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The survey of the Shalbatana Vallis course and the analysis of the distribution of the concentric peripheral and radial systems of faults east of Tharsis allow us to propose a model that demonstrates the effect of volcano–tectonic strains generating crossing fault systems where heat points allowed hydrothermal drainage of confined aquifers and generated headwater systems. This model is qualitatively consistent with the strain of the spherical buldge of Tharsis expressed by extensional faults intersected by radial faults in the upper lithosphere. The southward course of the channel is oriented parallel to extensional faults that are related to the wrinkle ridge field of Lunae Planum. The channel is perpendicular to the normal radial faults of eastern Tharsis between the equator and −20° lat. The intersection of these two fault systems generated weak points where magmatic material originating from the lithosphere entered the upper crust through the self-sealing ice-saturated cryosphere. We demonstrate that the system of fault intersections can explain: (a) the hydrothermal drainage of underground aquifers by convective heat flux and dike intrusion that formed chaotic terrains where source areas were generated beneath the cryosphere and (b) the linear course of the sharp-rimmed, U-shaped Shalbatana Vallis intersected by successive chaos developments as governed by the underlying peripheral fault system. The physics of thermal underpressure vapor diffusion and the condensation into water is described as the exchange of heat between the magmatic intrusion and the ice-rich confined layers beneath the chaotic terrains.

Based on a comprehensive study of the recent topographic map USGS MC-23 SE I-2119 (USGS, 1991, Topographical Map of Aeolis ) plus geomorphic and digital elevation, we have constructed a geometric model of Ma'adim Vallis. The sedimentary history of the valley was determined by the damming of the valley by two impact craters, one by the southern crater over Gusev rim and the second located 120 km upstream. The presence of an intravalley lake system is inferred from independent sedimentary analysis and from geomorphic evidence for valley benches. Peripheral and parallel topographic contour intervals of the putative intravalley lake shores suggest that the lake was covered by a thick ice sheet. The longitudinal profile of the valley floor shows a peculiar reverse slope located 300 km upstream from the southern Gusev crater rim. We conclude that this is an accumulation of sedimentary deposits adjacent to the crater rampart which defined the northern boundary of the lake. When the water level reached the lowest point of the west bank, which is 0.4 km below the top of the rim of Gusev crater, it began to floor the surrounding plateau. As the intravalley lake level raised, its ice cover formed a wide lateral bench. Later, this ponded water was funneled into Gusev crater through the breaching of the southern Gusev rampart by an impact crater. The paleolacustrine sediments transported by the lake water that channeled through the rampart debris slope mantled the floor of Gusev crater. The sublacustrine sediments are promising targets for future missions and of high priority in the search for past life.

Nathalie A.Cabrol,EdmondA.Grin, GuillermoChong,EdwinMinkley,AndrewN. Hock,Youngseob Yu, Leslie Bebout, Erich Fleming, Donat P. Ha¨der, Cecilia Demergasso,John Gibson, Lorena Escudero, Cristina Dorador, Darlene Lim, Clayton Woosley,Robert L. Morris, Cristian Tambley, Victor Gaete, Matthieu E. Galvez,Eric Smith, Ingrid Ukstins Peate, Carlos Salazar, G. Dawidowicz, and J. Majerowicz

[1] The High Lakes Project is a multidisciplinary astrobiological investigation studying high-altitude lakes between 4200 m and 6000 m elevation in the Central Andes of Bolivia and Chile. Its primary objective is to understand the impact of increased environmental stress on the modification of lake habitability potential during rapid climate change as an analogy to early Mars. Their unique geophysical environment and mostly uncharted ecosystems have added new objectives to the project, including the assessment of the impact of low-ozone/high solar irradiance in nonpolar aquatic environments, the documentation of poorly known ecosystems, and the quantification of the impact of climate change on lake environment and ecosystem. Data from 2003 to 2007 show that UV flux is 165% that of sea level with maximum averaged UVB reaching 4 W/m2. Short UV wavelengths (260–270 nm) were recorded and peaked at 14.6 mW/m2. High solar irradiance occurs in an atmosphere permanently depleted in ozone falling below ozone hole definition for 33–36 days and between 30 and 35% depletion the rest of the year. The impact of strong UVB and UV erythemally weighted daily dose on life is compounded by broad daily temperature variations with sudden and sharp fluctuations. Lake habitat chemistry is highly dynamical with notable changes in yearly ion concentrations and pH resulting from low and variable yearly precipitation. The year-round combination of environmental variables define these lakes as end-members. In such an environment, they host ecosystems that include a significant fraction of previously undescribed species of zooplankton, cyanobacterial, and bacterial populations.