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(a) A dilational band (provisionally named Phaidra Linea) in the equatorial trailing hemisphere. Note how features can be matched up to either side of the band. Although the dilational band is one of the younger features in this region, a number of younger ridges and troughs can be seen to crosscut the band (from Galileo observation 11ESREGMAP01). (b) A 17-km-wide ridged band in the northern leading hemisphere (from Galileo image mosaic 11ESMORPHY01). (c) The dark dilational band Yelland Linea in the "Wedges" region, Argadnel Regio (from Galileo image mosaics 12ESWEDGE_01/02/03 superimposed on C3ESWEDGES01).
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Europa has experienced significant tectonic disruption over its visible history. The descrip- tion, interpretation, and modeling of tectonic features imaged by the Voyager and Galileo mis- sions have resulted in significant developments in four key areas addressed in this chapter: (1) The characteristics and formation mechanisms of the various type...
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... Also referred to as pull-apart bands (see chapter by Prockter and Patterson), dilational bands represent clear evidence of prolonged dilation in the icy shell and hence a resurfacing process on the icy moon. A dilational band is a tabular zone of new crustal material that intruded between the progressively dilating walls of a tension fracture (Fig. 6a). The surface of this material ap- Fig. 5. Theoretical distribution of cycloids that grow from east to west (Hurford et al., 2007a) in a stress field composed of a diurnal component plus stress due to 1° of nonsynchronous rotation. East- growing cycloids will have an opposite curvature in each hemisphere but a similar overall ...
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... featureless in lower-resolution images, in which case the term smooth band can be used as a descriptor; however, if an internal geometry of fine lineations is observable (usu- ally at medium to high resolution, as in Fig. 6a), the term lineated band may be used. Complete separation and infill of the surface is evidenced by the fact that the ridged plains to either side of a dilational band typically match up, im- plying that the dilational band material represents new sur- face area (Schenk and McKinnon, 1989;Sullivan et al., 1998). The exact source of the ...
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... respon- sible for driving the dilation is unable to sustain spreading beyond a certain time and/or width limit. Nonetheless, a di- lational band formation event likely represents a prolonged period of uninterrupted tectonic extension. Discrete episodes of dilation are evidenced by an internal fabric of fine linea- ments within lineated bands (Fig. 6a) that commonly form a bilateral symmetry about a central axis and exhibit a con- sistent spacing on the order of ~500 m Prockter et al., 1999Prockter et al., , 2002Stempel et al., 2005). If these lineaments are normal faults, creating a type of lineated band called a faulted band, they may perhaps form a graben-like system centered ...
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... dilational bands have been noted to exhibit a maxi- mum dilation near the center of the length of the band, with dilation decreasing toward either tip, such as Thynia Linea (Pappalardo and Sullivan, 1996) and Yelland Linea (Fig. 6c) (Stempel et al., 2005). Such dilational bands resemble typi- cal cracks in an elastic layer that are dilated by a regional tensile stress acting perpendicular to the feature. Nonethe- less, some dilational bands are more rhomboidal where they occur in extensional stepover zones (i.e., pull-aparts) along strike-slip faults (see section ...
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... dilational bands (Marshall and Kattenhorn, 2005), including Thynia Linea ( Pappalardo and Sullivan, 1996;Tufts et al., 2000), wedge-shaped bands in Argadnel Regio (Prockter et al., 2002), and the prominent example of the "Sickle" (provisionally named Phaidra Linea) in the equatorial trailing hemisphere ( Tufts et al., 2000;Prockter et al., 2002) (Fig. 6a). In these examples, the open- ing vector across each dilational band is constant, result- ing in portions of the band having undergone oblique dila- tion relative to the ...
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... apparent type of lineated dilational band morphologi- cally similar to a faulted band, but composed of ridges, is a tabular spreading zone referred to here as a ridged band (cf. Greeley, 2000, 2004) (Fig. 6b). Stempel et al. (2005) used this same terminology to refer to faulted bands; however, we abandon that use of the nomenclature because ridges (which have an explicit meaning on Europa) are not the dominant feature in faulted bands. In low-reso- lution imagery, ridged bands may be mistaken for smooth bands if the internal lineated ...
