Numerical Modeling of the Formation of Hellas Planitia with Focus on Spatio-Temporal Scales Required for Hydrologic Equilibration

Cratering processes are ubiquitous throughout the solar system, and were particularly important after the systems formation when cratering rates were significantly higher and during the Late Heavy Bombardment, when the high early rate of impacts were further elevated. Four Ga, on Noachian Mars, several basin forming impacts occurred within a few million years of formation and contemporaneously with surficial water flow, as evidenced with remotely sensed geomorphic and mineralogical observations. Mars planetary evolution included the cessation of geomagnetic activity and subsequent loss of atmospheric pressure. This resulted in reduction of the weathering and erosion rates, and serendipitously preserved the large impact structures from early Martian history. In this work, we have numerically investigated the spatiotemporal scales associated with planet-wide disruption of hydrologic equilibrium as the largest impact structures form. Specifically, we have investigated the formation of Hellas Planitia, a large impact basin on the southern Martian highlands. Terrestrial impact structures, such as the Chicxulub impact structure, formed in marine environments and were inundated by resurgent sea water within minutes. Craters on the Martian highlands would not have been inundated by a putative Martian Ocean and therefore would have filled via combination of groundwater flow and precipitation. We considered parametric spaces for hydraulic conductivity, initial hydraulic head gradients, and precipitation rates for which numerous simulations were conducted. The geomorphic evidence from DiAchilles and Hynek (2010) as well as Salese et al. (2019) are used as constraints. A possible Hellas catchment has been delineated and modeled as a no-flow boundary condition with the Boussinesq model for both unconfined and confined groundwater flows using MOLA-derived approximations of a Noachian topography. On a spherical shell coordinate system, we have modeled the propagation of the initial disequilibrium front, from Kyr to Gyr time scales, as the transient effects of instantaneous basin formation propagated across the early Martian highlands.