Mars Reconnaissance Orbiter observations of light-toned layered deposits and associated fluvial landforms on the plateaus adjacent to Valles Marineris

We have used data from the Mars Reconnaissance Orbiter to study 30-80 m thick light-toned layered deposits on the plateaus adjacent to Valles Marineris at five locations: (1) south of Ius Chasma, (2) south of western Melas Chasma, (3) south of western Candor Chasma, (4) west of Juventae Chasma, and (5) west of Ganges Chasma. The beds within these deposits have unique variations in brightness, color, mineralogy, and erosional properties that are not typically observed in light-toned layered deposits within Valles Marineris or many other equatorial areas on Mars. Reflectance spectra indicate these deposits contain opaline silica and Fe-sulfates, consistent with low-temperature, acidic aqueous alteration of basaltic materials. We have found valley or channel systems associated with the layered deposits at all five locations, and the volcanic plains adjacent to Juventae, Ius, and Ganges exhibit inverted channels composed of light-toned beds. Valleys, channels, and light-toned layering along the walls of Juventae and Melas Chasmata are most likely coeval to the aqueous activity that affected the adjacent plateaus and indicate some hydrological activity occurred after formation of the chasmata. Although the source of water and sediment remains uncertain, the strong correlation between fluvial landforms and light-toned layered deposits argues for sustained precipitation, surface runoff, and fluvial deposition occurring during the Hesperian on the plateaus adjacent to Valles Marineris and along portions of chasmata walls.     

Hydrated mineral stratigraphy of Ius Chasma, Valles Marineris

New high-resolution spectral and morphologic imaging of deposits on walls and floor of Ius Chasma extend previous geomorphic mapping, and permit a new interpretation of aqueous processes that occurred during the development of Valles Marineris. We identify hydrated mineralogy based on visible-near infrared (VNIR) absorptions. We map the extents of these units with CRISM spectral data as well as morphologies in CTX and HiRISE imagery. Three cross-sections across Ius Chasma illustrate the interpreted mineral stratigraphy. Multiple episodes formed and transported hydrated minerals within Ius Chasma. Polyhydrated sulfate and kieserite are found within a closed basin at the lowest elevations in the chasma. They may have been precipitates in a closed basin or diagenetically altered after deposition. Fluvial or aeolian processes then deposited layered Fe/Mg smectite and hydrated silicate on the chasma floor, postdating the sulfates. The smectite apparently was weathered out of Noachian-age wallrock and transported to the depositional sites. The overlying hydrated silicate is interpreted to be an acid-leached phyllosilicate transformed from the underlying smectite unit, or a smectite/jarosite mixture. The finely layered smectite and massive hydrated silicate units have an erosional unconformity between them, that marks a change in surface water chemistry. Landslides transported large blocks of wallrock, some altered to contain Fe/Mg smectite, to the chasma floor. After the last episode of normal faulting and subsequent landslides, opal was transported short distances into the chasma from a few m-thick light-toned layer near the top of the wallrock, by sapping channels in Louros Valles. Alternatively, the material was transported into the chasma and then altered to opal. The superposition of different types of hydrated minerals and the different fluvial morphologies of the units containing them indicate sequential, distinct aqueous environments, characterized by alkaline, then circum-neutral, and finally very acidic surface or groundwater chemistry.     

Modeling the formation of bright slope deposits associated with gullies in Hale Crater, Mars: Implications for recent liquid water

Our study investigates possible formation mechanisms of the very recent bright gully deposits (BGDs) observed on Mars in order to assess if liquid water was required. We use two models in our assessment: a one-dimensional (1D) kinematic model to model dry granular flows and a two-dimensional (2D) fluid-dynamic model, FLO-2D (O’Brien et al., 1993, FLO Engineering), to model water-rich and wet sediment-rich flows. Our modeling utilizes a high-resolution topographic model generated from a pair of images acquired by the High Resolution Imaging Science Experiment (HiRISE) aboard the Mars Reconnaissance Orbiter. For the 1D kinematic modeling of dry granular flows, we examine a range of particle sizes, flow thicknesses, initial velocities, flow densities, and upslope initiation points to examine how these parameters affect the flow run-out distances of the center of mass of a flow. Our 1D modeling results show that multiple combinations of realistic parameters could produce dry granular flows that travel to within the observed deposits’ boundaries. We run the 2D fluid-dynamic model, FLO-2D, to model both water-rich and wet sediment-rich flows. We vary the inflow volume, inflow location, discharge rate, water-loss rate (water-rich models only), and simulation time and examine the resulting maximum flow depths and velocities. Our 2D modeling results suggest that both wet sediment-rich and water-rich flows could produce the observed bright deposits. Our modeling shows that the BGDs are not definitive evidence of recent liquid water on the surface of Mars.     

