A New Model of the Hydrologic Properties of the Martian Crust and Implications for the Formation of Valley Networks and Outflow Channels

Introduction: There is abundant geomorphic evidence for the importance of groundwater in the Martian hydrologic cycle. It appears likely that the valley networks formed as a result of groundwater sapping [1], and it is clear that the outflow channels are the manifestation of large-scale eruptions of groundwater to the surface [2]. Thus, our understanding of the formation of these features is only as good as our understanding of the hydrologic properties of the Martian crust. Clifford [3] modeled the Martian crust based upon interpretations of the lunar seismic data and general comparisons to terrestrial aquifers. MacKinnon and Tanaka [4] modeled the Martian curst as a layer of regolith overlying a fractured basement. The present model builds upon this groundwork to construct a more general and widely applicable model of the hydrologic properties of the Martian crust, based on the decomposition of the aquifer into its component parts and the detailed modeling of each component. Given the varying proportions of competent rock and breccia or sediments, as well as the fracture frequency, the porosity, hydraulic conductivity, and compressibility of an aquifer can then be calculated as functions of the depth and pore pressure. The individual aspects of this model are easily adjustable to match the inferred geologic environment of different areas and times in the planet’s history, as well as any combination of depth and pore pressure. Model. Ancient highlands crust. The Noachian aged highlands crust is assumed to be dominated by the effects of impacts. A simplified model of the crust beneath an impact crater includes a brecciated region and a fracture zone. Gravity studies of terrestrial and lunar impact craters suggest that the breccia lens beneath a crater of diameter D extends to a depth of D/3 [5]. A deep drill core into a terrestrial impact crater has revealed that the brecciated region for this crater is actually composed of large blocks of bedrock separated by zones of breccia, with a ratio of rock to breccia of 3:1 [6]. The fracture frequency within the fractured zone can be estimated based on fracture counts in a drill core into the upper portions of a terrestrial impact crater [7]. This simple crater model was applied to the Martian surface using the crater distribution in Noachian terrains to calculate the porosity, hydraulic conductivity, and compressibility of the top 10 km of crust (see Figure 1). It is assumed that with the essential saturation of craters in the southern highlands, the effects of the impacts overlap, creating a homogenous and isotropic distribution of fractures and breccia.