Early thermal profiles and lithospheric strength of Ganymede from extensional tectonic features

Estimates of the brittle lithosphere thickness derived from the width and spacing of extensional tectonic features, coupled with lithospheric strength envelopes (brittle and ductile yield stress versus depth) appropriate for ice, allow the quantitative determination of early thermal profiles and lithospheric strength and stability on Ganymede. Furrows and grooves indicate brittle lithospheric thicknesses of 5–10 and 2–5 km, respectively, assuming that their spacing is controlled by an extensional instability or that their width is controlled by the intersection depth of their bounding faults. Plots of the brittle and ductile yield stress versus depth for the icy lithosphere of Ganymede show a linear increase in brittle strength with depth to a maximum at the brittle-ductile transition, followed by an exponential decrease in ductile yield stress with depth. Because the depth to the brittle-ductile transition depends primarily on the thermal gradient, the thickness of the brittle lithosphere can be used to calculate early thermal profiles of 1.5–6 and 4.5–20°/km during the formation of the furrows and grooves, respectively. Lithospheric strength, the integral of the yield stress versus depth curve, varied from 30–125 GPa m when the furrows formed to 5–30 GPa m when the grooves formed, which correspond to maximum yield stresses of 6–11 and 2.5–6 MPa, respectively. These results indicate that the thermal gradient and lithospheric strength varied laterally by as much as a factor of 5 and that Ganymede cooled in a highly inhomogeneous manner with significant lateral thermal anomalies. Finally, this analysis provides a straightforward explanation for the stability of large remnants of cratered terrain such as Galileo Regio that had a low thermal gradient and strong lithosphere in contrast to small remnants of cratered terrain that were fractured and broken up by grooved terrain as a result of higher thermal gradients and weaker lithospheres.     

Topographic domes on Ganymede: Ice vulcanism or isostatic upwarping

Voyager images of Ganymede show two broad, gently sloping dome-shaped features. They lie in grooved terrain and have diameters of roughly 250 km. The one observed at high resolution has a summit elevation 2–2.5 km above the surrounding plains, and appears to be surrounded by a field of secondary craters. Two formation processes are considered: water vulcanism triggered by a major impact, and isostatic upwarping of a crater formed in a thin crust. Numerical simulation of nonadiabatic water vulcanism indicates that the volume of the domes is inconsistent with eruption through a conduit created by complete penetration of the crust by an impact. It is consistent, however, with eruption through fractures created by an impact that excavates partly through a thin crust. Isostatic upwarp rates calculated as a function of effective crustal temperature indicate that upwarping could also create such a dome if the impact excavated to depths where the crust was sufficiently warm and mobile. Both models require that the density of the crust slightly exceed that of a liquid water mantle for a short period of time. Morphologic evidence suggests that both processes may have been important. If either of the proposed models is correct, the situation of the domes in grooved terrain implies that grooved terrain formation occurred prior to the thickening and stiffening of Ganymede's crust.     

Surface temperatures and retention of H2O frost on Ganymede and Callisto

Surface temperatures and ice evaporation rates are calculated for Ganymede and Callisto as a function of latitude, time of day, and albedo. The model uses surface thermal properties determined by eclipse radiometry (Morrison and Cruikshank, 1973Icarus18 224–236) and albedos determined from photometrically decalibrated Voyager images. Daytime temperatures on Callisto are roughly 8°K warmer than those in Ganymede's cratered terrain and 11°K warmer than those in Ganymede's grooved terrain. Diurnal mean ice evaporation rates are high enough on both bodies that the surface material probably consists of a very low density lag deposit of primarily silicate dust overlying a denser regolith of silicates and ice. The difference in temperature between Ganymede and Callisto is not great enough to account for the lack of bright polar caps on Callisto. This lack seems instead to reflect a real deficiency in the amount of available H₂O frost relative to Ganymede. The temperature difference between Ganymede's grooved and cratered terrains also cannot account for the strong concentration of bright ray craters in grooved terrain. This concentration suggests instead that an internal geologic process has enriched the grooved terrain in ice relative to the cratered terrain.     

