Evidence for life in a martian meteorite?
The controversial hypothesis that the ALH84001 meteorite contains relics of ancient martian life has spurred new findings, but the question has not yet been resolved.  Organic matter probably results, at least in part, from terrestrial contamination by Antarctic ice meltwater.  The origin of nanophase magnetites and sulfides, suggested, on the basis of their sizes and morphologies, to be biogenic remains contested, as does the formation temperature of the carbonates that contain all of the cited evidence for life.  The reported nonfossils may be magnetite whiskers and platelets, probably grown from a vapor.  New observations, such as the possible presence of biofilms and shock metamorphic effects in the carbonates, have not yet been evaluated.  Regardless of the ultimate conclusion, this controversy continues to help define strategies and sharpen tools that will be required for a Mars exploration program focused on the search for life.
McSween
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Department of Geological Sciences, University of Tennessee, Knoxville 37996, USA.
D002254:Carbonates / Q000032:analysis* / Q000737:chemistry; D018559:Exobiology; D052203:Ferrosoferric Oxide; D005580:Fossils; D007501:Iron / Q000032:analysis* / Q000737:chemistry; D019369:Life; D018535:Mars; D019110:Meteoroids; D008854:Microscopy, Electron; D000069497:Origin of Life; D010087:Oxides / Q000032:analysis* / Q000737:chemistry; D011084:Polycyclic Aromatic Hydrocarbons / Q000032:analysis*; D013696:Temperature
D002254:Carbonates; D010087:Oxides; D011084:Polycyclic Aromatic Hydrocarbons; D007501:Iron; D052203:Ferrosoferric Oxide