Fresh Impact Craters Research Articles

The role of pre-impact topography in impact melt emplacement on terrestrial planets

On terrestrial planets, flow-like deposits of impact melt are commonly found exterior to fresh impact craters. Often, these deposits are asymmetric about the crater rim, and the direction of flow may relate to either the pre-impact topography or the impact azimuth. In this work, we seek to determine the primary mechanism responsible for impact melt emplacement on two very different terrestrial worlds: the Moon and Venus. Newly derived stereo topography data allows us to investigate the role of pre-impact topography in melt emplacement on Venus for the first time. We determine that pre-impact topography plays an important role in melt emplacement around complex craters on the Moon but not on Venus. This difference may relate to the differences in gravity (and hence, crater depth and melt volume) on the two worlds. Melt trapped in the relatively deep lunar craters may require additional momentum to be pushed over the crater rim, and therefore emerges preferentially from the region of rim crest low. This added momentum might come from uplift during the crater modification stage. For the shallower craters on Venus, the downrange momentum imparted in oblique impacts may be sufficient to push the melt up and over the crater rim, explaining the correlation between melt direction and impact azimuth. Understanding the emplacement of impact melt on terrestrial planets of different sizes and geologic histories provides added insight into the impact cratering process.

Geologic mapping of the Ac-2 Coniraya quadrangle of Ceres from NASA's Dawn mission: Implications for a heterogeneously composed crust

Since its arrival at Ceres, Dawn's Framing Camera has been imaging the dwarf planet at different altitudes, using 8 different filters. Based on these images, global clear filter mosaics, digital terrain models, and global color mosaics were produced. These datasets are basis for the derived photo-geologic map of the Ac-2 Coniraya quadrangle, located between 0° and 90° eastern longitude and between 20° and 40° northern latitude. Crater size-frequency distribution (CSFD) measurements have been applied to derive the stratigraphic history of the identified geologic units in the Ac-2 Coniraya quadrangle. Based on these investigations, we found that the Ac-2 Coniraya quadrangle shows evidence for a long geologic history lasting over 3.5 Ga. Most of the identified geologic units are related to impact cratering at different periods of time. While the oldest unit identified in the Ac-2 Coniraya quadrangle, the cratered terrain, shows an absolute model age (AMA) of 3.3–3.5 Ga, the youngest units, including fresh impact craters and lobate flows, show AMAs of < 100 Ma. The large number of different sized impact craters excavated a variety of materials with different colors and albedo, indicative of a heterogeneous crustal composition, including water or volatiles. Pits identified at Ikapati crater's smooth deposits might further indicate the presence of volatiles in the subsurface. Furthermore, our mapping of the Ac-2 Coniraya quadrangle shows that possible H2O ice in the crust is heterogeneously distributed, as lobate flows, bright spots, and pits are only found at certain locations, related to relatively fresh impact craters.

Revised constraints on absolute age limits for Mercury's Kuiperian and Mansurian stratigraphic systems

AbstractFollowing an approach similar to that used for the Moon, Mercury's surface units were subdivided into five time‐stratigraphic systems based on geologic mapping using Mariner 10 images. The absolute time scale originally suggested for the time periods associated with these systems was based on the assumption that the lunar impact flux history applied to Mercury. However, we find that the duration and onset of corresponding periods in the stratigraphic sequences on Mercury and the Moon are not the same. Using high‐resolution and multiband image data obtained by the MErcury Surface, Space ENviroment, GEochemistry, and Ranging (MESSENGER) spacecraft, we identify and catalog fresh impact craters interpreted to have formed during Mercury's two most recent periods, the Mansurian and Kuiperian. We use the densities of the inferred Kuiperian‐ and Mansurian‐aged crater populations to estimate new limits for the age boundaries of these time intervals. Results suggest that both the Mansurian and Kuiperian periods began more recently and extended for significantly shorter durations of time than previously suggested. The Kuiperian is estimated to have initiated as recently as ~280 ± 60 Ma and the Mansurian as recently as ~1.7 ± 0.2 Ga.

