Observations of martian gullies and constraints on potential formation mechanisms

The discovery of presumably geologically recent gully features on Mars (Malin and Edgett, 2000, Science 288, 2330-2335) has spawned a wide variety of proposed theories of their origin including hypotheses of the type of erosive material. To test the validity of gully formation mechanisms, data from the Mars Global Surveyor spacecraft has been analyzed to uncover trends in the dimensional and physical properties of the gullies and their surrounding terrain. We located 106 Mars Orbiter Camera (MOC) images that contain clear evidence of gully landforms, distributed in the southern mid and high latitudes, and analyzed these images in combination with Mars Orbiter Laser Altimeter (MOLA) and Thermal Emission Spectrometer (TES) data to provide quantitative measurements of numerous gully characteristics. Parameters we measured include apparent source depth and distribution, vertical and horizontal dimensions, slopes, orientations, and present-day characteristics that affect local ground temperatures. We find that the number of gully systems normalized to the number of MOC images steadily declines as one moves poleward of 30° S, reaches a minimum value between 60°-63° S, and then again rises poleward of 63° S. All gully alcove heads occur within the upper one-third of the slope encompassing the gully and the alcove bases occur within the upper two-thirds of the slope. Also, the gully alcove heads occur typically within the first 200 meters of the overlying ridge with the exception of gullies equatorward of 40° S where some alcove heads reach a maximum depth of 1000 meters. While gullies exhibit complex slope orientation trends, gullies are found on all slope orientations at all the latitudes studied. Assuming thermal conductivities derived from TES measurements as well as modeled surface temperatures, we find that 79% of the gully alcove bases lie at depths where subsurface temperatures are greater than 273 K and 21% of the alcove bases lie within the solid water regime. Most of the gully alcoves lie outside the temperature-pressure phase stability of liquid CO₂. Based on a comparison of measured gully features with predictions from the various models of gully formation, we find that models involving carbon dioxide, melting ground ice in the upper few meters of the soil, dry landslide, and surface snowmelt are the least likely to describe the formation of the martian gullies. Although some discrepancies still exist between prediction and observation, the shallow and deep aquifer models remain as the most plausible theories. Interior processes involving subsurface fluid sources are generally favored over exogenic processes such as wind and snowfall for explaining the origin of the martian gullies.     

Observations of martian gullies and constraints on potential formation mechanisms: II. The northern hemisphere

The formation process(es) responsible for creating the observed geologically recent gully features on Mars has remained the subject of intense debate since their discovery. We present new data and analysis of northern hemisphere gullies from Mars Global Surveyor data which is used to test the various proposed mechanisms of gully formation. We located 137 Mars Orbiter Camera (MOC) images in the northern hemisphere that contain clear evidence of gully landforms and analyzed these images in combination with Mars Orbiter Laser Altimeter (MOLA) and Thermal Emission Spectrometer (TES) data to provide quantitative measurements of numerous gully characteristics. Parameters we measured include apparent source depth and distribution, vertical and horizontal dimensions, slopes, orientations, and present-day characteristics that affect local ground temperatures. Northern hemisphere gullies are clustered in Arcadia Planitia, Tempe Terra, Acidalia Planitia, and Utopia Planitia. These gullies form in craters (84%), knobby terrain (4%), valleys (3%), other/unknown terrains (9%) and are found on all slope orientations although the majority of gullies are equator-facing. Most gullies (63%) are associated with competent rock strata, 26% are not associated with strata, and 11% are ambiguous. Assuming thermal conductivities derived from TES measurements as well as modeled surface temperatures, we find that 95% of the gully alcove bases with adequate data coverage lie at depths where subsurface temperatures are greater than 273 K and 5% of the alcove bases lie within the solid water regime. The average alcove length is 470 m and the average channel length is 690 m. Based on a comparison of measured gully features with predictions from the various models of gully formation, we find that models involving carbon dioxide, melting ground ice in the upper few meters of the soil, dry landslide, and surface snowmelt are the least likely to describe the formation of the martian gullies. Although some discrepancies still exist between prediction and observation, the shallow and deep aquifer models remain as the most plausible theories. Interior processes involving subsurface fluid sources are generally favored over exogenic processes such as wind and snowfall for explaining the origin of the martian gullies. These findings gleaned from the northern hemisphere data are in general agreement with analyses of gullies in the southern hemisphere [Heldmann, J.L., Mellon, M.T., 2004. Icarus 168, 285-304].     

