Microtektites on Mars: Volume and texture of distal impact ejecta deposits

Microtektites, small blobs of ejecta formed in the shock melt and vapor plu me of an impact, can be dispersed far from the source crater only if the im pact is violent enough for the ejecta plume to pierce the atmosphere; they are therefore formed in far smaller (and more numerous) impact events on Ma rs than on Venus and Earth, which have thicker atmospheres. Microtektite ab undances from the Chicxulub and Bosumtwi craters on Earth suggest that the volume of this material is similar to 5 x 10(-5)D(c)(3.74) km(3), with D-c the crater diameter in kilometers, similar to the observed volumes of the d ark parabolic ejecta deposits on Venus. Corresponding volumes on Mars are s imilar to 2.5 x smaller, but even so this result implies that even only a 1 5-km crater can produce a layer of microtektites with a global average thic kness 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 unmelte d 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 p reserved against abrasion. (C) 2000 academic Press.