Impact-generated winds on Mars

Bright and dark wind streaks across Mars record wind patterns related to atmospheric circulation. In some cases these streaks represent erosion of a surface veneer; in others, they indicate sand grains mobilized by strong vortices shed off of positive relief, such as crater rims. While many streaks change length or orientation over time, others not only remain unchanged but also may indicate a completely different wind direction. These permanent streaks could reflect past circulation patterns in response to conditions related to orbital forcing (e.g., Thomas and Veverka, 1979; Veverka et al., 1981). Here, however, we focus on a subset of permanent wind streaks unrelated to global circulation, rather to impact-generated winds that can extend more than 500km away from the crater. Nighttime images from the Mars Odyssey's Thermal Emission Imaging System (THEMIS) reveal certain large craters (> 15km in diameter) having sets of thermally bright streaks that radiate from certain fresh impact craters. These streaks extend from pre-existing topographic highs (crater rims, wrinkle ridges) beyond the continuous ejecta deposits to more than 6 crater radii, unrelated to secondary craters. For illustration, this contribution primarily focuses on the 20km-diameter Santa Fe crater in Chryse Planitia. Context Camera (CTX) images reveal that these streaks correspond to zones of erosion. The thermally bright rays in nighttime images correlate with regions where coarser materials have been exposed, not always resolved even in HiRISE images. Models of the impact process indicate impact-generated vapor most likely generated intense winds that scoured the region, well before arrival of secondary craters and later ejecta run-out flows. Pre-existing relief (such as crater rims) disturbed this flow and generated intense cross-flow instabilities resulting in long parallel streaks.

An investigation of the bluish material on Ceres

AbstractThe dwarf planet Ceres shows spatially well‐defined regions, which exhibit a negative (blue) spectral slope between 0.5 and 2.5 µm. Comparisons with planetary bodies known to exhibit a blue slope and spectral properties of materials identified on Ceres's surface based on infrared wavelength signatures indicate that the spectral changes could be related to physical properties of the surface material rather than variations in its composition. The close association of bluish surface regions to fresh impact craters implies a possible relationship to an impact‐triggered alteration and/or space weathering processes. The bluish regions could be linked with blankets of ultrafine grains and partly amorphous phyllosilicates, which form larger agglomerates due to the sticky behavior of impact‐induced phyllosilicate dust and/or the amorphization of the ejecta material during the impact process. Space weathering processes (micrometeoritic impacts, temperature changes) cause a reversal of the agglutination process and a recrystallization of the surface material with time resulting in a reddening of the spectral slope.

A sedimentary origin for intercrater plains north of the Hellas basin: Implications for climate conditions and erosion rates on early Mars

AbstractUnderstanding the origin (volcanic or sedimentary) and timing of intercrater plains is crucial for deciphering the geological evolution of Mars. We have produced a detailed geological map of the intercrater plains north of the Hellas basin, based on images from the Mars Express High‐Resolution Stereo Camera, the Mars Reconnaissance High‐Resolution Imaging Science Experiment, and Context. Erosional windows and fresh impact craters provide a way of studying the lithology of intercrater plain units. They are composed predominantly of light‐toned sedimentary rocks with subhorizontal bedding over a broad extent (greater than tens of kilometers), showing cross‐bedding stratifications locally. The broad extent, geometry, and flat topography of these sediments favor a formation by aqueous processes (alluvial and lacustrine) rather than airfall (eolian and volcaniclastic). The Late Noachian (~3.7 Ga) sedimentary plains are locally covered by dark‐toned, rough‐textured lava flows of Late Hesperian age (~3.3 Ga). Fe/Mg phyllosilicates were detected within sedimentary rocks, whereas volcanic rocks contain pyroxene and lack signatures of alteration, in agreement with interpretations made from texture and morphology. In erosional windows, the superimposition of sedimentary rocks by younger volcanic flows enables the estimation of an erosion rate of ~1000 nm yr−1 during the Hesperian period (3.3–3.7 Ga). Thus, our study shows that an intense sedimentary cycle occurred on the northern rim of the Hellas basin before and during the Late Noachian, leading to the formation of widespread sedimentary plains, which were then eroded, in agreement with a gradual change in the climatic conditions in this period, and later covered by volcanic flows.

