camera and NIMS being critical in testing these inferences at key eruptions. For example, data collected from the late Galileo flybys showed a probable lava lake at Pele (Radebaugh et al., 2004); insulated, pahoehoe-like flows at Prometheus and Amirani (Keszthelyi et al., 2001); open channel flows at Pillan (Williams et al., 2001a); and lava fountains at Pillan and in Tvashtar Paterae (Keszthelyi et al., 2001;Milazzo et al., 2005). The inferred styles of eruption on Io cover all the main types of mafic eruption on Earth, but often at spatial scales much larger than those witnessed terrestrially (e.g., Davies, 2007). ...
... The lava is somehow laterally confined, suggesting that Pele is most probably a large, active, overturning, pit-confined lava lake (Davies et al., 2001). High-spatial-resolution SSI observations showed that the margin and center of the lake are being constantly disrupted (McEwen et al., 2000;Radebaugh et al., 2004). To date, Pele is the only volcano on Io to exhibit such a consistent Fig. 3. ...
Volcanic eruptions on Io and Earth are monitored by a variety of thermal
remote sensing instruments. While higher resolution data are always
desirable, we have developed methodologies to constrain the style of
volcanic eruption using low spatial, spectral, and temporal resolution
data. For the volcanic moon Io, this is necessitated by the limits of
spacecraft and Earth-based telescopic observations. Eruption style can
be classified using the concept of "thermal signature" which focuses on
the temporal evolution of thermal emission spectra [1]. We find that the
ratio of the emission at 2 µm and 5 µm, and how this ratio
changes temporally, is often diagnostic of effusive eruption style, even
in low spatial resolution data [2]. Tests using ground-based thermal
data for terrestrial “ground truth” cases show that this
classification system is equally valid for Earth. A square meter of an
active lava lake on Io looks very similar to a square meter of an active
lava lake on Earth. The same goes for pahoehoe flows. This validation of
“thermal signature” means that appropriate physical models
can be selected to interpret the data. On Io, the scale of eruptions can
utterly dwarf their terrestrial counterparts. “Outburst”
eruptions, known to be caused by extensive lava fountaining, can radiate
>1013 W. The smallest thermal anomalies detected on Io in thermal
infrared data are still larger than any contemporaneous mafic volcanic
activity on Earth. The large volumes of lava erupted on Io (e.g., >56
km3 at Pillan in 1997) are an expression of internal tidal heating. It
may be that high compressive stresses in the lower lithosphere inhibit
magma ascent, and so only relatively large volumes of magma can overcome
this “stress barrier” and reach the surface. The results of
the “thermal signature” analysis [2] can be used as an aid
in the planning of future space-borne instruments that can be used for
volcano monitoring on Io, as well as on Earth. This work was performed
at the Jet Propulsion Laboratory-California Institute of Technology,
under NASA contract, with support from the NASA Outer Planets Research
Program. © 2009. All rights reserved. References: [1] Davies, A.
G., 2007, Volcanism on Io - A Comparison with Earth, Cambridge
University Press, 372 pages. [2] Davies, A. G., Keszthelyi L. P., and
Harris, A. J. L., 2009, The Thermal Signature of Volcanic Eruptions on
Io and Earth, JVGR, submitted.
... In Figure 1 we show four examples of the hotspots/lava lakes analysed in this study. The level of detail present in these images of thermal emissions on Io is unprecedented except in a few high-resolution Galileo images of Pele and another region Radebaugh et al. 2004). Note the continuous, bright "hot rings" that correlate with the outlines of paterae wall as seen in Galileo and Voyager images. ...
We report recent observations of lava lakes within patera on Io made by the JIRAM imager/spectrometer on board the Juno spacecraft, taken during close observation occurred in the extended mission. At least 40 lava lakes have been identified from JIRAM observations. The majority (>50%) of paterae have elevated thermal signatures when imaged at sufficiently high spatial resolution (a few km/pixel), implying that lava lakes are ubiquitous on Io. The annular width of the spattering region around the margins, a characteristic of lava lakes, is of the order of few meters to tens of meters, the diameter of the observed lava lakes ranges from 10 to 100 km. The thickness of the crust in the center of some lava lakes is of the order of 5-10 m; we estimate that this crust is a few years old. Also, the bulk of the thermal emission comes from the much larger crust and not from the smaller exposed lava, so the total power output cannot be calculated from the 5-um radiance alone. Eight of the proposed lava lakes have never been reported previously as active hotspots.
... Io has been previously observed by generations of groundand space-based missions, including the Keck Observatory at Maunakea (e.g., Marchis et al. 2005;de Pater et al. 2016), the Adaptive Optics Near Infrared System in Chile (e.g., Howell et al. 2001, and references therein), the Hubble Space Telescope (e.g., Ballester et al. 1994;Clarke et al. 1994;Wannawichian et al. 2010), Pioneer 10 and 11 (e.g., Anderson et al. 1974;Carlson & Judge 1974;Van Allen et al. 1974), Voyager 1 and 2 (e.g., Strom et al. 1981;McEwen & Soderblom 1983;Bagenal 1994), Galileo (e.g., Anderson et al. 1996;Frank et al. 1996;Kivelson et al. 1996;Geissler 2003), Cassini (e.g., Radebaugh et al. 2004), New Horizons (e.g., Spencer et al. 2007;Tsang et al. 2014), and Juno (e.g., Mura et al. 2020;Zambon et al. 2023). Despite the myriad of historical observations, many open questions remain regarding its interior structure, the processes through which tidal heat is dissipated from its interior, the nature of its volcanism, and the characteristics of its atmosphere and space environment. ...
A mission to Jupiter's moon Io, the most volcanically active body in the solar system, was suggested as a priority for the New Frontiers program in the 2013 Planetary Science Decadal Survey. We present a New Frontiers–class mission concept, Vulcan, that was designed as an educational exercise through the Jet Propulsion Laboratory’s 2022 Planetary Science Summer School. Vulcan would leverage an instrument suite consisting of wide- and narrow-angle cameras, a thermal infrared spectrometer, two fluxgate magnetometers, and ion and electron electrostatic analyzers to conduct the most thorough investigation of Io to date. Using 78 flybys over a 2 yr primary science mission, Vulcan would characterize the effects of tidal forces on the differentiation state, crustal structure, and volcanism of Io and investigate potential interactions between Io's volcanoes, surface features, and atmosphere. Although Vulcan was developed as an academic exercise, we show that a New Frontiers–class mission to Io could achieve transformative science in both geophysics and plasma physics, unifying typically disparate subfields of planetary science. A dedicated mission to Io, in combination with the Europa Clipper and Jupiter Icy Moons Explorer missions, would address fundamental questions raised by the 2023 Planetary Science Decadal Survey and could complete our understanding of the spectrum of planetary habitability. Lessons learned from Vulcan could be applied to a New Frontiers 5 Io mission concept in the near future.
... Io has been observed by other flyby missions: Cassini in 2000 and New Horizons in 2007. Both obtained data of Io's hot spots (e.g., Radebaugh et al. 2004;Spencer et al. 2007). Since 2017, Io has been imaged regularly by instruments on the JIRAM spacecraft (Adriani et al. 2014;Bolton et al. 2017;Hansen et al. 2017;Mura et al. 2020;Becker 2021;Davies et al. 2024). ...
By combining multiple spacecraft and telescope data sets, the first fully global volcanic heat flow map of Io has been created, incorporating data down to spatial resolutions of ∼10 km pixel ⁻¹ in Io’s polar regions. Juno Jovian Infrared Auroral Mapper data have filled coverage gaps in Io’s polar regions and other areas poorly imaged by Galileo instruments. A total of 343 thermal sources are identified in data up to mid-2023. While poor correlations are found between the longitudinal distribution of volcanic thermal emission and radially integrated end-member models of internal heating, the best correlations are found with shallow asthenospheric tidal heating and magma ocean models and negative correlations with the deep-mantle heating model. The presence of polar volcanoes supports, but does not necessarily confirm, the presence of a magma ocean on Io. We find that the number of active volcanoes per unit area in polar regions is no different from that at lower latitudes, but we find that Io’s polar volcanoes are smaller, in terms of thermal emission, than those at lower latitudes. Half as much energy is emitted from polar volcanoes as from those at lower latitudes, and the thermal emission from the north polar cap volcanoes is twice that of those in the south polar cap. Apparent dichotomies in terms of volcanic advection and resulting power output exist between sub- and anti-Jovian hemispheres, between polar regions and lower latitudes, and between the north and south polar regions, possibly due to internal asymmetries or variations in lithospheric thickness.
... was later observed in much greater detail by the NASA Galileo mission in the period 1996-2003 (Lopes-Gautier et al., 1999;McEwen et al., 2000), and was observed from a large distance by the NASA-ESA-ASI Cassini Orbiter (2000) on its way to Saturn (Radebaugh et al., 2004). In 2007, the NASA New Horizons spacecraft also acquired images of Io, highlighting the eruption of the volcano Tvashtar . ...
In this work, we present the most updated catalog of Io hot spots based on Juno/JIRAM data. We find 242 hot spots, including 23 previously undetected. Over the half of the new hot spots identified, are located at high northern and southern latitudes (>70°). We observe a latitudinal variability and a larger concentration of hot spots in the polar regions, in particular in the North. The comparison between JIRAM and the most recent Io hot spot catalogs listing power output (Veeder et al., 2015, https://doi.org/10.1016/j.icarus.2014.07.028; de Kleer, de Pater, et al., 2019, https://doi.org/10.3847/1538-3881/ab2380), shows JIRAM detected 63% and 88% of the total number of hot spots, respectively. Furthermore, JIRAM observed 16 of the 34 faint hot spots previously identified. JIRAM data revealed thermal emission from 5 dark pateræ inferred to be active from color ratio images, thus confirming that these are hot spots.
... At Pele, a Galileo imager observation revealed a sinuous feature >10 km long that had intermittently spaced hot spots. This has been interpreted as being fountaining activity along the edge of a lava lake (McEwen et al., 2000;Radebaugh et al., 2004). Alternatively, these hot spots may be lava tube skylights. ...
We provide the first solar system wide compendium of speleogenic processes and products. An examination of 15 solar system bodies revealed that six cave‐forming processes occur beyond Earth including volcanic (cryo and magmatic), fracturing (tectonic and impact melt), dissolution, sublimation, suffusion, and landslides. Although no caves (i.e., confirmed entrances with associated linear passages) have been confirmed, 3,545 SAPs (subsurface access points) have been identified on 11 planetary bodies and the potential for speleogenic processes (and thus SAPs) was observed on an additional four planetary bodies. The bulk of our knowledge on extraterrestrial SAPs is based on global databases for the Moon and Mars, which are bodies for which high‐resolution imagery and other data are available. To further characterize most of the features beyond the Moon and Mars, acquisition (preferably global coverage) and subsequent analysis of high‐resolution imagery will be required. The next few decades hold considerable promise for further identifying and characterizing caves across the solar system.
... Ethiopia's Erta Ale volcano hosts a 150 m diameter lava lake [19] within the 0.7 × 1.6 km elliptical caldera, active since at least the 1960s [19][20][21] and a useful analog for lava lakes on Io, a volcanically active moon of Jupiter [22,23]. The terrain surrounding the volcano is dominated by previous flows, evaporite deposits, and some ephemeral lakes and is generally devoid of vegetation [20]. ...
Identifying volcanic activity on worlds with optically thick atmospheres with passive microwave radiometry has been proposed as a means of skirting the atmospheric interference that plagues near infrared observations. By probing deeper into the surface, microwave radiometers may also be sensitive to older flows and thus amenable for investigations where repeat observations are infrequent. In this investigation we explore the feasibility of this tactic using data from the Soil Moisture Active Passive (SMAP) mission in three case studies: the 2018 Kilauea eruption, the 2018 Oct-Nov eruption at Fuego, and the ongoing activity at Erta Ale in Ethiopia. We find that despite SMAP’s superior spatial resolution, observing flows that are small fractions of the observing footprint are difficult to detect—even in resampled data products. Furthermore, the absorptivity of the flow, which can be temperature dependent, can limit the depths to which SMAP is sensitive. We thus demonstrate that the lower limit of detectability at L-band (1.41 GHz) is in practice higher than expected from first principles.
