MARS RAMPART CRATER EJECTA YIELDS REGOLITH WATER CONTENT

Rampart craters on Mars have long been hypothesized to form in material that contained significant near-surface volatiles, including possibly liquid water. New shock wave equation-of-state experiments demonstrate that under shock pressures above ~1 GPa, ice I transforms to ice VI (r₀~1.33 g/cm³), involving a density increase of over 40%, and above ~2 GPa, ice I transforms to ice VII (r₀~1.56 g/cm³), involving a density increase of over 60%. Moreover, these phase changes are highly hysteretic, resulting in high net entropy production upon dynamic compression and subsequent release. Therefore, partial melting of ice to liquid water occurs upon subjecting an H₂O ice-bearing Martian regolith to shock pressures of only 2-3 GPa. Our constitutive mixture model for an ice-silicate Martian regolith assumes that the onset of melting drastically reduces the mechanical strength of the regolith. This, in turn, causes a larger fraction of impact ejecta to be launched almost vertically and deposited closer to the crater rim compared to impacts onto a rock-only regolith. The resulting ejecta blankets contain a small fraction of liquid water that is shock-induced. Our results show that ejecta fluidized with liquid water may form by an impact onto a solid-ice bearing regolith and does not require the presence of initially liquid water in the regolith. Asteroidal impacts, typically at velocities of ~10 km/s, induce partial melting to a radius of ~7 projectile radii. A high fraction of Martian impact craters of all ages, and lying at all latitudes, have so-called rampart craters, i.e., have ejecta blankets that appear to have been fluidized. Assuming an ice-filled regolith with exponentially decaying ice content with depth, and decay constant of 2.8 km (e.g., Clifford, 1993), we estimate the minimum ice content of the regolith. Computational modeling of 10 km/s impacts onto a Martian regolith containing 5-15%_vol H₂O yields theoretical impact ejecta fields for rampart craters whose topography closely matches the Mars Global Surveyor, Orbiting Laser Altimeter data.

M. Clifford. A model for the hydrologic and climatic behavior of water on Mars. Journal of Geophysical Research, 98(E6):10,973-11,016, 1993.