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Geological analysis of Monad Regio, Triton: Possible evidence of endogenic and exogenic processes
Abstract
The surface morphology of Triton is believed to be predominantly influenced by endogenic processes, such as tectonism, volcanism, diapirism and geyser-like eruptions. To understand how some of these processes have reshaped the surface of the satellite during its evolutionary history, we performed a geological analysis of an extended area located in Monad Regio, where several of these endogenic processes may have taken place. We produced a geomorphological map at 1:1,000,000 scale derived from the joint analysis of the Voyager images, a digital elevation model and a roughness map of the study area. Such approach allowed us to delimit with higher accuracy the geomorphological units already known in literature, as planitiae, cantaloupe and smooth terraced terrains, paterae, knobby, and smooth terraced terrains, but it also enabled us to identify new ones (hummocky terrain, inner planitia, inner patera). We reconstructed the relative ages between such terrains, and we found that after an endogenic phase, an exogenic phase could have followed, during which several geological events reshaped the area. These comprise depositional and erosional processes, including the movement of ice and the subglacial flowing of liquid nitrogen. Such observations imply that Monad Regio could be strongly influenced by exogenic processes, perhaps still active today.
Observations of Triton’s rotational light curve from Table Mountain Observatory in 2017 show that volatile transport is continuing on its surface. Changes in the light curve are consistent with sustained increases in albedo in two patches near the equator at longitudes of ∼120° and ∼280°, as first reported by Bauer et al. based on observations obtained with the Hubble Space Telescope. An early observation of Triton’s visual albedo shows a substantially lower albedo in the past, when the northern polar region was visible. This result confirms the hypothesis of Moore & Spencer and Spencer & Moore of a net flux of volatile material into the southern hemisphere of Triton over much of the satellite’s history and up to the present. The averaged V -band geometric albedo of Triton has been stable over the past three decades. The color of Triton has also been stable during the past decade.
Neptune's moon Triton shares many similarities with Pluto, including volatile cycles of N2, CH4 and CO, and represents a benchmark case for the study of surface-atmosphere interactions on volatile-rich Kuiper Belt objects. The observations of Pluto by New Horizons acquired during the 2015 flyby and their analysis with volatile transport models (VTMs) shed light on how volatile sublimation-condensation cycles control the climate and shape the surface of such objects. Within the context of New Horizons observations as well as recent Earth-based observations of Triton, we adapt a Plutonian VTM to Triton, and test its ability to simulate its volatile cycles, thereby aiding our understanding of its climate.
Here we present numerical VTM simulations exploring the volatile cycles of N2, CH4 and CO on Triton over long-term and seasonal timescales (cap extent, surface temperatures, surface pressure, sublimation rates) for varying model parameters (including the surface ice reservoir, albedo, thermal inertia, and the internal heat flux). We explore what scenarios and model parameters allow for a best match of the available observations. In particular, our set of observational constraints include Voyager 2 observations (surface pressure and cap extent), ground-based near-infrared (0.8–2.4 μm) disk-integrated spectra (the relative surface area of volatile vs. non-volatile ice) and the evolution of surface pressure as retrieved from stellar occultations.
Our results show that Triton's poles act as cold traps for volatile ices and favor the formation of polar caps extending to lower latitudes through glacial flow or through the formation of thinner seasonal deposits. As previously evidenced by other VTMs, North-South asymmetries in surface properties can favor the development of one cap over the other. Our best-case simulations are obtained for a bedrock surface albedo of 0.6–0.7, a global reservoir of N2 ice thicker than 200 m, and a bedrock thermal inertia larger than 500 SI (or smaller but with a large internal heat flux). The large N2 ice reservoir implies a permanent N2 southern cap (several 100 m thick) extending to the equatorial regions with higher amounts of volatile ice at the south pole, which is not inconsistent with Voyager 2 images but does not fit well with observed full-disk near-infrared spectra. Our results also suggest that a small permanent polar cap exists in the northern (currently winter) hemisphere if the internal heat flux remains relatively low (e.g. radiogenic, < 3 mW m⁻²). A non-permanent northern polar cap was only obtained in some of our simulations with high internal heat flux (30 mW m⁻²). The northern cap will possibly extend to 30°N in the next decade, thus becoming visible by Earth-based telescopes. On the basis of our model results, we also discuss the composition of several surface units seen by Voyager 2 in 1989, including the bright equatorial fringe and dark surface patches.
Finally, we provide predictions for the evolution of ice distribution, surface pressure and CO and CH4 atmospheric mixing ratios in the next decades. According to our model, the surface pressure should slowly decrease but remain larger than 0.5 Pa by 2060. We also model the thermal lightcurves of Triton for different climate scenarios in 2022, which serve as predictions for future James Webb Space Telescope observations.
The topography of Neptune’s large icy moon Triton could reveal important clues to its internal evolution, but has been difficult to determine. New global digital color maps for Triton have been produced as well as topographic data for <40% of the surface using stereogrammetry and photoclinometry. Triton is most likely a captured Kuiper Belt dwarf planet, similar though slightly larger in size and density to Pluto, and a likely ocean moon that exhibited plume activity during Voyager 2′s visit in 1989. No surface features or regional deviations of greater than ±1 km amplitude are found. Volatile ices in the southern terrains may take the form of extended lobate deposits 300–500 km across as well as dispersed bright materials that appear to embay local topography. Limb hazes may correlate with these deposits, indicating possible surface–atmosphere exchange. Triton’s topography contrasts with high relief up to 6 km observed by New Horizons on Pluto. Low relief of (cryo)volcanic features on Triton contrasts with high-standing massifs on Pluto, implying different viscosity materials. Solid-state convection occurs on both and at similar horizontal scales but in very different materials. Triton’s low relief is consistent with evolution of an ice shell subjected to high heat flow levels and may strengthen the case of an internal ocean on this active body.
Triton is an important signpost in understanding the diverse populations of both ocean worlds and Kuiper Belt objects (KBOs). As a likely ocean world, it is unique by virtue of its kidnapped history from the Kuiper Belt: its large orbital inclination makes it the only ocean world thought to be primarily heated by obliquity tides. It is volatile-rich due to its formation in the outer solar system and its unusual surface geology may be the product of cryovolcanism. Observations from New Horizons and Cassini motivate re-examination of Triton data sets and models, with value for comparative planetology of ocean worlds and KBOs, most notably with Europa, Enceladus, Titan, and Pluto. We re-explore old data sets with the new perspective of the importance of ocean worlds in our solar system and the search for life.
Pluto, Titan and Triton all have low-temperature environments with an N2, CH4 and CO atmospheric composition in which solar radiation drives an intense organic photochemistry. Titan is rich in atmospheric hazes, and Cassini–Huygens observations showed that their formation is initiated with the production of large molecules through ion-neutral reactions. New Horizons revealed that optical hazes are also ubiquitous in Pluto’s atmosphere, and it is thought that similar haze formation pathways are active in this atmosphere as well. However, we show here that Pluto’s hazes may contain a major organic ice component (dominated by C4H2 ice) from the direct condensation of the primary photochemical products in this atmosphere. This contribution may imply that haze has a less important role in controlling Pluto’s atmospheric thermal balance compared to Titan. Moreover, we expect that the haze composition of Triton is dominated by C2H4 ice.