... Most hot spots showed short-timescale stability, but in at least one case, at Marduk Fluctus, there were large changes in thermal emission (both waxing and waning) in the space of a few minutes over wavelengths from 1 to 5 µm; this event was modeled as a large explosive eruption event that briefly generated large areas at very high temperatures and small clasts that rapidly cooled (Davies et al., 2018). The Cassini Imaging Science Subsystem (ISS) observed Io at low spatial resolution but high temporal resolution during a distant flyby in 1999 (e.g., Radebaugh et al., 2004). Some hot spots, including Pele, showed rapid variability, but this might be expected from an instrument sensitive only to the hottest (and therefore most rapidly cooling) surface present. ...
Tidal heating is key to the evolution and habitability of many worlds across our solar system and beyond. However, there remain fundamental gaps in our understanding of tidal heating and coupled orbital evolution, which motivated a Keck Institute for Space Studies (KISS) workshop on this topic. The Cassini mission has led to many recent results about ocean worlds and what may become a new paradigm for understanding orbital evolution with tidal heating, the model of resonance locking in the parent planet (Fuller et al., 2016). Resonance locking explains how subsurface oceans may persist over much of geologic time, even in tiny Enceladus. The discovery of the Laplace resonance of Io, Europa, and Ganymede orbiting Jupiter led to the prediction of intense tidal heating of Io (Peale et al., 1979); this system provides the greatest potential for advances in the next few decades. Europa Clipper and JUpiter ICy moons Explorer (JUICE) will provide in-depth studies of Europa and Ganymede in the 2030s. The easily observed heat flow of Io, from hundreds of continually erupting volcanoes, makes it an ideal target for further investigation, and the missing link—along with missions in development—to understand the Laplace system. We identified five key questions to drive future research and exploration: (Q1) What do volcanic eruptions tell us about the interiors of tidally heated bodies (e.g., Io, Enceladus, and perhaps Europa and Triton)? (Q2) How is tidal dissipation partitioned between solid and liquid materials? (Q3) Does Io have a melt-rich layer, or “magma ocean”, that mechanically decouples the lithosphere from the deeper interior? (Q4) Is the Jupiter/Laplace system in equilibrium (i.e., does the satellite’s heat output equal the rate at which energy is generated)? (Q5) Can stable isotope measurements inform long-term evolution of tidally heated bodies? The most promising avenues to address these questions include a new spacecraft mission making close flybys of Io, missions orbiting and landing on key worlds such as Europa and Enceladus, technology developments to enable advanced techniques, closer coupling between laboratory experiments and tidal heating theory, and advances in Earth-based telescopic observations of solar system and extrasolar planets and moons. All of these avenues would benefit from technological developments. An Io mission should: characterize volcanic processes (Q1); test interior models via a set of geophysical measurements coupled with laboratory experiments and theory (Q2 and Q3); measure the rate of Io’s orbital migration (to complement similar measurements expected at Europa and Ganymede) to determine if the Laplace resonance is in equilibrium (Q4); and determine neutral compositions and measure stable isotopes in Io’s atmosphere and plumes (Q5). No new technologies are required for such an Io mission following advances in radiation design and solar power realized for Europa Clipper and JUICE. Seismology is a promising avenue for future exploration, either from landers or remote laser reflectometry, and interferometric synthetic aperture radar (InSAR) could be revolutionary on these active worlds, but advanced power systems plus lower mass and power-active instruments are needed for operation in the outer solar system.
... Most hot spots showed short-timescale stability, but in at least one case, at Marduk Fluctus, there were large changes in thermal emission (both waxing and waning) in the space of a few minutes over wavelengths from 1 to 5 µm; this event was modeled as a large explosive eruption event that briefly generated large areas at very high temperatures and small clasts that rapidly cooled (Davies et al., 2018). The Cassini Imaging Science Subsystem (ISS) observed Io at low spatial resolution but high temporal resolution during a distant flyby in 1999 (e.g., Radebaugh et al., 2004). Some hot spots, including Pele, showed rapid variability, but this might be expected from an instrument sensitive only to the hottest (and therefore most rapidly cooling) surface present. ...
Tidal heating is key to the evolution and habitability of many worlds across our solar system and beyond. However, there remain fundamental gaps in our understanding of tidal heating and coupled orbital evolution, which motivated a Keck Institute for Space Studies (KISS) workshop on this topic. The Cassini mission has led to many recent results about ocean worlds and what may become a new paradigm for understanding orbital evolution with tidal heating, the model of resonance locking in the parent planet (Fuller et al., 2016). Resonance locking explains how subsurface oceans may persist over much of geologic time, even in tiny Enceladus. The discovery of the Laplace resonance of Io, Europa, and Ganymede orbiting Jupiter led to the prediction of intense tidal heating of Io (Peale et al., 1979); this system provides the greatest potential for advances in the next few decades. Europa Clipper and JUpiter ICy moons Explorer (JUICE) will provide in-depth studies of Europa and Ganymede in the 2030s. The easily observed heat flow of Io, from hundreds of continually erupting volcanoes, makes it an ideal target for further investigation, and the missing link—along with missions in development—to understand the Laplace system. We identified five key questions to drive future research and exploration: (Q1) What do volcanic eruptions tell us about the interiors of tidally heated bodies (e.g., Io, Enceladus, and perhaps Europa and Triton)? (Q2) How is tidal dissipation partitioned between solid and liquid materials? (Q3) Does Io have a melt-rich layer, or “magma ocean”, that mechanically decouples the lithosphere from the deeper interior? (Q4) Is the Jupiter/Laplace system in equilibrium (i.e., does the satellite’s heat output equal the rate at which energy is generated)? (Q5) Can stable isotope measurements inform long-term evolution of tidally heated bodies? The most promising avenues to address these questions include a new spacecraft mission making close flybys of Io, missions orbiting and landing on key worlds such as Europa and Enceladus, technology developments to enable advanced techniques, closer coupling between laboratory experiments and tidal heating theory, and advances in Earth-based telescopic observations of solar system and extrasolar planets and moons. All of these avenues would benefit from technological developments. An Io mission should: characterize volcanic processes (Q1); test interior models via a set of geophysical measurements coupled with laboratory experiments and theory (Q2 and Q3); measure the rate of Io’s orbital migration (to complement similar measurements expected at Europa and Ganymede) to determine if the Laplace resonance is in equilibrium (Q4); and determine neutral compositions and measure stable isotopes in Io’s atmosphere and plumes (Q5). No new technologies are required for such an Io mission following advances in radiation design and solar power realized for Europa Clipper and JUICE. Seismology is a promising avenue for future exploration, either from landers or remote laser reflectometry, and interferometric synthetic aperture radar (InSAR) could be revolutionary on these active worlds, but advanced power systems plus lower mass and power-active instruments are needed for operation in the outer solar system.
... Deformation on Io is probably thick-skinned rather than thinskinned because of the very nature of its crust. The crust is most likely dominated by volcanic and intrusive rocks of mafic and ultramafic compositions ( McEwen et al., 1989;Carr et al., 1998;Geissler et al., 1999;McEwen et al., 1998;McEwen et al., 20 0 0;Radebaugh et al., 2004 ). Io's high heat flow probably leads to recrystallization and metamorphism of these mafic protoliths, even at shallow depths, creating a strong "crystalline basement" akin to that of Earth's continents, which is then overlain with thin coverings of volcanic material. ...
... of < 20%, or ultramafic lavas at eruption temperatures exceeding 1800 K (our model uses 1870 K), implying extreme melting and a fluid magma ocean ( Keszthelyi et al., 2007 ). Analysis and reanalysis of primarily Galileo Solid State Imaging experiment (SSI) data yielded a minimum eruption temperature of ∼1400 K McEwen et al., 1998;Milazzo et al., 2005;Radebaugh et al., 2004 ). An Earth-based observation of an outburst in 2013 has been interpreted as suggestive of temperatures ≥1900 K ( de Kleer et al., 2014 ). ...
Determining the eruption temperature of Io's dominant silicate lavas would constrain Io's present interior state and composition. We have examined how eruption temperature can be estimated at lava tube skylights through synthesis of thermal emission from the incandescent lava flowing within the lava tube. Lava tube skylights should be present along Io's long-lived lava flow fields, and are attractive targets because of their temporal stability and the narrow range of near-eruption temperatures revealed through them. We conclude that these skylights are suitable and desirable targets (perhaps the very best targets) for the purposes of constraining eruption temperature, with a 0.9:0.7-μm radiant flux ratio ≤6.3 being diagnostic of ultramafic lava temperatures. Because the target skylights may be small – perhaps only a few m or 10s of m across – such observations will require a future Io-dedicated mission that will obtain high spatial resolution (<100 m/pixel), unsaturated observations of Io's surface at multiple wavelengths in the visible and near-infrared, ideally at night. In contrast to observations of lava fountains or roiling lava lakes, where accurate determination of surface temperature distribution requires simultaneous or near-simultaneous (<<0.1 s) observations at different wavelengths, skylight thermal emission data are superior for the purposes of temperature derivation, as emission is stable on much longer time scales (minutes, or longer), so long as viewing geometry does not greatly change during that time.
... For comparison 250 km 2 is the area of the dark region seen in Voyager images such asFig. 2 of Radebaugh et al. (2004), which they suggest may be a lava lake and the source of the Pele plume. ...
... Because cooling causes solidification and hence a change in rheology from liquid-like to solid-like, the interaction of the plates on the surface of lava lakes has also served as a model for global plate tectonics (e.g., Duffield, 1972). Finally, patterns on terrestrial lava lakes provide a template for interpreting surfaces features on Jupiter's moon Io (e.g., Radebaugh et al., 2004). ...
The distinctive rift patterns observed on newly formed lava lakes are very likely a product of interaction between heat transfer (cooling of lava) and deformation of the solid crust in response to applied stresses. One common pattern consists of symmetric "zigzag" rifts separating spreading plates. Zigzags can be characterized by two measurable parameters: an amplitude A, and an angle θ between segments that make up the zigzags. Similar patterns are observed in analog wax experiments in which molten wax acts as cooling and solidifying lava. We perform a series of these wax experiments to find the relationship between θ, A, and the cooling rate. We develop a model to explain the observed relationships: θ is determined by a balance of spreading and solidification speeds; the amplitude A is limited by the thickness of the solid wax crust. Theoretical predictions agree well with experimental data; this enables us to scale the model to basaltic lava lakes. If zigzag rifts are observed on the surface of lava lakes, and if physical properties of the lava crust can be measured or inferred by other means, measurements of θ and A make it possible to calculate crust-spreading velocity and crust thickness.
The MAJIS (Moons And Jupiter Imaging Spectrometer) instrument on board the ESA JUICE (JUpiter ICy moon Explorer) mission is an imaging spectrometer operating in the visible and near-infrared spectral range from 0.50 to 5.55 μm in two spectral channels with a boundary at 2.3 μm and spectral samplings for the VISNIR and IR channels better than 4 nm/band and 7 nm/band, respectively. The IFOV is 150 μrad over a total of 400 pixels. As already amply demonstrated by the past and present operative planetary space missions, an imaging spectrometer of this type can span a wide range of scientific objectives, from the surface through the atmosphere and exosphere. MAJIS is then perfectly suitable for a comprehensive study of the icy satellites, with particular emphasis on Ganymede, the Jupiter atmosphere, including its aurorae and the spectral characterization of the whole Jupiter system, including the ring system, small inner moons, and targets of opportunity whenever feasible. The accurate measurement of radiance from the different targets, in some case particularly faint due to strong absorption features, requires a very sensitive cryogenic instrument operating in a severe radiation environment. In this respect MAJIS is the state-of-the-art imaging spectrometer devoted to these objectives in the outer Solar System and its passive cooling system without cryocoolers makes it potentially robust for a long-life mission as JUICE is. In this paper we report the scientific objectives, discuss the design of the instrument including its complex on-board pipeline, highlight the achieved performance, and address the observation plan with the relevant instrument modes.
