Impact crater formation in icy layered terrains on Mars

Abstract— We present numerical simulations of crater formation under Martian conditions with a single near‐surface icy layer to investigate changes in crater morphology between glacial and interglacial periods. The ice fraction, thickness, and depth to the icy layer are varied to understand the systematic effects on observable crater features. To accurately model impact cratering into ice, a new equation of state table and strength model parameters for H₂O are fitted to laboratory data. The presence of an icy layer significantly modifies the cratering mechanics. Observable features demonstrated by the modeling include variations in crater morphometry (depth and rim height) and icy infill of the crater floor during the late stages of crater formation. In addition, an icy layer modifies the velocities, angles, and volumes of ejecta, leading to deviations of ejecta blanket thickness from the predicted power law. The dramatic changes in crater excavation are a result of both the shock impedance and the strength mismatch between layers of icy and rocky materials. Our simulations suggest that many of the unusual features of Martian craters may be explained by the presence of icy layers, including shallow craters with well‐preserved ejecta blankets, icy flow related features, some layered ejecta structures, and crater lakes. Therefore, the cratering record implies that near‐surface icy layers are widespread on Mars.     

Erosional modification and gully formation at Meteor Crater, Arizona: Insights into crater degradation processes on Mars

Hydrogeological modification of Meteor Crater produced a spectacular set of gullies throughout the interior wall in response to rainwater precipitation, snow melting, and possible groundwater discharge. The crater wall has an exceptionally well-developed centripetal drainage pattern consisting of individual alcoves, channels, and fans. Some of the gullies originate from the rim crest and others from the middle crater wall where a lithologic transition occurs; broad gullies occur along the crater corner radial faults. Deeply incised alcoves are well developed on the soft Coconino Sandstone exposed on the middle crater wall, beneath overlying dolomite. In general, the gully locations are along crater wall radial fractures and faults, which are favorable locales of erosion due to preferential rock breakup from faulting, and groundwater flow/discharge; these structural discontinuities are also the locales where the surface runoff from rain precipitation and snow melting can preferentially flow, causing erosion and crater degradation. Channels are well developed on the talus deposits and alluvial fans on the periphery of the crater floor. Caves exposed on the lower crater level point to percolation of surface runoff and selective discharge through fractures on the crater wall. In addition, lake sediments on the crater floor provide significant evidence of a past pluvial climate, when the water table was higher, and groundwater may have seeped from springs on the crater wall. Although these hydrological processes continue at Meteor Crater today, conditions at the crater are much more arid than they were soon after impact, reflecting a climatic shift. This climate shift and the hydrological modifications observed at Meteor Crater provide insights for landscape sculpturing on Mars during various parts of its history.     

Surficial properties in Gale Crater, Mars, from Mars Odyssey THEMIS data

We examine the nature of the surface layer in Gale Crater as determined from high-resolution thermal and visible Mars Odyssey Thermal Emission Imaging System (THEMIS) data as well as how our conclusions compare to past analyses. At THEMIS resolution, the thermal surface structure is dominated by local control, thus providing us with detailed images that contain thermophysical information as well. Using these data sets we have created a map of the area, defining units based primarily on their geomorphology as determined from the daytime thermal and visible images and then using the nighttime thermal data to interpret the nature of the surface layer within each unit. Seven units have been defined: (i) partially blanketed knobby plateaus, (ii) crater walls with terrain similar to that on the plateaus on the upper half and exposed, rocky surfaces on the lower half, (iii)-(v) three floor units with varying combinations of bedrock and indurated and/or particulate deposits, (vi) sand sheets, and (vii) a central mound, consisting of indurated and/or rocky material forming layers, terraces, and slides, covered by particulate material that tapers in thickness downslope. Additionally, dozens of channels have been observed on the crater walls and central mound. The results indicate that aeolian processes have played a major role in shaping much of the present surface layer within Gale and may still be active today. Because of the dramatic size and structure of Gale, the winds are most likely controlled by the local topography. Additionally, the presence and frequency of channels within Gale bolster hypotheses involving aqueous episodes in the history of the crater.     