Band ogives are a striking and enigmatic feature of Mer de Glace glacier flow. The surface mass balances (SMBs) of these ogives have been thoroughly investigated over a period of 12 years. We find similar cumulative SMBs over this period, ranging between −64.1 and −66.2 m w.e., on the dark and light ogives even though the dark ogive albedo is ~40% lower than that of the light ogives. We, therefore, looked for another process that could compensate for the large difference of absorbed short-wave radiation between dark and light ogives. Based on in situ roughness measurements, our numerical modeling experiments demonstrate that a significant difference in turbulent flux over the dark and light ogives due to different surface roughnesses could compensate for the difference in radiative forcing. Our results discard theories for the genesis of band ogives that are based on the assumption of a strong ice ablation contrast between dark and light ogives. More generally, our study demonstrates that future roughness changes are as important to analyze as the radiative impacts of a potential increase of aerosols or debris at the surface of glaciers.
Titan's lower atmosphere has condensed phases in equilibrium with it. It is well known that there is a liquid phase in lakes and liquid condensates in the lower troposphere that are part of the liquid cycle in Titan's system. This work analyzes further the phase equilibria using CRYOCHEM equation of state by accounting for solid phases that can exist at the extremely cold conditions on Titan, as well as the possibility of a second liquid phase in the deep lakes, all within the ternary system nitrogen-methane-ethane. Titan's fluid exhibits rich variation of phases that allow us to show multiphase equilibria in full action. The multiphase equilibria covered in this work are two-phase (vapor-liquid, liquid-liquid, solid-liquid, solid-vapor, solid-solid), three-phase (vapor-liquid-liquid, vapor-liquid-solid, vapor-solid-solid, liquid-liquid-solid), and four-phase equilibria (solid-liquid-liquid-vapor); issues associated with the unphysical phases in the calculations are also discussed. The results of multiphase-equilibria analysis describe new understandings about Titan's surface, troposphere, and subsurface lake liquids, as described in the conclusions.
We investigate the T-c% phase diagram of the binary system N2-CO. From changes in IR spectra of all kinds of mode excitations (phonons, vibrons) we were able to determine the temperature of phase transitions (solid-solid, solid-liquid). The improvements in comparison to structural investigations by x-rays or electrons are the following: sample growing and handling with perfect optical and thermodynamic quality; determination of actual concentration (N2)x(CO)y from optical spectra; reduction of thermal hysteresis by careful cooling-heating cycles of the samples.
Triton is one of the few bodies in the solar system with observed cryo-volcanic activity, in the form of plumes at its south pole, which suggests large-scale surface volatile transport over time. Triton's large variations in obliquity have motivated prior predictions of changing atmospheric column densities of several orders of magnitude, driven by seasonal evaporation of surface volatiles. Using the Hubble Space Telescope, we directly imaged Triton's surface and have detected large-scale differences in increased and decreased reflectance when compared with Voyager data at UV, visual, and methane-band wavelengths. Our surface map shows regions of increased brightness at near-equatorial latitudes and near the Neptune-facing side, and darkened regions near longitudes of ±180°, indicating the presence of ongoing seasonal volatile transport.
Spectra of Triton between 1.8 and 5.5 μm, obtained in 2007 May and 2009 November, have been analyzed to determine the global surface composition. The spectra were acquired with the grism and the prism of the Infrared Camera on board AKARI with spectral resolutions of 135 and 22, respectively. The data from 4 to 5 μm are shown in this Letter and compared to the spectra of N2, CO, and CO2, i.e., all the known ices on this moon that have distinct bands in this previously unexplored wavelength range. We report the detection of a 4σ absorption band at 4.76 μm (2101 cm–1), which we attribute tentatively to the presence of solid HCN. This is the sixth ice to be identified on Triton and an expected component of its surface because it is a precipitating photochemical product of Triton's thin N2 and CH4 atmosphere. It is also formed directly by irradiation of mixtures of N2 and CH4 ices. Here we consider only pure HCN, although it might be dissolved in N2 on the surface of Triton because of the evaporation and recondensation of N2 over its seasonal cycle. The AKARI spectrum of Triton also covers the wavelengths of the fundamental (1-0) band of β-phase N2 ice (4.296 μm, 2328 cm–1), which has never been detected in an astronomical body before, and whose presence is consistent with the overtone (2-0) band previously reported. Fundamental bands of CO and CO2 ices are also present.
Multibeam surveys can provide detailed bathymetry data for the continental slope from which quantitative descriptors of the seabed terrain (e.g., slope) may be obtained. We illustrate the value of these descriptors for benthic habitat mapping, and highlight the advantages of multiscale analysis. We examine the application of these descriptors as predictor variables for species distribution models, which are particularly valuable in the deep sea where opportunities to directly survey the benthic fauna remain limited. Our initial models are encouraging and suggest that wider adoption of these methods may assist the delivery of ecologically relevant information to marine resource managers.
On the basis of Voyager 2 observations, a simple model of Triton's thermosphere and ionosphere is built. Heating and ionization by solar EUV and precipitating electrons are computed. Our main conclusions are: (i) solar EUV flux at Triton is large enough to account for the observed exospheric temperature (95 K), but not for the observed electron densities (ii) Although N2+ is the principal product of ionization, it is rapidly quenched in collisions with CH4 or H2 and N+ is the major ion (iii) the ionosphere is in chemical equilibrium below 700 km and in diffusive equilibrium above (iv) Ionization by a maxwellian distribution of precipitating electrons with average energy 900 eV and total input 0.1 erg cm−3 s−1 can reasonably well account for the observed ionosphere.
At least two active plumes were observed on Neptune's moon Triton during the Voyager 2 flyby in 1989. Models for Triton's plumes have previously been grouped into five hypotheses, two of which are primarily atmospheric phenomena and are generally considered unlikely, and three of which include eruptive processes and are plausible. These hypotheses are compared, including new arguments, such as comparisons based on current understanding of Mars, Enceladus, and Pluto. An eruption model based on a solar-powered, solid-state greenhouse effect was previously considered the leading hypothesis for Triton's plumes, in part due to the proximity of the plumes to the subsolar latitude during the Voyager 2 flyby and the distribution of Triton's fans that are putatively deposits from former plumes. The other two eruption hypotheses are powered by internal heat, not solar insolation. Based on new analyses of the ostensible relation between the latitude of the subsolar point on Triton and the geographic locations of the plumes and fans, we argue that neither the locations of the plumes nor fans are strong evidence in favor of the solar-powered hypothesis. We conclude that all three eruption hypotheses should be considered further. Five tests are presented that could be implemented with remote sensing observations from future spacecraft to confidently distinguish among the eruption hypotheses for Triton's plumes. The five tests are based on the: (1) composition and thickness of Triton's southern hemisphere terrains, (2) composition of fan deposits, (3) distribution of active plumes, (4) distribution of fans, and (5) surface temperature at the locations of plumes and/or fans. The tests are independent, but complementary, and implementable with a single flyby mission such as the Trident mission concept. We note that, in the case of the solar-driven hypothesis, the 2030s and 2040s may be the last chance for approximately a century to observe actively erupting plumes on Triton.
