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S.T. Stewart (Caltech), J.D. O'Keefe (Caltech), T.J. Ahrens (Caltech)
We quantify the effects of interstitial H2O ice on Martian rampart crater formation and conclude that sufficient quantities of ice may be melted to form fluidized ejecta blankets. We calculate the composition of the ejecta blankets as a function of rampart crater diameter and subsurface distribution of ice.
For small volume fractions of subsurface ice (\phi\lesssim0.20), rocky projectiles impacting at 10 km/s vaporize and melt interstitial ice up to 2 and 7 projectile radii (Rp), respectively. Ice is partially melted to 16 Rp, well outside the final crater diameter. We calculate the amount of melted ice excavated and ballistically emplaced in the continuous ejecta blanket. From our hydrocode simulations of impact in a \phi=0.15 ice-rock mixture, the ejecta excavation angle is nearly constant at 70\arcdeg~in the melted ice zone, decreasing outside the melted ice zone to 45\arcdeg~near the crater rim. The mixture excavation angle is steeper compared to impacts into pure rock resulting in a larger volume of excavated material landing near the crater compared to dry Lunar craters.
The ice is shocked to higher temperatures than the rock, experiencing transient shock temperatures above 10,000 K in the vaporized zone and a few 1000 K in the melted zone. After releasing from the shock, most of the water in the ejecta blanket will be boiling. The ejecta is emplaced from the crater rim outward with the outer material preferentially containing more water. The continuous ejecta blanket is laid down within minutes, with a lateral velocity component of 10s-100s m/s. High lateral velocities are required for the observed ejecta flow morphologies, including cases of flow over and around pre-existing topography.
For exponentially decreasing ice content with depth, z, of the form \phi(z {\rm km})=0.15e-z/2, the fraction of the ejecta blanket containing melted ice peaks for craters with D=5-25 km and becomes negligible by D=60 km. This range agrees with the observed size distribution of rampart craters and suggested subsurface ice distribution (Clifford, S.M. JGR {\bf98}, 10937, 1993). This work is supported by NASA grant NAG5-8907.
The author(s) of this abstract have provided an email address for comments about the abstract: sstewart@gps.caltech.edu