Millochau crater, Mars: Infilling and erosion of an ancient highland impact crater

The geology and stratigraphy of Millochau crater (21.4° S, 275° W), located in the highlands of Tyrrhena Terra, Mars, are documented through geomorphic analyses and geologic mapping. Crater size-frequency distributions and superposition relationships are used to constrain relative ages of geologic units and determine the timing and duration of the geologic processes that modified Millochau rim materials and emplaced deposits on Millochau's floor. Crater size-frequency distributions show a Middle Noachian age for rim materials and Middle Noachian to Early Hesperian ages for most of the interior deposits. Valley networks and gullies incised within Millochau's rim materials and interior wall, respectively, indicate fluvial activity was an important erosional process. Millochau contains an interior plateau, offset northeast of Millochau's center, which rises up to 400 m above the surrounding crater floor and slopes downward to the south and west. Layers exposed along the northern and eastern scarp boundaries of the plateau are tens to hundreds of meters thick and laterally continuous in MOC images. These layers suggest most materials within Millochau were emplaced by sedimentary processes (e.g., fluvial or eolian), with the potential for lacustrine deposition in shallow transient bodies of water and contributions of volcanic airfall. Mass wasting may have also contributed significant quantities of material to Millochau's interior, especially to the deposits surrounding the plateau. Superposition relationships combined with impact crater statistics indicate that most deposition and erosion of Millochau's interior deposits is ancient, which implies that fluvial activity in this part of Tyrrhena Terra is much older than in the eastern Hellas region. Eolian processes mobilized sediment to form complicated patterns of long- and short-wavelength dunes, whose emplacement is controlled by local topography. These deposits are some of the youngest within Millochau (Amazonian) and eolian modification may be ongoing.     

Basalt–trachybasalt samples in Gale Crater,Mars

The ChemCam instrument on the Mars Science Laboratory (MSL) rover, Curiosity, observed numerous igneous float rocks and conglomerate clasts, reported previously. A new statistical analysis of single‐laser‐shot spectra of igneous targets observed by ChemCam shows a strong peak at ~55 wt% SiO₂ and 6 wt% total alkalis, with a minor secondary maximum at 47–51 wt% SiO₂ and lower alkali content. The centers of these distributions, together with the rock textures, indicate that many of the ChemCam igneous targets are trachybasalts, Mg# = 27 but with a secondary concentration of basaltic material, with a focus of compositions around Mg# = 54. We suggest that all of these igneous rocks resulted from low‐pressure, olivine‐dominated fractionation of Adirondack (MER) class‐type basalt compositions. This magmatism has subalkaline, tholeiitic affinities. The similarity of the basalt endmember to much of the Gale sediment compositions in the first 1000 sols of the MSL mission suggests that this type of Fe‐rich, relatively low‐Mg#, olivine tholeiite is the dominant constituent of the Gale catchment that is the source material for the fine‐grained sediments in Gale. The similarity to many Gusev igneous compositions suggests that it is a major constituent of ancient Martian magmas, and distinct from the shergottite parental melts thought to be associated with Tharsis and the Northern Lowlands. The Gale Crater catchment sampled a mixture of this tholeiitic basalt along with alkaline igneous material, together giving some analogies to terrestrial intraplate magmatic provinces.     

Evidence for subsurface water ice in Korolev crater, Mars

Following the work of Kieffer and Titus (2001, Icarus 154, 162-180), we present results of thermal IR observations of Korolev crater, located at ∼73° latitude in the martian northern polar region. Similar to techniques employed by Titus et al. (2003, Science 299, 1048-1050), we use infrared images from the Thermal Emission Imaging System (THEMIS) aboard Mars Odyssey to identify several regions within the crater basin with distinct thermal properties that correlate with topography. The THEMIS results show these regions exhibit temperature variations, spatially within the crater and throughout the martian year. In addition to the variations identified in the THEMIS observations, Mars Global Surveyor Thermal Emission Spectrometer (TES) observations show differences in albedo and temperature of these regions on both daily and seasonal cycles. Modeling annual temperature variations of the surface, we use TES observations to examine the thermal properties of these regions. This analysis reveals the crater interior deposits are likely thick layers (several meters) of high thermal inertia material (water ice, or extremely ice-rich regolith). Spatial variations of the physical properties of these regions are likely due to topography and possibly variations in the subsurface material itself. The nature of these deposits may help constrain polar processes, as well as provide context for the polar lander mission, Phoenix.     