The self‐secondary crater population of the Hokusai crater on Mercury

AbstractWhether or not self‐secondaries dominate small crater populations on continuous ejecta deposits and floors of fresh impact craters has long been a controversy. This issue potentially affects the age determination technique using crater statistics. Here the self‐secondary crater population on the continuous ejecta deposits of the Hokusai crater on Mercury is unambiguously recognized. Superposition relationships show that this population was emplaced after both the ballistic sedimentation of excavation flows and the subsequent veneering of impact melt, but it predated the settlement and solidification of melt pools on the crater floor. Fragments that formed self‐secondaries were launched via impact spallation with large angles. Complex craters on the Moon, Mercury, and Mars probably all have formed self‐secondaries populations. Dating young craters using crater statistics on their continuous ejecta deposits can be misleading. Impact melt pools are less affected by self‐secondaries. Overprint by subsequent crater populations with time reduces the predominance of self‐secondaries.

Recent shallow moonquake and impact-triggered boulder falls on the Moon: New insights from the Schrödinger basin

Shallow moonquakes are thought to be of tectonic origin. However, the geologic structures responsible for these moonquakes are unknown. Here we report sites where moonquakes possibly occurred along young lobate scarps in the Schrodinger basin. Our analysis of Lunar Reconnaissance Orbiter and Chandrayaan-1 images revealed four lobate scarps in different parts of the Schrodinger basin. The scarps crosscut small fresh impact craters ( 1500 boulders associated with trails and bouncing marks. Their origins are largely controlled by recent impact events. Ejecta rays and secondary crater chains from a 14 km diameter impact crater traversed Schrodinger and triggered significant boulder falls about 17 Ma. Therefore, a combination of recent shallow moonquakes and impact events triggered the boulder falls in the Schrodinger basin.

The lunar Gruithuisen silicic extrusive domes: Topographic configuration, morphology, ages, and internal structure

The Gruithuisen domes, situated on the western portion of the Imbrium basin rim, form three tall mountains (NW, Gamma, Delta) totaling ∼780km3 in volume. The shapes of the domes are significantly different from that of mare-type domes elsewhere on the Moon. We use data from the Lunar Reconnaissance Orbiter (LRO) and Kaguya missions (LRO Lunar Orbiter Laser Altimeter, Lunar Reconnaissance Orbiter Camera, Diviner, and the Kaguya imager) to characterize the domes and assess models for their origin. The configuration of the domes (steep slopes, up to ∼18–20°) and their specific remote sensing characteristics (strong downturn in the UV, and results from the M3 and Diviner instruments) suggest that the domes formed by eruptions of highly viscous lava. The estimated surface volumes of the domes vary from ∼20km3 (NW dome) to ∼290km3 (Gamma dome) to ∼470km3 (Delta dome). The domes occur on the portion of the Imbrium basin rim that is overlain by ejecta from the post-Imbrium Iridum crater. In some areas, relatively high albedo smooth volcanic plains are seen within the Iridum ejecta near the Gruithuisen domes, and low albedo mare deposits surround and embay the domes and Iridum crater. Dating of different units and features by crater counts indicates that impact melts from the Iridum basin are ∼3.9Ga old, the domes Gamma and Delta are ∼3.8Ga, and the ages of the plains near the domes vary from ∼2.3 to ∼3.6Ga. A fresh impact crater exposes the internal structure of the Gamma dome. The most prominent features on the wall of the crater are rough, blocky layers that are typical of volcanic plains in the highlands and maria around the domes. The layers are interleaved with fine-grained materials of higher and lower albedo and the visible orientation of the layers changes over short (a few hundred meters) distances. These characteristics of the internal structure of the dome are consistent with eruptions of high viscosity lava (rough layers) that alternated with possible explosive activity (fine-grained materials). The spatial association of the Gruithuisen domes with the highland lava plains resembles the situation in which bimodal volcanism occur on Earth. The terrestrial association can be due to either fractional crystallization in basaltic magma reservoirs or remelting of high-silica crustal materials. In the first case, the evolved melts appear in later stages of volcanic activity and in the second case these melts are formed near the beginning of evolution of the magmatic systems. The age estimates of the Gruithuisen domes and the surrounding volcanic plains are more consistent with the crustal remelting scenario. However, remelting of primary anorthositic crust cannot readily produce the silica-rich melts and requires the presence of pre-existing granite-like materials. Formation of the domes by fractional crystallization avoids this difficulty but requires explanation of the older age of the domes relative to the volcanic plains in the surroundings. A third option is that the domes are unrelated genetically to the mare deposits.