Citations
... However, the stress patterns associated with eccentricity do not change with either of these quantities, so our results are not dependent upon the exact values we chose. In contrast, stress magnitudes scale linearly with eccentricity and with semimajor axis to the −1/3 (e.g., Kattenhorn & Hurford, 2009). We picked an eccentricity value of 0.01, which is the current eccentricity of Europa and used Charon's current semimajor axis. ...
... There, we assumed that fractures in a given region were a combination of fractures formed there and fractures that propagated through the region but formed elsewhere. This assumption makes sense when looking at Europa's fractures, which can span hundreds to thousands of km (e.g., Kattenhorn & Hurford, 2009). We, thus, reported all orientations for which tidal stresses decomposed along a fracture of that orientation would be greater than a particular stress threshold. ...
... We have not evaluated a stress field that combines NSR stress with eccentricity driven stress, which could affect the overall pattern of fracture orientations. However, the lack of north-south trending fractures on Charon is inconsistent with patterns expected from NSR for equatorial or midlatitudes (e.g., Kattenhorn & Hurford, 2009). ...
In 2015, the New Horizons spacecraft flew past Pluto and its moon Charon, providing the first clear look at Charon's surface. New Horizons images revealed an ancient surface, a large, intricate canyon system, and many fractures, among other geologic features. Here, we assess whether tidal stresses played a significant role in the formation of Charon's tensile fractures. Although presently in a circular orbit, most scenarios for Charon's orbital evolution include an eccentric orbit for some period of time and possibly an internal ocean. Past work has shown that these conditions could have generated stresses comparable in magnitude to other tidally fractured moons, such as Europa and Enceladus. However, we find no correlation between observed fracture orientations and those predicted to form due to eccentricity driven tidal stress. It, thus, seems more likely that Charon's orbit circularized before its ocean froze and that either tidal stresses alone were insufficient to fracture the surface or subsequent resurfacing removed these ancient fractures.
... However, the stress patterns associated with eccentricity do not change with either of these quantities, so our results are not dependent upon the exact values we chose. In contrast, stress magnitudes scale linearly with eccentricity and with semimajor axis to the −1/3 (e.g., Kattenhorn and Hurford, 2009). We picked an eccentricity value of 0.01, which is the current eccentricity of Europa, and used Charon's current semimajor axis. ...
... This assumption makes sense when looking at Europa's fractures, which can span 100s to 1000s of km (e.g. Kattenhorn and Hurford, 2009). We, thus, reported all orientations for which tidal stresses decomposed along a fracture of that orientation would be greater than a particular stress threshold. ...
... However, the lack of north-south trending fractures on Charon is inconsistent with patterns expected from NSR for equatorial or midlatitudes (e.g. Kattenhorn and Hurford, 2009). ...
In 2015, the New Horizons spacecraft flew past Pluto and its moon Charon, providing the first clear look at the surface of Charon. New Horizons images revealed an ancient surface, a large, intricate canyon system, and many fractures, among other geologic features. Here, we assess whether tidal stresses played a significant role in the formation of tensile fractures on Charon. Although presently in a circular orbit, most scenarios for the orbital evolution of Charon include an eccentric orbit for some period of time and possibly an internal ocean. Past work has shown that these conditions could have generated stresses comparable in magnitude to other tidally fractured moons, such as Europa and Enceladus. However, we find no correlation between observed fracture orientations and those predicted to form due to eccentricity-driven tidal stress. It thus seems more likely that the orbit of Charon circularized before its ocean froze, and that either tidal stresses alone were insufficient to fracture the surface or subsequent resurfacing remove these ancient fractures.