Comparison of the crater morphology-size relationship for Mars,Moon, and Mercury

New central peak-crater size data for Mars shows that a higher percentage of relatively unmodified Martian craters have central peaks than do fresh lunar craters below a diameter of 30 km. For example, in the diameter range 10 to 20 km, 60% of studied Martian craters have central peaks compared to 26% for the Moon. Gault et al. (1975, J. Geophys. Res.80, 2444–2460) have demonstrated that central peaks occur in smaller craters on Mercury than on the Moon, and that this effect is due to the different gravity fields in which the craters formed. Similar differences when comparing Mars and the Moon show that gravity has affected the diameter at which central peaks form on Mars. Erosion on Mars, therefore, does not completely mask differences in crater interior structure that are caused by differences in gravity. Effects of Mars' higher surface gravity when compared to the Moon are not detected when comparing terrace and crater shape data. The morphology-crater size statistics also show that a full range of crater shapes occur on Mars, and craters tend to become more morphologically complex with increasing diameter. Comparisons of Martian and Mercurian crater data show differences which may be related to the greater efficacy of erosion on Mars.     

Interplanet variations in scale of crater morphology—Earth,Mars, Moon

Craters on the Earth, Mars, and the Moon show a spectrum of morphologies with diameter increasing from simple, bowl-shaped craters through craters with increasingly complex central peaks, to craters with “peak rings” and basins with multiple concentric scarps. In each category there is a range of diameters, centered around a characteristic diameter, D_c. It is found that D_c decreases as the size of the planet increases. Several possible explanations are considered. It is suggested that the effect results from a gravity scaling law derived here and having approximately from the D_c∂ 1/g^1.25, where g is the surface gravity. All geological structures in which gravity is the dominant parameter affecting the morphology should follow such a law.     

Stratigraphy, mineralogy, and origin of layered deposits inside Terby crater, Mars

The 174 km diameter Terby impact crater (28.0°S-74.1°E) located on the northern rim of the Hellas basin displays anomalous inner morphology, including a flat floor and light-toned layered deposits. An analysis of these deposits was performed using multiple datasets from Mars Global Surveyor, Mars Odyssey, Mars Express and Mars Reconnaissance Orbiter missions, with visible images for interpretation, near-infrared data for mineralogical mapping, and topography for geometry. The geometry of layered deposits was consistent with that of sediments that settled mainly in a sub-aqueous environment, during the Noachian period as determined by crater counts. To the north, the thickest sediments displayed sequences for fan deltas, as identified by 100 m to 1 km long clinoforms, as defined by horizontal beds passing to foreset beds dipping by 6-10° toward the center of the Terby crater. The identification of distinct sub-aqueous fan sequences, separated by unconformities and local wedges, showed the accumulation of sediments from prograding/onlapping depositional sequences, due to lake level and sediment supply variations. The mineralogy of several layers with hydrated minerals, including Fe/Mg phyllosilicates, supports this type of sedimentary environment. The volume of fan sediments was estimated as >5000 km³ (a large amount considering classical martian fan deltas such as Eberswalde (6 km³)) and requires sustained liquid water activity. Such a large sedimentary deposition in Terby crater is characteristic of the Noachian/Phyllosian period during which the environment favored the formation of phyllosilicates. The latter were detected by spectral data in the layered deposits of Terby crater in three distinct layer sequences. During the Hesperian period, the sediments experienced strong erosion, possibly enhanced by more acidic conditions, forming the current morphology with three mesas and closed depressions. Small fluvial valleys and alluvial fans formed subsequently, attesting to late fluvial processes dated as late Early to early Late Hesperian. After this late fluvial episode, the Terby impact crater was submitted to aeolian processes and permanent cold conditions with viscous flow features. Therefore, the Terby crater displays, in a single location, geologic features that characterize the three main periods of time on Mars, with the presence of one of the thickest sub-aqueous fan deposits reported on Mars. The filling of Terby impact crater is thus one potential “reference geologic cross-section” for Mars stratigraphy.     