Io’s high internal heat flow powers its dramatic silicate volcanism, and leads to a surface and atmosphere that are dominated by sulfurous volcanic products. Io’s surface is peppered with constantly-changing thermal hot spots, which are active volcanic features including lava lakes, lava fountains, and lava flow fields; such features are analogous to highly-mafic counterparts on Earth, albeit at much larger scales. Our understanding of Io’s hot spots and heat flow has progressed substantially since the end of the Galileo mission due to new telescopic datasets, continuing analyses of spacecraft data, and improvements in theoretical models. This chapter reviews advances in our understanding of Io’s thermal emission, both volcanic and passive, since the last major review in 2007. The major datasets and observational techniques are reviewed, and the results synthesized and discussed in terms of the volcanology of Io and the mechanisms of tidal heating in its interior.
Io is unlike any other body in the Solar System making questions about its chemical composition especially interesting and challenging. This chapter examines the many different, but frustratingly indirect, constraints we have on the bulk composition of this restless moon. A detailed consideration of Io’s lavas is used to illustrate how decades of research have bounded, but not pinned down, the chemistry of Io. A self-consistent model for the core, mantle and crust is constructed based on a conventional chondritic composition but exotic alternatives cannot be ruled out. The study of Io’s composition should provide a fertile and exciting realm for future scientists.
The upper crust of Io may be very rich in volatile sulfur and SO2. The surface is also highly volcanically active, and slopes may be warmed by radiant heat from the lava. This is particularly the case in paterae, which commonly host volcanic eruptions and long-lived lava lakes. Paterae slopes are highly variable, but some are greater than 70°. I model the heating of a volatile slope for two end-member cases: instantaneous emplacement of a large sheet flow, and persistent heating by a long-lived lava lake. In general, single flows can briefly raise sulfur to the melting temperature, or drive a modest amount of sublimation of SO2. Persistently lava-covered surfaces will drive much more significant geomorphic effects, with potentially significant sublimation and slope retreat. In addition to the direct effects, heating is likely to weaken slope materials and may trigger mass wasting. Thus, if the upper crust of Io is rich in these volatile species, future missions with high-resolution imaging are likely to observe actively retreating slopes around lava lakes and other locations of frequent eruptions.
Overview Volcanism is of primary importance in shaping the surfaces of many planets and satellites of the Solar System. In this chapter we show how models developed for volcanic processes on Earth can be adapted to model volcanism on other planetary bodies, including those displaying familiar silicate volcanism (such as Mars, Venus, and the Moon), as well as those with more exotic volcanic behavior (such as high-temperature volcanism on Io and “cryovolcanism” on the icy satellites). Due to space limitations, only certain “type example” worlds are detailed here, the intent is more to give an insight into how the volcanic process varies from body to body than to discuss each. Each planet or satellite possesses a unique combination of environmental factors (gravity, atmospheric properties, surface temperature, etc.) that influence almost every aspect of magma ascent and eruption. By incorporating these parameters into models of volcanic behavior it is possible to elucidate the causes of the diversity in volcanic expression on the surfaces of other planetary bodies and hence understand the eruptive history and evolution of our Solar System neighbors. Introduction Volcanism has affected all solid planets and most moons in the Solar System and even some of the earliest-forming asteroids, and is therefore of key importance for the study of the evolution of planets and moons. The discovery of numerous extra-terrestrial volcanoes, including active ones, has stretched our traditional definition of “volcano” (Lopes et al., 2010a) and prompted a new understanding of how volcanism, as a process, can operate.
The aim of this study is to contribute to a better understanding of Io's surface chemical composition. For this purpose, we combined a thermochemical model of Ionian volcanic gases with a spectroscopic and chemical experimental study of low temperature condensed molecules.
The thermochemical study is carried out using an improved volcanic gases model, in order to predict the most probable molecules ejected by Io's volcanoes and to follow them during their cooling and the condensation of some of them.
The experimental study concerns disulfur monoxide, S2O, a major component predicted by the thermochemical model, and its polymer, polysulfur oxide. We reproduced S2O low temperature condensation, followed its polymerization and measured its infrared spectra in laboratory, under conditions of temperature, pressure, mixing with SO2 and UV-visible radiation simulating Ionian ones.
On the one hand, this study improves our knowledge of S2O and of its polymerization mechanism and gives a better idea of polysulfur oxide's structure. On the other hand, our experiments and our spectroscopic results compared to Io's infrared and visible spectra lead us to the conclusion that S2O can not be responsible for red volcanic deposits on Io and that Io's surface is probably mainly composed of sulfur dioxide and a mixture of sulfur S8 and sulfur polymer. To these dominant components, some polysulfur oxide could be added, possibly localized in more restricted volcanic areas.
Io’s giant Pele plume rises high above the moon’s surface and produces a complex deposition pattern. We use the direct simulation Monte Carlo (DSMC) method to model the flow of SO2 gas and silicate ash from the surface of the lava lake, into the umbrella-shaped canopy of the plume, and eventually onto the surface where the flow leaves black “butterfly wings” surrounded by a large red ring. We show how the geometry of the lava lake, from which the gas is emitted, is responsible for significant asymmetry in the plume and for the shape of the red deposition ring by way of complicated gas-dynamic interactions between parts of the gas flow arising from different areas in the lava lake. We develop a model for gas flow in the immediate vicinity of the lava lake and use it to show that the behavior of ash particles of less than about 2 μm in diameter in the plume is insensitive to the details of how they are introduced into the flow because they are coupled to the gas at low altitudes.
We simulate dust particles in the plume to show how particle size determines the distance from the lava lake at which particles deposit on the surface, and we use this dependence to find a size distribution of black dust particles in the plume that provides the best explanation for the observed black fans to the east and west of the lava lake. This best-fit particle size distribution suggests that there may be two distinct mechanisms of black dust creation at Pele, and when two log-normal distributions are fit to our results we obtain a mean particle diameter of 88 nm. We also propose a mechanism by which the condensible plume gas might overlay black dust in areas where black coloration is not observed and compare this to the observed overlaying of Pillanian dust by Pele’s red ring.
This chapter reviews the origin and fate of sulfur (S) in silicate melts in the solar system, experiments bearing on the role of S in element partitioning among melts and solids in planets, and finally our current understanding of silicate melts and the role of sulfur in planetary evolution. Sulfur is an important component of undifferentiated meteorites that are precursors to planets. When planetary bodies differentiated into cores and mantles, metal and/or sulfides were removed from silicates. This process can be traced. Then, iron-sulfide cores differentiated into metal and metal-sulfide fractions, some of which are preserved as iron meteorites. The iron meteorites probably fractionated from silicate mantles at much lower pressures than the cores of Earth or Mars. Understanding the role of sulfur in silicate melts is critical to unraveling the history of Earth, the terrestrial planets, and the differentiated asteroids that were once parts of early planetesimals.
### Silicate melts and sulfur in primitive source materials
Primitive extraterrestrial samples available for laboratory study include 1–20 μm cometary grains collected by the United States’ (NASA) Stardust mission, asteroidal material collected by the Japanese (JAXA) Hyabusa mission, interplanetary dust particles (IDPs) collected from the stratosphere by airplanes, micrometeorites from various collection sites, and meteorites that fall to Earth and are recovered. Sources of primitive meteorites are parent bodies, primarily asteroids, that did not differentiate into silicate mantles and metal-rich cores. The oldest dated solar system rocks are not bulk meteorites, but are the high-temperature, melted components of undifferentiated meteorites, which are a kind of cosmic sedimentary rock. These “chondritic” meteorites are slightly younger than the components that accreted to form them. They have atomic ratios of rock-forming elements (e.g., Fe/Si) that are very similar to those measured in the solar photosphere using spectroscopy. The “primitive” nature of meteorites is established by their radiometric ages, and their lack of aqueous and …
Long-exposure visible-light images of Augustine Volcano were obtained using a charge-coupled device (CCD) camera during several nights of the 2006 eruption. The camera was located 105 km away, at Homer, Alaska, yet showed persistent bright emissions from the north fl ank of the volcano corre-sponding to steam releases, pyroclastic fl ows, and rockfalls originating near the summit. The apparent brightness of the emissions substantially exceeded that of the background night-time scene. The bright signatures in the images are shown to probably be thermal emissions detected near the long-wave-length limit (~1 μm) of the CCD. Modeling of the emissions as a black-body brightness yields an apparent temperature of 400 to 450°C that likely refl ects an unresolved combination of emissions from hot ejecta and cooler material.
In February 2007, the New Horizons spacecraft flew by the Jupiter system, obtaining images of Io, the most volcanically active body in the Solar System. The Multicolor Visible Imaging Camera (MVIC), a four-color (visible to near infrared) camera, obtained 17 sets of images. The Long-Range Reconnaissance Imager (LORRI), a high-resolution panchromatic camera, obtained 190 images, including many of Io eclipsed by Jupiter. We present a complete view of the discrete point-like emission sources in all images obtained by these two instruments. We located 54 emission sources and determined their brightnesses. These observations, the first that observed individual Ionian volcanoes on short timescales of seconds to minutes, demonstrate that the volcanoes have stable brightnesses on these timescales. The active volcanoes Tvashtar (63N, 124W) and E. Girru (22N, 245W) were observed by both LORRI and MVIC, both in the near-infrared (NIR) and methane (CH4) filters. Tvashtar was additionally observed in the red filter, which allowed us to calculate a color temperature of approximately 1200 K. We found that, with some exceptions, most of the volcanoes frequently active during the Galileo era continued to be active during the New Horizons flyby. We found that none of the seven volcanoes observed by New Horizons multiple times over short timescales showed substantial changes on the order of seconds and only one, E. Girru exhibited substantial variation over minutes to days, increasing by 25% in just over an hour and decreasing by a factor of 4 over 6 days. Observations of Tvashtar are consistent with a current eruption similar to previously observed eruptions and are more consistent with the thermal emission of a lava flow than the fire fountains inferred from the November 1999 observations. These data also present new puzzles regarding Ionian volcanism. Since there is no associated surface change or low albedo feature that could be identified nearby, the source of the emission from E. Girru is a mystery. Furthermore, the in-eclipse glows we observe over many paterae are likely to be gas emission from interaction with the magnetosphere, but the details of that process are not clear.
We have analysed high-spatial-resolution and high-temporal-resolution
temperature measurements of the active lava lake at Erta'Ale volcano,
Ethiopia, to derive requirements for measuring eruption temperatures at
Io's volcanoes. Lava lakes are particularly attractive targets because
they are persistent in activity and large, often with ongoing lava
fountain activity that exposes lava at near-eruption temperature. Using
infrared thermography, we find that extracting useful temperature
estimates from remote-sensing data requires (a) high spatial resolution
to isolate lava fountains from adjacent cooler lava and (b) rapid
acquisition of multi-color data. Because existing spacecraft data of
Io's volcanoes do not meet these criteria, it is particularly important
to design future instruments so that they will be able to collect such
data. Near-simultaneous data at more than two relatively short
wavelengths (shorter than 1 μm) are needed to constrain eruption
temperatures. Resolving parts of the lava lake or fountains that are
near the eruption temperature is also essential, and we provide a rough
estimate of the required image scale.
Cassini spacecraft images of Io obtained during its flyby of Jupiter in late 2000 and early 2001 were used to determine the lava composition and eruption style of three faint hot spots, Pillan, Wayland Patera, and Loki Patera. We found a maximum color temperature of 1130 ± 289 K for Pillan and maximum color temperatures of 1297 ± 289 K and 1387 ± 287 K for Wayland Patera and Loki Patera, respectively. These temperatures are suggestive of basaltic lava but an ultramafic composition cannot be ruled out. The temperatures with the best signal-to-noise ratios also suggested basaltic lava and were found to be 780 ± 189 K, 1116 ± 250 K, and 1017 ± 177 K for Pillan, Wayland Patera, and Loki Patera, respectively. Pillan showed constant thermal output within error over three days of observations. The data also suggest Pillan may be surrounded by topography that blocked emission in the middle of the observation and caused a more dramatic decrease in emission. Wayland Patera’s intensity decreased over the three eclipse observations, consistent with a cooling lava flow or decreasing effusion rate. Intensities at Loki Patera over the course of the observations varied, consistent with previous determinations that Loki Patera is an often quiescent lava lake with periods of overturning, fountaining, and crustal foundering.