Grooved terrain on Ganymede

The icy crust of Ganymede comprises bright and dark areas. Investigation of Voyager 1 and 2 images has shown that bright terrain is grooved and separates dark polygons of cratered terrain. The grooved terrain contains alternating ridges and grooves in straight and curvilinear sets, which are locally interrupted by smooth patches and swaths. Cratered terrain, where 'it occurs in small wedges and slivers, has a pervasive grain of narrowly spaced furrows, and thus is transitional to grooved terrain. An analysis of the morphology of terrain types, and of superposition and cross-cutting relations, suggests that grooved terrain grew at the expense of cratered terrain, that tracts of cratered terrain were converted into grooved terrain in situ, and that vertical tectonism and shear movements dominated in the restructuring of Ganymede's surface. It is postulated that during a period in the planet's history when the lithosphere was thin, upwelling convection currents caused incipient rifting accompanied by intensive normal faulting; where rifting went to completion, crustal segments separated, locally spread apart, and sheared past one another. In places subduction and compression may have occured, but the evidence is inconclusive. Thus, the grooved terrain on Ganymede may record an early phase of ice-plate tectonics that caused rifting and drifting of the icy lithosphere, but, unlike silicate plate tectonics on Earth, may have resulted in only minor vertical turnover.     

Tectonic framework of grooved terrain on ganymede

An analysis of the tectonic features (grooves and prominent troughs) comprising grooved terrain on Ganymede shows a global tectonic framework. Domains of grooves, bordered by deep structural boundaries that controlled their extent and orientation, developed along preferred orientations. Some of these directions are much more prominent than others and appear to coincide with at least two great circles with pole locations at 152° long. 55° lat. and 65° long. and 50° lat. The great circle hypothesis is in good agreement with several statistical tests of the data set. Several mechanisms for the emplacement of grooved areas along great circles are discussed. A plausible mechanism is a pattern of equatorial-symmetric rising and descending convection cells. Because the studied areas represent only a small portion of Ganymede, interpretations of global tectonic patterns and models of formation must remain tentative.     

Voyager photometry of surface features on Ganymede and Callisto

The photometric properties of selected surface features on Ganymede and Callisto have been studied using Voyager images over phase angles from 10 to 124° taken with the clear filter (effective wave wavelength ∽0.5 μm). Normal reflectences on Ganymede average 0.35 for the cratered terrain and 0.44 for the grooved terrain. The value for the ubiquitous cratered terrain on Callistro is 0.18. The photometric properties of these regions are described closely by a simple scattering function of the form I = Af(α)μ₀/(μ + μ₀), where A is a constant, μ is the cosine of the emission angle, μ₀ is the cosine of the incidence angle, and f(α) is a function of the phase angle, α, only. For these terrains the shape of f(α) is qualitatively similar to that for the moon—generally concave upward. By contrast, bright craters on both satellites have f(α)'s which are concave downward. The scattering properties of these bright features are definitely not Lambertian, but are described approximately by the scattering law given above. The brightest craters on Callisto have reflectances which are only 10% lower than the brightest craters on Ganymede; both have closely similar scattering laws. We estimate that the brightest craters on Ganymede may reach normal reflectances of 0.7. Our phase functions yield phase integrals of q = 0.8 and 0.6 for Ganymede and Callisto, respectively.     

Pedestal craters on Ganymede

The high-resolution Voyager images of Ganymede show a class of fresh craters 6–89 km in diameter which is distinguished by an ejecta blanket similar to those seen for some types of Martian craters. One hundred and eighty-five were identified and studied for trends with respect to latitude, longitude, and terrain type. No correlation of the ratio of ejecta diameter to crater diameter was found as a function of latitude or longitude, and there is only a suggestion of a trend in this ratio with respect to major terrain types. Central peak frequency is greatest for the smaller crater diameters. Central pit occurrence dominates central peak occurrence at crater diameters ?35 km. We conclude that the ejecta morphology probably results from impact into an icy target. The question of whether atmospheric ejecta-particle drag contributes to ejecta blanket morphologies on planets with an atmospheric cannot be resolved entirely from the Voyager images. The image resolution is insufficient to show diagnostic flow features on the ejecta, if they exist, or to detect evidence of any other ejecta deposits which would lie beyond the pedestal, predicted by some researchers to exist only on bodies with an atmosphere.     