Earth analog for high-latitude landforms and recent flows on Mars: Icy debris fans in the Wrangell Volcanic Field, Alaska

New Mars Reconnaissance Orbiter HiRise and CRISM imagery of polar layered terrain of Mars reveals striking similarities to icy debris fans along the base of steep escarpments in Alaska formed in high-latitude periglacial environments. Process and morphologic observations of a deglaciating site in the Wrangell Mountains reveal a complex suite of supraglacial processes involved in the construction of icy debris fans. Snow, ice, and sediment are delivered to the fans from degradation of an upper-level icecap. Alaskan icy debris fans were studied during an 8-day reconnaissance mission in July 2006. We directly observed 289 major depositional events dominated by dry snow/ice avalanches, but also including icy debris flows, rockfalls, small jokulhlaups, and glacial calving. Small fans with larger catchments receive episodic icy debris flows triggered by outburst flows that mobilize rockfall sediment temporarily stored in catchments above the fan apex. Large fans with smaller catchments have better linkage to the upper icecap, providing a direct pathway for frequent large avalanches. The large, avalanche-dominated fans thicken rapidly from an overabundance of snow/ice supply to the point where they become hybrid fan-glaciers. Surficial geology evolves rapidly in this high-latitude environment through both depositional events and solar-driven albedo changes that occur daily. Ground penetrating radar surveys show that subsurface sedimentary architecture and fan evolution is similar to the active surface processes and deposits observed on the fans. Direct field observations of active geomorphic processes provide unique insights on the pace and nature of high-latitude landscape evolution during climate changes on both planets.     

Modeling diffusion advection in the mass transfer of water vapor through martian regolith

Diffusion advection is an effect in diffusive multicomponent mass transfer that occurs when the flux vectors of the individual components do not add up to zero. This can be a significant effect for the mass transfer of water vapor from subsurface ice or liquid reservoirs through porous regolith at martian temperatures and pressures. Ignoring diffusion advection and using Fick's law alone to calculate the flux under these conditions will result in an erroneously small value while using a measured flux to calculate a diffusivity will result in an erroneously high value. The inaccuracy in both cases increases with temperature. The literature contains several examples of erroneous treatment of this effect. The correct approach is well-known from other applications of mass transfer and takes diffusion advection into account in the appropriate amount regardless of the temperature and pressure and reduces to the simple Fick's law when conditions warrant. In this way, there is no need to decide under what conditions diffusion advection is or is not important. It can be used in the transition region to pure Knudsen diffusion in a fashion similar to that used with the more limited Fickian approach.     

Groundwater discharge and gully formation on martian slopes

Young gullies and gully deposits on walls of martian craters have been cited as evidence that liquid water flowed on the surface of Mars relatively recently. Effects of variable environmental conditions at the surface of Mars are modeled and applied to the case of groundwater emergence from shallow aquifers to investigate whether groundwater is a viable source to enable the erosion of these gullies. The model includes detailed treatment of ice growth in the aquifer. Model results indicate that groundwater discharge can be maintained under the current environmental conditions if the aquifer permeability is like that of terrestrial gravel or higher, if the aquifer is 350 K or warmer, or if the aquifer is a brine with a freezing point depressed to 250 K or below. Groundwater discharge cannot be maintained for the conservative case of a cold, pure water, semi-pervious aquifer. Cold (275 K) pure water pervious (gravel) aquifers, warm (350 K) pure water semi-pervious aquifers, and cold (275 K) CaCl₂ brine semi-pervious aquifers all exhibit a dependence of discharge on season, latitude and slope orientation in our modeling. Seasonal, latitudinal and azimuthal discharge variations are strongest for cold CaCl₂ brine semi-pervious aquifers, with discharges from this aquifer type favoring equator-facing slopes at mid and high southern latitudes. At all latitudes and slope azimuths under our nominal conditions, the cold pure water pervious aquifer, the cold pure water semi-pervious aquifer and the cold CaCl₂ brine semi-pervious aquifer all freeze completely shortly after the simulations are started. Discharge restarts in the summer for the cold pure water pervious aquifer and the cold brine aquifer, but discharge does not restart for the cold pure water semi-pervious aquifer. The warm pure water semi-pervious aquifer maintains daily seeps throughout the year at all but high latitudes. In the case of the cold pure water pervious aquifer, approximately 500,000 m³ of water could be discharged from a mid-latitude, 150-m thick aquifer with a 20-m wide seepage face orientated towards the equator or the pole after a single undermining-induced event before ice growth seals the seepage face. For a brine semi-pervious aquifer with the same dimensions, 200-300 m³ of water could be released from a mid-latitude 20-m wide equator-facing seepage face before the fresh exposure is sealed for the fall and winter seasons. Our results do not rule out groundwater emergence as a means of creating some recent gullies, but they indicate that rather special and perhaps unusual conditions would be required.     