Using the NIMS Io Thermal Emission Database (NITED), a collection of over 1000 measurements of radiant flux from Io’s volcanoes (Davies, A.G. et al. [2012]. Geophys. Res. Lett. 39, L01201. doi:10.1029/2011GL049999), we have examined the variability of thermal emission from three of Io’s volcanoes: Pele, Janus Patera and Kanehekili Fluctus. At Pele, the 5-μm thermal emission as derived from 28 night time observations is remarkably steady at 37 ± 10 GW μm−1, re-affirming previous analyses that suggested that Pele an active, rapidly overturning silicate lava lake. Janus Patera also exhibits relatively steady 5-μm thermal emission (≈20 ± 3 GW μm−1) in the four observations where Janus is resolved from nearby Kanehekili Fluctus. Janus Patera might contain a Pele-like lava lake with an effusion rate (QF) of ≈40–70 m3 s−1. It should be a prime target for a future mission to Io in order to obtain data to determine lava eruption temperature. Kanehekili Fluctus has a thermal emission spectrum that is indicative of the emplacement of lava flows with insulated crusts. Effusion rate at Kanehekili Fluctus dropped by an order of magnitude from ≈95 m3 s−1 in mid-1997 to ≈4 m3 s−1 in late 2001.
The Pele region of Io has been the site of vigorous volcanic activity from the time of the first Voyager I observations in 1979 up through the final Galileo ones in 2001. There is high-temperature thermal emission from a visibly dark area that is thought to be a rapidly overturning lava lake, and is also the source of a large sulfur-rich plume. We present a new analysis of Voyager I visible wavelength images, and Galileo Solid State Imager (SSI) and Near Infrared Mapping Spectrometer (NIMS) thermal emission observations which better define the morphology of the region and the intensity of the emission. The observations show remarkable correlations between the locations of the emission and the features seen in the Voyager images, which provide insight into eruption mechanisms and constrain the longevity of the activity. We also analyze an additional wavelength channel of NIMS data (1.87 μm) which paradoxically, because of reduced sensitivity, allows us to estimate temperatures at the peak locations of emission. Measurements of eruption temperatures on Io are crucial because they provide our best clues to the composition of the magma. High color temperatures indicative of ultramafic composition have been reported for the Pillan hot spot and possibly for Pele, although recent work has called into question the requirement for magma temperatures above those expected for ordinary basalts. Our new analysis of the Pele emission near the peak of the hot spot shows color temperatures near the upper end of the basalt range during the I27 and I32 encounters. In order to analyze the observed color temperatures we also present an analytical model for the thermal emission from fire-fountains, which should prove generally useful for analyzing similar data. This is a modification of the lava flow emission model presented in Howell (Howell, R.R. [1997]. Icarus 127, 394–407), adapted to the fire-fountain cooling curves first discussed in Keszthelyi et al. (Keszthelyi, L., Jaeger, W., Milazzo, M., Radebaugh, J., Davies, A.G., Mitchell, K.L. [2007]. Icarus 192, 491–502). When applied to the I32 observations we obtain a fire-fountain mass eruption rate of 5.1 × 105 kg s−1 for the main vent area and 1.4 × 104 kg s−1 for each of two smaller vent regions to the west. These fire-fountain rates suggest a solution to the puzzling lack of extensive lava flows in the Pele region. Much of the erupted lava may be ejected at high speed into the fire-fountains and plumes, creating dispersed pyroclastic deposits rather than flows. We compare gas and silicate mass eruption rates and discuss briefly the dynamics of this ejection model and the observational evidence.
A simple model for the thermal emission from lava fountains helps explain the high lava temperatures seen at Io.
In order to determine the eruption temperature of Io's lavas, imagers need to obtain multispectral data very quickly in order to overcome wild variations in derived temperatures caused by rapid cooling and variation in volcanic activity.
We present a robust model for the crustal density structure of Io, which we use to constrain magma dynamics with emphasis on the Prometheus volcanic center. Preliminary results indicate that magma will pond at shallow depths under a volatile layer.
A model for patera formation is presented that uses our collective observations of Io's paterae and current understanding of Io's interior as the basis for the steps in the model.
The modeling of thermal emission from active lava flows must account for the cooling of the lava after solidification. Models of lava cooling applied to data collected by the Galileo spacecraft have, until now, not taken this into consideration. This is a flaw as lava flows on Io are thought to be relatively thin with a range in thickness from ˜1 to 13 m. Once a flow is completely solidified (a rapid process on a geological time scale), the surface cools faster than the surface of a partially molten flow. Cooling via the base of the lava flow is also important and accelerates the solidification of the flow compared to the rate for the `semi-infinite' approximation (which is only valid for very deep lava bodies). We introduce a new model which incorporates the solidification and basal cooling features. This model gives a superior reproduction of the cooling of the 1997 Pillan lava flows on Io observed by the Galileo spacecraft. We also use the new model to determine what observations are necessary to constrain lava emplacement style at Loki Patera. Flows exhibit different cooling profiles from that expected from a lava lake. We model cooling with a finite-element code and make quantitative predictions for the behavior of lava flows and other lava bodies that can be tested against observations both on Io and Earth. For example, a 10-m-thick ultramafic flow, like those emplaced at Pillan Patera in 1997, solidifies in ˜450 days (at which point the surface temperature has cooled to ˜280 K) and takes another 390 days to cool to 249 K. Observations over a sufficient period of time reveal divergent cooling trends for different lava bodies [examples: lava flows and lava lakes have different cooling trends after the flow has solidified (flows cool faster)]. Thin flows solidify and cool faster than flows of greater thickness. The model can therefore be used as a diagnostic tool for constraining possible emplacement mechanisms and compositions of bodies of lava in remote-sensing data.
The very high temperatures reported for Ionian lavas imply an unrealistic degree of melting of the interior. Improved analysis of the Galileo SSI data and superheating of the ascending magma help resolve this problem.
We produced the first complete, 1:15M-scale global geologic map of Jupiter’s moon Io, based on a set of monochrome and color Galileo–Voyager image mosaics produced at a spatial resolution of 1km/pixel. The surface of Io was mapped into 19 units based on albedo, color and surface morphology, and is subdivided as follows: plains (65.8% of surface), lava flow fields (28.5%), mountains (3.2%), and patera floors (2.5%). Diffuse deposits (DD) that mantle the other units cover ∼18% of Io’s surface, and are distributed as follows: red (8.6% of surface), white (6.9%), yellow (2.1%), black (0.6%), and green (∼0.01%). Analyses of the geographical and areal distribution of these units yield a number of results, summarized below. (1) The distribution of plains units of different colors is generally geographically constrained: Red–brown plains occur >±30° latitude, and are thought to result from enhanced alteration of other units induced by radiation coming in from the poles. White plains (possibly dominated by SO2+contaminants) occur mostly in the equatorial antijovian region (±30°, 90–230°W), possibly indicative of a regional cold trap. Outliers of white, yellow, and red–brown plains in other regions may result from long-term accumulation of white, yellow, and red diffuse deposits, respectively. (2) Bright (possibly sulfur-rich) flow fields make up 30% more lava flow fields than dark (presumably silicate) flows (56.5% vs. 43.5%), and only 18% of bright flow fields occur within 10km of dark flow fields. These results suggest that secondary sulfurous volcanism (where a bright-dark association is expected) could be responsible for only a fraction of Io’s recent bright flows, and that primary sulfur-rich effusions could be an important component of Io’s recent volcanism. An unusual concentration of bright flows at ∼45–75°N, ∼60–120°W could be indicative of more extensive primary sulfurous volcanism in the recent past. However, it remains unclear whether most bright flows are bright because they are sulfur flows, or because they are cold silicate flows covered in sulfur-rich particles from plume fallout. (3) We mapped 425 paterae (volcano-tectonic depressions), up from 417 previously identified by Radebaugh et al. (Radebaugh, J., Keszthelyi, L.P., McEwen, A.S., Turtle, E.P., Jaeger, W., Milazzo, M. [2001]. J. Geophys. Res. 106, 33005–33020). Although these features cover only 2.5% of Io’s surface, they correspond to 64% of all detected hot spots; 45% of all hot spots are associated with the freshest dark patera floors, reflecting the importance of active silicate volcanism to Io’s heat flow. (4) Mountains cover only ∼3% of the surface, although the transition from mountains to plains is gradational with the available imagery. 49% of all mountains are lineated and presumably layered, showing evidence of linear structures supportive of a tectonic origin. In contrast, only 6% of visible mountains are mottled (showing hummocks indicative of mass wasting) and 4% are tholi (domes or shields), consistent with a volcanic origin. (5) Initial analyses of the geographic distributions of map units show no significant longitudinal variation in the quantity of Io’s mountains or paterae, in contrast to earlier studies. This is because we use the area of mountain and patera materials as opposed to the number of structures, and our result suggests that the previously proposed anti-correlation of mountains and paterae (Schenk, P., Hargitai, H., Wilson, R., McEwen, A., Thomas, P. [2001]. J. Geophys. Res. 106, 33201–33222; Kirchoff, M.R., McKinnon, W.B., Schenk, P.M. [2011]. Earth Planet. Sci. Lett. 301, 22–30) is more complex than previously thought. There is also a slight decrease in surface area of lava flows toward the poles of Io, perhaps indicative of variations in volcanic activity. (6) The freshest bright and dark flows make up about 29% of all of Io’s flow fields, suggesting active emplacement is occurring in less than a third of Io’s visible lava fields. (7) About 47% of Io’s diffuse deposits (by area) are red, presumably deriving their color from condensed sulfur gas, and ∼38% are white, presumably dominated by condensed SO2. The much greater areal extent of gas-derived diffuse deposits (red+white, 85%) compared to presumably pyroclast-bearing diffuse deposits (dark (silicate tephra)+yellow (sulfur-rich tephra), 15%) indicates that there is effective separation between the transport of tephra and gas in many Ionian explosive eruptions. Future improvements in the geologic mapping of Io can be obtained via (a) investigating the relationships between different color/material units that are geographically and temporally associated, (b) better analysis of the temporal variations in the map units, and (c) additional high-resolution images (spatial resolutions ∼200m/pixel or better). These improvements would be greatly facilitated by new data, which could be obtained by future missions.
We modified the MAGMA chemical equilibrium code developed by Fegley and Cameron (1987, Earth Planet. Sci. Lett. 82, 207–222) and used it to model vaporization of high temperature silicate lavas on Io. The MAGMA code computes chemical equilibria in a melt, between melt and its equilibrium vapor, and in the gas phase. The good agreement of MAGMA code results with experimental data and with other computer codes is demonstrated. The temperature-dependent pressure and composition of vapor in equilibrium with lava is calculated from 1700 to 2400 K for 109 different silicate lavas in the ONaKFeSiMgCaAlTi system. Results for five lavas (tholeiitic basalt, alkali basalt, Barberton komatiite, dunite, and a molten type B1 Ca, Al-rich inclusion) are discussed in detail. The effects of continuous fractional vaporization on chemistry of these lavas and their equilibrium vapor are presented. The predicted abundances (relative to Na) of K, Fe, Si, Al, Ca, and Ti in the vapor equilibrated with lavas at 1900 K are lower than published upper limits for Io's atmosphere (which do not include Mg). We predict evaporative loss of alkalis, Fe, and Si during volcanic eruptions. Sodium is more volatile than K, and the Na/K ratio in the gas is decreased by fractional vaporization. This process can match Io's atmospheric Na/K ratio of 10±3 reported by Brown (2001, Icarus 151, 190–195). Silicon monoxide is an abundant species in the vapor above lavas. Spectroscopic searches are recommended for SiO at IR and mm wavelengths. Reactions of metallic vapors with S- and Cl-bearing volcanic gases may form other unusual gases including MgCl2, MgS, MgCl, FeCl2, FeS, FeCl, and SiS.