Bright dust devil tracks on Earth: Implications for their formation on Mars

We report on the first observations of bright dust devil tracks (BDDTs) on Earth, observed in the Turpan depression desert in northwestern China, where raindrop impacts on sand surfaces form aggregates of sand, silt and clay resulting in rough surface textures, which are destroyed by passages of dust devils leading to smooth surface textures within the tracks. The differences in photometric properties between the track and outside the tracks cause the albedo differences leading to the formation of BDDTs and similar processes might lead to BDDTs on Mars in areas with thick dust covers.     

Light-toned layered deposits in Juventae Chasma, Mars

We examine hypotheses for the formation of light-toned layered deposits in Juventae Chasma using a combination of data from Mars Global Surveyor's Mars Orbiter Camera (MOC), Mars Orbiter Laser Altimeter (MOLA), and Thermal Emission Spectrometer (TES), as well as Mars Odyssey's Thermal Emission Imaging System (THEMIS). We divide Juventae Chasma into geomorphic units of (i) chasm wall rock, (ii) heavily cratered hummocky terrain, (iii) a mobile and largely crater-free sand sheet on the chasm floor, (iv) light-toned layered outcrop (LLO) material, and (v) chaotic terrain. Using surface temperatures derived from THEMIS infrared data and slopes from MOLA, we derive maps of thermal inertia, which are consistent with the geomorphic units that we identify. LLO thermal inertias range from ∼400 to 850 J m⁻² K⁻¹ s^−1/2. Light-toned layered outcrops are distributed over a remarkably wide elevation range () from the chasm floor to the adjacent plateau surface. Geomorphic features, the absence of small craters, and high thermal inertia show that the LLOs are composed of sedimentary rock that is eroding relatively rapidly in the present epoch. We also present evidence for exhumation of LLO material from the west wall of the chasm, within chaotic and hummocky terrains, and within a small depression in the adjacent plateau. The data imply that at least some of the LLO material was deposited long before the adjacent Hesperian plateau basalts, and that Juventae Chasma underwent, and may still be undergoing, enlargement along its west wall due to wall rock collapse, chaotic terrain evolution, and exposure and removal of LLO material. The new data allow us to reassess possible origins of the LLOs. Gypsum, one of the minerals reported elsewhere as found in Juventae Chasma LLO material, forms only at low temperatures () and thus excludes a volcanic origin. Instead, the data are consistent with either multiple occurrences of lacustrine or airfall deposition over an extended period of time prior to emplacement of Hesperian lava flows on the plateau above the chasm.     

Evidence for paleolakes in Erythraea Fossa, Mars: Implications for an ancient hydrological cycle

There is now widespread agreement that the surface of Mars underwent some degree of fluvial modification, but there is not yet full understanding of its surface hydrological cycle and the nature of standing bodies of water, rivers, and precipitation that affected its surface. In this paper we explore Erythraea Fossa (31.5 W, 27.3 S), a graben adjacent to Holden crater, which exhibits strong evidence that it once housed a chain of three lakes, had overland water flow, and was subject to precipitation. The inlet valley, outlet valley, and fan morphologies in the paleolakes are used to qualitatively discern the hydrologic history of the paleolakes; based on topography constraints, the three basins combined once held 56 km³ of water. Depositional features within the basins that change with drainage area and nearby valleys that start near drainage divides indicate that the paleolakes may have been fed by precipitation driven runoff. This suggests the presence of an atmosphere, at least locally, that was capable of supporting a hydrological cycle.     

