Tharsis-driven Hydrology and the Martian Outflow Channels

Introduction: The outflow channels of Mars provide evidence for a dramatic subsurface hydrology during the Hesperian epoch (~3.7 to 3.0 Ga). These channels appear to have been carved by the eruption of catastrophic floods from groundwater aquifers. The large total flood volumes inferred to have been necessary to carve the outflow channels [1] suggest that a global scale hydrologic system is required in order to supply their source regions, despite limited evidence for surficial recharge at the time of outflow channel formation. Phillips et al. [2] noted that the majority of the outflow channel source regions are found in the eastern portion of the Tharsis trough, the flexural depression surrounding the Tharsis rise (Figure 1a). Recently, Hanna and Phillips [3] presented evidence for a system of older and degraded outflow channels in the vicinity of Mangala Valles in the west Tharsis trough. This spatial correlation is suggestive of a causal relationship between Tharsis and the outflow channels. We outline the major hydrologic processes likely to be operating in early Mars history, with a focus on Tharsis-centered processes, and present preliminary results of a global hydrologic model. Overview: The characteristic timescale of globalscale flow can be calculated as τ = Ss·K·(π RMars), where Ss is the aquifer specific storage, and K is the hydraulic conductivity. For values typical of deep aquifers on Mars [5], this timescale is approximately 360 Myr, roughly half the duration of the Hesperian epoch. Thus, the patterns of hydrologic flow during the Hesperian cannot be understood without first considering the hydrologic state of the planet at the close of the Noachian. A growing body of evidence suggests a climate during the Noachian that was at least transiently within the stability field of liquid water [4]. Precipitation in the mid-latitudes during the Noachian would have charged aquifers at high elevations in the southern highlands and on the incipient Tharsis rise, creating hydraulic gradients that would have persisted into the Hesperian. The resulting hydraulic gradients along the dichotomy boundary and the Tharsis rise would have resulted in a focusing of flow in the Tharsis trough, where many of the outflow channel sources are found. The construction of the Tharsis rise during the Noachian played a dominant role in the geophysical and hydrological evolution of Mars [2]. Tharsis formation would have buried any preexisting aquifers in the region to great depths beneath the growing volcanic load. A limit on the maximum depth to which aquifers are stable is dictated by the brittleplastic transition (BPT) [5]. During the growth of the Tharsis rise, the BPT would migrate upwards through the crust. As the buried aquifers transitioned from the brittle to the plastic regime, the water within would be placed under the burden of the full lithostatic pressure and be expelled, resulting in a steady upwards flux of water beneath Tharsis. There would be a lag between growth of the rise and BPT migration, limited by the thermal diffusion timescale in the deep crust, extending the duration of the hydrologic response beyond the main period of Tharsis construction. Simple calculations suggest that up to 40x10 km of water could be mobilized by this mechanism. Tharsisrelated flexural and membrane deformation of the planet would have also driven the migration of groundwater, though for most cases considered the water table in mid-latitudes at the end of the Noachian is in equilibrium with the surface topography and such flexure-driven flow does not have a significant effect. Hanna and Phillips [6] demonstrated the effectiveness of extensional tectonism in pressurizing aquifers on Mars, and suggested that the proximity of the outflow channels to the Valles Marineris canyons may indicate the importance of tectonic aquifer pressurization in this region. The volume of water