Constraints on the origin and evolution of the layered mound in Gale Crater, Mars using Mars Reconnaissance Orbiter data

Gale Crater contains a 5.2 km-high central mound of layered material that is largely sedimentary in origin and has been considered as a potential landing site for both the MER (Mars Exploration Rover) and MSL (Mars Science Laboratory) missions. We have analyzed recent data from Mars Reconnaissance Orbiter to help unravel the complex geologic history evidenced by these layered deposits and other landforms in the crater. Results from imaging data from the High Resolution Imaging Science Experiment (HiRISE) and Context Camera (CTX) confirm geomorphic evidence for fluvial activity and may indicate an early lacustrine phase. Analysis of spectral data from the CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) instrument shows clay-bearing units interstratified with sulfate-bearing strata in the lower member of the layered mound, again indicative of aqueous activity. The formation age of the layered mound, derived from crater counts and superposition relationships, is ∼3.6-3.8 Ga and straddles the Noachian-Hesperian time-stratigraphic boundary. Thus Gale provides a unique opportunity to investigate global environmental change on Mars during a period of transition from an environment that favored phyllosilicate deposition to a later one that was dominated by sulfate formation.     

The nature and emplacement of distal aqueous-rich ejecta deposits from Hale crater,Mars

Hale crater formed in the Early to Middle Amazonian and is one of the best preserved large craters on Mars. We focus on the emplacement of previously mapped distal continuous ejecta and newly recognized discontinuous distal ejecta deposits reaching up to 450 km northeast of Hale. The distal continuous ejecta deposits are typically tens of meters thick, likely water-rich, and subsequent dewatering of some resulted in flow along gradients of 10 m km^-1 for distances of tens of kilometers. The discontinuous distal ejecta are typically <10 m thick with volumes generally <0.5 km³ and embay Hale secondaries, which occur up to ~600 km from Hale. Both continuous and discontinuous distal ejecta deposits are typically smooth at scales of tens to hundreds of meters, relatively dark-toned, devoid of eolian bedforms, inferred to be mostly fine-grained, and were likely emplaced within hours to 1–2 days after impact. The occurrence of well-preserved discontinuous distal ejecta at Hale is unusual compared to other large Martian craters and could be due to impact into an ice-rich substrate that enabled their formation and (or) their survival after minimal postimpact degradation relative to older craters. The pristine nature of distal continuous and discontinuous distal deposits at Hale and the preservation of associated secondaries imply (1) low erosion rates after the Hale impact, comparable to those estimated elsewhere during the Amazonian; (2) the impact did not significantly influence long-term global or regional scale geomorphic activity or climate; and (3) the Hale impact occurred after late alluvial fan activity in Margaritifer Terra.     

Observations at terrestrial impact structures: Their utility in constraining crater formation

Abstract— Hypervelocity impact involves the near instantaneous transfer of considerable energy from the impactor to a spatially limited near‐surface volume of the target body. Local geology of the target area tends to be of secondary importance, and the net result is that impacts of similar size on a given planetary body produce similar results. This is the essence of the utility of observations at impact craters, particularly terrestrial craters, in constraining impact processes. Unfortunately, there are few well‐documented results from systematic contemporaneous campaigns to characterize specific terrestrial impact structures with the full spectrum of geoscientific tools available at the time. Nevertheless, observations of the terrestrial impact record have contributed substantially to fundamental properties of impact. There is a beginning of convergence and mutual testing of observations at terrestrial impact structures and the results of modeling, in particular from recent hydrocode models. The terrestrial impact record provides few constraints on models of ejecta processes beyond a confirmation of the involvement of the local substrate in ejecta lithologies and shows that Z‐models are, at best, first order approximations. Observational evidence to date suggests that the formation of interior rings is an extension of the structural uplift process that occurs at smaller complex impact structures. There are, however, major observational gaps and cases, e.g., Vredefort, where current observations and hydrocode models are apparently inconsistent. It is, perhaps, time that the impact community as a whole considers documenting the existing observational and modeling knowledge gaps that are required to be filled to make the intellectual breakthroughs equivalent to those of the 1970s and 1980s, which were fueled by observations at terrestrial impact structures. Filling these knowledge gaps would likely be centered on the later stages of formation of complex and ring structures and on ejecta.     