The effect of target properties on crater morphology: Comparison of central peak craters on the Moon and Ganymede

Abstract— We examine the morphology of central peak craters on the Moon and Ganymede in order to investigate differences in the near‐surface properties of these bodies. We have extracted topographic profiles across craters on Ganymede using Galileo images, and use these data to compile scaling trends. Comparisons between lunar and Ganymede craters show that crater depth, wall slope and amount of central uplift are all affected by material properties. We observe no major differences between similar‐sized craters in the dark and bright terrain of Ganymede, suggesting that dark terrain does not contain enough silicate material to significantly increase the strength of the surface ice. Below crater diameters of ?12 km, central peak craters on Ganymede and simple craters on the Moon have similar rim heights, indicating comparable amounts of rim collapse. This suggests that the formation of central peaks at smaller crater diameters on Ganymede than the Moon is dominated by enhanced central floor uplift rather than rim collapse. Crater wall slope trends are similar on the Moon and Ganymede, indicating that there is a similar trend in material weakening with increasing crater size, and possibly that the mechanism of weakening during impact is analogous in icy and rocky targets. We have run a suite of numerical models to simulate the formation of central peak craters on Ganymede and the Moon. Our modeling shows that the same styles of strength model can be applied to ice and rock, and that the strength model parameters do not differ significantly between materials.     

The Galilean satellites: New near-infrared spectral reflectance measurements (0.65–2.5 μm) and a 0.325–5 μm summary

New near-infrared (0.65–2.5 μm) reflectance spectra of the Galilean satellites with 1.5% spectral resolution and ≈2% intensity precision are presented. These spectra more precisely define the water ice absorption features previously identified on Europa, Ganymede, and Callisto at 1.55 and 2.0 μm. In addition, previously unreported spectral features due to water ice are seen at 1.25, 1.06, 0.90, and 0.81 μm on Europa, and at 1.25, 1.04, and possibly 0.71 μm on Ganymede. Unreported absorption features in Callisto's spectrum occur at 1.2 μm, probably due to H₂O, and a weak, broad band extending from 0.75 to 0.95 μm, due possibly to other minerals. The spectrum of Io has only weak absorption features at 1.15 μm and between 0.8 and 1.0 μm. No water absorptions are positively identified in the Io spectra, indicating an upper limit of areal water frost coverage of 2% (leading and trailing sides). It is found for Callisto, Ganymede, and Europa that the water ice absorption features are due to free water and not to water bound or absorbed onto minerals. The areal coverage of water frost is ≈ 100% on Europa (trailing side), ≈65% on Ganymede (leading side), and 20–30% on Callisto (leading side). An upper limit of ≈5% bound water (in addition to the 20–30% ice) may be present on Callisto, based on the strong 3-μm band seen by other investigators. A summary of spectra of the satellites from 0.325 to about 5 μm to aid in laboratory and interpretation studies is also presented.     

Thermal evolution of a differentiated Ganymede and implications for surface features

Thermal evolution models are presented for Ganymede, assuming a mostly differentiated initial state of a water ocean overlying a rock layer. The only heat sources are assumed to be primordial heat (provided by accretion) and the long-lived radiogenic heat sources in the rock component. As Ganymede cools, the ocean thins, and two ice layers develop, one above composed of ice I, and the other below composed of high-pressure polymorphs of ice. Subsolidus convection proceeds separately in each ice layer, its transport of heat calculated using a simple parameterized convection scheme and the most recent data on ice rheology. The model requires that the average entropy of the deep ice layer exceeds that of the ice I layer. If the residual ocean separating these layers becomes thin enough, then a Rayleigh-Taylor-like (“diapiric”) instability may ensue, driven by the greater entropy of the deeper ice and merging the two ice mantles into a single convective layer. This instability is not predicted by linear analysis but occurs for plausible finite amplitude perturbations associated with large Rayleigh number convection. The resulting warm ice diapirs may lead to a dramatic “heat pulse” at the surface and to fracturing of the lithosphere, and may be directly or indirectly responsible for resurfacing and grooved terrain formation on Ganymede. The timing of this event depends rather sensitively on poorly known rheological parameters, but could be consistent with chronologies deduced from estimated cratering rates. Irrespective of the occurrence or importance of the heat pulse, we find that lithospheric fracturing requires rapid stress loading (on a time scale ⪅10⁴ years). Such a time scale can be realized by warm ice diapirism, but not directly by gradual global expansion. In the absence of any quantitative and self-consistent model for the resurfacing of Ganymede by liquid water, we favor resurfacing by warm ice flows, which we demonstrate to be physically possible, a plausible consequence of our models, compatible with existing observations, and a hypothesis testable by Galileo. We discuss core formation as an alternative driver for resurfacing, and conclude that it is less attractive. We also consider anew the puzzle of why Callisto differs so greatly from Ganymede, offering several possible explanations. The models presented do not provide a compelling explanation for all aspects of Ganymedean geological evolution, since we have identified several potential problems, most notably the apparently extended period of grooved terrain formation (several hundred million years), which is difficult to reconcile with the heat pulse phenomenon.     

