Ring furrows: Inversion of topography in Martian highland terrains

Ring furrows are flat-floored trenches, circular in plan view, surrounding a central, flat-topped, mesa or plateau. The internal plateau is about the same elevation or lower than the plain outside the ring. The outer wall is often breached by valley drainage or opened to low, degraded surfaces. Related landforms range from ring furrows with fractured central plateaus to those with isolated circular mesas without depressed rings. Ring furrows are superposed on many types of materials, but they are most common on cratered plateau-type materials which are interpreted as volcanic flows overlying ancient cratered terrain. Most rings occur in or near regions of fretted terrain. Ring furrows are formed by preferential removal of the exposed rims of partially buried craters. Ground ice decay and sapping followed by fluvial erosion are proposed for removal of the least resistant rim materials. Thus, differential erosion has caused an inversion of topography in which the originally elevated rim is reduced to negative relief.     

Inverted channel deposits on the floor of Miyamoto crater, Mars

Morphological features on the western floor of Miyamoto crater in southwestern Meridiani Planum, Mars, are suggestive of past fluvial activity. Imagery from the High Resolution Imaging Science Experiment (HiRISE) gives a detailed view of raised curvilinear features that appear to represent inverted paleochannel deposits. The inverted terrain appears to be capped with a resistant, dark-toned deposit that is partially covered by unconsolidated surficial materials. Subsequent to deposition of the capping layer, erosion of the surrounding material has left the capping materials perched on pedestals of uneroded basal unit material. Neither the capping material nor the surrounding terrains show any unambiguous morphological evidence of volcanism or glaciation. The capping deposit may include unconsolidated or cemented stream deposits analogous to terrestrial inverted channels in the Cedar Mountain Formation near Green River, Utah. In addition to this morphological evidence for fluvial activity, phyllosilicates have been identified in the basal material on the floor of Miyamoto crater by orbital spectroscopy, providing mineralogical evidence of past aqueous activity. Based on both the morphological and mineralogical evidence, Miyamoto crater represents an excellent site for in situ examination and sampling of a potentially habitable environment.     

Ancient wet aeolian environments on Earth: clues to presence of fossil/live microorganisms on Mars

Ancient wet aeolian (wet-sabkha) environments on Earth, represented in the Entrada and Navajo sandstones of Utah, contain pipe structures considered to be the product of gas/water release under pressure. The sediments originally had considerable porosity allowing the ingress of living plant structures, microorganisms, clay minerals, and fine-grained primary minerals of silt and sand size from the surface downward in the sedimentary column. Host rock material is of a similar size and porosity and presumably the downward migration of fine-grained material would have been possible prior to lithogenesis and final cementation. Recent field emission scanning electron microscopy (FESEM) and EDS (energy-dispersive spectrometry) examination of sands from fluidized pipes in the Early Jurassic Navajo Sandstone reveal the presence of fossil forms resembling fungal filaments, some bearing hyphopodium-like structures similar to those produced by modern tropical leaf parasites. The tropical origin of the fungi is consistent with the paleogeography of the sandstone, which was deposited in a tropical arid environment. These fossil fungi are silicized, with minor amounts of CaCO₃ and Fe, and in some cases a Si/Al ratio similar to smectite. They exist as pseudomorphs, totally depleted in nitrogen, adhering to the surfaces of fine-grained sands, principally quartz and orthoclase. Similar wet aeolian paleoenvironments are suspected for Mars, especially following catastrophic sediment-charged floods of enormous magnitudes that are believed to have contributed to rapid formation of large water bodies in the northern plains, ranging from lakes to oceans. These events are suspected to have contributed to a high frequency of constructional landforms (also known as pseudocraters) related to trapped volatiles and water-enriched sediment underneath a thick blanket of materials that were subsequently released to the martian surface, forming piping structures at the near surface and constructional landforms at the surface. This constructional process on Mars may help unravel the complex history of some of the piping structures observed on Earth; on Earth, evidence for the constructional landforms has been all but erased and the near-surface piping structures exposed through millions of years of differential erosion and topographic inversion now occur as high-standing promontories. If the features on both Earth and Mars formed by similar processes, especially involving water and other volatiles, and since the piping structures of Earth provided suitable environments for life to thrive in, the martian features in the northern plains should be considered as prime targets for physico/mineral/chemical/microbiological analyses once the astrobiological exploration of the red planet begins in earnest.     