Spectral unmixing for mineral identification in pancam images of soils in Gusev crater, Mars

The objective of this work is to propose an automated unmixing technique for the analysis of 11-channel Mars Exploration Rover Panoramic Camera (MER/Pancam) spectra. Our approach is to provide a screening tool for identifying unique/distinct reflectance spectra. We demonstrate the utility of this unmixing technique in a study of the mineralogy of the bright salty soils exposed by the rover wheels in images of Gusev crater regions known as Paso Robles (Sols 400,426), Arad (Sol 721), and Tyrone (Sol 790). The unmixing algorithm is based on a novel derivation of the Nonnegative Matrix Factorization technique and includes added features that preclude the adverse effects of low abundance materials that would otherwise skew the unmixing. In order to create a full 11-channel spectrum out of the left and right eye stereo pairs, we also developed a new registration procedure that includes rectification and disparity calculation of the images. We identified two classes of endmember spectra for the bright soils imaged on Sols 426 and 790. One of these endmember classes is also observed for soils imaged on Sols 400 and 721 and has a unique spectral shape that is distinct from most iron oxide, sulfate and silicate spectra and differs from typical martian surface spectra. Instead, its unique spectral character resembles the spectral shape of the ferric sulfate minerals fibroferrite (Fe³⁺(SO₄)(OH) · 5H₂O) and ferricopiapite and the phosphate mineral ferristrunzite . The other endmember class is less consistent with specific minerals and is likely a mixture of altered volcanic material and some bright salts. Further analyses of data from Sols 400 and 790 using an anomaly detection algorithm as a tool for detecting low abundance materials additionally suggests the identification of the sulfate mineral paracoquimbite (Fe₂(SO₄)₃ · 9H₂O). This spectral study of Pancam images of the bright S- and P-enriched soils of Gusev crater identifies specific ferric sulfate and ferric phosphate minerals that are consistent with the unique spectral properties observed here in the 0.4-1 μm range.     

Chesapeake Bay impact structure: Morphology,crater fill,and relevance for impact structures on Mars

Abstract— The late Eocene Chesapeake Bay impact structure (CBIS) on the Atlantic margin of Virginia is one of the largest and best‐preserved “wet‐target” craters on Earth. It provides an accessible analog for studying impact processes in layered and wet targets on volatile‐rich planets. The CBIS formed in a layered target of water, weak clastic sediments, and hard crystalline rock. The buried structure consists of a deep, filled central crater, 38 km in width, surrounded by a shallower brim known as the annular trough. The annular trough formed partly by collapse of weak sediments, which expanded the structure to ?85 km in diameter. Such extensive collapse, in addition to excavation processes, can explain the “inverted sombrero” morphology observed at some craters in layered targets. The distribution of crater‐fill materials in the CBIS is related to the morphology. Suevitic breccia, including pre‐resurge fallback deposits, is found in the central crater. Impact‐modified sediments, formed by fluidization and collapse of water‐saturated sand and silt‐clay, occur in the annular trough. Allogenic sediment‐clast breccia, interpreted as ocean‐resurge deposits, overlies the other impactites and covers the entire crater beneath a blanket of postimpact sediments. The formation of chaotic terrains on Mars is attributed to collapse due to the release of volatiles from thick layered deposits. Some flat‐floored rimless depressions with chaotic infill in these terrains are impact craters that expanded by collapse farther than expected for similar‐sized complex craters in solid targets. Studies of crater materials in the CBIS provide insights into processes of crater expansion on Mars and their links to volatiles.     

The Carancas meteorite impact crater,Peru: Geologic surveying and modeling of crater formation and atmospheric passage