We report observations of Io that were conducted on UT 12 November 2002 with the NIRSPEC spectrometer, coupled to the adaptive optics system, on the 10-m Keck II telescope. We detected a bright eruption in the Ra Patera area, with a color (H–K′ band) temperature of 1031±110 K over an effective area of 1.5±0.2 km2. The eruption was associated with a hot plume, which revealed itself through SO emission at a rate of (9±3)×1025 photonss−1, about 10–15% of Io's total SO flux at the time. The rotational temperature was 700±150 K. No significant SO emission was received from Io's northern hemisphere (north of Ra Patera/Loki); roughly 50% of the total SO emission came from the equatorial region (including Ra Patera, Janus and Loki), and ∼40% came from the south. The rotational temperatures typically measured between 600 and 1000 K. The emissions are most likely produced by SO molecules ejected out of volcanic vents in the excited a1Δ state. Our narrow band images that span the SO emission band suggest that a source near Io's south pole, Nemea, may be a source of some of the southern SO emissions.
Color temperature analyses were conducted on three hotspots using Cassini ISS data of the surface of Io in eclipse by Jupiter. The data for Pillan, Loki, and Wayland will be presented.
In December 2005 we observed strombolian activity at an active lava lake in Antarctica, ground-truthing VIS/IR spacecraft observations (four instruments on two spacecraft) using Forward Looking Infrared (FLIR) cameras.
The most powerful volcanoes in the Solar System are not on Earth, but on Io, a tiny moon of Jupiter. Whilst Earth and Io are the only bodies in the Solar System to have active, high-temperature volcanoes, those found on Io are larger, hotter, and more violent. This, the first book dedicated to volcanism on Io, contains the latest results from Galileo mission data analysis. As well as investigating the different styles and scales of volcanic activity on Io, it compares these volcanoes to their contemporaries on Earth. The book also provides a background to how volcanoes form and how they erupt, and explains quantitatively how remote-sensing data from spacecraft and telescopes are analysed to reveal the underlying volcanic processes. This richly illustrated book will be a fascinating reference for advanced undergraduates, graduate students and researchers in planetary sciences, volcanology, remote sensing and geology.
Io, the volcanically active innermost large moon of Jupiter, was a target of intense study during the recently completed NASA Galileo mission to Jupiter (1989–2003). Galileo's suite of instruments obtained unprecedented observations of Io, including high spatial resolution imaging in the visible and infrared. This paper reviews the insights gained about Io's surface, atmosphere and space environment during the Galileo mission. Io is thought to have a large Fe–FeS core, whose radius is slightly less than half the radius of Io and whose mass is 20% of the moon. The lack of an intrinsic magnetic field implies that the core is either completely solid or completely liquid. The mantle of Io appears to undergo a high degree of partial melting (20–50% molten) that produces ultramafic lavas dominated by Mg-rich orthopyroxene in an apparent 'mushy magma ocean', suggesting an undifferentiated mantle. The crust of Io is thought to be rigid, 20–30 km thick, cold away from volcanic heat sources and composed of mafic to ultramafic silicates. Tidal flexing due to Io's orbital resonance produces ~100 m tides at the surface, generating heat that powers Io's volcanism. Silicate volcanism appears to be dominant at most hot spots, although secondary sulfur volcanism may be important in some areas. The key discoveries of the Galileo era at Io were: (1) the detection of high-temperature volcanism (ultramafic, superheated mafic or 'ceramic'); (2) the detection of both S2 and SO2 gas in Ionian plumes; (3) the distinction between eruption styles, including between Pelean plumes (originating from central vents) and Promethean plumes (originating from silicate lava flow fronts); (4) the relationship between mountains and paterae, which indicates that many paterae are formed as magma preferentially ascends along tectonic faults associated with mountain building; (5) the lack of detection of an intrinsic magnetic field; (6) a new estimate of global heat flow; and (7) increased understanding of the relationship between Io, its plasma torus and Jupiter's magnetic field. There is an apparent paradox between Io's potentially ultramafic volcanism (suggestive of a primitive, undifferentiated mantle) and the widespread intensity of the volcanism on Io (which should have produced a volume of lava ~140 times the volume of Io over the last 4.5 Ga, resulting in more silicic materials). The resolution of this paradox requires either an Io that only recently (geologically) entered its tidal resonance and became volcanically active or a response of Io's lithosphere–mantle to tidal heating that has in some way prevented extreme differentiation. Understanding this problem is one of many important issues about Io that remain unresolved. We conclude this paper with a discussion of the types of future observations, from the ground and from space, that will be needed to address these issues.
Since before the beginning of the Galileo spacecraft’s Jupiter orbital tour, we have observed Io from the ground using NASA’s Infrared Telescope Facility (IRTF). We obtained images of Io in reflected sunlight and in-eclipse at 2.3, 3.5, and 4.8 μm. In addition, we have measured the 3.5 μm brightness of an eclipsed Io as it is occulted by Jupiter. These lightcurves enable us to measure the brightness and one-dimensional location of active volcanoes on the surface. During the Galileo era, two volcanoes were observed to be regularly active: Loki and either Kanehekili and/or Janus. At least 12 other active volcanoes were observed for shorter periods of time, including one distinguishable in images that include reflected sunlight. These data can be used to compare volcano types and test volcano eruption models, such as the lava lake model for Loki.
A 1978 revision of Alfred Rittmann's 1936 classification of volcanoes is
further augmented. Cratered terrestrial edifices now are divided into 25 relatively distinct types. The acircular crater formed in the May 1980 eruption of Mount St. Helens is diagnostic of one of the newly created classes. Five topographic measurements — height; flank width; and diameter, depth, and circularity of the summit depression — are presented for 697 terrestrial volcanoes and for 33 probable volcanoes on the Moon and Mars (six additional classes). Calculated averages of circularity and edifice volume partition the 31 classes of volcanoes into eight genetic categories.
Models indicate that the high heat flow from Io would result in a very thin (approximately 5 km) silicate lithosphere overlying a molten interior, if all heat was transported through the lithosphere via conduction. However, the presence of mountains with relief in excess of 10 km would seem to demand a thick lithosphere, at least locally. A significant fraction of Io's heat flow may be transported via advection through volcanoes. Advective heat transfer permits a thicker lithosphere than does pure conduction, possibly reconciling Io's high heat flow with the rugged topography.
The solid-state imaging subsystem (SSI) on NASA's Galileo Jupiter orbiter spacecraft has already demonstrated its superior performance as a scientific imager by returning stunning pictures of several planetary bodies as well as detailed inflight calibration data during its cruise to Jupiter. The SSI inflight performance remains excellent; the instrument calibration is stable and accurate. Improved determinations of the SSI's absolute spectral radiometric response and scattered-light properties have been made. Evaluation of the camera's point spread function suggests that the focus setting may be slightly nonoptimum, but the spatial resolution in returned images is still very good. The shielding of the SSI's CCD detector against energetic particle radiation appears to be adequate for operation in Jupiter's intense radiation field. New camera modes, onboard editing and data compression capabilities, and an adaptive mission operations plan have been implemented for the Jupiter orbital mission phase in order to mitigate the effects of a spacecraft anomaly that limits the allowable data return rate from Jupiter. These new capabilities are expected to allow the accomplishment of a historic scientific investigation of the Jupiter system using the SSI. (C) 1997 Society of Photo-Optical instrumentation Engineers.
A high-spatial-resolution, multi-wavelength observation by the Galileo NIMS instrument has been analysed to determine the temperature and area distribution of a large portion of the ionian volcano Loki Patera. The temperatures of the cooler components from a two-temperature fit to the data can be used to determine ages of the surface. The age of the floor along a profile across the floor of the caldera ranges from ~10 to ~80 days. This puts the start of the resurfacing in July/early August 2001, yielding a resurfacing rate of approximately 1 km/day, with the new lava spreading from the SW corner of the caldera in a NE direction. This rate is consistent with resurfacing by foundering of the crust on a lava lake. However, the temperature distribution may also result from the emplacement of flows. Implied crust thicknesses (derived using a lava cooling model) range from 2.6 to 0.9 m.
The Galileo spacecraft has had five successful close Io flybys and many distant observations of Io, innermost of the four Galilean satellites of Jupiter. Scientists knew from Voyager nearly a quarter century ago and Earth-based astronomy that Io is by far the most volcanically active object in the solar system. Our new estimate reaffirms that it is more volcanically active than the rest of the solar system combined. Perhaps the most astonishing discovery about Io since Voyager is that some of its lavas possess emission temperatures of about 1870 K (about 2900°F) or more. Not only are some lavas hotter than any now erupted on Earth, but they might be hotter than any in Earth's geologic history. The Io community has identified three alternative interpretations of the message written in the glow from Io's hottest lavas: they may be ultramafics similar to komatiite; superheated by an exotic mechanism; or an ultra-refractory substance deficient in silica and rich in Ca-Al oxides. New advances in Earth-orbital and ground-based observing systems will (a) allow continued monitoring of Io's volcanism and improve lava temperature determinations and (b) improve analysis of volcanic gases and deduction of lava.
During three close flybys in late 1999 and early 2000 the Galileo spacecraft acquired new observations of the mountains that tower above Io's surface. These images have revealed surprising variety in the mountains' morphologies. They range from jagged peaks several kilometers high to lower, rounded structures. Some are very smooth, others are covered by numerous parallel ridges. Many mountains have margins that are collapsing outward in large landslides or series of slump blocks, but a few have steep, scalloped scarps. From these observations we can gain insight into the structure and material properties of Io's crust as well as into the erosional processes acting on Io. We have also investigated formation mechanisms proposed for these structures using finite-element analysis. Mountain formation might be initiated by global compression due to the high rate of global subsidence associated with Io's high resurfacing rate; however, our models demonstrate that this hypothesis lacks a mechanism for isolating the mountains. The large fraction (∼40%) of mountains that are associated with paterae suggests that in some cases these features are tectonically related. Therefore we have also simulated the stresses induced in Io's crust by a combination of a thermal upwelling in the mantle with global lithospheric compression and have shown that this can focus compressional stresses. If this mechanism is responsible for some of Io's mountains, it could also explain the common association of mountains with paterae.
Paterae, defined by the International Astronomical Union as ``irregular crater[s], or complex one[s] with scalloped edges,'' are some of the most prominent topographic features on Io. Paterae on Io are unique, yet in some aspects they resemble calderas known and studied on Earth, Mars, and Venus. They have steep walls, flat floors, and arcuate margins and sometimes exhibit nesting, all typical of terrestrial and Martian basalt shield calderas. However, they are much larger, many are irregular in shape, and they typically lack shields. Their great sizes (some > 200 km diameter) and lack of associated volcanic edifices beg comparison with terrestrial ash flow calderas; however, there is no convincing evidence on Io for the high-silica erupted products or dome resurgence associated with this type of caldera. Ionian paterae seem to be linked with the eruption of large amounts of mafic to ultramafic lavas and colorful sulfur-rich materials that cover the floors and sometimes flow great distances away from patera margins. They are often angular in shape or are found adjacent to mountains or plateaus, indicating tectonic influences on their formation. A database of 417 paterae on Io measured from images with
Remotely sensed and field data can be used to estimate heat and mass fluxes at active lava lakes. Here we use a three thermal component pixel model with three bands of Landsat thematic mapper (TM) data to constrain the thermal structure of, and flux from, active lava lakes. Our approach considers that a subpixel lake is surrounded by ground at ambient temperatures and that the surface of the lake is composed of crusted and/or molten material. We then use TM band 6 (10.42-12.42 gm) with bands 3 (0.63-0.69 gm) or 4 (0.76-0.90 gm) and 5 (1.55-1.75 gm) or 7 (2.08-2.35 gm), along with field data (e.g., lava lake area), to place limits on the size and temperature of each thermal component. Previous attempts to achieve this have used two bands of TM data with a two-component thermal model. Using our model results with further field data (e.g., petrological data) for lava lakes at Erebus, Erta 'Ale, and Pu'u 'O'o, we calculate combined radiative and convective fluxes of 11-20, 14-27 and 368-373 MW, respectively. These yield mass fluxes, of 30-76, 44-104 and 1553-2079 kg s -i, respectively. We also identify a hot volcanic feature at Nyiragongo during 1987 from which a combined radiative and convective flux of 0.2-0.6 MW implies a mass flux of 1-2 kg s -1. We use our mass flux estimates to constrain circulation rates in each reservoir-conduit-lake system and consider four models whereby circulation results in intrusion within or beneath the volcano (leading to endogenous or cryptic growth) and/or magma mixing in the reservoir (leading to recycling). We suggest that the presence of lava lakes does not necessarily imply endogenous or cryptic growth: lava lakes could be symptomatic of magma recycling in supraliquidus reservoirs.