Global geological mapping of Ganymede

We have compiled a global geological map of Ganymede that represents the most recent understanding of the satellite based on Galileo mission results. This contribution builds on important previous accomplishments in the study of Ganymede utilizing Voyager data and incorporates the many new discoveries that were brought about by examination of Galileo data. We discuss the material properties of geological units defined utilizing a global mosaic of the surface with a nominal resolution of 1 km/pixel assembled by the USGS with the best available Voyager and Galileo regional coverage and high resolution imagery (100-200 m/pixel) of characteristic features and terrain types obtained by the Galileo spacecraft. We also use crater density measurements obtained from our mapping efforts to examine age relationships amongst the various defined units. These efforts have resulted in a more complete understanding of the major geological processes operating on Ganymede, especially the roles of cryovolcanic and tectonic processes in the formation of might materials. They have also clarified the characteristics of the geological units that comprise the satellite’s surface, the stratigraphic relationships of those geological units and structures, and the geological history inferred from those relationships. For instance, the characteristics and stratigraphic relationships of dark lineated material and reticulate material suggest they represent an intermediate stage between dark cratered material and light material units.     

A technique for processing of planetary images with heterogeneous characteristics for estimating geodetic parameters of celestial bodies with the example of Ganymede

The new technique for generation of coordinate control point networks based on photogrammetric processing of heterogeneous planetary images (obtained at different time, scale, with different illumination or oblique view) is developed. The technique is verified with the example for processing the heterogeneous information obtained by remote sensing of Ganymede by the spacecraft Voyager-1, -2 and Galileo. Using this technique the first 3D control point network for Ganymede is formed: the error of the altitude coordinates obtained as a result of adjustment is less than 5 km. The new control point network makes it possible to obtain basic geodesic parameters of the body (axes size) and to estimate forced librations. On the basis of the control point network, digital terrain models (DTMs) with different resolutions are generated and used for mapping the surface of Ganymede with different levels of detail (Zubarev et al., 2015b).     

Color photometry of surface features on Ganymede and Callisto

Voyager imaging data demonstrate that the scattering properties (“phase curves”) of all major terrain types on Ganymede and callisto are not significantly wavelength dependent between 0.4 and 0.6 μm. Our data suggest that the phase curves may be slightly steeper at the shorter wavelengths, consistent with the trend of telescopic observations near opposition. However, the differences are small and entirely within the uncertainties of our analysis. Our result indicates that the phase integrals (0.8 for Ganymede and 0.6 for Callisto) derived by S. W. Squyres and J. Veverka [Icarus46, 137–155 (1981)] from the abundant Voyager clear filter observations are reliable measures of the radiometric phase integrals. The corresponding values of the Bond albedo turn out to be 0.35 for Ganymede and 0.11 for Callisto.     

Central pit craters on Ganymede

Central pit craters are common on Mars, Ganymede and Callisto, and thus are generally believed to require target volatiles in their formation. The purpose of this study is to identify the environmental conditions under which central pit craters form on Ganymede. We have conducted a study of 471 central pit craters with diameters between 5 and 150 km on Ganymede and compared the results to 1604 central pit craters on Mars (diameter range 5-160 km). Both floor and summit pits occur on Mars whereas floor pits dominate on Ganymede. Central peak craters are found in similar locations and diameter ranges as central pit craters on Mars and overlap in location and at diameters <60 km on Ganymede. Central pit craters show no regional variations on either Ganymede or Mars and are not concentrated on specific geologic units. Central pit craters show a range of preservation states, indicating that conditions favoring central pit formation have existed since crater-retaining surfaces have existed on Ganymede and Mars. Central pit craters on Ganymede are generally about three times larger than those on Mars, probably due to gravity scaling although target characteristics and resolution also may play a role. Central pits tend to be larger relative to their parent crater on Ganymede than on Mars, probably because of Ganymede’s purer ice crust. A transition to different characteristics occurs in Ganymede’s icy crust at depths of 4-7 km based on the larger pit-to-crater-diameter relationship for craters in the 70-130-km-diameter range and lack of central peaks in craters larger than 60-km-diameter. We use our results to constrain the proposed formation models for central pits on these two bodies. Our results are most consistent with the melt-drainage model for central pit formation.     