The largest moon of Neptune, Triton, possesses a cold and hazy atmosphere. Since the discovery of the near-surface haze layer during the Voyager fly in 1989, the haze formation mechanism has not been investigated in detail. Here we provide the first haze microphysical model on Triton. Our model solves the evolution of both size and porosity distributions of haze particles in a self-consistent manner. We simulated the formation of sphere and aggregate hazes with and without condensation of the C2H4 ice. The haze particles can grow into fractal aggregates with mass-equivalent sphere sizes of ∼0.1-1 μm and fractal dimensions of D f = 1.8-2.2. The ice-free hazes cannot simultaneously explain both UV and visible observations of Voyager 2, while including the condensation of C2H4 ices provides two better solutions. For ice aggregates, the required total haze mass flux is ∼2 10-15 g cm-2 s-1. For the icy sphere scenario, the column-integrated C2H4 production rate is ∼8 10-15 g cm-2 s-1, and the ice-free mass flux is ∼6 10-17 g cm-2 s-1. The UV occultation observations at short wavelengths, <0.15 μm, may slightly favor the icy aggregates. Observations of the haze optical depth and the degree of forward scattering in UV and visible should be able to distinguish whether Triton's hazes are icy spheres or ice aggregates in future Triton missions. © 2021. The American Astronomical Society. All rights reserved..
Viscous flow features, including lobate debris aprons and lineated valley fill, are common ice-related features found across the mid-latitudes of Mars. These features are commonly found along the dichotomy boundary and around massifs where snow and ice can accumulate and flow to topographic lows during periods of higher obliquity. We have identified and mapped glacial-related features reminiscent of lineated valley fill and lobate debris aprons in the lower mid-latitudes of Arcadia Planitia based on morphology, albedo, thermal infrared reflectance, thermal inertia, and subsurface radar reflections. Three sinuous features show surface morphologies and supporting spectral properties indicative of massive buried ice, including distinct boundaries, arcuate and linear banding, deep radar reflections, dielectric constants similar to water-ice, expanded secondary craters, and proximity to lobate debris aprons. We interpret the sinuous and lobate features of Arcadia Planitia to be buried debris-covered glaciers formed during higher obliquity periods in Mars' recent past. These sinuous features appear to be channelized ice that once flowed but currently reside in a flat-lying region. Although their origin is unclear, we consider an interpretation analogous to stagnated terrestrial ice streams due to a lack of obvious topographic dependence. The abundant evidence of widespread near-surface ice in a flat-lying lower mid-latitude region of Mars makes Arcadia Planitia a potentially favorable site for future missions due to the high potential for in situ resource utilization.
The NASA Ames Stereo Pipeline is a suite of free and open source automated geodesy and stereogrammetry tools designed for processing stereo images captured from satellites (around Earth and other planets), robotic rovers, aerial cameras, and historical images, with and without accurate camera pose information. It produces cartographic products, including digital terrain models, ortho-projected images, 3-D models, and bundle-adjusted networks of cameras. Ames Stereo Pipeline's data products are suitable for science analysis, mission planning, and public outreach.
Triton, the largest Solar System satellite beyond Saturn, was probably captured from the Transneptunian population by Neptune. It is mainly covered by N2, CO, CO2, CH4 and H2O in solid state and, except for H2O and CO2, these species are also present in gas phase. Sublimation and recondensation of the volatile species may lead to geographical and temporal variation on the surface composition, and could participate to the formation of complex chemical compounds formed from photochemistry occurring in the atmosphere (Krasnopolsky and Cruikshank, 1999) or from irradiation of N2 : CH4 : CO layers (Moore and Hudson, 2003). We present new near-IR observations performed at the VLT-ESO with SINFONI in 2010, 2011 and 2013, from which band depths, areas and positions of the main ice features are determined at different longitudes. We also re-reduce and re-analyze earlier data obtained during the last 20 years. Models based on the Hapke theory (Hapke, 1993) are developed to constrain the abundance, grain size, temperature, and state of mixing of the different ices (N2, CH4, CO, CO2, H2O) as well as attempt to identify other species. For this purpose, we present and use new optical constants of CO2 measured at 35 and 54 K. Our analyses confirm the longitudinal variation of the N2 and CO surface abundances previously evidenced, and suggest additional latitudinal and/or temporal variability of these two species. We confirm the presence of deep N2 layers in which CO and CH4 are diluted. In particular, we demonstrate that CO is present in diluted (as opposed to pure ice) form. In contrast, our models support the presence of small and longitudinally variable amounts of pure CH4 ice, providing an explanation to the enhanced atmospheric CH4/N2 ratio over expectations based on an ideal N2-CH4-CO mixture. They also suggest significant smaller particles of H2O and CO2 than reported previously in Quirico et al. (1999), with CO2 being probably distributed over a large area of the surface. We find that the 2.40 µm band is primarily due to ¹³CO ice, with a telluric value of the ¹³CO/¹²CO ratio, and not to ethane. We infer a N2 ice temperature of 37.5 ± 1K, suggesting that the atmospheric pressure over 2010–2013 was similar to that during the Voyager 1989 epoch.
The 2015 New Horizons flyby has produced the first high-resolution maps of morphology and topography of Pluto and Charon, the most distant objects so mapped. Global integrated mosaics of Pluto were produced using both LORRI framing camera and MVIC line scan camera data, showing the best resolution data obtained for all areas of the illuminated surface, ∼78% of the body. A unique feature of the Pluto imaging data set is the observation of terrains illuminated only by light scattered from atmospheric haze, allowing us to map terrains in the southern hemisphere that would otherwise have been in darkness. MVIC 4-color data were combined with the panchromatic map to produce full color global maps. Digital elevation models (DEMs) over ∼42% of Pluto were produced using combinations of MVIC hemispheric scans and LORRI mosaics, from which slopes at scales of ∼1 km can be determined. Pluto can be divided into regions each with distinct topographic signatures, corresponding with major physiographic terrain types. Large areas of Pluto are comprised of low-relief moderately cratered plains units. Deeply pitted and glaciated plains east of Sputnik Planitia are elevated ∼0.7 km. The most dominant topographic feature on Pluto is the 1200-by-2000-km wide depression enclosing the bright Sputnik Planitia ice sheet, the surface of which is 2.5-to-3.5 km deep (relative to the rim) and ∼2 km deep relative to the mean radius. The partial ring of steep-sided massifs, several of which are more than 5 km high, along the western margins of Sputnik Planitia produce some of the locally highest and steepest relief on Pluto, with slopes of 40–50°. The second major topographic feature is a complex, eroded, ridge-trough system ∼300–400 km wide and at least 3200 km long extending north-to-south along the 155° meridian. This enormous structure has several kilometers of relief. It may predate the large impact event forming the basin, though some post-Sputnik Planitia deformation is evident. The large depressed, partially walled plain, Hyecho Palus, lies due southwest of Sputnik Planitia. Near the center of Hyecho Palus lie the circular constructional edifices Wright and Piccard Montes. Wright Mons rises 4.5 km above these plains, with a central depression ∼4.5 km deep, whereas Piccard Mons, best observed in haze-light, rises ∼5.5 km above the plains but has a bowl-shaped central depression ∼5.5 km below the plains for a total relief of up to 11 km, the greatest observed on Pluto. Both of these features are interpreted as constructional (volcanic?) in nature. Additional prominent topographic features include a 2–3 km high and ∼600 km wide dome centered on the illuminated IAU pole and the amoeboidal plateaus of “bladed” terrains in the equatorial region, which rise 2–5 km above local terrains and are the highest standing geologic units on the encounter hemisphere. The mean elevations in the integrated DEM for the two radio occultation areas are consistent with the 5–6 km difference in elevation as determined independently by the radio experiment, and a limb profile near the egress point confirms the presence of elevated bladed terrains in that area. Local relief of 3–5 km at massifs, troughs and pits supports conclusions that the icy shell of Pluto is relatively rigid. Numerous examples of topographic control of ice or frost deposition occur across Pluto, including the distinct coloration of the polar dome, the elevated terrains of eastern Tombaugh Regio, and along the ridge-trough system, where ridge tops and fossae rims are covered in different ices than at lower elevations. The topographic hypsogram of Pluto's encounter hemisphere is strongly bimodal due to the large Sputnik Planitia depression. Otherwise the topographic signature of Pluto is controlled by deviations from the otherwise dominant low plains, including elevated bladed terrain plateaus and the depressed volcanic province including Wright and Piccard Montes.
