Martian subsurface cryosalt expansion and collapse as trigger for landslides.

On Mars, seasonal martian flow features known as recurring slope lineae (RSL) are prevalent on sun-facing slopes and are associated with salts. On Earth, subsurface interactions of gypsum with chlorides and oxychlorine salts wreak havoc: instigating sinkholes, cave collapse, debris flows, and upheave. Here, we illustrate (i) the disruptive potential of sulfate-chloride reactions in laboratory soil crust experiments, (ii) the formation of thin films of mixed ice-liquid water "slush" at -40° to -20°C on salty Mars analog grains, (iii) how mixtures of sulfates and chlorine salts affect their solubilities in low-temperature environments, and (iv) how these salt brines could be contributing to RSL formation on Mars. Our results demonstrate that interactions of sulfates and chlorine salts in fine-grained soils on Mars could absorb water, expand, deliquesce, cause subsidence, form crusts, disrupt surfaces, and ultimately produce landslides after dust loading on these unstable surfaces.

A closer look at water-related geologic activity on Mars.

Water has supposedly marked the surface of Mars and produced characteristic landforms. To understand the history of water on Mars, we take a close look at key locations with the High-Resolution Imaging Science Experiment on board the Mars Reconnaissance Orbiter, reaching fine spatial scales of 25 to 32 centimeters per pixel. Boulders ranging up to approximately 2 meters in diameter are ubiquitous in the middle to high latitudes, which include deposits previously interpreted as finegrained ocean sediments or dusty snow. Bright gully deposits identify six locations with very recent activity, but these lie on steep (20 degrees to 35 degrees) slopes where dry mass wasting could occur. Thus, we cannot confirm the reality of ancient oceans or water in active gullies but do see evidence of fluvial modification of geologically recent mid-latitude gullies and equatorial impact craters.

Episodic ocean-induced CO2 greenhouse on Mars: implications for fluvial valley formation.

Pulses of CO2 injected into the martian atmosphere more recently than 4 Ga can place the atmosphere into a stable, higher pressure, warmer greenhouse state. One to two bar pulses of CO2 added to the atmosphere during the past several billion years are sufficient to raise global mean temperatures above 240 or 250 K for tens to hundreds of millions of years, even when accounting for CO2 condensation. Over time, the added CO2 is lost to carbonates, the atmosphere collapses and returns to its buffered state. A substantial amount of water could be transported during the greenhouse periods from the surface of a frozen body of water created by outflow channel discharges to higher elevations, despite global temperatures well below freezing. This water, precipitated as snow, could ultimately form fluvial valleys if deposition sites are associated with localized heat sources, such as magmatic intrusions or volcanoes. Thus, if outflow channel discharges were accompanied by the release of sufficient quantities of CO2, a limited hydrological cycle could have resulted that would have been capable of producing geomorphic change sufficient for fluvial erosion and valley formation. Glacial or periglacial landforms would also be a consequence of such a mechanism.

The circum-Chryse region as a possible example of a hydrologic cycle on Mars: geologic observations and theoretical evaluation.

The transection and superposition relationships among channels, chaos, surface materials units, and other features in the circum-Chryse region of Mars were used to evaluate relative age relationships and evolution of flood events. Channels and chaos in contact (with one another) were treated as single discrete flood-carved systems. Some outflow channel systems form networks and are inferred to have been created by multiple flood events. Within some outflow channel networks, several separate individual channel systems can be traced to a specific chaos which acted as flood-source area to that specific flood channel. Individual flood-carved systems were related to widespread materials units or other surface features that served as stratigraphic horizons. Chryse outflow channels are inferred to have formed over most of the perceivable history of Mars. Outflow channels are inferred to become younger with increasing proximity to the Chryse basin. In addition, outflow channels closer to the basin show a greater diversity in age. The relationship of subsequent outflow channel sources to the sources of earlier floods is inferred to disfavor episodic flooding due to the progressive tapping of a juvenile near-surface water supply. Instead, we propose the circum-Chryse region as a candidate site of past hydrological recycling. The discharge rates necessary to carve the circum-Chryse outflow channels would have inevitably formed temporary standing bodies of H2O on the Martian surface where the flood-waters stagnated and pooled (the Chryse basin is topographically enclosed). These observations and inferences have led us to formulate and evaluate two hypotheses: (1) large amounts of the sublimated H2O off the Chryse basin flood lakes precipitated (snowed) onto the flood-source highlands and this H2O was incorporated into the near surface, recharging the H2O sources, making possible subsequent deluges; and (2) ponded flood-water in Chryse basin drained back down an anti basinward dipping subsurface layer accessed long the southern edge of the lake, recharging the flood-source aquifers. H2O not redeposited in the flood-source region was largely lost to the hydrologic cycle. This loss progressively lowered the vitality of the cycle, probably by now killing it. Our numerical evaluations indicate that of the two hypotheses we formulated, the groundwater seep cycle seems by far the more viable. Optimally, approximately 3/4 of the original mass of an ice-covered cylindrical lake (albedo 0.5, 1 km deep, 100-km radius, draining along its rim for one quarter of its circumference into substrata with a permeability of 3000 darcies) can be modeled to have moved underground (on timescales of the order of 10(3) years) before the competing mechanisms of sublimation and freeze down choked off further water removal. Once underground, this water can travel distances equal to the separation between Chryse basin and flood-source sites in geologically short (approximately 10(6) year-scale) times. Conversely, we calculate that optimally only approximately 40% of the H2O carried from Chryse can condense at the highlands, and most of the precipitate would either collect at the base of the highlands/lowlands scarp or sublimate at rates greater than it would accumulate over the flood-source sites. Further observations from forthcoming missions may permit the determination of which mechanisms may have operated to recycle the Chryse flood-waters.