Modelling Esker Formation on Mars

International audience<p><strong>Introduction:</strong>  Eskers are sinuous sedimentary ridges that are widespread across formerly glaciated landscapes on Earth. They form when sediment in subglacial tunnels is deposited by meltwater. Some sinuous ridges on Mars have been identified as eskers; whilst some are thought to have formed early in Mars’ history beneath extensive ice sheets, smaller, younger systems associated with extant glaciers in Mars’ mid latitudes have also been identified. Elevated geothermal heating and formation during periods with more extensive glaciation have been suggested as possible prerequisites for recent Martian esker deposition.</p><p>Here, we adapt a model of esker formation with g and other constants altered to Martian values, using it initially to investigate the impact of Martian conditions on subglacial tunnel systems, before investigating the effect of varying water discharge on esker deposition.</p><p><strong>Methods:</strong> To investigate the effect of these values on the operation of subglacial tunnel systems we first conduct a series of model experiments with steady water discharge, varying the assumed liquid density (r<sub>w</sub>) from 1000 kgm<sup>-3</sup> to 1980 kgm<sup>-3</sup> (the density of saturated perchlorate brine) and ice hardness (A) from 2.4x10<sup>-24</sup> Pa<sup>-3</sup>s<sup>-1</sup> to 5x10<sup>-27</sup> Pa<sup>-3</sup>s<sup>-1</sup> (a temperature range of 0°C to -50°C). We then investigate the impact of variable water discharge on esker formation to simulate very simply a possible release of meltwater from an assumed geothermal event beneath a Martian glacier or ice cap.</p><p><strong>Results and Discussion:</strong>  A key aspect of model behaviour is the decrease in sediment carrying capacity towards the ice margin due to increased tunnel size as ice thins. Our results suggest that Martian parameters emphasise this effect, making deposition more likely over a greater length of the conduit. Lower gravity has the largest impact; it reduces the modeled closure rate of subglacial tunnels markedly as this varies with overburden stress (and hence g) cubed. Frictional heating from flowing water also drops, but much less sensitively. Thus, for a given discharge, the tunnels tend to be larger, leading to lower water pressure and a reduction in flow power. This effect is amplified for harder ice. Higher inferred fluid density raises the flow power, but by a smaller amount.</p><p>These effects are clearly seen in the variable discharge experiments. Sediment is deposited on the falling limb of the hydrograph, when the tunnels are larger than the equivalent steady-state water discharge would produce. Sediment deposition occurs much further upglacier from the glacier snout, and occurs earlier on the falling limb leading to longer periods in which deposition occurs.</p><p><strong>Conclusions:</strong> Our results suggest that esker formation within a subglacial meltwater tunnel would be more likely on Mars than Earth, primarily because subglacial tunnels tend to be larger for equivalent water discharges, with consequent lower water flow velocities. This allows sediment deposition over longer lengths of tunnel, and to greater depths, than for terrestrial systems. Future work will use measured bed topography of a mid-latitude esker to assess the impact of topography on deposition patterns and esker morphology, and we will expand the range of discharges and sediment supply regimes investigated.</p&gt

Eskers on Mars: Morphometric comparisons to eskers on Earth and implications for sediment-discharge dynamics of subglacial drainage

Mars’ present climate is extremely cold and arid. Until recently, it was widely thought that debris-covered glaciers in Mars’ mid-latitudes have been pervasively cold-based since their formation 10s–100s Myr ago. However, we recently discovered eskers associated with ~110–150 Myr old glaciers in the Phlegra Montes [1] and NW Tempe Terra [2] regions of Mars’ northern mid-latitudes. Eskers are sinuous ridges comprising sediments deposited in glacial meltwater conduits. Therefore, eskers associated with existing mid-latitude glaciers on Mars indicate that localised wet-based glaciation did occur during Mars’ most recent geological period. Eskers are important tools for reconstructing the nature, extent, and dynamics of wet-based glaciation on Earth, and have similar potential for Mars. We used 1–2 m/pixel resolution digital elevation models derived from 25–50 cm/pixel High Resolution Imaging Science Experiment stereo-pair images to measure the planform and 3D morphometries of the Phlegra Montes and NW Tempe Terra eskers, and compare them with the morphometries of Quaternary-aged eskers in Canada [3] and SW Finland [4]. We found that the Martian eskers have remarkably similar lengths, sinuosities and heights to terrestrial eskers, but that the Martian eskers are typically wider and have lower side slopes. Large width-height ratios of the Martian eskers are consistent with our previous measurements of ancient (~3.5 Ga) eskers close to Mars’ south pole [5], and may arise from differences in either: esker degradation state, or fundamental glacio-hydrological controls on esker formation between Mars and Earth. Portions of the two Martian eskers with comparable crest morphologies (e.g., sharp- or round-crested) have similar width-height relationships, suggesting that glacio-hydrological processes may exert controls upon the observed relationships between esker morphology and morphometry. Our morphometric analyses also reveal that the Martian esker in NW Tempe Terra has a ‘stacked’ morphology: the crest of a wide, round-crested underlying ridge is superposed by a narrow, sharp- to multi-crested ridge. Based on morpho-sedimentary relationships observed along terrestrial eskers [6], we interpret this transition to represent waning sediment supply and meltwater discharge towards the end of the esker-forming drainage episode(s). Direct sedimentary insights into Martian eskers are not yet possible so we emphasise that such inferences should be rigorously grounded in observations of analogous landforms on Earth. This work was funded by STFC grant ST/N50421X/1. References: [1] Gallagher, C., and Balme, M.R., (2015), Earth. Planet. Sci. Lett. 431, 96-109, [2] Butcher, F.E.G., et al. (2017), J. Geophys. Res. Planets. 122(12), 2445-2468, [3] Storrar, R.D., et al. (2014) Quat. Sci. Rev. 105, 1-25, [4] Storrar, R.D., and Jones, A., Unpublished, [5] Butcher, F.E.G., et al. (2016), Icarus 275, 65-84, [6] Burke, M.J., et al. (2010) Geol. Soc. Am. Bull. 122, 1637-1645