A combination of geological interpretations and thermal-orbital evolution models imply that Pluto's large moon, Charon, had a subsurface water (and possibly ammonia) ocean that eventually froze. Ocean freezing generates large tensile stresses in the upper part of the ice shell and pressurizes the ocean below, perhaps leading to the formation of Charon's large canyons and putative cryovolcanic flows. Here, we identify the conditions in which a freezing ocean could create fractures that fully penetrate its ice shell, linking Charon's surface with its ocean and facilitating ocean-sourced cryovolcanism. We find that current models of Charon's interior evolution predict ice shells that are far too thick to be fully cracked by the stresses associated with ocean freezing. Either Charon's ice shell was <10 km thick when the flows occurred (as opposed to >100 km) or the surface was not in direct communication with the ocean as part of the eruptive process. If Charon's ice shell had been thin enough to be fully cracked, it would imply substantially more ocean freezing than is indicated by the canyons, Serenity and Mandjet Chasma. Due to the low radiogenic heating within Charon and the loss of tidal heating early in its history, a thin ice shell should have been short-lived, implying that ocean-sourced cryovolcanic flows would have ceased relatively early in Charon's history, consistent with interpretations of its surface geology. An additional (and perhaps implausibly large) heat source would be required to generate the substantially larger ocean implied by through-going fractures. We also find that ocean freezing can easily generate deep fractures that do not fully penetrate to the ocean, which may be the foundation of Charon's canyons.
Plain language summary
When ocean-bearing moons begin to cool down, their oceans can freeze. As new ice accretes to the bottom of the existing ice shell, the added volume of the ice can stress the shell. Pluto's largest moon, Charon, has canyons and cryovolcanic flows that may have formed in response to a freezing ocean. Here, we model the formation of fractures within Charon's ice shell as the ocean underneath it freezes to explore the evolution of Charon's interior and surface. We find that an ocean source for cryovolcanic flows is unlikely because the ice shell would have had to be much thinner than current thermal evolution models imply. However, freezing the ocean may have produced the stresses that formed canyons later in Charon's history.
Large fractures on Europa's surface have been extensively studied for the insight they provide into the evolution of the satellite's ice shell. The presence of small fractures has not been characterized on a global or regional scale because of the resolution limits in the current image data. However, if smaller fractures are present they could significantly increase the overall fracture abundance and influence the mechanical properties of the ice shell. Regional variability in this abundance would also impact the spatial heterogeneity of ice shell characteristics. To investigate the possible contribution of smaller fractures to the influence of the fracture network on the behavior of the ice lithosphere, we measure the surface length of visible fractures in regions on Europa's surface and calculate their length distributions and fracture intensities. These outcomes are extrapolated, using best-fit functions, to consider the possible abundance and role of smaller fractures in these networks. The analysis of the visible fractures sampled different geologic terrains and geographic regions to consider the effect of terrain type and location. Fracture abundance in observable populations shows some variation with terrain type. However, observable abundance also changes with image resolution, indicating that current data limit the interpretation of visible fracture populations. Typical log-normal, exponential, and power-law fracture network distributions, based on analysis of terrestrial fracture networks, were considered as methods for extrapolating the observed fracture populations. Power-law was found to be the most appropriate extrapolation choice. It was also found to predict observed fracture abundance in one sample region with a smaller sub-region that had locally higher image resolution. Applying a power-law extrapolation for the fracture abundance as a function of length, we predict fracture abundance characteristics that are up to two orders of magnitude higher than abundance of observed fractures, and which vary by an order of magnitude between regions. Thus, the predicted fracture populations indicate that abundant small fractures would contribute significantly to the mechanical behavior of the ice lithosphere and create regional variations in fracture abundance. For example, where they are abundant, these smaller fractures could contribute to fracture networks providing localized migration pathways for plume activity on Europa. New image data from the Europa Clipper mission should enable fracture mapping at smaller scales to refine the roles of these networks and the smaller fractures on Europan lithospheric processes and rheology.