Methane ice dunes on Pluto
Wind-blown sand or ice dunes are known on Earth, Mars, Venus, Titan, and comet 67P/Churyumov-Gerasimenko. Telfer et al. used images taken by the New Horizons spacecraft to identify dunes in the Sputnik Planitia region on Pluto (see the Perspective by Hayes). Modeling shows that these dunes could be formed by sand-sized grains of solid methane ice transported in typical Pluto winds. The methane grains could have been lofted into the atmosphere by the melting of surrounding nitrogen ice or blown down from nearby mountains. Understanding how dunes form under Pluto conditions will help with interpreting similar features found elsewhere in the solar system.
Science , this issue p. 992 ; see also p. 960
Pluto's landscape is shaped by the cycles of the volatile ices covering its surface. In particular, the Sputnik Planitia (SP) ice sheet displays a large diversity of terrains, with bright and dark plains, pits, topographic depressions and evidences of recent and past glacial flows. Outside SP, New Horizons also revealed numerous N2 ice deposits, in Tombaugh Regio and at mid-northern latitudes. These observations suggest a complex history involving volatile and glacial processes on different timescales. We present numerical simulations of volatile transport on Pluto performed with a model able to simulate the N2 cycle over millions of years (Myrs), taking into account the changes of obliquity and orbital parameters as experienced by Pluto. Results show that over one obliquity cycle, the latitudes of SP between 25{\deg}S-30{\deg}N are dominated by N2 condensation, while the latitudes between 30-50{\deg}N are dominated by N2 sublimation. We find that a net amount of 1 km of ice has sublimed at the northern edge of SP during the last 2 Myrs. By comparing these results with the observed geology of SP, we can relate the formation of the pits and the brightness of the ice to the ice flux occurring at the annual timescale, while the glacial flows at its eastern edge and the erosion of the water ice mountains all around the ice sheet are related to the astronomical timescale. We also perform simulations with a glacial flow scheme which shows that SP is currently at its minimum extent. We also explore the stability of N2 ice outside SP. Results show that it is not stable at the poles but rather in the equatorial regions, in particular in depressions, where thick deposits may persist over tens of Myrs, before being trapped in SP. Finally, another key result is that the minimum and maximum surface pressures obtained over the simulated Myrs remain in the range of mm-Pa and Pa, respectively.
From Earth based observations Pluto is known to be the host of N2, CH4 and CO ices and also a dark red material. Very limited spatial distribution information is available from rotational visible and near-infrared spectral curves obtained from hemispheric measurements. In July 2015 the New Horizons spacecraft reached Pluto and its satellite system and recorded a large set of data. The LEISA spectro-imager of the RALPH instruments are dedicated to the study of the composition and physical state of the materials composing the surface. In this paper we report a study of the distribution and physical state of the ices and non-ice materials on Pluto's illuminated surface and their mode and degree of mixing. Principal Component analysis as well as various specific spectral indicators and correlation plots are used on the first set of 2 high resolution spectro-images from the LEISA instrument covering the whole illuminated face of Pluto at the time of the New Horizons encounter. Qualitative distribution maps have been obtained for the 4 main condensed molecules, N2, CH4, CO, H2O as well as for the visible-dark red material. Based on specific spectral indicators, using either the strength or the position of absorption bands, these 4 molecules are found to indicate the presence of 3 different types of ices: N2-rich:CH4:CO ices, CH4-rich(:CO:N2?) ices and H2O ice. The mixing lines between these ices and with the dark red material are studied using scatter plots between the various spectral indicators. CH4 is mixed at the molecular level with N2, most probably also with CO, thus forming a ternary molecular mixture that follows its phase diagram with low solubility limits. The occurrence of a N2-rich – CH4-rich ices mixing line associated with a progressive decrease of the CO/CH4 ratio tells us that a fractionation sublimation sequence transforms one type of ice to the other forming either a N2-rich – CH4-rich binary mixture at the surface or an upper CH4-rich ice crust that may hide the N2-rich ice below. The strong CH4-rich – H2O mixing line witnesses the subsequent sublimation of the CH4-rich ice lag left behind by the N2:CO sublimation (N spring-summer), or a direct condensation of CH4 ice on the cold H2O ice (S autumn). The weak mixing line between CH4-containing ices and the dark red material and the very sharp spatial transitions between these ices and this non-volatile material are probably due to thermal incompatibility. Finally the occurrence of a H2O ice – red material mixing line advocates for a spatial mixing of the red material covering H2O ice, with possibly a small amount intimately mixed in water ice. From this analysis of the different materials distribution and their relative mixing lines, H2O ice appears to be the substratum on which other ices condense or non-volatile organic material is deposited from the atmosphere. N2-rich ices seem to evolve to CH4-dominated ices, possibly still containing traces of CO and N2, as N2 and CO sublimate away. The spatial distribution of these materials is very complex.
Pluto's Sputnik Planitia is a bright, roughly circular feature that resembles a polar ice cap. It is approximately 1,000 kilometres across and is centred on a latitude of 25 degrees north and a longitude of 175 degrees, almost directly opposite the side of Pluto that always faces Charon as a result of tidal locking. One explanation for its location includes the formation of a basin in a giant impact, with subsequent upwelling of a dense interior ocean. Once the basin was established, ice would naturally have accumulated there. Then, provided that the basin was a positive gravity anomaly (with or without the ocean), true polar wander could have moved the feature towards the Pluto-Charon tidal axis, on the far side of Pluto from Charon. Here we report modelling that shows that ice quickly accumulates on Pluto near latitudes of 30 degrees north and south, even in the absence of a basin, because, averaged over its orbital period, those are Pluto's coldest regions. Within a million years of Charon's formation, ice deposits on Pluto concentrate into a single cap centred near a latitude of 30 degrees, owing to the runaway albedo effect. This accumulation of ice causes a positive gravity signature that locks, as Pluto's rotation slows, to a longitude directly opposite Charon. Once locked, Charon raises a permanent tidal bulge on Pluto, which greatly enhances the gravity signature of the ice cap. Meanwhile, the weight of the ice in Sputnik Planitia causes the crust under it to slump, creating its own basin (as has happened on Earth in Greenland). Even if the feature is now a modest negative gravity anomaly, it remains locked in place because of the permanent tidal bulge raised by Charon. Any movement of the feature away from 30 degrees latitude is countered by the preferential recondensation of ices near the coldest extremities of the cap. Therefore, our modelling suggests that Sputnik Planitia formed shortly after Charon did and has been stable, albeit gradually losing volume, over the age of the Solar System. © 2016 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
Pluto has a variety of surface frosts and landforms as well as a complex atmosphere. There is ongoing geological activity related to the massive Sputnik Planum glacier, mostly made of nitrogen (N2) ice mixed with solid carbon monoxide and methane, covering the 4-kilometre-deep, 1,000-kilometre-wide basin of Sputnik Planumnear the anti-Charon point. The glacier has been suggested to arise from a source region connected to the deep interior, or from a sink collecting the volatiles released planetwide. Thin deposits of N2 frost, however, were also detected at mid-northern latitudes and methane ice was observed to cover most of Pluto except for the darker, frost-free equatorial regions. Here we report numerical simulations of the evolution of N2, methane and carbon monoxide on Pluto over thousands of years. The model predicts N2 ice accumulation in the deepest low-latitude basin and the threefold increase in atmospheric pressure that has been observed to occur since 1988. This points to atmospheric-topographic processes as the origin of Sputnik Planum's N2 glacier. The same simulations also reproduce the observed quantities of volatiles in the atmosphere and show frosts of methane, and sometimes N2, that seasonally cover the mid- and high latitudes, explaining the bright northern polar cap reported in the 1990sand the observed ice distribution in 2015. The model also predicts that most of these seasonal frosts should disappear in the next decade.