Gridmapping the northern plains of Mars: Geomorphological, Radar and Water-Equivalent Hydrogen results from Arcadia Plantia

A project of mapping ice-related landforms was undertaken to understand the role of sub-surface ice in the northern plains. This work is the first continuous regional mapping from CTX (“ConTeXt Camera”, 6 m/pixel; Malin et al., 2007) imagery in Arcadia Planitia along a strip 300 km across stretching from 30°N to 80°N centred on the 170° West line of longitude. The distribution and morphotypes of these landforms were used to understand the permafrost cryolithology. The mantled and textured signatures occur almost ubiquitously between 35° N and 78° N and have a positive spatial correlation with inferred ice stability based on thermal modelling, neutron spectroscopy and radar data. The degradational features into the LDM (Latitude Dependent Mantle) include pits, scallops and 100 m polygons and provide supporting evidence for sub-surface ice and volatile loss between 35-70° N in Arcadia with the mantle between 70-78° N appearing much more intact. Pitted terrain appears to be much more pervasive in Arcadia than in Acidalia and Utopia suggesting that the Arcadia study area had more wide-spread near-surface sub-surface ice, and thus was more susceptible to pitting, or that the ice was less well-buried by sediments. Correlations with ice stability models suggest that lack of pits north of 65-70° N could indicate a relatively young age (~1Ma), however this could also be explained through regional variations in degradation rates. The deposition of the LDM is consistent with an airfall hypothesis however there appears to be substantial evidence for fluvial processes in southern Arcadia with older, underlying processes being equally dominant with the LDM and degradation thereof in shaping the landscape

Eskers in a complete, wet-based glacial system in the Phlegra Montes region, Mars

Although glacial landsystems produced under warm/wet based conditions are very common on Earth, even here, observations of subglacial landforms such as eskers emerging from extant glaciers are rare. This paper describes a system of sinuous ridges emerging from the in situ but now degraded piedmont terminus of a Late Amazonian-aged (∼150 Ma) glacier-like form in the southern Phlegra Montes region of Mars. We believe this to be the first identification of martian eskers that can be directly linked to their parent glacier. Together with their contextual landform assemblage, the eskers are indicative of significant glacial meltwater production and subglacial routing. However, although the eskers are evidence of a wet-based regime, the confinement of the glacial system to a well-defined, regionally significant graben, and the absence of eskers elsewhere in the region, is interpreted as evidence of sub-glacial melting as a response to locally enhanced geothermal heat flux rather than climate-induced warming. These observations offer important new insights to the forcing of glacial dynamic and melting behaviour on Mars by factors other than climate

Perspective‐Taking and Depth of Theory‐of‐Mind Reasoning in Sequential‐Move Games

Theory‐of‐mind (ToM) involves modeling an individual’s mental states to plan one’s action and to anticipate others’ actions through recursive reasoning that may be myopic (with limited recursion) or predictive (with full recursion). ToM recursion was examined using a series of two‐player, sequential‐move matrix games with a maximum of three steps. Participants were assigned the role of Player I, controlling the initial and the last step, or of Player II, controlling the second step. Appropriate for the assigned role, participants either anticipated or planned Player II’s strategy at the second step, and then determined Player I’s optimal strategy at the first step. Participants more readily used predictive reasoning as Player II (i.e., planning one’s own move) than as Player I (i.e., anticipating an opponent’s move), although they did not differ when translating reasoning outcome about the second step to optimal action in the first step. Perspective‐taking influenced likelihood of predictive reasoning, but it did not affect the rate at which participants acquired it during the experimental block. We conclude that the depth of ToM recursion (related to perspective‐taking mechanisms) and rational application of belief–desire to action (instrumental rationality) constitute separate cognitive processes in ToM reasoning.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/91101/1/j.1551-6709.2012.01238.x.pd