Impact melts and glasses on Mars

[1] Dark mobile materials on Mars are widely considered to be volcanically derived windblown particulates. Such a conclusion is consistent with the large volcanic edifices on Mars and the assumption that a volatile-rich crust would not favor significant impact melt generation. Several post-Viking discoveries, however, provide new perspectives for reconsidering alternative origins not only for some of the mobile dark veneers but also certain materials at the Pathfinder and Viking landing sites. First, certain dark materials on Mars do not exhibit the expected mafic absorption features in their reflectance spectra. Second, the Pathfinder mission revealed unexpected feldspathic compositions of vesicular blocks that lead to a preliminary conclusion that they represent andesites or basaltic andesites, rather than basalts. Third, recent discoveries of Holocene to Miocene impacts in Argentine loessoid deposits provide new insights for the nature of melts generated from eolian sediments. Fourth, global dispersal of impact melt products are now recognized in the terrestrial record throughout the Cenozoic. Consequently, the generation and preservation of impact glass on Mars should be reconsidered. Possible occurrences include possible tektite-like strewnfields downrange from recent oblique impacts, dark avalanches emerging from backwasting cliffs of easily eroded unconformable deposits, global dispersal of glassy ejecta from Hesperian and Amazonian craters of Chicxulub scale, and dark mobile veneers emerging from Noachian craters. We propose that the mobile dark materials comprising many of the named features of Mars as well as some blocks at the Pathfinder/Viking landing sites may represent impact-melt products.