Galileo's Near-Infrared Mapping Spectrometer (NIMS) observed Io during the spacecraft's three flybys in October 1999, November 1999, and February 2000. The observations, which are summarized here, were used to map the detailed thermal structure of active volcanic regions and the surface distribution of SO2 and to investigate the origin of a yet unidentified compound showing an absorption feature at ---1 • m. We present a summary of the observations and results, focusing on the distribution of thermal emission and of SO2 deposits. We find high eruption temperatures, consistent with ultramafic volcanism, at Pele. Such temperatures may be present at other hot spots, but the hottest areas may be too small for those temperatures to be detected at the spatial resolution of our observations. Loki is the site of frequent eruptions, and the low thermal emission may represent lavas cooling on the caldera's surface or the cooling crust of a lava lake. High-resolution spectral observations of Emakong caldera show thermal emission and SO2 within the same pixels, implying that patches of SO2 frost and patches of cooling lavas or sulfur flows are present within a few kilometers from one another. Thermal maps of Prometheus and Amirani show that these two hot spots are characterized by long lava flows. The thermal profiles of flows at both locations are consistent with insulated flows, with the Amirani flow field having more breakouts of fresh lava along its length. Prometheus and Amirani each show a white ring at visible wavelengths, while SO2 distribution maps show that the highest concentration of SO2 in both ring deposits lies outside the white portion. Visible measurements at high phase angles show that the white deposit around Prometheus extends into the SO2 ring. This suggests that the deposits are thin and that compositional or grain size variations may occur in the radial direction. SO2 mapping of the Chaac region shows that the interior of a caldera adjacent to Chaac has almost pure SO2. The deposit appears to be topographically controlled, suggesting a possible origin by liquid flow.
The shield volcano Erta'Ale, situated in the Danakil Depression, Ethiopia, is known for its active lava lake. In February 2001, our team visited this lake, located inside an 80-m-deep pit, to perform field temperature measurements. The distribution and variation of temperature inside the lake were obtained on the basis of infrared radiation measurements performed from the rim of the pit and from the lake shores. The crust temperature was also determined from the lake shores with a thermocouple to calibrate the pyrometer. We estimated an emissivity of the basalt of 0.74 from this experiment. Through the application of the Stefan-Boltzmann law, we then obtained an estimate of the total radiative heat flux, constrained by pyrometer measurements of the pit, and visual observations of the lake activity. Taking into account the atmospheric convective heat flux, the convected magma mass flux needed to balance the energy budget was subsequently derived and found to represent between 510 and 580 kg s-1. The surface circulation of this mass flux was also analyzed through motion processing techniques applied to video images of the lake. Electronic supplementary material to this paper can be obtained by using the Springer LINK server located at http://dx.doi.org/10.1007/s00445-002-0224-3.
Active volcanism on Io has been monitored during the nominal Galileo satellite tour from mid 1996 through late 1997. The Solid State Imaging (SSI) experiment was able to observe many manifestations of this active volcanism, including (1) changes in the color and albedo of the surface, (2) active airborne plumes, and (3) glowing vents seen in eclipse.About 30 large-scale (tens of kilometers) surface changes are obvious from comparison of the SSI images to those acquired by Voyager in 1979. These include new pyroclastic deposits of several colors, bright and dark flows, and caldera-floor materials. There have also been significant surface changes on Io during the Galileo mission itself, such as a new 400-km-diameter dark pyroclastic deposit around Pillan Patera. While these surface changes are impressive, the number of large-scale changes observed in the four months between the Voyager 1 and Voyager 2 flybys in 1979 suggested that over 17 years the cumulative changes would have been much more impressive. There are two reasons why this was not actually the case. First, it appears that the most widespread plume deposits are ephemeral and seem to disappear within a few years. Second, it appears that a large fraction of the volcanic activity is confined to repeated resurfacing of dark calderas and flow fields that cover only a few percent of Io's surface.The plume monitoring has revealed 10 active plumes, comparable to the 9 plumes observed by Voyager. One of these plumes was visible only in the first orbit and three became active in the later orbits. Only the Prometheus plume has been consistently active and easy to detect. Observations of the Pele plume have been particularly intriguing since it was detected only once by SSI, despite repeated attempts, but has been detected several times by the Hubble Space Telescope at 255 nm. Pele's plume is much taller (460 km) than during Voyager 1 (300 km) and much fainter at visible wavelengths. Prometheus-type plumes (50–150 km high, long-lived, associated with high-temperature hot spots) may result from silicate lava flows or shallow intrusions interacting with near-surface SO2.A major and surprising result is that ∼30 of Io's volcanic vents glow in the dark at the short wavelengths of SSI. These are probably due to thermal emission from surfaces hotter than 700 K (with most hotter than 1000 K), well above the temperature of pure sulfur volcanism. Active silicate volcanism appears ubiquitous. There are also widespread diffuse glows seen in eclipse, related to the interaction of energetic particles with the atmosphere. These diffuse glows are closely associated with the most active volcanic vents, supporting suggestions that Io's atmopshere is dominated by volcanic outgassing.Globally, volcanic centers are rather evenly distributed. However, 14 of the 15 active plumes seen by Voyager and/or Galileo are within 30° of the equator, and there are concentrations of glows seen in eclipse at both the sub- and antijovian points. These patterns might be related to asthenospheric tidal heating or tidal stresses. Io will continue to be observed during the Galileo Europa Mission, which will climax with two close flybys of Io in late 1999.
We present the observations of Io acquired by the Solid State Imaging (SSI) experiment during the Galileo Millennium Mission (GMM) and the strategy we used to plan the exploration of Io. Despite Galileo's tight restrictions on data volume and downlink capability and several spacecraft and camera anomalies due to the intense radiation close to Jupiter, there were many successful SSI observations during GMM. Four giant, high-latitude plumes, including the largest plume ever observed on Io, were documented over a period of eight months; only faint evidence of such plumes had been seen since the Voyager 2 encounter, despite monitoring by Galileo during the previous five years. Moreover, the source of one of the plumes was Tvashtar Catena, demonstrating that a single site can exhibit remarkably diverse eruption styles—from a curtain of lava fountains, to extensive surface flows, and finally a ∼400 km high plume—over a relatively short period of time (∼13 months between orbits I25 and G29). Despite this substantial activity, no evidence of any truly new volcanic center was seen during the six years of Galileo observations. The recent observations also revealed details of mass wasting processes acting on Io. Slumping and landsliding dominate and occur in close proximity to each other, demonstrating spatial variation in material properties over distances of several kilometers. However, despite the ubiquitous evidence for mass wasting, the rate of volcanic resurfacing seems to dominate; the floors of paterae in proximity to mountains are generally free of debris. Finally, the highest resolution observations obtained during Galileo's final encounters with Io provided further evidence for a wide diversity of surface processes at work on Io.
The Galileo spacecraft has been periodically monitoring volcanic activity on Io since June 1996, making it possible to chart the evolution of individual eruptions. We present results of coanalysis of Near-Infrared Mapping Spectrometer (NIMS) and solid-state imaging (SSI) data of eruptions at Pele and Pillan, especially from a particularly illuminating data set consisting of mutually constraining, near-simultaneous NIMS and SSI observations obtained during orbit C9 in June 1997. The observed thermal signature from each hot spot, and the way in which the thermal signature changes with time, tightly constrains the possible styles of eruption. Pele and Pillan have very different eruption styles. From September 1996 through May 1999, Pele demonstrates an almost constant total thermal output, with thermal emission spectra indicative of a long-lived, active lava lake. The NIMS Pillan data exhibit the thermal signature of a "Pillanian" eruption style, a large, vigorous eruption with associated open channel, or sheet flows, producing an extensive flow field by orbit C10 in September 1997. The high mass eruption rate, high liquidus temperature (at least 1870 K) eruption at Pillan is the best candidate so far for an active ultramafic (magnesium-rich, "komatiitic") flow on Io, a style of eruption never before witnessed. The thermal output per unit area from Pillan is, however, consistent with the emplacement of large, open-channel flows. Magma temperature at Pele is ≥1600 K. If the magma temperature is 1600 K, it suggests a komatiitic-basalt composition. The power output from Pele is indicative of a magma volumetric eruption rate of ∼250 to 340 m3 s-1. Although the Pele lava lake is considerably larger than its terrestrial counterparts, the power and mass fluxes per unit area are similar to active terrestrial lava lakes.
High-temperature hot spots on Io have been imaged at ∼50 km spatial resolution by Galileo's CCD imaging system (SSI). Images were acquired during eclipses (Io in Jupiter's shadow) via the SSI clear filter (∼0.4-1.0 μm), detecting emissions from both small intense hot spots and diffuse extended glows associated with Io's atmosphere and plumes. A total of 13 hot spots have been detected over ∼70% of Io's surface. Each hot spot falls precisely on a low-albedo feature corresponding to a caldera floor and/or lava flow. The hot-spot temperatures must exceed ∼700 K for detection by SSI. Observations at wavelengths longer than those available to SSI require that most of these hot spots actually have significantly higher temperatures (∼1000 K or higher) and cover small areas. The high-temperature hot spots probably mark the locations of active silicate volcanism, supporting suggestions that the eruption and near-surface movement of silicate magma drives the heat flow and volcanic activity of Io.
Io, innermost of Jupiter's large moons, is one of the most unusual objects in the Solar System. Tidal heating of the interior produces a global heat flux 40 times the terrestrial value, producing intense volcanic activity and a global resurfacing rate averaging perhaps 1 cm yr-1. The volcanoes may erupt mostly silicate lavas, but the uppermost surface is dominated by sulfur compounds including SO2 frost. The volcanoes and frost support a thin, patchy SO2 atmosphere with peak pressure near 10-8 bars. Self-sustaining bombardment of the surface and atmosphere by Io-derived plasma trapped in Jupiter's magnetosphere causes escape of material from Io (predominantly sulfur, oxygen, and sodium atoms, ions, and molecules) at a rate of about 103 kg s-1. The resulting Jupiter-encircling torus of ionized sulfur and oxygen dominates the Jovian magnetosphere and, together with an extended cloud of neutral sodium, is readily observable from Earth.
Infrared wavelength observations of Io by the Galileo spacecraft show that at least 12 different vents are erupting lavas
that are probably hotter than the highest temperature basaltic eruptions on Earth today. In at least one case, the eruption
near Pillan Patera, two independent instruments on Galileo show that the lava temperature must have exceeded 1700 kelvin and
may have reached 2000 kelvin. The most likely explanation is that these lavas are ultramafic (magnesium-rich) silicates, and
this idea is supported by the tentative identification of magnesium-rich orthopyroxene in lava flows associated with these
high-temperature hot spots.