Ages of Valles Marineris (Mars) landslides and implications for canyon history

The chronology of landslides of Valles Marineris, the equatorial trough system of Mars, has been investigated by a crater population study. Valles Marineris landslides have widespread debris aprons which offer a remarkable opportunity to study the crater population with high resolution images from Mars Orbiter Camera (MOC) and from Mars Odyssey Thermal Emission Imaging System (THEMIS). Sixty-six ages were determined within Valles Marineris including 56 landslide ages and 10 ages of the canyon floor. Results reveal that landslides of Valles Marineris system of canyons occurred during a widespread period of time between 3.5 Gy and 50 My. In some locations, the canyon floor has an apparent age of 3.5 Gy suggesting that at least locally within Valles Marineris no major refreshing processes have occurred for 3.5 Gy. The temporal repetitivity of landslides implies that the triggering mechanisms of the landslides are reproducible in time. Landslides have the same features whatever their age. The dynamic of these landslides is probably the same either with intervention of water up to recently (the last 100 My) or without water since 3.5 Gy.     

Evidence for ponding and catastrophic floods in central Valles Marineris, Mars

The Valles Marineris canyon system of Mars is closely related to large flood channels, some of which emerge full born from chaotic terrain in canyon floors. Coprates Chasma, one of the largest Valles Marineris canyons, is connected at its west end to Melas Chasma and on its east end to chaotic terrain-filled Capri and Eos Chasmata. The area from central Melas to Eos Chasmata contains a 1500 km long and about 1 km deep depression in its floor. Despite the large volumes of groundwater that likely discharged from chaotic terrain in this depression, no evidence of related fluvial activity has thus far been reported. We present an analysis of the regional topography which, together with photogeologic interpretation of available imagery, suggests that ponding due to late Hesperian discharge of water possibly produced a lake (mean depth 842 m) spanning parts of the Valles Marineris depression (VMD). Overflow of this lake at its eastern end resulted in delivery of water to downstream chaos regions and outflow channels. Our ponding hypothesis is motivated primarily by the identification of scarp and terrace features which, despite a lateral spread of about 1500 km, have similar elevations. Furthermore, these elevations correspond to the maximum ponding elevation of the region (−3560 m). Simulated ponding in the VMD yields an overflow point at its eastern extremity, in Eos Chasma. The neighborhood of this overflow point contains clear indicators of fluvial erosion in a consistent east-west orientation.     

Discovery of jarosite within the Mawrth Vallis region of Mars: Implications for the geologic history of the region

Analysis of visible to near infrared reflectance data from the MRO CRISM hyperspectral imager has revealed the presence of an ovoid-shaped landform, approximately 3 by 5 km in size, within the layered terrains surrounding the Mawrth Vallis outflow channel. This feature has spectral absorption features consistent with the presence of the ferric sulfate mineral jarosite, specifically a K-bearing jarosite (KFe₃(SO₄)₂(OH)₆). Terrestrial jarosite is formed through the oxidation of iron sulfides in acidic environments or from basaltic precursor minerals with the addition of sulfur. Previously identified phyllosilicates in the Mawrth Vallis layered terrains include a basal sequence of layers containing Fe-Mg smectites and an upper set of layers of hydrated silica and aluminous phyllosilicates. In terms of its fine scale morphology revealed by MRO HiRISE imagery, the jarosite-bearing unit has fracture patterns very similar to that observed in Fe-Mg smectite-bearing layers, but unlike that observed in the Al-bearing phyllosilicate unit. The ovoid-shaped landform is situated in an east-west bowl-shaped depression superposed on a north sloping surface. Spectra of the ovoid-shaped jarosite-bearing landform also display an anomalously high 600 nm shoulder, which may be consistent with the presence of goethite and a 1.92 μm absorption which could indicate the presence of ferrihydrite. Goethite, jarosite, and ferrihydrite can be co-precipitated and/or form through transformation of schwertmannite, both processes generally occurring under low pH conditions (pH 2-4). To date, this location appears to be unique in the Mawrth Vallis region and could represent precipitation of jarosite in acidic, sulfur-rich ponded water during the waning stages of drying.     