Preliminary mariner 9 report on the geology of Mars

Mariner 9 pictures indicate that the surface of Mars has been shaped by impact, volcanic, tectonic, erosional and depositional activity. The moonlike cratered terrain, identified as the dominant surface unit from the Mariner 6 and 7 flyby data, has proven to be less typical of Mars than previously believed, although extensive in the mid- and high-latitude regions of the southern hemisphere. Martian craters are highly modified but their size-frequency distribution and morphology suggest that most were formed by impact. Circular basins encompassed by rugged terrain and filled with smooth plains material are recognized. These structures, like the craters, are more modified than corresponding features on the Moon and they exercise a less dominant influence on the regional geology. Smooth plains with few visible craters fill the large basins and the floors of larger craters; they also occupy large parts of the northern hemisphere where the plains lap against higher landforms. The middle northern latitudes of Mars from 90 to 150† longitude contain at least four large shield volcanoes each of which is about twice as massive as the largest on Earth. Steep-sided domes with summit craters and large, fresh-appearing volcanic craters with smooth rims are also present in this region. Multiple flow structures, ridges with lobate flanks, chain craters, and sinuous rilles occur in all regions, suggesting widespread volcanism. Evidence for tectonic activity postdating formation of the cratered terrain and some of the plains units is abundant in the equatorial area from 0 to 120° longitude.Some regions exhibit a complex semiradial array of graben that suggest doming and stretching of the surface. Others contain intensity faulted terrain with broader, deeper graben separated by a complex mosaic of flat-topped blocks. An east-west-trending canyon system about 100–200 km wide and about 2500 km long extends through the Coprates-Eos region. The canyons have gullied walls indicative of extensive headward erosion since their initial formation. Regionally depressed areas called chaotic terrain consist of intricately broken and jumbled blocks and appear to result from breaking up and slumping of older geologic units. Compressional features have not been identified in any of the pictures analyzed to data. Plumose light and dark surface markings can be explained by eolian transport. Mariner 9 has thus revealed that Mars is a complex planet with its own distinctive geologic history and that it is less primitive than the Moon.     

Possible liquid water origin for Atacama Desert mudflow and recent gully deposits on Mars

Evidence of recent gully activity on Mars has been reported based on the formation of new light toned deposits within the past decade, the origin of which remains controversial. Analogous recent light toned gully features have formed by liquid water activity in the Atacama Desert on Earth. These terrestrial deposits leave no mineralogical trace of water activity but rather show an albedo difference due to particle size sorting within a fine-grained mudflow. Therefore, spectral differences indicating varying mineralogy between a recent gully deposit and the surrounding terrain may not be the most relevant criteria for detecting water flow in arid environments. Instead, variation in particle size between the deposit and surrounding terrain is a possible discriminator to identify a water-based flow. We show that the Atacama deposit is similar to the observed Mars gully deposits, and both are consistent with liquid water activity. The light-toned Mars gully deposits could have formed from dry debris flows, but a liquid water origin cannot be ruled out because not all liquid water flows leave hydrated minerals behind on the surface. Therefore, the Mars deposits could be remnant mudflows that formed on Mars within the last decade.     

The geology of Australian Mars analogue sites

Australia has numerous landforms and features, some unique, that provide a useful reference for interpreting the results of spacecraft orbiting Mars and exploring the martian surface. Examples of desert landforms, impact structures, relief inversion, long-term landscape evolution and hydrothermal systems that are relevant to Mars are outlined and the relevant literature reviewed. The Mars analogue value of Australia's acid lakes, hypersaline embayments and mound spring complexes is highlighted along with the Pilbara region, where the oldest convincing evidence of life guides exploration for early life on Mars. The distinctive characteristics of the Arkaroola Mars Analogue Region are also assessed and opportunities for future work in Australia are outlined.     

Meteorites at Meridiani Planum provide evidence for significant amounts of surface and near‐surface water on early Mars

Abstract– Six large iron meteorites have been discovered in the Meridiani Planum region of Mars by the Mars Exploration Rover Opportunity in a nearly 25 km‐long traverse. Herein, we review and synthesize the available data to propose that the discovery and characteristics of the six meteorites could be explained as the result of their impact into a soft and wet surface, sometime during the Noachian or the Hesperian, subsequently to be exposed at the Martian surface through differential erosion. As recorded by its sediments and chemical deposits, Meridiani has been interpreted to have undergone a watery past, including a shallow sea, a playa, an environment of fluctuating ground water, and/or an icy landscape. Meteorites could have been encased upon impact and/or subsequently buried, and kept underground for a long time, shielded from the atmosphere. The meteorites apparently underwent significant chemical weathering due to aqueous alteration, as indicated by cavernous features that suggest differential acidic corrosion removing less resistant material and softer inclusions. During the Amazonian, the almost complete disappearance of surface water and desiccation of the landscape, followed by induration of the sediments and subsequent differential erosion and degradation of Meridiani sediments, including at least 10–80 m of deflation in the last 3–3.5 Gy, would have exposed the buried meteorites. We conclude that the iron meteorites support the hypothesis that Mars once had a denser atmosphere and considerable amounts of water and/or water ice at and/or near the surface.     

