Microtektites on Mars: Volume and Texture of Distal Impact Ejecta Deposits

Microtektites, small blobs of ejecta formed in the shock melt and vapor plume of an impact, can be dispersed far from the source crater only if the impact is violent enough for the ejecta plume to pierce the atmosphere; they are therefore formed in far smaller (and more numerous) impact events on Mars than on Venus and Earth, which have thicker atmospheres. Microtektite abundances from the Chicxulub and Bosumtwi craters on Earth suggest that the volume of this material is [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], with [D.sub.c] the crater diameter in kilometers, similar to the observed volumes of the dark parabolic ejecta deposits on Venus. Corresponding volumes on Mars are ~2.5 x smaller, but even so this result implies that even only a 15-km crater can produce a layer of microtektites with a global average thickness on Mars of 40 microtektites per square centimeter. I use a trajectory code and a thermal model to show that these particles are easily dispersed globally on Mars and that micrometeoroids of the same size will be unmelted by reentry heating. The uniform size and glassy texture of microtektites may allow such ejecta layers to be identified by the remote arm cameras on Mars landers, particularly in the polar layered terrain where they may be preserved against abrasion. Key Words: Impact Processes; Mars, Surface; Mars, Atmosphere; Meteorites.