Observations of Pluto's surface made by the New Horizons spacecraft indicates present-day nitrogen ice glaciation in and around the basin known as Sputnik Planum. Motivated by these observations, we have developed an evolutionary glacial flow model of solid nitrogen ice taking into account its published thermophysical and rheologies properties. This model assumes that glacial ice layers flow laminarly and have low aspect ratios which permits a vertically integrated mathematical formulation. We assess the conditions for the validity of laminar nitrogen ice motion by revisiting the problem of the onset of solid-state buoyant convection of nitrogen ice for a variety of bottom thermal boundary conditions. Subject to uncertainties in nitrogen ice rheology, nitrogen ice layers are estimated to flow laminarly for thicknesses less than 400-1000 meters. The resulting mass-flux formulation for when the nitrogen ice flows as a laminar dry glacier is characterized by an Arrhenius-Glen functional form. The flow model developed is used here to qualitatively answer some questions motivated by observed glacial flow features found on Sputnik Planum. We find that the wavy transverse dark features found along the northern shoreline of Sputnik Planum may be a transitory imprint of shallow topography just beneath the ice surface suggesting the possibility that a major shoreward flow event happened relatively recently within the last few hundred years. Model results also support the interpretation that the prominent darkened features resembling flow lobes observed along the eastern shoreline of the Sputnik Planum basin may be a result of wet nitrogen glacial ice flowing into the basin from the pitted highlands of eastern Tombaugh Regio.
New Horizons unveils the Pluto system
In July 2015, the New Horizons spacecraft flew through the Pluto system at high speed, humanity's first close look at this enigmatic system on the outskirts of our solar system. In a series of papers, the New Horizons team present their analysis of the encounter data downloaded so far: Moore et al. present the complex surface features and geology of Pluto and its large moon Charon, including evidence of tectonics, glacial flow, and possible cryovolcanoes. Grundy et al. analyzed the colors and chemical compositions of their surfaces, with ices of H 2 O, CH 4 , CO, N 2 , and NH 3 and a reddish material which may be tholins. Gladstone et al. investigated the atmosphere of Pluto, which is colder and more compact than expected and hosts numerous extensive layers of haze. Weaver et al. examined the small moons Styx, Nix, Kerberos, and Hydra, which are irregularly shaped, fast-rotating, and have bright surfaces. Bagenal et al. report how Pluto modifies its space environment, including interactions with the solar wind and a lack of dust in the system. Together, these findings massively increase our understanding of the bodies in the outer solar system. They will underpin the analysis of New Horizons data, which will continue for years to come.
Science , this issue pp. 1284 , 10.1126/science.aad9189 , 10.1126/science.aad8866 , 10.1126/science.aae0030 , & 10.1126/science.aad9045
Compositional analyses of Pluto's surface ice in the literature typically include large areas on the body where CH4 and other volatiles are segregated in the pure form from the solid solution N2:CH4 in which CH4 is diluted. However, the existence of continent-size areas of pure CH4 are in conflict with both of the alternative models that successfully explain the enhancement of CH4 in Pluto's atmosphere, the Detailed Balancing thermal equilibrium model and the Hot Methane Patch model. Pluto's spectrum includes an apparently unshifted CH4 component while Triton's does not, and 93% of the concentration range of the binary phase diagram at 38 K shows that these species exist as a mixture of two saturated solid solution phases. Recognizing this, we propose that both of these saturated phases are present on Pluto and the CH4-rich phase of the mixture, CH4:N2, is the source of the relatively unshifted CH4 spectrum attributed to pure CH4. We also propose that CH4 is less abundant in Triton's ice to the point where either the ice is not saturated or the saturated CH4:N2 phase has not been detected. In this scenario, the partial vapor pressures do not change when the relative proportions of these saturated phases are varied in the mixture. Thus, the partial vapor pressures are independent of N2-CH4 concentrations if both saturated phases are present. Accordingly, the longitudinal and seasonal variations of CH4 and N2 features in Pluto's spectrum would be attributed to spatial variations in the relative proportions of these species. This may occur during volatile transport in the sublimation wind through extensive influences. The lower, unsaturated, values of the mole fraction of CH4 in the ice reported by Owen et al. (Owen et al. [1993]. Science 261, 745-748) and Cruikshank et al. (Cruikshank, D.P., Rush, T.L., Owen, T.C., Quirico, E., de Bergh, C. [1998]. The surface compositions of Triton, Pluto, and Charon. In: Solar System Ices. Astrophysics and Space Science Library Series, vol. 227. Kluwer Academic Publishers, Dordrecht), and by Doute et al. (Doute, S., Schmitt, B., Quirico, E., Owen, T.C., Cruikshank, D.P., de Bergh, C., Geballe, T.R., Roush, T.L. [1999]. Icarus 142, 421-444) based on a compositional analysis of Pluto's surface, were not obtained using optical constants for components consistent with the constraints of the phase diagram.
We investigate the origins of Triton’s deformed and young surface. Assuming Triton was captured early in Solar System history, the bulk of the energy released during capture will have been lost, and cannot be responsible for its present-day activity. Radiogenic heating is sufficient to maintain a long-lived ocean beneath a conductive ice shell, but insufficient to cause convective deformation and yielding at the surface. However, Triton’s high inclination likely causes a significant (≈0.7°) obliquity, resulting in large heat fluxes due to tidal dissipation in any subsurface ocean. For a 300 km thick ice shell, the estimated ocean heat production rate (≈0.3 TW) is capable of producing surface yielding and mobile-lid convection. Requiring convection places an upper bound on the ice shell viscosity, while the requirement for yielding imposes a lower bound. Both bounds can be satisfied with an ocean temperature ≈240 K for our nominal temperature-viscosity relationship, suggesting the presence of an antifreeze such as . In our view, Triton’s geological activity is driven by obliquity tides, which arise because of its inclination. In contrast, Pluto is unlikely to be experiencing significant tidal heating. While Pluto may have experienced ancient tectonic deformation, we do not anticipate seeing the kind of young, deformed surfaces seen at Triton.
We present a study of coupled thermal and structural evolution of
Neptune's moon, Triton, driven by tidal dissipation and radiogenic
heating. Triton's orbital history likely involves capture from a binary
system by Neptune, followed by a period of circularization. This work
investigates Triton's evolution past its circularization. We examine the
rate of ice shell growth as a function of different orbital
eccentricities, in the presence of radiogenic heating. Tidal dissipation
in the ice shell, proportional to orbital eccentricity squared,
concentrates heating near the base, reducing the basal heat flux. As the
growth of the ice shell is proportional to the basal heat flux,
increased tidal heating creates a blanketing effect, reducing the rate
of ice shell growth. Radiogenic heating from Triton's core is the other,
more dominant, source of heat to the shell. Despite being several orders
of magnitude higher than the tidal dissipation, radiogenic heating alone
fails to sustain an ocean within Triton over 4.5 Ga. For orbital
eccentricities of 5 × 10-7 and 3 ×
10-5 it takes approximately 2 Ga and 3 Ga, respectively, to
completely freeze the ocean. For higher values of orbital
eccentricities, an ocean can be sustained in Triton's interior over 4.5
Ga. If Triton's history past circularization involves a slow decrease in
orbital eccentricity to the current value, a thin, possibly
NH3-rich ocean exists beneath Triton's icy shell.