The first images of Jupiter's moon Europa from the Voyager missions sparked the curiosity about the lineament system appearing at the surface. Curiosity quickly turned into profound interest, following the discovery of its subsurface ocean, which may harbour life below the thick ice crust. This study revisits Europa and reinvestigates its surface using high-resolution Galileo data and mapping one of its most characteristic surface patterns: the complex network of lineaments. The analysis of the morphological type of over two hundred lineaments and their crosscutting relationship based on relative age indicate the influence of cyclical tidal force, orbital forcing, and the nonsynchronous rotation triggered periodic stress in the formation of three characteristic lineaments-generations, along with additional and unaccounted forces that may contribute to surface renewal. Despite ongoing debate, which suggests that Earth-like tectonism (e.g., subduction) is less likely plausible on Europa, the result of this study calls for some process acting along with tidal forces, causing shortening, horizontal movement and increasing extension in the ice plate located in the region of a suspected subduction (or low-relief subsumption) zone and may be part of the “subducting” crust. This study raises new questions and encourages additional scientific discussions, which, along with the Europa Clipper and JUICE missions, will help to understand the nature of Europa's ocean and ice-tectonic processes.
Bodily tides provide key information on the interior structure, evolution, and origin of the planetary bodies. Our Solar system harbors a very diverse population of planetary bodies, including those composed of rock, ice, gas, or a mixture of all. While a rich arsenal of geophysical methods has been developed over several years to infer knowledge about the interior of the Earth, the inventory of tools to investigate the interiors of other Solar-system bodies remains limited. With seismic data only available for the Earth, the Moon, and Mars, geodetic measurements, including the observation of the tidal response, have become especially valuable and therefore, played an important role in understanding the interior and history of several Solar system bodies. To use tidal response measurements as a means to obtain constraints on the interior structure of planetary bodies, appropriate understanding of the viscoelastic reaction of the materials from which the planets are formed is needed. Here, we review the fundamental aspects of the tidal modeling and the information on the present-day interior properties and evolution of several planets and moons based on studying their tidal response. We begin with an outline of the theory of viscoelasticity and tidal response. Next, we proceed by discussing the information on the tidal response and the inferred structure of Mercury, Venus, Mars and its moons, the Moon, and the largest satellites of giant planets, obtained from the analysis of the data that has been provided by space missions. We also summarize the upcoming possibilities offered by the currently planned missions.
Raman spectroscopic imaging facilitates the detection of submicron‐scale features to identify their composition with the added benefit of visualizing their spatial relationships and textures. This powerful technique has proven useful in ascertaining the origins of and reconstructing the evolution of solar system materials through the characterization of macromolecular carbon and mineral assemblages in meteorites, interplanetary dust particles (IDPs), and terrestrial rocks. In Raman imaging microscopy the Raman instrument is mated with an optical microscope to achieve a spatial resolution ∼300 nm with the capability of resolving sample features on the micron and submicron spatial scales. In preparation for surface exploration and sample return missions of solar system bodies, planetary scientists rely on comparative analog studies of samples from terrestrial environments. Geologic deposits on Earth and other planetary bodies undergo several iterations of geochemical change over time resulting in often subtle structural and compositional differences that can obscure the origins of those deposits. The origins, formation mechanisms, and evolution of both terrestrial and planetary deposits may be recorded in different forms of organic compounds and mineral assemblages, often as barely discernible nanocrystalline phases, inclusions, and mineral intergrowths. Raman spectroscopic imaging is an excellent tool with which to visualize the relationships between different planetary minerals and to detect the subtle differences, especially when many of the same organic compounds and minerals occur in so many different types of planetary environments. It will certainly continue to play a significant role in new discoveries pertaining to understanding the evolution of the solar system, potentially habitable worlds, and life beyond Earth.
