IMPACT DISPERSAL AND WEATHERING OF SULFIDES ON EARLY MARS: A SOURCE OF ACID SULFATES AND A POSSIBLE HABITAT FOR MICROBES

Much of the surface of Mars appears to be enriched in sulfate minerals, and much is heavily cratered. These observations may be related to each other (cf. Knauth et al., 2005), especially as regards the cross-bedded, sulfate-rich deposits discovered by the Opportunity Rover in Meridiani Planum and local Fe-sulfate enrichments discovered by the Spirit Rover in Gusev Crater. This impact excavation hypothesis represents an updating of the original “gossan hypothesis” of the late Roger Burns of MIT. Burns (in 5 papers, 1986-1990) proposed that the color and mineralogy of Mars might be related to the weathering of Fe-sulfides in martian igneous rocks. He noted that martian rocks were at least 2x richer in Fe than terrestrial equivalents, and therefore also richer in cumulate Fe,Ni- sulfides (immiscible droplets that “rain out” of magma). Weathering of such sulfide deposits on Earth typically produces Mars-colored, acid sulfate-rich gossans (rusty areas sought by prospectors). In discussing gossans, Burns never considered Mars impact cratering (or sulfide shattering and dispersal). We suggest that a terrestrial analog for Mars sulfate formation might be large open-pit sulfide mines (analogous to impact craters), surrounding mine dumps (analogous to impact-excavated breccias), and associated sandy, layered mill tailings (analogous to distal impact surge and fall deposits, presumed to be widespread on Mars: Burt et al., 2005 GSA). This simple hypothesis (impact excavation of readily-shattered sulfides, followed by in situ oxidation to acid sulfates) provides an alternative to competing suggestions involving planet-wide volcanogenic sulfuric acid mists, acid lakes and streams, or acid groundwaters, proposed by others to have formed acid sulfates (such as jarosite) on Mars. By the impact hypothesis, the surface of early Mars dried up and froze down while impacting continued. Impacts into a partly frozen, salty, locally sulfide-rich regolith, followed by moist weathering, produced all of the features seen at Meridiani and probably in many layered deposits elsewhere on Mars. Mars sediments are then analogous to weathered S-rich mine waste, but with the mining and milling having been performed by impacts. On Mars as in mine waste, did intense microbial activity accompany sulfide oxidation?