The structure of Triton's lower atmosphere is determined primarily through interactions with its surface. The temperature of N2 frost on the surface determines the atmospheric pressure and temperature near the surface. Transport of latent heat by winds in the atmosphere acts to keep the N2 surface frosts at a single temperature. Condensation of N2 and photochemically produced hydrocarbons creates discrete clouds and a pervasive haze in the atmosphere. Winds in the atmosphere have a large effect on the thermal structure and are likely responsible for a negative temperature gradient near the surface, defining a troposphere. At an altitude of 8 km, thermal conduction becomes an important process and the temperature gradient becomes positive. Condensation of the atmosphere is probably responsible for what appears to be a large polar cap covering the southern hemisphere.
Triton, with a diameter of ≡2700 km, is Neptune's only planet-class satellite. The complexity of Triton's surface and the variety of surface features is unequaled among the satellites of the solar system. From a geologic viewpoint, some of Triton's features have apparently familiar morphologies and general interpretative agreement exists. However, many of its landforms have novel morphologies and geologic settings, which have given rise to a number of innovative and competing interpretations. The first portion of this chapter describes Triton's surface in primarily nongenetic terms. The authors then review various models and speculations regarding geologic processes that have operated on Triton, followed by an interpretive stratigraphy and geologic history.
We investigate the near-surface properties of Europa's ice shell by examining small crater morphology; we use both primary and secondary craters for our analysis. For primary craters, the simple-to-complex transition provides an estimate of 0.19–0.36 MPa for the cratering effective strength. Through the aggregate statistics of over 100 profiles, we find that secondary craters on Europa tend toward smaller depth-to-diameter (d/D) ratios than primary craters, consistent with observations on other cratered surfaces (the Moon and Mars). In addition, we find that adjacent secondaries tend to be more shallow than distant secondaries, also consistent with trends seen on the Moon and Mars. The presence of numerous and far-flung secondaries requires a solid, competent surface for spallation that is inconsistent with weak ice. Although the effective strength evaluated by crater scaling laws does not have a direct quantitative relationship to lab-based measurements of shear or tensile strength, comparison to other observationally derived effective strengths and geologic materials suggests that the mechanical properties of Europa's surface ice should be consistent with terrestrial measurements of cold ice.
The polar terrains of Mars are covered in many places with irregular
pits and retreating scarps, as are some of the surfaces of the
outer-planet satellites (e.g., Io, Europa, and Triton). These features
are diagnostic of exogenic degradation due to the loss of a volatile
rock-forming matrix or cement. It has gone generally unrecognized that
the same (or very similar) geologic process responsible for the martian
polar terrains also operates on some of the outer-planet satellites. The
development of many of the scarps on Triton, and the depressions they
surround, appears to involve scarp recession, as the planimetric traces
of these scarps are inconsistent with formation either by faulting or as
flow fronts. Scarp recession can occur on the Earth when a structural or
stratigraphic inhomogeneity near the scarp base is mechanically
weakened. On Triton, where conditions are unfavorable for some of the
processes which cause scarp retreat such as erosion by abrasion from
wind, rainfall, or channelized running fluid, mechanical weakening of
material exposed in the face of a scarp probably involves the loss of a
cementing or matrix-forming material by sublimation.
Voyager 2 observed that, at the height of a 'major' southern summer, the southern 'cap' of Triton extends nearly to the equator, and the northern temperature regions (where they can be seen) are relatively dark even though nitrogen frost should be accumulating at those latitudes. This observation suggests a hypothesis that there has been a net nonreversable flux of surficial volatiles on Triton from the northern hemisphere to the southern hemisphere over much of the satellite's history and up to the present. The north-south hemispherical albedo difference is permanent and overwhelms the seasonal effect on volatile transport. This hypothesis assumes that volatile erosion, transport, and deposition have been in the form of sublimation, advection, and precipitation.
The Interactive Spectral Interpretation System (ISIS) is designed to
facilitate the interpretation and analysis of high resolution X-ray
spectra like those obtained using the grating spectrographs on Chandra
and XMM and the microcalorimeter on Astro-E. It is being developed as an
interactive tool for studying the physics of X-ray spectrum formation,
supporting measurement and identification of spectral features, and
interaction with a database of atomic structure parameters and plasma
emission models. The current version uses the atomic data and
collisional ionization equilibrium models in the Astrophysical Plasma
Emission Database (APED) of Brickhouse et.al., and also provides access
to earlier plasma emission models including Raymond-Smith and MEKAL.
Although the current version focuses on collisional ionization
equilibrium plasmas, the system is designed to allow use of other
databases to provide better support for studies of non-equilibrium and
photoionized plasmas. To maximize portability between Unix operating
systems, ISIS is being written entirely in ANSI C using free-software
components (CFITSIO, PGPLOT and S-Lang).
New spectra of Pluto were obtained with the Gemini Near-Infrared
Spectrometer (GNIRS) on the Gemini South 8-m telescope covering the
region 1.9-2.5 µm. We have analyzed these data and two spectra of
Triton with particular emphasis on a weak absorption feature detected at
2.405 μm. While this wavelength is coincident with a 13CO
absorption band that is the isotopic variant of the 12CO band
(2.35 μm) seen on both Pluto and Triton, our analysis, supported by
new lab spectra of CO, shows that the strength of the 2.405-μm band
is much too great to be attributed to any plausible abundance of
13CO. Instead, we identify this band as the 2.4045 μm
absorption of pure ethane in solid form (Quirico & Schmitt Icarus
127, 354, 1997). Published models of the spectra of Triton (Quirico et
al. Icarus 139, 159, 1999) and Pluto (Douté et al. Icarus 142,
421, 1999) show small variations from the data at 2.28 μm. The
addition of absorption from the ethane band at 2.274 μm removes this
small discrepancy. We do not see evidence for the 2.461 μm ethane
band, although this is a somewhat noisy region of both spectra. Other
investigators (Nakamura et al. P.A.S. Japan 52, 551, 2000) noted that
Pluto's absorption bands at 2.28 and 2.32 μm are best fit with
ethane, but their 2.405 μm region is discrepant with ethane. At
longer wavelengths, Sasaki et al. (Ap.J. 618, L57, 2005) noted that
models fit their Pluto data best when ethane was added, but they did not
clearly identify ethane bands. Estimates of the abundances of ethane on
Triton and Pluto suggest that this ice is deposited on relatively short
time-scales by precipitation from the atmosphere, where it is produced
by photochemistry (Krasnopolsky & Cruikshank JGR 100, 21271, 1995;
JGR 104, 21979, 1999).