The dispersal of pyroclasts from Apollinaris Patera, Mars: Implications for the origin of the Medusae Fossae Formation

The Medusae Fossae Formation (MFF) has long been thought to be of Amazonian age, but recent studies propose that a significant part of its emplacement occurred in the Hesperian and that many of the Amazonian ages represent modification (erosional and redepositional) ages. On the basis of the new formational age, we assess the hypothesis that explosive eruptions from Apollinaris Patera might have been the source of the Medusae Fossae Formation. In order to assess the likelihood of this hypothesis, we examine stratigraphic relationships between Apollinaris Patera and the MFF and analyze the relief of the MFF using topographic data. We predict the areal distribution of tephra erupted from Apollinaris Patera using a Mars Global Circulation Model (GCM) combined with a semi-analytical explosive eruption model for Mars, and compare this with the distribution of the MFF. We conclude that Apollinaris Patera could have been responsible for the emplacement of the Medusae Fossae Formation.     

Evidence for episodic alluvial fan formation in far western Terra Tyrrhena, Mars

A Late Noachian-aged alluvial fan complex within Harris Crater in far western Terra Tyrrhena, Mars, is comprised of two well-defined source regions and associated discrete depositional lobes. Three fan units were recognized based on common morphological characteristics, thermal properties and spectral signatures. Although the entire fan complex has been subjected to extensive erosional degradation, the preserved morphologies record episodic fan formation and indicate the type of flow processes that occurred; the bulk of the fan surface has morphology consistent with fluvial emplacement while one fan unit exhibits a rugged surface texture with boulders consistent with a debris flow. This transition from fluvial to late-stage debris flow(s) suggests a decline in available water and/or change in sediment supply. The thermal inertia values obtained for all three fan surface units (mean values ranged from 318 to 344 J m⁻² K⁻¹ s^−1/2) are typical for coarse-grained and/or well-indurated materials on Mars, but subtle variations point to important distinctions. Variations in aeolian bedform coverage as well as the density of ridges (inferred inverted channels) and boulders contribute to these subtle fan thermophysical differences and likely reflect changes in the fan depositional mechanisms and variations in post-depositional modification histories. The majority of the alluvial fan surface has a spectral signature that is broadly similar to TES “Surface Type 2” (ST2), with some important exceptions at long wavelengths. However, a unique spectral component was identified in one of the fan units (unit 3), that likely reflects lithological differences from other fan materials. This spectral attribute of unit 3 matched locations within the western catchment providing confirmation of provenance and supporting the contention that sediment supply changed over time as the fan developed. Finally, we applied simple modeling to a well preserved subsection of the fan complex to quantify the developmental history. Using the computed eastern fan volume (32 km³), significant water, likely from precipitation, was involved in fan construction (>50 km³) and an extensive period of fan formation occurred over millennia or longer.     

Paleolakes, paleofloods, and depressions in Aurorae and Ophir Plana, Mars: Connectivity of surface and subsurface hydrological systems

The plains of Aurorae and Ophir in the equatorial region of Mars display geomorphic evidence indicative of extensive but generally short-lived paleohydrological processes. Elaver Vallis in Aurorae Planum south of Ganges Chasma is an outflow channel system >180 km long, and here inferred to have formed by cataclysmic spillover flooding from a paleolake(s) contained in the Morella crater basin. Ganges Cavus is an enormous 5-km-deep depression of probable collapse origin located in the Morella basin. The fluid responsible for the infilling of the Morella basin likely emerged at least partially through Ganges Cavus or its incipient depression, and it may have been supplied also from small-scale springs in the basin. Similar paleohydrological processes are inferred also in Ophir Planum. It is reasonable to assume that water, sometimes sediment-laden and/or mixed with gases, was the responsible fluid for these phenomena although some of the observed features could be explained by non-aqueous processes such as volcanism. Water emergence may have occurred as consequences of ground ice melting or breaching of cryosphere to release water from the underlying hydrosphere. Dike intrusion is considered to be an important cause of formation for the cavi and smaller depressions in Aurorae and Ophir Plana, explaining also melting of ground ice or breaching of cryosphere. Alternatively, the depressions and crater basins may have been filled by regional groundwater table rising during the period(s) when cryosphere was absent or considerably thin. The large quantities of water necessary for explaining the paleohydrological processes in Aurorae and Ophir Plana could have been derived through crustal migration from the crust of higher plains in western Ophir Planum where water existed in confined aquifers or was produced by melting of ground ice due to magmatic heating or climatic shift, or from a paleolake in Candor Chasma further west.     