Evaporation effects on the formation of martian gullies

In order to investigate the formation of martian gullies and the stability of fluids on Mars, we examined about 120 gully images. Twelve HiRISE images contained a sufficient number of Transverse Aeolian Ridges (TARs) associated with the gullies to make the following measurements: overall gully length, length of the alcove, channel and apron, and we also measured the frequency of nearby TARs. Six of the 12 images examined showed a statistically significant negative correlation between overall gully length (alcove, channel and apron length) and TAR frequency. Previous experimental work from our group has shown that at temperatures below ∼200 K, evaporation rate increases by about an order of magnitude as wind speed increases from 0 to ∼15 m/s. Thus the negative correlations we observe between gully length and dune frequency can be explained by formation at temperatures below ∼200 K where wind speed/evaporation is a factor governing gully length. In these cases evaporation of the fluid carving the gully was a constraint on their dimensions. Cases where there is no correlation between gully length and TAR frequency, can be explained by formation at temperatures >200 K. The temperatures are consistent with Global Circulation Model and Thermal Emission Spectrometer (TES) data for these latitudes. The temperatures suggested by these trends are consistent with the fluid responsible for gully formation being a strong brine, such as Fe₂(SO₄)₃ which has a eutectic temperature of ∼200 K. We also find that formation timescales for gullies are 10⁵-10⁶ years.     

Evaluation of carbonate abundance in putative martian paleolake basins

Carbonate deposits have not been found so far on Mars, although there appears to have been sufficient water to have supported their formation. Many hypotheses have been proposed in order to explain this. In the present work we explore the possibility that the missed detection of carbonate deposits on the martian surface could be simply due to the fact that the concentration of carbonates, when mixed with other materials present in the sedimentary deposits, may be below the detection limit of the various instruments used so far in this search. In the present study we consider 21 putative paleolacustrine basins and use a sediment transport model to estimate the abundance of carbonates which could be present in the sediments deposited on the basin floor. In this way we find that for all the selected basins the estimated carbonate abundances are in general less than a few percent, and such values are below (or at best comparable to) the detection limits of the spectrometers flown around Mars during the recent space missions. Furthermore, applying the sediment transport model to the well studied Eberswalde crater, we conclude that the fluvio-lacustrine activity in this basin should have lasted for a period on the order of 10³–10⁴ years, in good agreement with previous work. Our results suggest that a hydrological cycle, able to move large volumes of water and to create relatively stable lakes, could have been active intermittently on Mars in the past, producing carbonate deposits that could escape detection by the instruments that have flown to date.     

Catalogue of large alluvial fans in martian impact craters

We conducted a systematic, global survey using Thermal Emission Imaging System Infrared (THEMIS IR) coverage (∼100 m/pixel) to search for large alluvial fans in impact craters on Mars. Our survey has focused on large fans (apron areas greater than ∼40 km², usually located in craters greater than 20 km in diameter) due to the resolution of the THEMIS images and Mars Orbiter Laser Altimeter (MOLA) coverage. We find that the host craters are found to have a distinctive diameter range from 30-150 km. The fans generally cluster in three geographic areas—southern Margaritifer Terra, southwestern Terra Sabaea, and southwestern Tyrrhena Terra, however several outliers do exist. The alluvial fans do not form in a particular orientation along the crater rim nor are they associated with the location of current high rim topography. Fan area magnitude and variability increase with crater diameter while fan concavity magnitude and variability increase with decreasing crater diameter. Smaller fan aprons in general have higher, more variable concavity. The source of the water forming these fans is uncertain given the challenges of accommodating the global distribution pattern and formation patterns within the craters.     