The detection of induced magnetic fields in the vicinity of the jovian satellites Europa, Ganymede, and Callisto is one of the most surprising
findings of the Galileo mission to Jupiter. The observed magnetic signature cannot be generated in solid ice or in silicate rock. It rather suggests
the existence of electrically conducting reservoirs of liquid water beneath the satellites’ outermost icy shells that may contain even more water
than all terrestrial oceans combined. The maintenance of liquid water layers is closely related to the internal structure, composition, and thermal
state of the corresponding satellite interior. In this study we investigate the possibility of subsurface oceans in the medium-sized icy satellites and
the largest trans-neptunian objects (TNO’s). Controlling parameters for subsurface ocean formation are the radiogenic heating rate of the silicate
component and the effectiveness of the heat transfer to the surface. Furthermore, the melting temperature of ice will be significantly reduced by
small amounts of salts and/or incorporated volatiles such as methane and ammonia that are highly abundant in the outer Solar System. Based on the
assumption that the satellites are differentiated and using an equilibrium condition between the heat production rate in the rocky cores and the heat
loss through the ice shell, we find that subsurface oceans are possible on Rhea, Titania, Oberon, Triton, and Pluto and on the largest TNO’s 2003
UB313, Sedna, and 2004 DW. Subsurface oceans can even exist if only small amounts of ammonia are available. The liquid subsurface reservoirs
are located deeply underneath an ice-I shell of more than 100 km thickness. However, they may be indirectly detectable by their interaction with
the surrounding magnetic fields and charged particles and by the magnitude of a satellite’s response to tides exerted by the primary. The latter is
strongly dependent on the occurrence of a subsurface ocean which provides greater flexibility to a satellite’s rigid outer ice shell.
We present here a search for solid ethane, C2H6, on the surfaces of Pluto and Triton, based on near-infrared spectral observations in the H and K bands (1.4–2.45 μm) using the Very Large Telescope (VLT) and the United Kingdom Infrared Telescope (UKIRT). We model each surface using a radiative transfer model based on Hapke theory (Hapke, B. [1993]. Theory of Reflectance and Emittance Spectroscopy. Cambridge University Press, Cambridge, UK) with three basic models: without ethane, with pure ethane, and with ethane diluted in nitrogen. On Pluto we detect weak features near 2.27, 2.405, 2.457, and 2.461 μm that match the strongest features of pure ethane. An additional feature seen at 2.317 μm is shifted to longer wavelengths than ethane by at least 0.002 μm. The strength of the features seen in the models suggests that pure ethane is limited to no more than a few percent of the surface of Pluto. On Triton, features in the H band could potentially be explained by ethane diluted in N2, however, the lack of corresponding features in the K band makes this unlikely (also noted by Quirico et al. (Quirico, E., Doute, S., Schmitt, B., de Bergh, C., Cruikshank, D.P., Owen, T.C., Geballe, T.R., Roush, T.L. [1999]. Icarus 139, 159–178)). While Cruikshank et al. (Cruikshank, D.P., Mason, R.E., Dalle Ore, C.M., Bernstein, M.P., Quirico, E., Mastrapa, R.M., Emery, J.P., Owen, T.C. [2006]. Bull. Am. Astron. Soc. 38, 518) find that the 2.406-μm feature on Triton could not be completely due to 13CO, our models show that it could not be accounted for entirely by ethane either. The multiple origin of this feature complicates constraints on the contribution of ethane for both bodies.
The sublimation and condensation of ices play a very important role in the formation of planetary systems, in the evolution of some solar system bodies as well as in the equilibrium and matter exchanges between surface and atmosphere of most planets and satellites. The precise knowledge of vapour pressure of molecular solids at all relevant temperatures is mandatory, but the published sublimation relations are not always accurate enough. All published experimental measurements, and empirical and thermodynamical relations for the vapour pressure of 53 different species in their pure solid phases are reviewed. For several species, we also calculate the vapour pressure with accurate thermodynamic formulae from the triple point down to low temperatures. For 27 species (H2O, O2, O3, CO, CO2, CH3OH, HCOOH, CH4, C2H2, C2H4, C2H6, C6H6, HCN, HC3N, C2N2, C4N2, N2, NH3, NO, N2O, H2S, SO2, AsH3, Ne, Ar, Kr, and Xe) we are able to propose vapour pressure relations, either empirical or theoretical, valid over a large range of temperatures, representative of astrophysical environments. All these relations are more accurate than those currently used in the astrophysical literature. In particular, most of the relations commonly used in the astrophysical literature are based on the data reported by Lide (2006) in the CRC Handbook of Chemistry and Physics, which are inaccurate for several compounds. The most problematic case is CO ice, for which a sublimation relation extrapolated from the liquid–gas equilibrium (Fanale and Salvail, 1990) is used in most of the models simulating the activity of comet nuclei.
This paper presents the analysis of near-infrared observations of the icy surface of Triton, recorded on 1995 September 7, with the cooled grating spectrometer CGS4 at the United Kingdom Infrared Telescope (Mauna Kea, HI). This analysis was performed in two steps. The first step consisted of identifying the molecules composing Triton's surface by comparing the observations with laboratory transmission spectra (direct spectral analysis); this also gives information on the physical state of the components. Most of the bands in Triton's spectrum were assigned to specific vibration bands of the CH4, N2, CO, and CO2 molecules previously discovered. A detailed comparison of the frequencies of the CH4 bands confidently indicated that this molecule exists in a diluted state in solid β-N2. Three new bands peaking at 5717, 5943, and 6480 cm−1 (1.749, 1.683, and 1.543 μm, respectively) were also observed. Laboratory experiments have shown that C2H6 isolated in solid N2 fits well the second band, but this would imply the appearance of unobserved bands and thus rules out this assignment. However, C2H6 may exist in another physical state, and more experiments are necessary. No plausible candidate was found for these three bands when comparing with the spectra of nine molecules (C2H2, C2H4, C3H8, NH3, SO2, HC3N, CH3OH, NO, NO2).
Ice streams influence ice sheet mass balance and stability but key aspects of their behaviour remain poorly understood. This paper reviews and discusses one very important aspect: what controls their location in an ice sheet? Seven potential controls on ice stream location are identified from the literature: topographic focusing, topographic steps, macro-scale bed roughness, calving margins, subglacial geology, geothermal heat flux and subglacial meltwater routing. For each control, the theoretical basis for its link to rapid ice flow is introduced, followed by discussion of the evidence of its influence on the location of both contemporary and palaeo-ice streams. Based on this new synthesis, topographic focusing, subglacial geology, meltwater routing and calving margins appear to be most commonly associated with fast ice flow. It is clear that rather than a single control, however, there exist a number of potential controls of varying influence. We propose a hierarchy of factors, with those occurring at the top of the hierarchy exerting a stronger influence on ice stream location, and where present beneath an ice sheet, are very likely to be associated with fast flow. Those factors occurring lower down the hierarchy are less commonly associated with ice streaming but appear to be influential in the absence of more common controls. In such a hierarchy topographic focusing in the presence of a calving margin is the primary control. In the absence of this, ice streams will preferentially occur in areas with favourable subglacial meltwater routing and subglacial geology. In the absence of these, bed roughness, geothermal heat flux and topographic steps may promote ice streaming. Significantly, the primary controls on a given ice stream location are likely to influence its spatial and temporal dynamics. Ice streams governed by the presence of meltwater routing and/or calving processes might exhibit more variable behaviour because these controls can vary over relatively short time-scales compared to controls that vary over longer time-scales, e.g. geothermal heat flux, subglacial roughness, geology and topography. Identifying the controls on ice stream location is therefore of paramount importance when understanding ice stream longevity and their past and future activity.