On 15 October 1997, the most capable interplanetary spacecraft ever
built was launched from Cape Canaveral Air Station near Cocoa Beach,
Florida, atop a Titan 4B/Centaur launch vehicle. The Cassini spacecraft
has used flyby gravitational assists at Venus (twice), Earth (once), and
Jupiter (once) on its way to reaching its target nearly seven years
after launch. On July 1, 2004, the spacecraft will be inserted into
orbit around the ringed planet; its primary mission concludes four years
later on July 1, 2008. This article provides an overview of the design
of the Cassini spacecraft (including its Huygens probe), a description
of the mission, and a brief outline of the scientific objectives. While
the Voyager 1 and 2 flybys in 1980 and 1981 provided a first close
examination of the giant planet Saturn, Cassini's 27 scientific
investigations will do an in-depth four-year orbital study of the
system. Early in its orbital tour around Saturn, Cassini will release
its Huygens probe (built by the European Space Agency), which will
traverse the atmosphere of Saturn's largest moon, Titan, and collect
data from its surface. Titan is the second largest moon in the solar
system and the only one that possesses a substantial atmosphere. Six
scientific instrument packages are part of the Huygens probe. The
Cassini orbiter carries an additional 12 instrument packages. Nine
interdisciplinary investigations utilize data from two or more
instruments to complete their scientific studies. In addition to Titan,
the scientific objectives address better understanding of the atmosphere
and interior of Saturn, of the complex ring system, of the seventeen
presently known icy moons (excluding the 12 recently discovered outer
moons), and of the huge magnetic bubble surrounding Saturn known as the
magnetosphere. Scientists from at least 16 nations are involved in
these studies.
In July 1996, with the Hubble Space Telescope (HST), we observed the Pele plume silhouetted against Jupiter at a wavelength of 0.27µm, the first definitive observation of an Io plume from Earth. The height, 420 ± 40 km, was greater than any plume observed by Voyager. The plume had significantly smaller optical depth at 0.34 and 0.41µm, where it was not detected. The wavelength dependence of the optical depth can be matched by a plume either of fine dust, with minimum mass of 1.2 × 109 g and maximum particle size of 0.08µm, or of SO2 gas with a column density of 3.7 × 1017 cm−2 and total mass of 1.1 × 1011 g. Our models suggest that early Voyager imaging estimates of the minimum mass of the Loki plume [Collins, 1981] may have been too large by a factor of ∼ 100. We may have detected the Pele plume in reflected sunlight, at 0.27µm, in July 1995, but did not see it 21 hours earlier, so the plume may be capable of rapid changes.
Cassini's flyby of the Jupiter system in December 2000 and January 2001provided us with three sets of images of Io in eclipse by Jupiter. Taking the ratio of fluxes through the clear and 1 micron filters yields hotspot temperatures. These wavelengths are sensitive to the highest-temperature components, corresponding to lava exposed to the surface within 10 minutes of each observation. Pele, often seen as an active hotspot and plume source by Voyager, Galileo, and ground-based observers, showed maximum temperatures of 1500 K +-70 K and minimum temperatures of 1300 K +- 40 K, over three eclipses observed by Cassini. Temperatures from 26 eclipse image pairs were typically about 1370 K, which is consistent with previous temperature measurements of Pele. Cassini's high data rate made it possible to view change in Pele's temperatures over short time scales; ten image pairs per eclipse were taken approximately 11 minutes apart. Temperatures during the first half of the first eclipse were relatively constant, then they varied over at least 100 K over several tens of minutes. Temperatures for the last eclipse showed a similar pattern of being relatively constant for the first half, then varying over at least 30 K near the end of the eclipse, when the view of Pele was more oblique. We also observed that the total intensity falls off at about cosine(emission angle)1.6, consistent with exposure of the hottest material in cracks and low areas. Pele has been postulated to be an active lava lake, based on Galileo observations of consistently high temperatures, shape of the thermal emission spectrum, lack of extensive lava flows, and possible presence of a glowing lava lake margin. These new Cassini measurements of consistently high temperatures (with small variations) may be explained by a vigorously and continuously overturning lava lake.
The Galileo spacecraft has been periodically monitoring volcanic activity on Io since June 1996, making it possible to chart the evolution of individual eruptions. We present results of coanalysis of Near‐Infrared Mapping Spectrometer (NIMS) and solid‐state imaging (SSI) data of eruptions at Pele and Pillan, especially from a particularly illuminating data set consisting of mutually constraining, near‐simultaneous NIMS and SSI observations obtained during orbit C9 in June 1997. The observed thermal signature from each hot spot, and the way in which the thermal signature changes with time, tightly constrains the possible styles of eruption. Pele and Pillan have very different eruption styles. From September 1996 through May 1999, Pele demonstrates an almost constant total thermal output, with thermal emission spectra indicative of a long‐lived, active lava lake. The NIMS Pillan data exhibit the thermal signature of a “Pillanian” eruption style, a large, vigorous eruption with associated open channel, or sheet flows, producing an extensive flow field by orbit C10 in September 1997. The high mass eruption rate, high liquidus temperature (at least 1870 K) eruption at Pillan is the best candidate so far for an active ultramafic (magnesium‐rich, “komatiitic”) flow on Io, a style of eruption never before witnessed. The thermal output per unit area from Pillan is, however, consistent with the emplacement of large, open‐channel flows. Magma temperature at Pele is ≥1600 K. If the magma temperature is 1600 K, it suggests a komatiitic‐basalt composition. The power output from Pele is indicative of a magma volumetric eruption rate of ∼250 to 340 m ³ s ⁻¹ . Although the Pele lava lake is considerably larger than its terrestrial counterparts, the power and mass fluxes per unit area are similar to active terrestrial lava lakes.
We present observations and initial modeling of the lava-SO2 interactions at the flow fronts in the Prometheus region of Io. Recent high-resolution observations of Prometheus reveal a compound flow field with many active flow lobes. Many of the flow lobes are associated with bright streaks of what is interpreted to be volatilized and recondensed SO2 radiating away from the hot lava. Lower-resolution color data show diffuse blue to violet areas, also near the active flow front, perhaps from active venting of SO2. Not clearly visible in any of the images is a single source vent for the active plume. While the size of the proposed vent is probably near the limit of the resolution, we expected to see radial or concentric albedo patterns or other evidence for gas and entrained particles above the flow field. The lack of an obvious plume vent, earlier suggestions that the Prometheus-type plumes may originate from the advancing flow lobes, and the high-resolution images showing evidence for large-scale volatilization of the SO2-rich substrate at Prometheus encouraged us to develop a model to quantify the heat transfer between a basaltic lava flow and a substrate of SO2 snow. We calculate that the vaporization rate of SO2 snow is 2.5×10-6ms-1 per unit area. Using an estimated 5 m2s-1 lava coverage rate (from change detection images), we show that the gas production rate of SO2 at the flow fronts is enough to produce a resurfacing rate of ~0.24 cm yr-1 at the annulus of Prometheus. This is much less than other estimates of resurfacing by the Prometheus plume. While not easily explaining the main Prometheus plume, our model readily accounts for the bright streaks.
The thermal emission from cracks in lava flows dominates the spectrum in the near-IR. Using a new model I show that the near-IR spectrum is a strong function of emission angle.
Mountains are distributed across the surface of Io, the fiercely tidally heated moon of Jupiter. The large crustal thicknesses implied by their great heights can be reconciled with Io's high heat flow, if most of the heat escapes directly via volcanic eruptions (the heat-pipe model), but the origin of the mountains has remained obscure. Recent images show that many of Io's mountains are tilted blocks undergoing tectonic collapse, and we propose here that the volcanic heat-pipe (and continuous terrain burial) model naturally leads to such unstable topography. That is, burial (1) generates horizontal tensile stresses as the volcanic crustal stack is loaded, (2) creates large horizontal compressive confining stresses as Io's crust subsides (moves to a smaller effective radius), and importantly, (3) allows for potentially large horizontal compressive thermal stresses as the base of the crust reheats owing to fluctuations in the efficiency of the volcanic heat piping. Faulting associated with these stresses may raise mountain scarps directly or in concert with thermal uplift due to the crustal reheating; continued crustal heating and melting then lead to mountain collapse (all over
We catalog 143 Ionian mountains (montes) and mountain-like features (mensae, tholi, plana, and small peaks) in order to investigate orogenic tectonism on Io. From this comprehensive list, we select 96 mountains for which there are sufficient coverage and resolution to discern spatial relationships with surrounding geologic features. Three of the 96 mountains are probably volcanoes, 92 appear to be tectonic massifs, and 1 is ambiguous. Of the 92 tectonic mountains, 38 abut paterae (volcanic or volcano-tectonic craters with irregular or scalloped margins). This juxtaposition is unlikely to be a coincidence as the probability of it occurring by chance is ∼0.1%. We propose instead that orogenic faults may act as conduits for magma ascent, thus fueling patera formation near mountains. As resurfacing buries a shell of material from Io's surface to the base of the lithosphere, its effective radius is reduced and it heats up. We calculate the lithospheric volume change due to subsidence and thermal expansion as a function of lithospheric thickness. Conservation of volume dictates that this material must be uplifted at Io's surface. By estimating the total volume of the mountains, we are able to place a lower limit of 12 km on Io's lithospheric thickness. We hypothesize that, in some cases, mountain formation may be facilitated by asthenospheric diapirs impinging on the base of the lithosphere. The resulting lithospheric swell could focus the compressive stresses that drive orogenic tectonism. This model is one of several possible mechanisms for uplifting isolated mountains such as are observed on Io.
In this study, we analyze a series of images of Io obtained with the European Southern Observatory adaptive optics system (Adaptive Optics Near Infrared System, ADONIS) at 3.8 mum from 1996 to 1999, with particular emphasis on the observations carried out in late 1999 in support of the Galileo flybys of Io. Use of a new myopic deconvolution method, Myopic Iterative Step Preserving Algorithm (MISTRAL), especially designed for planetary objects, significantly improves the quality and the reliability of the reconstructed images. Using a simulation of artificial images of Io, we estimate the extent to which this algorithm is able to prevent noise amplification, to better restore sharp edges, and to preserve the initial photometry. Once this algorithm has been applied to our data and the solar-reflected background has been subtracted, we have 89 images of Io available for search and temporal survey of the hot spots over 4 years. In most cases, the data, acquired during two consecutive nights, provide an almost global view of the surface of Io. We identify 20 hot spots for which we determine the coordinates and the brightness at 3.8 mum (as a function of time). More than half of the hot spots were detected on all the images and were considered as persistent. Particular emphasis has been put on the brightest two hot spots, Loki and Pele, for which we obtain a center-to-limb variation curve (a cosine curve for Loki and a curve decreasing slightly faster than a cosine law for Pele with possible physical interpretations). The temporal variations of their activity have been derived over these 4 years and, for Loki, compared with the results of other ground-based surveys. The variability of the other, fainter, sources has also been derived but with less accuracy. We have also identified a few new hot spots, some of which are transient, such as a diffuse emission seen in September and November 1999 at the location of the upcoming Tvashtar outburst. The data are based on observations (58.F-1003, 63.S-0260, and 64.S-0661) collected at the European Southern Observatory in La Silla, Chile.
Analyses of thermal infrared outbursts from the jovian satellite Io indicate that at least some of these volcanic events are due to silicate lava. Analysis of the January 9, 1990 outburst indicates that this was an active eruption consisting of a large lava flow (with mass eruption rate of order 105m3sec−1) and a sustained area at silicate liquidus temperatures. This is interpreted as a series of fire fountains along a rift zone. A possible alternative scenario is that of an overflowing lava lake with extensive fire fountaining. The January 9, 1990 event is unique as multispectral observations with respect to time were obtained. In this paper, a model is presented for the thermal energy lost by active and cooling silicate lava flows and lakes on Io. The model thermal emission is compared with Earth-based observations and Voyager IRIS data. The model (a) provides an explanation of the thermal anomalies on Io's surface; (b) provides constraints on flow behavior and extent and infers some flow parameters; and (c) determines flow geometry and change in flow size with time, and the temperature of each part of the flow or lava lake surface as a function of its age. Models of heat output from active lava flows or inactive but recently emplaced lava flows or overturning lava lakes alone are unable to reproduce the observations. If the January 9, 1990 event is the emplacement of a lava flow, the equivalent of 27 such events per year would yield a volume of material sufficient, if uniformly distributed, to resurface all of Io at a rate of 1 cm/year.