Abstract— The recent Carancas meteorite impact event caused a worldwide sensation. An H4–5 chondrite struck the Earth south of Lake Titicaca in Peru on September 15, 2007, and formed a crater 14.2 m across. It is the smallest, youngest, and one of two eye‐witnessed impact crater events on Earth. The impact violated the hitherto existing view that stony meteorites below a size of 100 m undergo major disruption and deceleration during their passage through the atmosphere and are not capable of producing craters. Fragmentation occurs if the strength of the meteoroid is less than the aerodynamic stresses that occur in flight. The small fragments that result from a breakup rain down at terminal velocity and are not capable of producing impact craters. The Carancas cratering event, however, demonstrates that meter‐sized stony meteoroids indeed can survive the atmospheric passage under specific circumstances. We present results of a detailed geologic survey of the crater and its ejecta. To constrain the possible range of impact parameters we carried out numerical models of crater formation with the iSALE hydrocode in two and three dimensions. Depending on the strength properties of the target, the impact energies range between approximately 100–1000 MJ (0.024–0.24 t TNT). By modeling the atmospheric traverse we demonstrate that low cosmic velocities (12–14 kms^?1) and shallow entry angles (<20 °) are prerequisites to keep aerodynamic stresses low (<10 MPa) and thus to prevent fragmentation of stony meteoroids with standard strength properties. This scenario results in a strong meteoroid deceleration, a deflection of the trajectory to a steeper impact angle (40–60 °), and an impact velocity of 350–600 ms^?1, which is insufficient to produce a shock wave and significant shock effects in target minerals. Aerodynamic and crater modeling are consistent with field data and our microscopic inspection. However, these data are in conflict with trajectories inferred from the analysis of infrasound signals.     

Surficial Geologic Surveys of Gale Crater and Melas Chasma, Mars: Integration of Remote-Sensing Data

Remote-sensing observations of Gale Crater and Melas Chasma are synthesized to better understand the present surface layer in these two scientifically interesting regions. Data sets analyzed include geologic maps, Mars Orbiter Laser Altimeter (MOLA) elevation, Thermal Emission Spectrometer (TES) albedo, TES thermal inertia, MOLA residual pulse width, TES rock abundance, and Viking and Mars Orbiter Camera images. Using these data sets, we constrain the properties of and processes acting on the present surface and create self-consistent models for the surface layer. Surface properties within Gale Crater are variable and complex, and interpreting the surface layer is not straightforward. Observations indicate that aeolian processes have shaped some of the intracrater surface, but other areas do not comply with this model and have counterintuitive surface characteristics. In contrast, aeolian activity appears to have played a large role in shaping the present-day physical structure of all surfaces in Melas Chasma and is reflected in the remote-sensing data sets. Here we summarize our analysis and discuss the surface attributes as determined from these data sets.     

Sinton crater, Mars: Evidence for impact into a plateau icefield and melting to produce valley networks at the Hesperian-Amazonian boundary

The majority of martian valley networks are found on Noachian-aged terrain and are attributed to be the result of a ‘warm and wet’ climate that prevailed early in Mars' history. Younger valleys have been identified, though these are largely interpreted to be the result of localized conditions associated with the melting of ice from endogenic heat sources. Sinton crater, a 60 km diameter impact basin in the Deuteronilus Mensae region of the dichotomy boundary, is characterized by small anastomosing valley networks that are located radial to the crater rim. Large scale deposits, interpreted to be the remains of debris covered glaciers, have been identified in the area surrounding Sinton, and our observations have revealed the occurrence of an ice rich fill deposit within the crater itself. We have conducted a detailed investigated into the Sinton valley networks with all the available remote data sets and have dated their formation to the Amazonian/Hesperian boundary. The spatial and temporal association between Sinton crater and the valley networks suggest that the impact was responsible for their formation. We find that the energy provided by an asteroid impact into surficial deposits of snow/ice is sufficient to generate the required volumes of melt water needed for the valley formation. We therefore interpret these valleys to represent a distinct class of martian valley networks. This example demonstrates the potential for impacts to cause the onset of fluvial erosion on Mars. Our results also suggest that periods of glacial activity occurred throughout the Amazonian and into the Hesperian in association with variations in spin orbital parameters.     

Distribution and characteristics of Adirondack-class basalt as observed by Mini-TES in Gusev crater,Mars and its possible volcanic source

