Late Noachian “icy Highlands” Mars: Implications for Melting and Ground- Water Recharge across the Tharsis Rise

Introduction: Results from modeling of an early CO2 martian atmosphere and late Noachian climate predict that at pressures greater than a fraction of a bar, atmosphere-surface thermal coupling occurs, resulting in adiabatic cooling of high areas across Mars [1]. This effect promotes the transport of water from warmer low-lying areas to the highlands where deposition and accumulation of water ice result in an “icy highlands” Late Noachian Mars [2]. Deposition of water ice by this process at predicted accumulation rates will quickly exceed the currently observed polar/near-surface water inventory on Mars [3,4]. Therefore, we assume the growth of the regional ice sheets throughout the highlands to be a supply limited process constrained by the available surface water reservoir [4,5]; we adopt a reservoir of 2 times the current available water ice in the polar/near-surface reservoirs (~60 m GEL). Distribution of this water supply across the predicted areas of deposition leads to an average ice sheet thickness of approximately 200 meters and thus predominantly cold base glaciation [3,4]. Of particular interest in the “icy highlands” model is the regional accumulation of snow and ice in the Tharsis rise volcanic provinces. The deposition of snow and ice throughout the Tharsis region coincides with a period of widespread volcanic activity [6] raising the possibility for melting through both top-down heating (climate modified by ideal insolation geometry, impacts, volcanism) and bottom-up heating (increased geothermal heat flux, thick ice accumulation). The Tharsis Montes have been previously examined as possible centers for groundwater recharge through basal melting of ice deposits resulting from the increased geothermal heat flux [e.g. 7]. Here we assess the nature of regional ice accumulation across the Tharsis rise in the presence of an elevated geothermal heat flux and the implications that this may have for possible melting mechanisms and meltwater transport. We examine the response of regional ice deposits to a baseline Noachian heat flux [8,9,10], an elevated geothermal heat flux representative of the broader Tharsis region, and two highly elevated heat fluxes predicted to occur on specific volcanic edifices [11]. For the geothermal heat flux of the broader Tharsis region we adopt a mean value of typical geothermal heat fluxes observed in terrestrial volcanic regions [12,13,14,15,16]. In addition, we assess the configuration of the martian cryosphere under the outlined conditions in order to determine the possibility of localized vertical integration of the hydrologic system in the form of groundwater aquifer recharge. Methods: To predict the temperature increase at the base of the regional ice deposits across Tharsis we implement the University of Maine Ice Sheet Model (UMISM) [17,18,19]. The UMISM was parameterized for Late Noachian climate conditions and used to calculate the basal temperature for ice sheets of various thicknesses at several selected geothermal heat flux values. The UMISM was also used to determine the amount of time a punctuated heating event would need to be sustained in order for the temperature increase to propagate to the ice sheet base and initiate basal melting. This places constraints on the necessary conditions that a punctuated heating event must meet in order to cause top-down induced basal melting. To assess the potential for localized vertical integration of the hydrologic system we implement the onedimensional heat equation, parameterized to the same conditions, to estimate cryosphere thicknesses. Results: Table 1 summarizes the temperature increases calculated at the ice sheet base relative to the surface as a function of geothermal heat flux. We account for the possibility of asymmetrical ice accumulation within Tharsis [e.g., 20] by performing the calculations for ice sheet thicknesses of up to 1 km.