The nature of coarse-grained crystalline hematite and its implications for the early environment of Mars

The Thermal Emission Spectrometer (TES) on the Mars Global Surveyor spacecraft has detected deposits of coarse-grained, gray crystalline hematite in Sinus Meridiani, Aram Chaos, and Vallis Marineris. We argue that the key to the origin of gray hematite is that it requires crystallization at temperatures in excess of about 100 °C. We discuss thermal crystallization (1) as diagenesis at a depth of a few kilometers of sediments originally formed in low-temperature waters, or (2) as precipitation from hydrothermal solution. In Aram Chaos, a combination of TES data, Mars Orbiter Camera images, and Mars Orbiter Laser Altimeter (MOLA) topography suggests that high concentrations of hematite were formed in planar strata and have since been exposed by erosion of an overlying light-toned, caprock. Lesser concentrations of hematite are found adjacent to these strata at lower elevations, which we interpret as perhaps due to accumulation from physical weathering. The topography and the collapsed nature of the chaotic terrain favor a hydrothermally charged aquifer as the original setting where the hematite formed. Concentration of iron into such an ore-like body would be chemically favored by saline, Cl-rich hydrothermal fluids. An alternative sedimentary origin requires post-depositional burial to a depth of ∼3-5 km to induce thermally driven recrystallization of fine-grained iron oxides to coarse-grained hematite. This depth of burial and re-exposure is difficult to reconcile with commonly inferred martian geological processes. However, shallow burial accompanied by post-burial hydrothermal activity remains plausible. When the hematite regions originally formed, redox balance requires that much hydrogen must have been evolved to complement the extensive oxidation. Finally, we suggest that the coexistence of several factors required to form the gray hematite deposits would have produced a favorable environment for primitive life on early Mars, if it ever existed. These factors include liquid water, abundant electron donors in the form of H₂, and abundant electron acceptors in the form of Fe³⁺.     

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.     

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.     

Br/Cl partitioning in chloride minerals in the Burns formation on Mars

Within Gusev Crater and Meridiani Planum on Mars, the Mars exploration rovers have found Br concentrations in soils and rocks in the hundreds of ppm range. Relative to Earth compositions, these are high Br concentrations. Because of low Br concentrations on Earth, Br largely precipitates from seawater as a minor constituent in halite crystals rather than as a separate phase mineral. This is also likely to be the case for Mars. But given that the surface chemistries on Mars are significantly different than on Earth, minerals other than halite could serve as sinks for Br. The specific objectives of this paper were to (1) incorporate Br solution phase chemistries into the FREZCHEM model, (2) integrate the Siemann–Schramm Br/Cl mineral model into FREZCHEM, and (3) apply this mineral model to Br/Cl partitioning in Burns formation rocks as an indicator of past environments in the Meridiani Planum region of Mars. We showed that: (1) a molar-based model for Br substitution into halite and bischofite provided a better fit to experimental data than the standard mass-based model; (2) the concentrations of all of the soluble salts (mainly of Na, Mg, Ca, Cl, Br, and SO₄) in the Burns formation, except for Ca, were significantly related to stratigraphic depth; (3) the likely precipitation of Ca as gypsum on Mars precluded Ca precipitating as a CaCl₂ salt and thus impacts the possible minimum eutectic brine temperatures relevant to the Burns formation; (4) bischofite (MgCl₂⋅6H₂O) was a much more important sink for Br than halite; (5) Br/Cl patterns in the Burns formation, and within the three formation layers, argued in support of salt upwelling through groundwater evaporation; and (6) the high concentrations of Br in the surface layers of the Burns formation suggested that there was little water leaching and removal of soluble phases from the upper part of the stratigraphic succession.     

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.     