The evolution of tectonic features on Ganymede

The bands of bright resurfaced terrain on Ganymede are probably broad grabens formed by global expansion and filled with deposits of ice. Grooves within the bands are thought to be extensional features formed during the same episode of expansion. The crust of Ganymede is modeled as a viscoelastic material subjected to extensional strain. With sufficiently high strain rates and stresses, deep normal faulting will occur, creating broad grabens that may then be filled. Continuing deformation at high strain rates and stresses will cause propagation of deep faults up into the flood deposits and normal faulting at the surface, while lower strain rates and stresses will cause formation of open extension fractures or, if the crustal strength is very low, grabens at the surface. The spacing between adjacent fractures may reflect the geothermal gradient at the time of deformation. Surface topography resulting from fracturing and normal faulting will decay with time as a result of viscous relaxation and mass-wasting.     

Analysis and interpretation of Cassini Titan radar altimeter echoes

The Cassini spacecraft has acquired 25 radar altimeter elevation profiles along Titan's surface as of April 2008, and we have analyzed 18 of these for which there are currently reconstructed ephemeris data. Altimeter measurements were collected at spatial footprint sizes from 6-60 km along ground tracks of length 400-3600 km. The elevation profiles yield topographic information at this resolution with a statistical height accuracy of 35-50 m and kilometer-scale errors several times greater. The data exhibit significant variations in terrain, from flat regions with little topographic expression to very rugged Titanscapes. The bandwidth of the transmitted waveform admits vertical resolution of the terrain height to 35 m at each observed location on the surface. Variations in antenna pointing and changes in surface statistics cause the range-compressed radar echoes to exhibit strong systematic and time-variable biases of hundreds of meters in delay. It is necessary to correct the received echoes for these changes, and we have derived correction algorithms such that the derived echo profiles are accurate at the 100 m level for off-nadir pointing errors of 0.3° and 0.6°, for leading edge and echo centroid estimators, respectively. The leading edge of the echo yields the elevation of the highest points on the surface, which we take to be the peaks of any terrain variation. The mean value of the echo delay is more representative of the mean elevation, so that the difference of these values gives an estimate of any local mountain heights. Finding locations where these values diverge indicates higher-relief terrain. Elevation features are readily seen in the height profiles. Several of the passes show mountains of several hundred m altitude, spread over 10's or even 100's of km in spatial extent, so that slopes are very small. Large expanses of sub-100 m topography are commonplace on Titan, so it is rather smooth in many locations. Other areas exhibit more relief, although the overall observed variation in surface height on any pass is less than about 1 km. Some elevation features correspond to observed changes in brightness in Cassini infrared images, but many do not. Correspondence between the imaging SAR ground tracks and the altimeter paths is limited, so that identifying elevation changes with higher resolution SAR features is premature at present.     

Geology of the Selk crater region on Titan from Cassini VIMS observations

Observations of Titan obtained by the Cassini Visual and Infrared Mapping Spectrometer (VIMS) have revealed Selk crater, a geologically young, bright-rimmed, impact crater located ∼800 km north-northwest of the Huygens landing site. The crater rim-crest diameter is ∼90 km; its floor diameter is ∼60 km. A central pit/peak, 20-30 km in diameter, is seen; the ratio of the size of this feature to the crater diameter is consistent with similarly sized craters on Ganymede and Callisto, all of which are dome craters. The VIMS data, unfortunately, are not of sufficient resolution to detect such a dome. The inner rim of Selk crater is fluted, probably by eolian erosion, while the outer flank and presumed ejecta blanket appear dissected by drainages (particularly to the east), likely the result of fluvial erosion. Terracing is observed on the northern and western walls of Selk crater within a 10-15 km wide terrace zone identified in VIMS data; the terrace zone is bright in SAR data, consistent with it being a rough surface. The terrace zone is slightly wider than those observed on Ganymede and Callisto and may reflect differences in thermal structure and/or composition of the lithosphere. The polygonal appearance of the crater likely results from two preexisting planes of weakness (oriented at azimuths of 21° and 122° east of north). A unit of generally bright terrain that exhibits similar infrared-color variation and contrast to Selk crater extends east-southeast from the crater several hundred kilometers. We informally refer to this terrain as the Selk “bench.” Both Selk and the bench are surrounded by the infrared-dark Belet dune field. Hypotheses for the genesis of the optically bright terrain of the bench include: wind shadowing in the lee of Selk crater preventing the encroachment of dunes, impact-induced cryovolcanism, flow of a fluidized-ejecta blanket (similar to the bright crater outflows observed on Venus), and erosion of a streamlined upland formed in the lee of Selk crater by fluid flow. Vestigial circular outlines in this feature just east of Selk’s ejecta blanket suggest that this might be a remnant of an ancient, cratered crust. Evidently the southern margin of the feature has sufficient relief to prevent the encroachment of dunes from the Belet dune field. We conclude that this feature either represents a relatively high-viscosity, fluidized-ejecta flow (a class intermediate to ejecta blankets and long venusian-style ejecta flows) or a streamlined upland remnant that formed downstream from the crater by erosive fluid flow from the west-northwest.     

