Modeling the Effect of Obliquity on Mars Elliptical Crater Orientations

Introduction: Obliquity is a strong control on post-Noachian Martian climate [1]. However, Martian obliquity is chaotic and dynamical models produce a wide range of instantaneous and long-term average values [2]. Here, we model the effect of Mars mean obliquity on the distribution of fresh elliptic crater orientations. To achieve this we used an N-body simulation to estimate the time-averaged distribution of asteroid close encounter inclinations and speeds. These close encounters seeded an analytic forward model of asteroid trajectories that tracked impact orientations and angles. Preliminary comparison of model output and elliptic crater orientation data from the Robbins database [3] suggest that mean Mars obliquity after the Mars impactor population stabilized was low, < ~30. Close Encounter Simulation: We started with the Minor Planet Center database of asteroid orbital elements and selected Mars crossing objects defined as having perihelion distance less than a × (1 + e) and aphelion distance greater than a × (1 e) where a = 1.52AU is the semi-major axis of Mars and e = 0.15 was chosen to include the entire range of Mars eccentricities observed in our simulations. We also required that the objects have magnitude less than 14 to ensure population completeness and more than one opposition to ensure accuracy of orbital elements. Using the Mercury N-body hybrid symplectic code [4], we integrated the solar system forward 10Myr with these objects as massless test particles. Each instance of a test particle passing within 1 Hill radius of Mars was counted as a close encounter, and from these close encounters we obtained a distribution of impactor speeds and inclinations relative to Mars’s orbital plane (Figure1).