A GLIMPSE OF MARTIAN HABITABILITY FROM METEORITE ALLAN HILLS (ALH) 84001 (Invited Presentation)

The martian meteorite ALH 84001 formed before ~4.0 Ga, and so can preserve information about martian habitability from then until nearly the present. ALH 84001 hosts several putative tracers or markers of ancient martian life, all of which have been shown to be incorrectly interpreted or ambiguous. However, absence of habitation does not equal absence of habitability! ALH 84001 is particularly important because several of these putative tracers or markers were deposited from liquid water, and thus suggest potentially habitable environments. ALH 84001 does provide evidence for three distinct episodes of potentially habitable environments on early Mars. First is Sr-isotopic evidence that the meteorite’s precursors interacted with clay-rich material, which formed approximately at 4.2 Ga. Second, is that igneous olivine crystals in ALH 84001 were partially dissolved and removed, presumably by liquid water. Third is, of course, the deposition of the carbonate globules (and associated minerals), from near-neutral to alkaline waters at ~15-25°C. The environments of olivine dissolution and carbonate deposition are not known precisely; hydrothermal and soil environments are possible. By analogies with similar alteration minerals and sequences in the nakhlite martian meteorites and in volcanic rocks from Spitzbergen (Norway), I favor deposition of the ALH 84001 carbonate globules (etc.) in a hydrothermal environment. The hydrothermal system could reasonably have been initiated by a meteoroid impact event (at ~3.95 Ga). ALH 84001 preserves no evidence of subsequent habitable environments, i.e. interaction with water. The meteorite contains severely strained pyroxene and plagioclase-composition glass (formed by impact shock at ~3.9 Ga) that would have reacted rapidly to form hydrous silicates, had the rock been in contact with liquid water for any length of time. ALH 84001 represents only a single point on Mars, and its lack of evidence for recent habitable environments does not mean that none existed anywhere on Mars. Even so, any model of Mars’ environment and habitability must include places where ALH 84001 could have been cold and dry for the last 3.9 Ga.