Sources of Water for Groundwater-fed Outflow Channels on Mars: Implications of the Late Noachian “icy Highlands” Model for Melting and Groundwater Recharge on the Tharsis Rise

IMPLICATIONS OF THE LATE NOACHIAN “ICY HIGHLANDS” MODEL FOR MELTING AND GROUNDWATER RECHARGE ON THE THARSIS RISE. J. P. Cassanelli, J. W. Head, J. L. Fastook, Brown University Department of Earth, Environmental and Planetary Sciences, Providence, RI 02912 USA, University of Maine, Orono, ME 04469 USA. ([email protected]). Introduction: During the Late Noachian (LN), Hesperian, and early Amazonian periods, large outflow channels were carved into the surface of Mars [1-3]. Many are interpreted to have originated through catastrophic discharge of groundwater from pressurized aquifers [4,5]. Formation of outflow channels by this mechanism requires a predominantly cold climate during the time of aquifer discharge because a thick and globally extensive layer of perennially frozen ground (cryosphere) is needed to create an impermeable confining unit (in order for the aquifers to become pressurized). However, the impermeable cryosphere prohibits the operation of typical groundwater recharge mechanisms by preventing infiltration of surface water. Therefore, the water needed to prime the aquifers for outflow channel formation must have been supplied prior to the discharge events. While the climate of early Mars has been suggested by some to have been “warm and wet” [e.g. 6], which would allow for groundwater recharge, recent Global Climate Modeling (GCM) efforts [7,8] predict an early Mars climate dominated by “cold and icy” conditions. The GCM modeling of an early, thicker CO2 martian atmosphere indicates that, when coupled with a full water cycle, the atmosphere of Mars will behave adiabatically causing temperatures to decrease with elevation. As a result high standing areas across Mars are cooled, leading to preferential accumulation of snow and ice, and the formation of regional ice sheets throughout the Tharsis region and the Southern highlands. These predictions outline the Late Noachian “icy uplands” (LNIH) early Mars climate model [8]. Under the conditions predicted by the LNIH model, a thick global cryosphere would exist, preventing groundwater recharge. Therefore, the LNIH early Mars climate scenario appears incompatible with the later formation of groundwater sourced outflow channels through near-simultaneous recharge mechanisms. Here we adopt the assumption that the LNIH scenario is the actual representation of the LN Mars climate in order to test this model: Can LNIH ice sheet basal melting produce sufficient groundwater recharge to provide a mechanism for the near-simultaneous formation of outflow channels by cryospheric cracking and groundwater release? Previous work has shown that groundwater recharge from the Tharsis region could have supplied the hydraulic head needed to explain the formation of some outflow channels [9], though further assessment showed that ice sheet basal melting was only likely to occur at very significant ice sheet thicknesses or geothermal heat flux values [10,11]. However, the regional ice sheet formation predicted by the LNIH model coincides with a time of intense volcanic and magmatic activity in the Tharsis region [12], which is likely to have contributed to an elevated geothermal heat flux. Here, we revisit the Tharsis region as a center for groundwater recharge given the predicted deposition of LNIH sheets in the presence of a regionally elevated geothermal heat. Ice Sheet Basal Melting: In order for ice sheet basal melting to occur, a critical thickness of insulating ice must be accumulated for a given mean annual surface temperature and geothermal heat flux. 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 [13]. We adopt a reservoir 5X the currently observed polar/near-surface water inventory on Mars. Distribution of this reservoir across the high standing areas, above the predicted equilibrium line altitude (+1 km; [8]), gives an average ice sheet thickness of ~700m [14]. Given these predicted average thicknesses, and LN mean annual surface temperatures (~225 K; [7,8]), we assess the geothermal heat flux conditions required to initiate basal melting. Geothermal Heat Flux: During the LN, widespread volcanic and magmatic activity throughout the Tharsis region [12,15] would have contributed to an elevation of local and regional geothermal heat fluxes. The extent to which these geothermal heat fluxes may have been elevated is unclear, but estimates of the regional geothermal heat flux in the Tharsis region during this time range from ~60-100 mW/m [16]. Due to local variations, the geothermal heat flux can vary considerably from the regional average, particularly near active volcanic features. Here we assume that the geothermal heat flux values measured from active terrestrial volcanic regions (Fig. 1; [11]) are comparable to what would have occurred in the Tharsis region during the LN. We assess the potential for ice sheet basal melting in response to both regional and localized heat flux values.