New mapping reveals 100 probable impact craters on Triton wider than 5 km diameter. All of the probable craters are within 90° of the apex of Triton's orbital motion (i.e., all are on the leading hemisphere) and have a cosine density distribution with respect to the apex. This spatial distribution is difficult to reconcile with a heliocentric (Sun-orbiting) source of impactors, be it ecliptic comets, the Kuiper Belt, the scattered disk, or tidally-disrupted temporary satellites in the style of Shoemaker–Levy 9, but it is consistent with head-on collisions, as would be produced if a prograde population of planetocentric (Neptune-orbiting) debris were swept up by retrograde Triton. Plausible sources include ejecta from impact on or disruption of inner/outer moons of Neptune. If Triton's small craters are mostly of planetocentric origin, Triton offers no evidence for or against the existence of small comets in the Kuiper Belt, and New Horizons observations of Pluto must fill this role. The possibility that the distribution of impact craters is an artifact caused by difficulty in identifying impact craters on the cantaloupe terrain is considered and rejected. The possibility that capricious resurfacing has mimicked the effect of head-on collisions is considered and shown to be unlikely given current geologic constraints, and is no more probable than planetocentrogenesis. The estimated cratering rate on Triton by ecliptic comets is used to put an upper limit of ∼50 Myr on the age of the more heavily cratered terrains, and of ∼6 Myr for the Neptune-facing cantaloupe terrain. If the vast majority of cratering is by planetocentric debris, as we propose, then the surface everywhere is probably less than 10 Myr old. Although the uncertainty in these cratering ages is at least a factor ten, it seems likely that Triton's is among the youngest surfaces in the Solar System, a candidate ocean moon, and an important target for future exploration.
This report arises from an ongoing program to monitor Neptune’s largest moon Triton spectroscopically in the 0.8 to 2.4 μm range using IRTF/SpeX. Our objective is to search for changes on Triton’s surface as witnessed by changes in the infrared absorption bands of its surface ices N2,CH4,H2O, CO, and CO2. We have recorded infrared spectra of Triton on 53 nights over the ten apparitions from 2000 to 2009. The data generally confirm our previously reported diurnal spectral variations of the ice absorption bands (Grundy and Young, 2004). Nitrogen ice shows a large amplitude variation, with much stronger absorption on Triton’s Neptune-facing hemisphere. We present evidence for seasonal evolution of Triton’s N2 ice: the 2.15 μm absorption band appears to be diminishing, especially on the Neptune-facing hemisphere. Although it is mostly dissolved in N2 ice, Triton’s CH4 ice shows a very different longitudinal variation from the N2 ice, challenging assumptions of how the two ices behave. Unlike Triton’s CH4 ice, the CO ice does exhibit longitudinal variation very similar to the N2 ice, implying that CO and N2 condense and sublimate together, maintaining a consistent mixing ratio. Absorptions by H2O and CO2 ices show negligible variation as Triton rotates, implying very uniform and/or high latitude spatial distributions for those two non-volatile ices.
Infrared spectra and radiation chemical behavior of N2-dominated ices relevant to the surfaces of Triton and Pluto are presented. This is the first systematic IR study of proton-irradiated N2-rich ices containing CH4 and CO. Experiments at 12 K show that HCN, HNC, and diazomethane (CH2N2) form in the solid phase, along with several radicals. NH3 is also identified in irradiated N2 + CH4 and N2 + CH4 + CO. We show that HCN and HNC are made in irradiated binary ice mixtures having initial N2/CH4 ratios from 100 to 4, and in three-component mixtures have an initial N2/(CH4 + CO) ratio of 50. HCN and HNC are not detected in N2-dominated ices when CH4 is replaced with C2H6, C2H2, or CH3OH.The intrinsic band strengths of HCN and HNC are measured and used to calculate G(HCN) and G(HNC) in irradiated N2 + CH4 and N2 + CH4 + CO ices. In addition, the HNC/HCN ratio is calculated to be ∼1 in both icy mixtures. These radiolysis results reveal, for the first time, solid-phase synthesis of both HCN and HNC in N2-rich ices containing CH4.We examine the evolution of spectral features due to acid–base reactions (acids such as HCN, HNC, and HNCO and a base, NH3) triggered by warming irradiated ices from 12 K to 30–35 K. We identify anions (OCN−, CN−, and N3−) in ices warmed to 35 K. These ions are expected to form and survive on the surfaces of Triton and Pluto. Our results have astrobiological implications since many of these products (HCN, HNC, HNCO, NH3, NH4OCN, and NH4CN) are involved in the syntheses of biomolecules such as amino acids and polypeptides.
THE recent Voyager encounter established certain facts about Triton's atmosphere: the surface pressure is in the range 1.5-1.9 Pa (15-19 μbar)1; the surface temperature is 38±3 K (ref. 2); molecular nitrogen is the dominant atmospheric constituent3; hazes and clouds are visible not only on the limb but also against the surface4; the wind in the southern hemisphere is to the north-east at low altitudes (as shown by streaks on the surface4) and to the west at high altitudes (as shown by geyser-like plume tails4). Triton rotates with a period of 5.877 days in a right-hand sense about the south pole, where the season now is late spring4. Here we argue that these features can be explained if Triton, like Mars5, has a global, well-structured atmosphere in equilibrium with surface frosts. The subliming frost cap produces a polar anticyclone at low altitudes, with northeastward winds of ∼5 m s-1 within the Ekman boundary layer. The temperature contrast between the cold frost-covered pole and the warm unfrosted equator produces westward winds at high altitudes.
The model of Summers and Strobel (1989) for photochemical reactions in the Uranus atmosphere was modified and used for quantitative calculations of methane in the atmosphere of Triton. The principal adjustable parameters in the new model are the surface CH4 concentrations and the vigor of vertical mixing in Triton's lower atmosphere. It is shown the rate of methane photolysis that was calculated is sufficient to generate a smog of condensed C2H2, C2H4, C2H6, and C4H2 particles in the lowest 30 km of Triton's atmosphere, with an optical depth consistent with the Voyager imaging results.
A quantitative analysis of error sources in 1D planetary photoclinometry is presented. The technique is affected by error sources arising from the spacecraft image, the planetary body, and the scan line orientation. Slope errors are calculated for each of these sources, using examples of Voyager imaging of Ganymede and Viking orbiting imaging of Mars. Slope errors are investigated for a variety of viewing and lighting geometries, slope angles, and slope orientations. The results are broken down into nonsystematic and systematic errors. Derivations that allow the calculation of photoclinometric slope errors for any photometric function are presented, and the implications of these results for 2D photoclinometric techniques are discussed.
Cantaloupe terrain on Neptune's large, icy satellite Triton comprises an organized cellular pattern of noncircular dimples that structurally and geologically most closely resemble salt diapirs exposed on Earth. The mean separation of these cells is 47 km. Modeling of the cells as compositionally driven diapirs suggests that cantaloupe terrain forms by gravity-driven overturn within an ice crust about 20 km thick with a maximum viscosity of 10 exp 22 Pa s. These diapirs probably formed as a result of a density inversion in a layered crust composed partly of ice phases other than water ice.
Several possible configurations for volatiles on Triton are assessed. It is concluded that the simplest volatile configuration which best satisfies the constraints with the least number of ad hoc assumptions is N2 and CH4 both in solid forms, perhaps partly as a microscopic mixture, but more probably as a disequilibrium assemblage, nonuniformly distributed. Thermodynamic equilibrium is then limited by seasonal transport and the finite diffusion time of CH4 in crystalline N2. Although a nitrogen ocean cannot be excluded, it requires very restrictive assumptions.