Metastable methane clathrate particles as a source of methane to the martian atmosphere

The observations of methane made by the PFS instrument onboard Mars Express exhibit a definite correlation between methane mixing ratio, water vapor mixing ratio, and cloud optical depth. The recent data obtained from ground-based telescopes seem to confirm the correlation between methane and water vapor. In order to explain this correlation, we suggest that the source of gaseous methane is atmospheric, rather than at the solid surface of the planet, and that this source may consist of metastable submicronic particles of methane clathrate hydrate continuously released to the atmosphere from one or several clathrate layers at depth, according to the phenomenon of “anomalous preservation” evidenced in the laboratory. These particles, lifted up to middle atmospheric levels due to their small size, and therefore filling the whole atmosphere, serve as condensation nuclei for water vapor. The observed correlation between methane and water vapor mixing ratios could be the signature of the decomposition of the clathrate crystals by condensation-sublimation processes related to cloud activity. Under the effect of water condensation on crystal walls, metastability could be broken and particles be eroded, resulting in a subsequent irreversible release of methane to the gas phase. Using PFS data, and according to our hypothesis, the lifetime of gaseous methane is estimated to be smaller than an upper limit of 6 ± 3 months, much smaller than the lifetime of 300 yr calculated from atmospheric chemical models. The reason why methane has a short lifetime might be the occurrence of heterogeneous chemical decomposition of methane in the subsurface, where it is known since Viking biology experiments that oxidants efficiently decompose organic matter. If true, it is shown by using existing models of H₂O₂ penetration in the regolith that methane could prevent H₂O₂ from penetrating in the subsurface, and further oxidizing the soil, at depths larger than a few millimeters. The present source of methane clathrate, acting over the last few hundred thousand or million years, could have given rise to the thin CO₂-ice layer covering the permanent water ice south polar cap. The hypothesis proposed in this paper requires, to be validated, a number of laboratory experiments studying the stability of methane clathrates in martian atmospheric conditions, and the kinetics and amplitude of clathrate particle erosion in presence of condensing water vapor. Detailed future observations of methane, and associated modeling, will allow to more accurately quantify the production rate of methane clathrate, its temporal variability at seasonal scale, and possibly to locate the source(s) of clathrates at the surface.     

Modification of martian slope streaks by eolian processes

Recent images from the High Resolution Imaging Science Experiment (HiRISE) camera have shown that slope streaks have relief on the order of a meter or less. This study presents observations of transverse bedforms and infill deposits within slope streak beds that were not previously identified or were uncommon from earlier analyses of HiRISE images. Transverse bedforms are linear to slightly arcuate features oriented transverse to the slope streak bed which may be analogous to terrestrial splash or coarse-grained ripples based on their morphology, wavelength, and amplitude. In addition to the bedforms, there is also evidence that slope streak beds gradually shallow over time by infilling of material. The presence of ripples within slope streaks implies that saltation-capable material is available on the surface today and/or was available in the recent past. Although airfall dust is not a capable saltation source material, aggregates of electrostatically-bound dust that are possibly later cemented by salts may serve as a source. From the results of this study, we hypothesize a sequence of events in a slope streak formation and modification cycle where grains saltate to form ripples along the bed of a slope streak, airfall dust mantling causes gradual fading of the streak, and infill material buries the ripples, eventually reaching the pre-avalanche surface that removes all traces of relief.     

Sorted stone circles in Elysium Planitia, Mars: Implications for recent martian climate

We have found sorted stone circles and polygons near the equator of Mars, using new 25 cm/pixel NASA HiRISE (High Resolution Imaging Science Experiment) images. The sorted circles occur in geologically recent catastrophic flood deposits in the equatorial Elysium Planitia region, and are diagnostic of periglacial processes: sorted polygons do not form from volcanic activity, as has been suggested for non-sorted polygons in this region. These landforms indicate that (i) a long-lived, geologically recent, active cryoturbation layer of ground ice was present in the regolith, (ii) there was some degree of freeze-thaw, and thus (iii) there were sustained period(s), likely within the last 10 Ma, in which the martian climate was 40 to 60 K warmer than current models predict.     