Crater Boguslawsky on the moon: Geological structure and an estimate of the degree of rockiness of the floor

The paper considers the results of a study of the geological structure of the floor of the crater Boguslawsky selected as a primary target for the Luna-Glob mission. The deplanate floor of the crater is covered by the material ejected from remote craters and the crater Boguslawsky-D on the eastern inner slope of the crater Boguslawsky. It is highly probable that the sampling of the crater Boguslawsky-D ejecta will provide the unique possibility to detect and analyze the material that predates the formation of the largest and most ancient currently known basin on the Moon—the South Pole–Aitken basin. The rockiness degree of the Boguslawsky crater floor has been estimated from the radar data and the manual boulder counts in the superresolution images (0.5 m/pixel obtained with the Narrow Angle Camera from the Lunar Reconnaissance Orbiter). Comparison of the radar data to the results of the photo-geological analysis shows that the main contributor to the radar signal is the rock debris located in the subsurface layer sounded by radar (1–1.5 m), while there are practically no boulders on the surface. The two most rocky regions on the crater Boguslawsky floor are associated with the relatively fresh impact craters 300–400 m in diameter. The spatial density of boulders near the craters suggests that one of them is 30–50 Myr older than the other. For both of these craters, the spatial density of boulders drops with the distance from their rims. The rate of the decrease in the boulder spatial density is the same for both craters, which points to the constant-in-time intensity of the fragmentation of boulders. The size distribution of boulders versus the distance from a rim of the older crater is approximated by the curve with a slope of–0.02, while the curve slope for the younger crater is–0.05. The gentler curve slope for the older crater is obviously connected with the equalization of sizes of the rock debris with time. The size-frequency distribution of all rock fragments for the both craters, regardless of the distance from the rim, shows that mainly large boulders first crumble away as the surface age increases. Some large boulders near the young crater demonstrate the traces of rolling, while such traces are absent for the boulders near the older crater. This allows us to estimate the intensity of the reworking of a thin surface layer at 0.01 m/Myr.

Impact spallation processes on the Moon: A case study from the size and shape analysis of ejecta boulders and secondary craters of Censorinus crater

Impact spallation is a fundamental process responsible for formation of ejecta boulders from impact craters. Although theoretical spallation models were developed about three decades ago, only limited geological observations have been made so far to test these models. The 3.8km Censorinus crater on the Moon provides an excellent opportunity for studying the impact spallation processes associated with a fresh simple crater formed by oblique impact. Using the Lunar Reconnaissance Orbiter Narrow Angle Camera images, we prepared the ejecta boulder distribution map of Censorinus crater and measured the boulder sizes and shapes. Mapping of about 242,000 ejecta boulders enabled us to document the size distribution of boulders both radial and concentric to the impact crater. Larger size boulders dominate the crater rim areas, while they become smaller away from the crater. The boulder distribution exhibits a radial asymmetry suggesting Censorinus is a oblique impact, in which the uprange ejecta have smaller ranges with larger concentration of boulders near the southwestern crater rim, while the downrange ejecta are in general characterized by smaller boulders with high spatial dispersion. The cumulative size–frequency distribution (CSFD) of boulders shows a highly variable fragmentation history in which the uprange boulders suffered more complex fragmentation. The ejecta boulders also exhibit a variety of shapes that are gleaned from their axial ratios and edge angle characteristics. There is a general decrease of axial ratios away from the crater rim. Rectangular boulders dominate the crater rim and they become more equant away from the crater. In addition to the boulder sizes, the boulder shape distribution also exhibits a mild asymmetry in response to the oblique impact. Small size fresh impact craters (84,000 craters) are abundant on the Censorinus ejecta and post-date Censorinus. These craters are found in two morphologic types in which a large majority of craters have subdued ejecta (rayless craters), while some possess bright-rayed ejecta (bright-rayed craters). The CSFD of rayless craters show a steep power-law slope with a b-value of −4.0, similar to the secondary craters produced by the impact of ejecta from primary craters. We therefore interpret the rayless craters as the secondary craters of Censorinus. On the other hand, the CSFD of bright-rayed craters have smaller power-law slope (b value −2.7) which is a characteristic of primary craters, and thus provide 3Ma age for Censorinus crater. When the characteristics of Censorinus boulders are compared with the theoretical spallation models that are sensitive to the petrophysical properties of the target (lunar highland), the models generally agree with the Censorinus boulders. However, the observed shape and size characteristics of the Censorinus boulders are found to be more complex than the theoretical spallation models. The ejecta boulders suffered more complex fragmentation and asymmetric distribution in response to the oblique impact. The spallation models accounting oblique impacts have not yet been developed. Therefore, our Censorinus boulder observations can be used to develop and validate the new theoretical spallation models for the effects of oblique impacts.