Cassini measurements of Mimas' physical libration can be explained by either a non-hydrostatic core or a global, liquid water ocean beneath a 24–31 km thick ice shell. An ocean within Mimas would be surprising, given the lack of comparable geologic activity to that observed on other ocean-bearing moons like Europa and Enceladus, and thus has important implications for the prevalence and identification of ocean worlds. Here, we calculate the tidal heating that would be generated within an ocean-bearing Mimas and determine the ice shell thicknesses that would result. Our model accounts for tidal heating caused by Mimas' eccentricity and libration and uses a depth-dependent rheological profile in the ice. We find that the ability of Mimas to host a 24–31 km ice shell over an ocean depends on the rheology of the ice, the surface temperature, and the basal heat flux. We find that, using the most reasonable assumptions, Mimas would have the suggested ocean and ice shell thicknesses today. We report corresponding average surface heat fluxes for these cases and discuss observational data that could further assess whether Mimas is, indeed, a present-day ocean world.
Jupiter’s moon Europa harbors one of the most likely environments for extant extraterrestrial life. Determining whether Europa is truly habitable requires understanding the structure and thickness of its ice shell, including the existence of perched water or brines. Stereo-derived topography from images acquired by NASA Galileo’s Solid State Imager (SSI) of Europa are often used as a constraint on ice shell structure and heat flow, but the uncertainty in such topography has, to date, not been rigorously assessed. To evaluate the current uncertainty in Europa’s topography we generated and compared digital terrain models (DTMs) of Europa from SSI images using both the open-source Ames Stereo Pipeline (ASP) software and the commercial SOCET SET® software. After first describing the criteria for assessing stereo quality in detail, we qualitatively and quantitatively describe both the horizontal resolution and vertical precision of the DTMs. We find that the horizontal resolution of the SOCET SET® DTMs is typically 8–11× the root mean square (RMS) pixel scale of the images, whereas the resolution of the ASP DTMs is 9–13× the maximum pixel scale of the images. We calculate the RMS difference between the ASP and SOCET SET® DTMs as a proxy for the expected vertical precision (EP), which is a function of the matching accuracy and stereo geometry. We consistently find that the matching accuracy is ~0.5 pixels, which is larger than well-established “rules of thumb” that state that the matching accuracy is 0.2–0.3 pixels. The true EP is therefore ~1.7× larger than might otherwise be assumed. In most cases, DTM errors are approximately normally distributed, and errors that are several times the derived EP occur as expected. However, in two DTMs, larger errors (differences) occur and correlate with real topography. These differences primarily result from manual editing of the SOCET SET® DTMs. The product of the DTM error and the resolution is typically 4–8 pixel2 if calculated using the RMS image scale for SOCET SET® DTMs and the maximum images scale for the ASP DTMs, which is consistent with recent work using martian data sets and suggests that the relationship applies more broadly. We evaluate how ASP parameters affect DTM quality and find that using a smaller subpixel refinement kernel results in DTMs with smaller (better) resolution but, in some cases, larger gaps, which are sometimes reduced by increasing the size of the correlation kernel. We conclude that users of ASP should always systematically evaluate the choice of parameters for a given dataset.
One of the clearest but unresolved questions for Europa is the thickness of its icy shell. Europa's surface is resplendent with geological features that bear on this question, and ultimately on its interior, geological history, and astrobiological potential. We characterize the size and topographic expression of circular and subcircular features created by endogenic thermal and tectonic disturbances on Europa: pits, uplifts, and small, subcircular chaos. We utilize the medium-resolution Galileo regional maps (RegMaps), as well as high-resolution regions, digital elevation models derived from albedo-controlled photoclinometry, and in some cases stereo-controlled photoclinometry. While limited in extent, the high-resolution images are extremely valuable for detecting smaller features and for overall geomorphological analysis. A peak in the size-distribution for all features is found at ~5–6 km in diameter and no pits smaller than 3.3 km in diameter were found in high resolution images. Additionally, there is a trend for larger pits to be deeper, and larger uplifts to be higher. Our data support a diapiric or intracrustal sill interpretation (as opposed to purely non-intrusive, melt-through models) and place a lower limit on ice shell thickness at the time of feature formation of 3-to-8 km, assuming isostasy and depending on the composition of the ice and underlying ocean.