In March 1994, we used the newly refurbished Hubble Space Telescope (HST) to obtain global imaging of Io at five wavelengths between 0.34 and 1.02 μm, with a spatial resolution of 160 km. The images provided the clearest view of Io since Voyager and the first systematic observations in the wavelength range 0.7–1.0 μm. We have produced absolutely calibrated global mosaics of Io's reflectance in all our five wavelengths. The near-infrared images reveal that the 0.55- to 0.7-μm absorption edge seen in Io's disk-integrated spectrum has a very different spatial distribution from the better-known 0.40- to 0.50-μm absorption edge studied by Voyager, and must be generated by a different chemical species. The 0.55- to 0.7-μm absorption edge is strongly concentrated in the pyroclastic ejecta blanket of the volcano Pele, at a few much smaller discrete spots, and probably also in the polar regions. The Pele ejecta spectrum is consistent with the idea that S2O, partially decomposed to S4(and probably S3), may be the species responsible for the 0.55- to 0.7-μm absorption edge at Pele and elsewhere on Io, though S4generated by other processes may also be a possibility. S2O can be produced by high-temperature decomposition of SO2gas, and the high temperature of the Pele volcano may account for its concentration there. Spectral anomalies of comparable size and prominence are not seen around the other “Pele-type” volcanos Surt and Aten (A. S. McEwen and L. A. Soderblom, 1983,Icarus55, 191–217), suggesting that these volcanos, if chemically similar to Pele, are much less active. The spectrum of high-latitude regions is similar to that of quenched red sulfur glass, and if this similarity is not coincidental, the glass may be preserved here by the low polar surface temperatures. Alternatively, the low polar temperatures may preserve sulfur that has been reddened by radiation. There are many changes in albedo patterns in the 15 years between Voyager and these HST observations, but these are generally subtle at HST resolution and are not strongly concentrated in longitude; however there was a major brightening of a 400-km-diameter region centered on Ra Patera between March 1994 and repeat HST observations in July 1995, which was a larger albedo change than any seen in the previous 15 years. This was presumably due to a large eruption at Ra Patera, as confirmed by Galileo images. Long-exposure eclipse images of Io at 1.02 μm on March 6, 1994, place strong limits on the area of exposed silicate magma on Io at the time of the observations.
High-temperature hot spots on Io have been imaged at approximately 50 km spatial resolution by Galileo's CCD imaging system (SSI). Images were acquired during eclipses (Io in Jupiter's shadow) via the SSI clear filter (approximately 0.4-1.0 micron), detecting emissions from both small intense hot spots and diffuse extended glows associated with Io's atmosphere and plumes. A total of 13 hot spots have been detected over approximately 70% of Io's surface. Each hot spot falls precisely on a low-albedo feature corresponding to a caldera floor and/or lava flow. The hot-spot temperatures must exceed approximately 700 K for detection by SSI. Observations at wavelengths longer than those available to SSI require that most of these hot spots actually have significantly higher temperatures (approximately 1000 K or higher) and cover small areas. The high-temperature hot spots probably mark the locations of active silicate volcanism, supporting suggestions that the eruption and near-surface movement of silicate magma drives the heat flow and volcanic activity of Io.
We are documenting characteristics of Io's paterae, mountains, and hotspots in a relational database in support of an extensive analysis of patterns in their distribution and how the features are interrelated.
Observational evidence and theoretical arguments suggest that Jupiter's
satellite Europa could be geologically active and possess an ``ocean''
of liquid water beneath its surface at the present time. We have
searched for evidence of current geologic activity on Europa in the form
of active plumes venting material above the surface and by comparison of
Voyager and Galileo images to look for any changes on the surface. So
far, we have observed no plumes and have detected no definitive changes.
The lack of observed activity allows us to estimate a maximum steady
state surface alteration rate of 1 km2 y-1 in the
regions analyzed, assuming alterations will cover contiguous areas of at
least 4 km2 over a period of 20 years. Assuming this as a
constant, globally uniform resurfacing rate leads to a minimum average
surface age of 30 million years. Lava flows and plumes are the two main
types of volcanic activity that resurface Io. We have used the Galileo
Io dataset to observe the detailed sequences of interconnected plume
activity, hotspot activity, and new surface deposits at a number of
volcanic centers on Io. Red material has faded on a timescale of less
than a year, and a green coating has formed on a caldera over a time
period of about 3 months. Change detection maps can illustrate the
percentage of the surface newly covered by plume deposits and lava
flows, and constrain volume and mass resurfacing rates. Areal
resurfacing is dominated by plume deposits, but volume resurfacing is
dominated by lava flows. Estimates of resurfacing from these change maps
range from 0.4 to 12.9 cm/year, assuming a flow thickness of 1 to 10
meters. The minimum resurfacing rate required for the lack of impact
craters on Io's surface is about 0.02 cm/year. If high-magnesium
(komatiitic) lavas dominate the observed Io heat flux, the maximum
resurfacing rate is about 0.69 cm/year. Basaltic lavas could produce a
rate of 1.3 cm/year. The komatiitic rate produces an average flow
thickness of about half a meter. Thus, we suggest that the average
resurfacing rate of lo is between 0.1 and 1 cm/year.
New Galileo images and Galileo and Cassini temperature data lend
credence to previous proposals that some of the paterae on Io contain
lava lakes, similar in some ways to those observed on Earth. Galileo's
October 2001 I32 flyby produced spectacular new high resolution
observations of Io's paterae, their margins, and floors. Images reveal
where lavas have filled Emakong Patera and overtopped its margins.
Landslides from the peaks of Tohil Mons are not present on the adjacent
floor of a dark patera, perhaps because they have slumped into a molten
lava pit. Dark lavas have filled and drained back from colorful Tupan
Patera, leaving a ring of material on its walls. This patera also shows
evidence of interaction between molten sulfur and silicate lavas, a
relationship observed at the terrestrial Poas Volcano (Francis et al.,
1980, Nature 283, 754-756; Oppenheimer and Stevenson, 1990, La Recherche
21,1088-1090). The extremely uniformly dark materials in many other
paterae could also be lava lakes. Pele Volcano on Io, in particular, has
previously been considered a lava lake based on several characteristics
(Davies et al., 2001, JGR 106,33,079-33,103). Recent analyses of eclipse
images of Pele from Cassini reveal average temperatures of 1375 K, with
variations on short (~10 minute) timescales, consistent with active
fountaining in a lava lake. Similar oscillations around high
temperatures over these time scales are seen in terrestrial lava lakes,
such as at Kupaianaha (Flynn et al., GRL, 19,6461-6476, 1993) and Erta
Ale (Bessard, Caillet and others, in progress). Nightside high
resolution (60 m/pixel) images from Galileo I32 reveal a region of
overturning and convection, with some areas reaching in excess of 1800
K, verifying very high-temperature components identified in
high-resolution NIMS data (Lopes et al., 2001, JGR, 106, 33,053-33,078).
This region is ringed with small hotspots, comparable to locations of
breakup and fountaining at the margins of many terrestrial lava lakes.
The presence of giant lava lakes within these large paterae (up to 200
km diameter) has implications for the transfer of internal heat to the
surface, as the paterae require direct links to comparably large, well
supplied magma chambers (Harris et al., 1999, JGR, 104, 7117-7136) in
order to maintain their vigorous activity over the observed timescales
of tens of years. In addition, if much of Io's heat flow is restricted
to these large lava lakes, then Io's resurfacing may be extremely
spatially confined.
▪ Abstract Io, innermost of Jupiter's large moons, is one of the most unusual objects in the Solar System. Tidal heating of the interior produces a global heat flux 40 times the terrestrial value, producing intense volcanic activity and a global resurfacing rate averaging perhaps 1 cm yr−1. The volcanoes may erupt mostly silicate lavas, but the uppermost surface is dominated by sulfur compounds including SO2 frost. The volcanoes and frost support a thin, patchy SO2 atmosphere with peak pressure near 10−8 bars. Self-sustaining bombardment of the surface and atmosphere by Io-derived plasma trapped in Jupiter's magnetosphere causes escape of material from Io (predominantly sulfur, oxygen, and sodium atoms, ions, and molecules) at a rate of about 103 kg s−1. The resulting Jupiter-encircling torus of ionized sulfur and oxygen dominates the Jovian magnetosphere and, together with an extended cloud of neutral sodium, is readily observable from Earth.
The thermal emission from Loki, Ulgen, and Amaterasu Paterae can be modeled remarkably accurately with a simple two-parameter model based on analytical expressions for the cooling of a silicate lava flow. In this model the flow is assumed to produce new surface at a constant rate and then the surface cools purely by radiation. The highly nonlinear (T4) dependence of radiative flux on temperature paradoxically produces simple approximations for the temperature of the surface vs time and also for the distribution function which gives the fraction of the surface at any given temperature. This work is based upon a modification of the Stefan model which describes the solidification of lava flows. The first of the two parameters which describe the system,R′A, is the product of a flow rateRA(given in m2sec−1) and a term set by material properties and related to thermal inertia. The second parametert′0is the product of a time scalet0and another term set by material properties. The timet0can be interpreted either as the time the flow has been active or the time it takes for old surface to be covered by new flow. The analytical model predicts a spectrum withFλ∝ λ3in the near infrared, with an exponential cutoff at short enough wavelengths and a slower turnover at long wavelengths. Slightly more elaborate versions of the model predict the way in which the spectrum should change when the flow rate varies with time. Measurements of such variations will provide a critical test of the model.
The Solid-State Imaging (SSI) instrument provided the first high- and medium-resolution views of Io as the Galileo spacecraft closed in on the volcanic body in late 1999 and early 2000. While each volcanic center has many unique features, the majority can be placed into one of two broad categories. The "Promethean" eruptions, typified by the volcanic center Prometheus, are characterized by long-lived steady eruptions producing a compound flow field emplaced in an insulating manner over a period of years of decades. In contrast, "Pillanian" eruptions are characterized by large pyroclastic deposits and short-lived but high effusion rate eruptions from fissures feeding open-channel or open-sheet flows. Both types of eruptions commonly have ~100-km-tall, bright, SO2-rich plumes forming near the flow fronts and smaller deposits of red material that mark the vent for the silicate lavas.
The combination of Voyager images and newly acquired Galileo images with low illumination and resolutions ranging from 2 to 6 km/pixel now allows determination of the global distribution of mountains and volcanic centers on Io. The mountains generally do not have characteristics typical of terrestrial volcanic landforms, they are evenly distributed across the surface and show no obvious correlation with known hot spots or plumes. Relative elevations, determined by shadow measurements and stereoscopy, indicate that mountains in the newly imaged area range in elevation up to at least 7.6 km. The origin of the mountains remains uncertain. Some appear to be multitiered volcanic constructs; others enclosing the partial remains of large circular depressions appear to be remnants of old volcanoes; yet others show extensive tectonic disruption. Volcanic centers also appear to be distributed evenly across the surface except for an apparently somewhat lower density at high latitudes. The low latitudes have one volcanic center per 7 × 104km2, and, on average, the centers are spaced roughly 250 km apart. The global distribution of high mountains suggests that the lithosphere over most of Io is thick. Although the thickness cannot be calculated, the previously suggested 30 km appears reasonable as a lower limit. The high rates of resurfacing combined with the likely dissipation of most of the tidal energy in the asthenosphere and underlying mantle implies a very low temperature gradient in the upper part of the lithosphere and steep gradients in the lower lithosphere. The slow rate of separation of melt from host rock in the magma source regions as a consequence of the low gravity on Io, coupled with the high rate of magma production, will likely result in larger melt fractions than is typical for source regions on Earth. The variety of volcanic landforms suggests that volcanic products with a range of compositions are deposited on the surface. This mixture will be carried downward through the lithosphere as a consequence of the 0.5–1.5 cm/yr resurfacing rates. During descent, the more volatile components will tend to be driven off early, but complete or near-complete melting at the base of the lithosphere may result in rehomogenization of the silicate mixture that remains.
If Io has been volcanically active through much of its history, it must be highly differentiated. We present an initial attempt to quantify the differentiation of the silicate portion of Io. We suggest that, on average, each part of Io has undergone about 400 episodes of partial melting. We employ a widely used thermodynamic model of silicate melts to examine the effect of such repeated differentiation. Despite many caveats, including a grossly ove