The current impact flux on Mars and its seasonal variation

We calculate the present-day impact flux on Mars and its variation over the martian year, using the current data on the orbital distribution of known Mars-crossing minor planets. We adapt the Öpik–Wetherill formulation for calculating collision probabilities, paying careful attention to the non-uniform distribution of the perihelion longitude and the argument of perihelion owed to secular planetary perturbations. We find that, at the current epoch, the Mars crossers have an axial distribution of the argument of perihelion, and the mean direction of their eccentricity vectors is nearly aligned with Mars’ eccentricity vector. These previously neglected angular non-uniformities have the effect of depressing the mean annual impact flux by a factor of about 2 compared to the estimate based on a uniform random distribution of the angular elements of Mars-crossers; the amplitude of the seasonal variation of the impact flux is likewise depressed by a factor of about 4–5. We estimate that the flux of large impactors (of absolute magnitude H<16) within ±30° of Mars’ aphelion is about three times larger than when the planet is near perihelion. Extrapolation of our results to a model population of meter-size Mars-crossers shows that if these small impactors have a uniform distribution of their angular elements, then their aphelion-to-perihelion impact flux ratio would be 11–15, but if they track the orbital distribution of the large impactors, including their non-uniform angular elements, then this ratio would be about 3. Comparison of our results with the current dataset of fresh impact craters on Mars (detected with Mars-orbiting spacecraft) appears to rule out the uniform distribution of angular elements.

Variegation and space weathering on asteroid 21 Lutetia

During the flyby in 2010, the OSIRIS camera on-board Rosetta acquired hundreds of high-resolution images of asteroid Lutetia's surface through a range of narrow-band filters. While Lutetia appears very bland in the visible wavelength range, Magrin et al. (2012) tentatively identified UV color variations in the Baetica cluster, a group of relatively young craters close to the north pole. As Lutetia remains a poorly understood asteroid, such color variations may provide clues to the nature of its surface. We take the color analysis one step further. First we orthorectify the images using a shape model and improved camera pointing, then apply a variety of techniques (photometric correction, principal component analysis) to the resulting color cubes. We characterize variegation in the Baetica crater cluster at high spatial resolution, identifying crater rays and small, fresh impact craters. We argue that at least some of the color variation is due to space weathering, which makes Lutetia's regolith redder and brighter.

Global occurrence trend of high-Ca pyroxene on lunar highlands and its implications

We present details of the global distribution of high-Ca pyroxene (HCP)-rich sites in the lunar highlands based on the global data set of hyperspectral reflectance obtained by the SELENE Spectral Profiler. Most HCP-rich sites in the lunar highlands are found at fresh impact craters. In each crater, most of the detection points are distributed on the ejecta, rim, and floor of the impact craters rather than the central peaks, while the central peaks are dominated by purest anorthosite (PAN). This indicates that HCP-rich materials originate from relatively shallower regions of the lunar crust than PAN. In addition, while all ray craters with sizes larger than ∼40 km possess HCP-rich materials, small fresh craters with sizes less than ∼6–10 km do not, indicating that the uppermost mixing layers in the lunar crust are not dominated by HCP. Based on these results, we propose that in the upper lunar crust, a HCP-rich zone overlying the PAN layer exists below the uppermost mixing layer. This HCP-rich zone may originate from interstitial melt during the formation of the flotation anorthositic cumulate, while an impact ejecta origin, impact melt origin, and/or magmatic intrusion into the upper lunar crust may also account for the occurrence of HCP-rich sites in the highlands.

