Atmospheric effects on Martian Ejecta Emplacement

Abstract

Analytical descriptions of crater growth and numerical calculations of aerodynamic drag have been combined to evaluate the possible effects of drag on impact crater ejecta emplacement on Mars. Below a critical particle size, ejecta deposition is severely restricted in range. This critical particle size increases with increasing crater size because ejection velocity at the same relative position within a crater increases with size and because aerodynamic deceleration increases as the square of the velocity. Numerical models of ejecta trajectories in the current Martian atmosphere under hydrostatic equilibrium reveal that craters of 1, 5, 10, and 20 km exhibit critical particle diameters of 0.4–0.8, 2–4, 5–10, and 10–20 cm. Ejecta approaching the critical particle size may impact within a crater radius of the excavation crater rim. Once velocities are reduced sufficiently, ejecta paths can be modified by turbulence and winds, thereby setting up conditions for ejecta flow from the rim region. However, ejecta larger than the critical particle size are relatively undecelerated and form secondary impact craters subsequently modified by the later arriving decelerated ejecta cloud. Consequently, ejecta emplacement will be multiphased, but the importance of this process depends on the distribution of ejecta sizes.