Frosted granular flow: A new hypothesis for mass wasting in martian gullies

Recent gully deposits on Mars have been attributed to both wet and dry mass wasting processes. In this paper frosted granular flow (FGF) is presented as a new hypothesis for recent mass wasting activity in martian gullies. FGF is a rare type of granular flow observed on a talus slope in the Province of Québec, Canada [Hétu, B., van Steijn, H., Vandelac, P., 1994. Géogr. Phys. Quat. 48, 3-22]. Frost reduces dynamic inter-particle friction, enabling flows to mobilize onto relatively low slope gradients (25-30°) compared to those involving dry granular flow of the same material (35-41°). Resulting erosional and depositional features include straight to sinuous channels, levees and digitate to branching arrangements of terminal deposits. Similar features are commonly found in association with geologically-young gully systems on Mars. Based on terrestrial observations of FGF processes the minimum criteria required for their occurrence on Mars include: (i) readily mobilized, unconsolidated sediment at the surface; (ii) an upper slope gradient at or near the angle of repose; (iii) frost accumulation at the surface; and (iv) triggering by rock fall. All four conditions appear to be met in many areas on present-day Mars though triggering mechanisms may vary. Compared to terrestrial FGFs, which are lubricated by thin liquid films at inter-particle contacts, those occurring on Mars are more likely lubricated by vaporization of CO₂ and small amounts of H₂O frost that becomes incorporated in the translating mass. Some recent mass wasting activity in martian gullies, therefore, could be interpreted as the product of FGF.     

A historical search for habitable ice at the Phoenix landing site

A time-resolved energy balance model in the latitude range targeted by Phoenix, and extending back in time over the past 10 Ma, has been developed and used to predict the time-varying temperature field in ground ice over scales ranging from minutes to millions of years. The temperature history is compared to the population doubling times of terrestrial psychrophiles as a function of temperature, and the lifetime of analog microbe spores against de-activation by galactic cosmic rays (GCR), in order to assess the habitability of ground ice and surrounding materials that may be sampled by Phoenix. Metrics are derived to quantify “habitability” and compare different model configurations, including total and maximum continuous time, per year, that ground ice temperatures exceed various thresholds, maximum and average dormancy periods, and maximum and average consecutive growing seasons. The key unknowns in assessing the position, and hence the temperature, of the ground ice table at high northern latitude is the fate of the perennial north polar cap at high obliquity. If enough H₂O ice can persist at polar latitudes to buffer at least the high-latitude atmosphere at all orbital configurations, ground ice is found to be relatively shallow over much of the past 10 Ma, and regularly achieves temperatures in excess of those required for the growth of terrestrial psychrophiles. The dry overburden expected at the landing site can easily be sampled by Phoenix, and includes the “sweet spot” that is characterized by the optimal habitability metrics over the past 10 Ma. If the atmosphere is buffered only by low-latitude ice deposits at obliquities greater than about 30°, the frequency and duration of habitable ice is considerably diminished, and the intervening dormancy periods, during which cosmic ray damage accumulates, are correspondingly longer. In all cases, the maximum dormancy period that must be survived by putative martian psychrophiles is at least an order of magnitude greater than the amount of time required to reduce terrestrial psychrophile spore viability by 10⁻⁶ (∼7×¹⁰⁴ years). Depending on the fate of high-obliquity polar ice, the maximum dormancy period can exceed 4×¹⁰⁶ years, a factor of 60 longer than terrestrial psychrophile spore lifetimes. Habitability of martian ground ice is therefore dependent on putative martian psychrophiles developing robustness against GCR deactivation at least an order of magnitude greater than their terrestrial counterparts. Simulations of ground ice throughout the 65° N-72° N latitude range accessible to Phoenix suggest that higher-latitude ground ice has better habitability metrics, although the discrepancy is less than an order of magnitude for all metrics and across the entire latitude range.     

Constraints on martian lobate debris apron evolution and rheology from numerical modeling of ice flow