Geological context of potential landing site of the Luna-Glob mission

The region planned for performing the Luna-Glob mission is located in the southern part of the swell surrounding the largest South Pole-Aitken (SPA) basin. The photogeological analysis of the surface topography of this region using the LRO-WAC (resolution of 100 Mpxl) photomaps made it possible to define the following groups of morphological units (area types): (1) related to the formation of relatively fresh impact craters; (2) associated with larger (>100 km across) degraded craters including (2a) external and (2b) inner facies; and (3) occupying intercrater spaces. The comparison of the geological map with the map illustrating the distribution of the epithermal neutron flow (Mitrofanov et al., 2012) shows no correlation between them. Consequently, one should not expect development of rock complexes, which would be characterized by elevated concentrations of water in the region chosen for the Luna-Glob mission and, thus, considered among the first-priority targets. The comparison of the neutron flow distribution with the map of circular polarization of the Mini-RF radar beam also shows no correlation. This means that high values of circular polarization reflect elevated concentrations of rock fragments rather than water accumulations. Even though ice fragments are present, their sizes should only slightly be less as compared with the radar wavelength (12.6 cm). The region planned for investigations in the scope of the Luna-Glob mission corresponds to the swell of the largest (and, likely, oldest) preserved basin and offers a potential opportunity to analyze ancient material of this planet and introduce important constraints into the spectrum of models proposed for explaining the Moon’s origin.

Compositional evidence of magmatic activity on Vesta

Like the Earth and other terrestrial planets, the asteroid Vesta has a basaltic crust and a large iron core; its surface is dominated by impact craters of all sizes and has tectonic features. The presence of basaltic compositions, olivine, howardite-eucrite-diogenite meteorites, and models of Vesta's formation, suggests that volcanic and/or magmatic activity could have occurred on Vesta. A global search for lobate structures did not find unequivocal evidence of volcanic features. Nevertheless, several morphological properties of Brumalia Tholus on Vestalia Terra suggest that this topographic high most likely formed as a magmatic intrusion. The presence of more orthopyroxene-rich material relative to surrounding terrain in the ejecta of Teia, a fresh impact crater on the northern face of Brumalia Tholus, supports the hypothesis of magmatic intrusions on Vesta.

Small fresh impact craters on asteroid 4 Vesta: A compositional and geological fingerprint

AbstractSmall morphologically fresh impact craters (&lt;10 km in diameter) on Vesta's surface with a photometrically distinct ejecta blanket are expected to represent fresh surface material and thus provide the opportunity to study the composition of the unweathered surface. Dawn‐Framing Camera and Visual and Infrared Spectrometer (VIR) data reveal impact craters with bright, dark, and mixed, i.e., partly bright and dark, ejecta existing on Vesta's surface, which not only differ in the visible albedo from their surroundings but also in their composition. Differences in the composition are related to the visible albedo and/or the geographic location of the impact craters. Bright ejecta, only seen in the southern Vestan hemisphere, are dominated by howardite/eucrite‐like material as expected for Vesta's upper crust. Dark ejecta associated with dark impact craters are dominated by a strongly absorbing, spectrally neutral compound, supporting an origin from carbon‐rich impactors. Few impact craters of intermediate albedo in Vesta's southern hemisphere contain material resembling diogenites, which are expected to exist in the deeper parts of Vesta's interior. The geological settings suggest that the diogenite‐like material represents a part of a layer of diogenitic material surrounding the Rheasilvia basin or local concentrations of diogenitic material as part of the ejecta excavated during the latter stage of the Rheasilvia impact event. The spectral differences between eucrite‐ and diogenite‐dominated materials also could be verified due to spin‐forbidden absorptions in the visible spectral range, which are known from laboratory spectra of pyroxenes, but, which have been identified in the VIR spectra of Vesta for the first time.