Geomorphic knobs of Candor Chasma, Mars: New Mars Reconnaissance Orbiter data and comparisons to terrestrial analogs

High Resolution Imaging Science Experiment (HiRISE) imagery and digital elevation models of the Candor Chasma region of Valles Marineris, Mars, reveal prominent and distinctive positive-relief knobs amidst light-toned layers. Three classifications of knobs, Types 1, 2, and 3, are distinguished from a combination of HiRISE and Thermal Emission Imaging System (THEMIS) images based on physical expressions (geometries, spatial relationships), and spectral data from Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). Type 1 knobs are abundant, concentrated, topographically resistant features with their highest frequency in West Candor, which have consistent stratigraphic correlations of the peak altitude (height). These Type 1 knobs could be erosional remnants of a simple dissected terrain, possibly derived from a more continuous, resistant, capping layer of pre-existing material diagenetically altered through recrystallization or cementation. Types 2 and 3 knobs are not linked to a single stratigraphic layer and are generally solitary to isolated, with variable heights. Type 3 are the largest knobs at nearly an order of magnitude larger than Type 1 knobs. The variable sizes and occasional pits on the tops of Type 2 and 3 knobs suggest a different origin, possibly related to more developed erosion, preferential cementation, or textural differences from sediment/water injection or intrusion, or from a buried impact crater. Enhanced color HiRISE images show a brown coloration of the knob peak crests that is attributable to processing and photometric effects; CRISM data do not show any detectable spectral differences between the knobs and the host rock layers, other than albedo. These intriguing knobs hold important clues to deducing relative rock properties, timing of events, and weathering conditions of Mars history.     

Mars: Evidence for dynamic processes from mariners 6 and 7

Mariners 6 and 7 photographs of the equatorial region of Mars document a three-stage evolution of that part of the Martian surface: (1) High- and intermediate-albedo cratered terrains in Meridiani Sinus, Margaritifer Sinus-Thymiamata, Deucalionis Regio-Sabaeus Sinus, and Hellespontus; (2) low-albedo moderately cratered terrain and dark crater fill in Meridiani Sinus, Thymiamata, and Deucalionis Regio-Sabaeus Sinus and possible volcanism in the Hellas-Hellespontus border; and (3) high-albedo surficial deposits, banked-up crater fill, a possible bright-ray crater in Meridiani Sinus, chaotic terrain on the edge of the Margaritifer Sinus mesa, featureless terrain in Hellas and Edom, sinuous channel-like reentrants on scarps at the Hellas-Hellespontus boundary. Regional faulting seems to have occurred following formation of the old cratered plains and prior to formation of low-albedo plains in Meridiani Sinus and also prior to formation of canyon-like reentrants and featureless terrain along the Hellas-Hellespontus boundary.Mars has had a complex history of dynamic evolution, possibly analogous to the more stable regions of Earth. Its geochemical differentiation and thermal regime should account for long-term postaccretional tectonic and volcano-tectonic processes as well as for fluid media on its surface sufficient to cause erosion, including the cutting of large canyons.     

Top layers charaterization of the Martian surface: Permittivity estimation based on geomorphology analysis

Mars Express spacecraft inserted successfully Martian orbit at the end of 2003. On board this probe, a radar instrument called MARSIS (for Mars Advanced Radar for Surface and Ionosphere Sounding) is looking for water inside the first kilometers of Martian crust. To support MARSIS planning and data inversion, Laboratoire de Planétologie de Grenoble developed a MARSIS signal simulator.We show in this paper that MARSIS can also characterize some surface features, in addition to subsurface water and ionosphere sounding. We study a Martian surface region of special interest: Nilokeras Mensae, inside Acidalia Planitia. We discuss the previous geological studies of this region, and show the geomorphologies analyze of this surface area could lead to a simple terrain model. Then, we present a possible data inversion scheme and applying the MARSIS simulator, we test a first radar data inversion.Finally, we will show that complete dielectric characteristics of surface top layers can be retrieved, at least as often Mars Express flies over some layered terrain (at wavelength scale).     