Radar observations in the Deuteronilus Mensae region by Mars Reconnaissance Orbiter have constrained the thickness and dust concentration found within mid-latitude ice deposits, providing an opportunity to more accurately estimate the rheology of ice responsible for the formation of lobate debris aprons based on their apparent age of ∼100 Myr. We developed a numerical model simulating ice flow under martian conditions using results from ice deformation experiments, theory of ice grain growth based on terrestrial ice cores, and observational constraints from radar profiles and laser altimetry. By varying the ice grain size, the ice temperature, the subsurface slope, and the initial ice volume we determine the combination of parameters that best reproduce the observed LDA lengths and thicknesses over a period of time comparable to the apparent ages of LDA surfaces (90-300 Myr). We find that an ice temperature of 205 K, an ice grain size of 5 mm, and a flat subsurface slope give reasonable ages for many LDAs in the northern mid-latitudes of Mars. Assuming that the ice grain size is limited by the grain boundary pinning effect of incorporated dust, these results limit the dust volume concentration to less than 4%. However, assuming all LDAs were emplaced by a single event, we find that there is no single combination of grain size, temperature, and subsurface slope which can give realistic ages for all LDAs, suggesting that some or all of these variables are spatially heterogeneous. Based on our model we conclude that the majority of northern mid-latitude LDAs are composed of clean (?4 vol%), coarse (?1 mm) grained ice, but regional differences in either the amount of dust mixed in with the ice, or in the presence of a basal slope below the LDA ice must be invoked. Alternatively, the ice temperature and/or timing of ice deposition may vary significantly between different mid-latitude regions. Either eventuality can be tested with future observations.     

Origin of martian northern hemisphere mid-latitude lobate debris aprons

Lobate debris aprons in the martian mid- to high-latitudes (northern and southern hemispheres) have been interpreted as ice-related features that indicate periglacial climate conditions as recently as late Amazonian. Using MOLA topographic profiles perpendicular to apron flow fronts, we surveyed 36 debris aprons in the northern hemisphere found in the regions of Mareotis, Protonilus, and Deuteronilus Mensae and Acheron Fossae. The profiles of these aprons were compared with idealized simple plastic and viscous power law models for ice-rock mixtures. All aprons studied exhibit convex profiles similar to a simple plastic model. This confirms previous interpretations that debris aprons are ice-rich mixtures with rheologies similar to stagnant ice sheets, thus indicating high ice concentrations (>40% by volume). About 60% of the surveyed debris apron population significantly deviates from the idealized simple plastic model profile; this may be due to locally reduced ice content, which primarily controls apron topography. Although post-emplacement modification due to near-surface ice sublimation plays a secondary role in defining the overall shape of aprons, it causes conspicuous surface textures. Degradation by ice sublimation probably results in pitted and ridge-and-furrow surface textures revealed by high resolution MOC images. Such textures may indicate decreased near-surface ice stability since the formation of the aprons, possibly due to Mars' current low obliquity after their emplacement. High ice content inferred from topography suggests some debris aprons have ice cores: potentially exploitable water resources for future robotic/human operations that could prove invaluable for missions remote from polar regions.     

Modeling sublimation of ice exposed by new impacts in the martian mid-latitudes

New impacts in the martian mid-latitudes have exposed near-surface ice. This ice is observed to slowly fade over timescales of months. In the present martian climate, exposed surface ice is unstable during summer months in the mid-latitudes and will sublimate. We model the sublimation of ice at five new impact sites and examine the implications of its persistence. Even with generally conservative assumptions, for most reasonable choices of parameters it is likely that over a millimeter of sublimation occurred in the period during which the ice was observed to fade. The persistence of visible ice through such sublimation suggests that the ice is relatively pure rather than pore-filling. Such ice could be analogous to the nearly pure ice observed by the Phoenix Lander in the “Dodo-Goldilocks” trench and suggests that the high ice contents reported by the Mars Odyssey Gamma Ray Spectrometer at high latitudes extend to the mid-latitudes. Our observations are consistent with a model of the martian ice table in which a layer with high volumetric ice content overlies pore-filling ice, although other structures are possible.     

Evidence for ice flow prior to trough formation in the martian north polar layered deposits

The relative importance of surface mass fluxes and ice flow in shaping the north polar layered deposits (NPLD), now or in the past, remains a fundamental and open question. Motivated by observation of an apparent ice divide on Gemina Lingula (also known as Titania Lobe), we propose a two-stage evolution leading to the present-day topography on that lobe of the NPLD. Ice flow approximately balances surface mass fluxes in the first stage, but in the second stage ice flow has minimal influence and topography is modified predominantly by the formation of troughs. We focus here on evidence for the first stage, by testing the fit of topography between troughs to an ice-flow model. We find that independent model fits on distinct flow paths closely match inter-trough topography, uniformly over a broad region on Gemina Lingula, with mutually consistent and physically reasonable fitting parameters. However, our model requires ice to occupy and flow in spaces where troughs currently incise the ice. We therefore infer that the troughs (and the distribution of mass balance that caused them) post-date deposition of the inter-trough material and its modification by flow. Because trough formation has apparently altered inter-trough topography very little, we infer that trough formation must have been rapid in comparison to the (still unknown) time-scale of flow since troughs began to form. We view the evidence for past flow as strong, but we do not think that topographic evidence alone can be conclusive. Observations of englacial stratigraphy using orbital sounding radars will yield conclusive tests of our inferred mechanism for the formation of inter-trough topography.     