Dynamic Landmarking for Surface Feature Identification and Change Detection

Given the large volume of images being sent back from remote spacecraft, there is a need for automated analysis techniques that can quickly identify interesting features in those images. Feature identification in individual images and automated change detection in multiple images of the same target are valuable for scientific studies and can inform subsequent target selection. We introduce a new approach to orbital image analysis called dynamic landmarking. It focuses on the identification and comparison of visually salient features in images. We have evaluated this approach on images collected by five Mars orbiters. These evaluations were motivated by three scientific goals: to study fresh impact craters, dust devil tracks, and dark slope streaks on Mars. In the process we also detected a different kind of surface change that may indicate seasonally exposed bedforms. These experiences also point the way to how this approach could be used in an onboard setting to analyze and prioritize data as it is collected.

Fluvial landforms on fresh impact ejecta on Mars

Fluvial valleys provide critical clues to the distribution and state of water throughout the history of the planet Mars. Early in Mars’ history (<3.7Gy), the climate may have been warmer than at present leading to the development of valley networks. Younger valleys formed on volcanic and glacial landforms under colder conditions than experienced in Mars’ early history. Only rare examples of fluvial valleys over fresh impact craters have been reported. In the present study, a survey of hundreds of fresh post-Noachian impact craters (of 12 to 150km in diameter) has been done to identify fluvial landforms, especially in regions lacking ancient valleys, using images from the High Resolution Stereo Camera (HRSC) instrument onboard Mars Express and from the Context Camera (CTX) instrument onboard Mars Reconnaissance Orbiter. Observations show that these valleys are locally sinuous, display isolated channels, a poor connectivity and frequent braiding. Valleys were most likely formed over a short duration with high discharge rates, estimated from 500 to 40,000m3s−1. In Arabia Terra, a total of 27 out of the 204 surveyed craters were found to have fluvial landforms on the ejecta blanket, exclusively in the mid-latitude band (25–45°). Dating of impact ejecta gives young ages from the Late Hesperian to the Middle Amazonian, thus providing a temporal constraint for the fluvial activity. Late climatic episodes of snow deposition and subsequent melting scattered in space and time could explain observations. Alternatively, the thermal anomaly of impacts and their ejecta over ice-bearing terrains is a possible triggering mechanism for the observed fluvial valleys. Calculations show that the thermal anomaly can persist in the ejecta over several hundreds of years for mid-size craters (20–40km). Such a process would not explain all Martian fluvial activity because of the marked difference between the pristine landforms described and Late Noachian valley networks. Nevertheless, fluvial landforms on preserved ejecta blankets can be used as a new proxy for the temporal distribution of water on Mars.

The Saturnian satellite Rhea as seen by Cassini VIMS

Since the arrival of the Cassini spacecraft at Saturn in June 2004, the Visual and Infrared Mapping Spectrometer has obtained new spectral data of the icy satellites of Saturn in the spectral range from 0.35 to 5.2μm. Numerous flybys were performed at Saturn’s second largest satellite Rhea, providing a nearly complete coverage with pixel-ground resolutions sufficient to analyze variations of spectral properties across Rhea’s surface in detail. We present an overview of the VIMS observations obtained so far, as well as the analysis of the spectral properties identified in the VIMS spectra and their variations across its surface compared with spatially highly resolved Cassini ISS images and digital elevation models.Spectral variations measured across Rhea’s surface are similar to the variations observed in the VIMS observations of its neighbor Dione, implying similar processes causing or at least inducing their occurrence. Thus, magnetospheric particles and dust impacting onto the trailing hemisphere appear to be responsible for the concentration of dark rocky/organic material and minor amounts of CO2 in the cratered terrain on the trailing hemisphere. Despite the prominent spectral signatures of Rhea’s fresh impact crater Inktomi, radiation effects were identified that also affect the H2O ice-rich cratered terrain of the leading hemisphere. The concentration of H2O ice in the vicinity of steep tectonic scarps near 270°W and geologically fresh impact craters implies that Rhea exhibits an icy crust at least in the upper few kilometers. Despite the evidence for past tectonic events, no indications of recent endogenically powered processes could be identified in the Cassini data.