Modeling the morphological diversity of impact craters on icy satellites

Impact craters on icy satellites display a wide range of morphologies, some of which have no counterpart on rocky bodies. Numerical simulation studies have struggled to reproduce the diversity of features, such as central pits and transitions in crater depth with increasing diameter, observed on the icy Galilean satellites. The transitions in crater depth (at diameters of about 26 and 150 km on Ganymede and Callisto) have been interpreted as reflecting subsurface structure. Using the CTH shock physics code, we model the formation of craters with diameters between 400 m and about 200 km on Ganymede using different subsurface temperature profiles. Our calculations include recent improvements in the model equation of state for H₂O and quasi-static strength parameters for ice. We find that the shock-induced formation of dense high-pressure polymorphs (ices VI and VII) creates a gap in the crater excavation flow, which we call discontinuous excavation. For craters larger than about 20 km, discontinuous excavation concentrates a hot plug of material (>270 K and mostly on the melting curve) in the center of the crater floor. The size and occurrence of the hot plug are in good agreement with the observed characteristics of central pit craters, and we propose that a genetic link exists between them. We also derive depth versus diameter curves for different internal temperature profiles. In a 120 K isothermal crust, calculated craters larger than about 30 km diameter are deeper than observed and do not reproduce the transition at about 26 km diameter. Calculated crater depths are shallower and in good agreement with observations between about 30 and 150 km diameter using a warm thermal gradient representing a convective interior. Hence, the depth-to-diameter transition at about 26 km reflects thermal weakening of ice. Finally, simulation results generally support the hypothesis that the anomalous interior morphologies for craters larger than 100 km are related to the presence of a subsurface ocean.     

Global variations in regolith properties on asteroid Vesta from Dawn's low‐altitude mapping orbit

We investigate the depth, variability, and history of regolith on asteroid Vesta using data from the Dawn spacecraft. High‐resolution (15–20 m pixel^?1) Framing Camera images are used to assess the presence of morphologic indicators of a shallow regolith, including the presence of blocks in crater ejecta, spur‐and‐gully–type features in crater walls, and the retention of small (<300 m) impact craters. Such features reveal that the broad, regional heterogeneities observed on Vesta in terms of albedo and surface composition extend to the physical properties of the upper ~1 km of the surface. Regions of thin regolith are found within the Rheasilvia basin and at equatorial latitudes from ~0–90°E and ~260–360°E. Craters in these areas that appear to excavate material from beneath the regolith have more diogenitic (Rheasilvia, 0–90°E) and cumulate eucrite (260–360°E) compositions. A region of especially thick regolith, where depths generally exceed 1 km, is found from ~100–240°E and corresponds to heavily cratered, low‐albedo surface with a basaltic eucrite composition enriched in carbonaceous chondrite material. The presence of a thick regolith in this area supports the idea that this is an ancient terrain that has accumulated a larger component of exogenic debris. We find evidence for the gardening of crater ejecta toward more howarditic compositions, consistent with regolith mixing being the dominant form of “weathering” on Vesta.     

2.7- to 4.1-μm spectrophotometry of icy satellites of Saturn and Jupiter

We present spectrophotometry in the 27–41 μm spectral region for icy satellites of Saturn (Tethys, Dione, Rhea, Iapetus, and Hyperion) and Jupiter (Europa, Ganymede, and Callisto). The 3.6-μm reflectance peak characteristic of fine-grained water ice is observed prominently on the satellites of Saturn, faintly on the leading side of Europa, and not all on Ganymede, Callisto, or the dark side of Iapetus. The spectral reflectances of these icy satellites may be affected by their equilibrium surface temperatures and magnetospheric effects.