Valley formation on martian volcanoes in the Hesperian: Evidence for melting of summit snowpack, caldera lake formation, drainage and erosion on Ceraunius Tholus

Ceraunius Tholus, a Hesperian-aged volcano in the Tharsis region, is characterized by small radial valleys on its flanks, and several larger valleys originating near its summit caldera. All of these large valleys drain from near the lowest present portion of the caldera rim and down the flanks of the volcano. The largest valley debauches into Rahe Crater (an oblique impact crater), forming a depositional fan. Recent study of climate change on Mars suggests that many low-latitude regions (especially large volcanic edifices) were periodically the sites of snow accumulation, likely triggered by variations in spin orbital parameters. We apply a conductive heat flow model to Ceraunius Tholus that suggests that following magmatic intrusion, sufficient heating would be available to cause basal melting of any accumulated summit snowpack and produce sufficient meltwater to cause the radial valleys. The geometry of the volcano summit caldera suggests that meltwater would also accumulate in a volumetrically significant caldera lake. Analysis of the morphology and volumes of the largest valley, as well as depositional features at its base, suggest that fluvial erosion due to drainage of this summit caldera lake formed the large valleys, in a manner analogous to how valleys were formed catastrophically from a lake in Aniakchak caldera in Alaska. Moreover, the event which carved the largest valley on Ceraunius Tholus appears to have led to the formation of a temporary lake within Rahe Crater, at its base. The more abundant, small valleys on the flanks are interpreted to form by radial drainage of melted ice or snow from the outside of the caldera rim. Comparison of Ceraunius Tholus with the volcano-capping Icelandic ice sheet Myrdalsjokull provides insight into the detailed mechanisms of summit heating, ice-cap accumulation and melting, and meltwater drainage. These observations further underline the importance of a combination of circumstances (i.e., climate change to produce summit snowpack and an active period of magmatism to produce melting) to form the valley systems on some martian volcanoes and not on others.     

Dating martian climate change

Geological evidence indicates that low-latitude polygonally-patterned grounds on Mars, generally thought to be the product of flood volcanism, are periglacial in nature and record a complex signal of changing climate. By studying the martian surface stratigraphically (in terms of the geometrical relations between surface landforms and the substrate) rather than genetically (by form analogy with Earth), we have identified dynamic surfaces across one-fifth of martian longitude. New stratigraphical observations in the Elysium-Amazonis plains have revealed a progressive surface polygonisation that is destructive of impact craters across the region. This activity is comparable to the climatically-driven degradation of periglacial landscapes on Earth, but because it affects impact craters—the martian chronometer—it can be dated. Here, we show that it is possible to directly date this activity based on the fraction of impact craters affected by polygon formation. Nearly 100% of craters (of all diameters) are superposed by polygonal sculpture: considering the few-100 Ma age of the substrate, this suggests that the process of polygon formation was active within the last few million years. Surface polygonisation in this region, often considered to be one of the signs of young, ‘plains-forming’ volcanism on Mars, is instead shown to postdate the majority of impact craters seen. We therefore conclude that it is post-depositional in origin and an artefact of thermal cycling of near-surface ground ice. Stratigraphically-controlled crater counts present the first way of dating climate change on a planet other than Earth: a record that may tell us something about climate change on our own planet. Parallel climate change on these two worlds—an ice age Mars coincident with Earth’s glacial Quaternary period—might suggest a coupled system linking both. We have previously been unable to generalise about the causes of long-term climate change based on a single terrestrial example—with the beginnings of a chronology for climate change on our nearest planetary neighbour, we can.     

Mineral alteration induced by sand transport: A source for the reddish color of martian dust

The iron mineral thought to give the characteristic reddish color to martian dust could have formed through mechanical activation during sand transport. This has been demonstrated experimentally under conditions which are known to occur on Mars.