Effects of impacts on the atmospheric evolution: Comparison between Mars, Earth, and Venus

Classified as a terrestrial planet, Venus, Mars, and Earth are similar in several aspects such as bulk composition and density. Their atmospheres on the other hand have significant differences. Venus has the densest atmosphere, composed of CO₂ mainly, with atmospheric pressure at the planet's surface 92 times that of the Earth, while Mars has the thinnest atmosphere, composed also essentially of CO₂, with only several millibars of atmospheric surface pressure. In the past, both Mars and Venus could have possessed Earth-like climate permitting the presence of surface liquid water reservoirs. Impacts by asteroids and comets could have played a significant role in the evolution of the early atmospheres of the Earth, Mars, and Venus, not only by causing atmospheric erosion but also by delivering material and volatiles to the planets. Here we investigate the atmospheric loss and the delivery of volatiles for the three terrestrial planets using a parameterized model that takes into account the impact simulation results and the flux of impactors given in the literature. We show that the dimensions of the planets, the initial atmospheric surface pressures and the volatiles contents of the impactors are of high importance for the impact delivery and erosion, and that they might be responsible for the differences in the atmospheric evolution of Mars, Earth and Venus.     

Ridges and scarps in the equatorial belt of Mars

The morphology and distribution of ridges and scarps on Mars in the ± 30° latitude belt were investigated. Two distinct types of ridges were recognized. The first is long and linear, resembling mare ridges on the Moon; it occurs mostly in plains areas. The other is composed of short, anastomosing segments and occurs mostly in ancient cratered terrain and intervening plateaus. Where ridges are eroded, landscape configurations suggest that they are located along regional structures. The age of ridges is uncertain, but some are as young as the latest documented volcanic activity on Mars. The origins of ridges are probably diverse-they may result from wrinkling due to compression or from buckling due to settling over subsurface structures. The similar morphologic expressions of ridge types of various origins may be related to a similar deformation mechanism caused by two main factors: (1) most ridges are developed in thick layers of competent material and (2) ridges formed under stresses near a free surface.     

An atmospheric blast/thermal model for the formation of high‐latitude pedestal craters

Abstract— Although tenuous, the atmosphere of Mars affects the evolution of impact‐generated vapor. Early‐time vapor from a vertical impact expands symmetrically, directly transferring a small percentage of the initial kinetic energy of impact to the atmosphere. This energy, in turn, induces a hemispherical shock wave that propagates outward as an intense airblast (due to high‐speed expansion of vapor) followed by a thermal pulse of extreme atmospheric temperatures (from thermal energy of expansion). This study models the atmospheric response to such early‐time energy coupling using the CTH hydrocode written at Sandia National Laboratories. Results show that the surface surrounding a 10 km diameter crater (6 km “apparent” diameter) on Mars will be subjected to intense winds (?200 m/s) and extreme atmospheric temperatures. These elevated temperatures are sufficient to melt subsurface volatiles at a depth of several centimeters for an ice‐rich substrate. Ensuing surface signatures extend to distal locations (?4 apparent crater diameters for a case of 0.1% energy coupling) and include striations, thermally armored surfaces, and/or ejecta pedestals—all of which are exhibited surrounding the freshest high‐latitude craters on Mars. The combined effects of the atmospheric blast and thermal pulse, resulting in the generation of a crater‐centered erosion‐resistant armored surface, thus provide a new, very plausible formation model for high‐latitude Martian pedestal craters.     

Aeolian bedforms, yardangs, and indurated surfaces in the Tharsis Montes as seen by the HiRISE Camera: Evidence for dust aggregates

HiRISE images of Mars with ground sampling down to 25 cm/pixel show that the dust-rich mantle covering the surfaces of the Tharsis Montes is organized into ridges whose form and distribution are consistent with formation by aeolian saltation. Other dusty areas near the volcanoes and elsewhere on the planet exhibit a similar morphology. The material composing these “reticulate” bedforms is constrained by their remote sensing properties and the threshold curve combined with the saltation/suspension boundary, both of which vary as a function of elevation (atmospheric pressure), particle size, and particle composition. Considering all of these factors, dust aggregates are the most likely material composing these bedforms. We propose that airfall dust on and near the volcanoes aggregates in situ over time, maybe due to electrostatic charging followed by cementation by salts. The aggregates eventually reach a particle size at which saltation is possible. Aggregates on the flanks are transported downslope by katabatic winds and form linear and “accordion” morphologies. Materials within the calderas and other depressions remain trapped and are subjected to multidirectional winds, forming an interlinked “honeycomb” texture. In many places on and near the volcanoes, light-toned, low thermal inertia yardangs and indurated surfaces are present. These may represent “duststone” formed when aggregates reach a particle size below the threshold curve, such that they become stabilized and subsequently undergo cementation.     