Correlations between hematite and sulfates in the chaotic terrain east of Valles Marineris

We have identified the presence of polyhydrated sulfates in association with crystalline gray hematite in outcrop units of the chaotic terrain east of Valles Marineris. The hematite is found in abundances of up to ∼18%, and is usually associated with thin (∼10's of meters) cliff-forming layers of intermediate-toned outcrops (albedo ∼0.15-0.20) as well as mantling deposits adjacent to the outcrops. The polyhydrated sulfates are usually restricted to the bedrock unit, and are not found in the adjacent mantling units. In analogy to the observations performed at the Opportunity landing site, we hypothesize that erosion of the sulfate/hematite-bearing outcrops leaves the hematite behind as a lag and breaks the sulfates down to wind-transportable sizes. We also find that the layered outcrops present, for the most part, embayment or on-lap relationships with respect to the hummocks that constitute the chaotic terrain, suggesting that these units were emplaced via subaqueous or aeolian deposition and/or flow after the event that formed the associated chaos. These morphological observations, in conjunction with the correlation between hematite and polyhydrated sulfates also suggest an aqueous genesis for the crystalline gray hematite in these chaotic units, and presents evidence for the action of aqueous processes after the formation of at least some of the chaotic units on Mars.     

Flood-formed dunes in Athabasca Valles, Mars: morphology, modeling, and implications

Estimates of discharge for martian outflow channels have spanned orders of magnitude due in part to uncertainties in floodwater height. A methodology of estimating discharge based on bedforms would reduce some of this uncertainty. Such a methodology based on the morphology and granulometry of flood-formed (‘diluvial’) dunes has been developed by Carling (1996b, in: Branson, J., Brown, A.G., Gregory, K.J. (Eds.), Global Continental Changes: The Context of Palaeohydrology. Geological Society Special Publication No. 115, London, UK, 165-179) and applied to Pleistocene flood-formed dunes in Siberia. Transverse periodic dune-like bedforms in Athabasca Valles, Mars, have previously been classified both as flood-formed dunes and as antidunes. Either interpretation is important, as they both imply substantial quantities of water, but each has different hydraulic implications. We undertook photoclinometric measurements of these forms, and compared them with data from flood-formed dunes in Siberia. Our analysis of those data shows their morphology to be more consistent with dunes than antidunes, thus providing the first documentation of flood-formed dunes on Mars. Other reasoning based on context and likely hydraulics also supports the bedforms' classification as dunes. Evidence does not support the dunes being aeolian, although a conclusive determination cannot be made with present data. Given the preponderance of evidence that the features are flood-formed instead of aeolian, we applied Carling's (1996b, in: Branson, J., Brown, A.G., Gregory, K.J. (Eds.), Global Continental Changes: The Context of Palaeohydrology. Geological Society Special Publication No. 115, London, UK, 165-179) dune-flow model to derive the peak discharge of the flood flow that formed them. The resultant estimate is approximately 2×10⁶ m³/s, similar to previous estimates. The size of the Athabascan dunes' in comparison with that of terrestrial dunes suggests that these martian dunes took at least 1-2 days to grow. Their flattened morphology implies that they were formed at high subcritical flow and that the flood flow that formed them receded very quickly.     

The Utopia/Isidis overlap: Possible conduit for mud volcanism on Mars

The largest areal concentration of pitted cones on Mars is located in the southwest section of Utopia basin. This particular area of pitted cones has been attributed to mud volcanism; several factors may have facilitated extensive mud volcanism at this location. The concentration of pitted cones is located where Utopia basin intersects Isidis basin; both features are multi-ring impact basins. On Earth, seismic investigations have shown that the outer rings of the Chicxulub multi-ring impact basin extend to the Mohorovi?i? discontinuity (Moho). If this is true on Mars as well, the fractures could act as conduits for water from Utopia Planitia, the site of a large, putative water body. It has been shown that methane can be generated at the mantle on Earth. On Mars this possible source of methane could combine with the infiltrated water to generate clathrates. While methane is not currently being released at the location of the pitted cones it could have been in the past. Three locations of methane release have been observed on Mars, two of which are located on the same outer ring of Isidis basin that intersects the pitted cone population. The area of Utopia basin that contains the large population of pitted cones is adjacent to the highland/lowland boundary where extensive deposition would have occurred. Extensive deposition combined with the potential for methane release may have contributed to the large population of pitted cones in this area of the Utopia basin.