We identified 104 unique rock targets belonging to the olivine-rich Adirondack class using Mini-TES data. Rare rocks on the West Spur of the Columbia Hills and on the plains east of the Hills also belong to this class. We present evidence that Adirondack-class basaltic lavas may have had their origin at Apollinaris Tholus. Linear modeling of Adirondack-class rock spectra shows only minor variations in mineralogy and the primary phases identified are consistent with olivine basalt having an average olivine composition of ～Fo₄₅. We used factor analysis and target transformation to identify variability within a single class of rocks for the first time, and we find that olivine abundance varies independently of a basaltic matrix. A spectral component previously attributed to downwelling radiance is proposed here to be attributable to optically thin dust on rocks, which has a greater effect on Mini-TES spectra than previously recognized, but can explain the sloping continuum observed in Adirondack-class rock spectra as well as the apparent distortion of an olivine absorption. Spectral mixing is not completely linear, leading to the overestimation of sulfate and olivine fractions and the underestimation of plagioclase feldspar, although linear mixing appears to successfully replicate the majority of the observed signal. Published TES spectra of low albedo, low dust cover, olivine-bearing materials do not exhibit the spectral telltales of non-linear behavior, probably because orbital spectra represent mostly mobile regolith that has not accumulated a sufficient dust cover over ～3 × 6 km areas.     

Outflow channel sources, reactivation, and chaos formation, Xanthe Terra, Mars

The undulating, warped, and densely fractured surfaces of highland regions east of Valles Marineris (located north of the eastern Aureum Chaos, east of the Hydraotes Chaos, and south of the Hydaspis Chaos) resulted from extensional surface warping related to ground subsidence, caused when pressurized water confined in subterranean caverns was released to the surface. Water emanations formed crater lakes and resulted in channeling episodes involved in the excavation of Ares, Tiu, and Simud Valles of the eastern part of the circum-Chryse outflow channel system. Progressive surface subsidence and associated reduction of the subsurface cavernous volume, and/or episodes of magmatic-driven activity, led to increases of the hydrostatic pressure, resulting in reactivation of both catastrophic and non-catastrophic outflow activity. Ancient cratered highland and basin materials that underwent large-scale subsidence grade into densely fractured terrains. Collapse of rock materials in these regions resulted in the formation of chaotic terrains, which occur in and near the headwaters of the eastern circum-Chryse outflow channels. The deepest chaotic terrain in the Hydaspis Chaos region resulted from the collapse of pre-existing outflow channel floors. The release of volatiles and related collapse may have included water emanations not necessarily linked to catastrophic outflow. Basal warming related to dike intrusions, thermokarst activity involving wet sediments and/or dissected ice-enriched country rock, permafrost exposed to the atmosphere by extensional tectonism and channel incision, and/or the injection of water into porous floor material, may have enhanced outflow channel floor instability and subsequent collapse. In addition to the possible genetic linkage to outflow channel development dating back to at least the Late Noachian, clear disruption of impact craters with pristine ejecta blankets and rims, as well as preservation of fine tectonic fabrics, suggest that plateau subsidence and chaos formation may have continued well into the Amazonian Period. The geologic and paleohydrologic histories presented here have important implications, as new mechanisms for outflow channel formation and other fluvial activity are described, and new reactivation mechanisms are proposed for the origin of chaotic terrain as contributors to flooding. Detailed geomorphic analysis indicates that subterranean caverns may have been exposed during chaos formation, and thus chaotic terrains mark prime locations for future geologic, hydrologic, and possible astrobiologic exploration.     

Periodicity in the crater formation rate and implications for astronomical modeling

Claim for periodicity in the crater formation rate is reinvestigated using a criterion proposed by Broadbent, and data sets of Rampino and Stothers and of Grieve are shown to satisfy the periodicity criterion (P 30 Myr).On the other hand, currently observed impactors are mainly asteroids, while long and short periodic comets whose fluxes may vary by external disturbances occupy only a small fraction. Using a Monte Carlo simulation, constraints are obtained for the dispersion Q(Myr) from an exact periodicity and for the periodic components (F _tp) in the signals for their periodicity to be detected. It is found that for = 5, 6 and 7 Myr, F _tp, would have to be 40% or greater, 60% or greater and 80% or greater, respectively. These constraints are used to discuss whether the giant molecular cloud perturbations can give rise to the periodicity in the impact events. The amplitude of the solar Z-motion need to be some 100pc for = 6 Myr, which requires the periodic component (SP and LP comets, if the former originate from the latter) to be 60%, while for = 7 Myr, the periodic component need to be 80%. The GMC perturbation model consistent with the periodicity appears to be the one where the amplitude is 100pc and the periodic component - 60% of the impactors. If SP comets mainly originate from a source such as the hypothetical Kuiper belt, the GMC perturbation would not be consistent with the periodicity.