Comparison of Earth and Mars as differentiated planets

Histories of the terrestrial planets are traceable to combinations of to five large-scale postaccretion processes: planetary differentiation, crustal differentiation, outgassing, plate tectonics, and recycling. All have operated on Earth to make a planet that was early differentiated into core, mantle, and crust and at very nearly the same time outgassed to form a differentiated crust, atmosphere and oceans. This gave rise to plate tectonics, recycling and thus two-way communication of the surface crust-atmosphere-ocean system with lower crust and upper mantle. Recycling of the Martian surface is probably restricted to limited chemical weathering of thin alteration surfaces of primary minerals because of the extreme slowness of diffusion controlled alteration where surfaces are not stripped by solution. There is evidence for neither subsidence of sedimentary basins nor subduction zones; thus internal recycling and two-way surface-interior communication is improbable. All sedimentary particles produced by mechanical erosion on Mars through its history are still at the surface or shallowly buried by later sediment. Any atmospheric components reacted with weathering crust are removed from the atmosphere. These and exospheric escape processes must have early reduced an original denser atmosphere to its present pressure after an early episode of planetary differentiation coupled to crustal differentiation and out-gassing. The early history of Mars may have been something like that of Earth until weathering and gas escape drew down its atmosphere.     

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.     

Latitudinal variation of wind erosion of crater ejecta deposits on Mars

Wind erosion seems to be the dominant process eroding crater ejecta deposits and sorrounding materials on Mars. In the equatorial zone, ejecta deposits are eroded back by scarp recession, where scarp heights appear to be approximately equivalent to ejecta thickness. In mantled areas, escarpments develop by relatively rapid deflation of sorrounding aeolian debris, leaving the ejecta deposit (continuous deposit and zone of high density of secondary craters) standing high above sorrounding terrain. If the rate of scarp recession is controlled by the rate of aeolian undercutting of escarpment bases, then recession rates may scale roughly as the inverse with respect to scarp height. Thus, preferential preservation of ejecta deposits emplaced in thickest aeolian debris may occur. An empirical model developed for wind erosion of ejecta deposits in nonmantled areas suggests that removal of ejecta materials on the average is exceedingly slow (～10^?5m/yr for 10m high scarp). On the other hand, rapid deflation of aeolian debris around crater ejecta is implied. Results suggest high differential aeolian erosion rates that are a function of both grain sizes and large-scale surface roughness. Aeolian activity on Mars has probably been dominated by rapid recycling of fine-grained debris, the bulk of which formed under more erosive conditions prevalent in the early history of Mars.     

The texture of condensed CO2 on the martian polar caps

The widespread deposition of CO₂ ice on the martian polar caps in winter is readily visible from Earth and has been extensively studied from orbit. As the surface cools during polar night, CO₂ condenses directly out of the atmosphere at a rate that establishes equilibrium between radiative loss and latent heat of condensation. Since radiative loss is strongly geometry-dependent, the CO₂ frost will grow most rapidly on exposed surfaces and more slowly in depressions. Positive feedback will cause a dramatic enhancement of the relief of the underlying topography and a corresponding reduction in the average bulk density. The resulting surface will be highly textured and riddled with perforations.     

Scalloped depressions and small-sized polygons in western Utopia Planitia,Mars: A new formation hypothesis

Flat-floored depressions with scalloped-shapes and spatially associated small-sized polygons (diameter <～100 m) dot the landscape of western Utopia Planitia (centered at 45°N–95°E). The scalloped depressions are thought to be the result of ice-rich regolith undergoing degradation by sublimation or thaw. Current models suggest that the formation and development of the depressions occur in a poleward direction due to the enhanced sublimation of their equator-facing slopes. By contrast, we propose a conceptual model that shows the equatorward growth of depressions due to preferential degradation by sublimation of their pole-facing slopes. Our model is based on a geomorphological study of the depressions and small-sized polygons in western Utopia Planitia (80°–110°E, 35°–50°N), using images from the High Resolution Imaging Science Experiment (HiRISE) and topographical data from the Mars Orbiter Laser Altimeter (MOLA) and a HiRISE stereo Digital Elevation Model (DEM). Here we describe (i) a morphological evolution of small-sized polygons within the depressions, from low-centered to high-centered, that facilitates one's understanding of depression growth and development; and (ii) occurrence of v-shaped alcoves, failure cracks and semicircular hollows that point to a retrogressive degradation of the pole-facing slopes of depressions. We propose that the development of the depressions is due to heightened insolation of their pole-facing slopes, leading to enhanced sublimation of ground-ice. Based upon the inferred asymmetric insolation, we suggest that the equatorward expansion of depressions occurred during recent high-obliquity periods of Mars.