A Martian meteorite is said to hold fossils
Bill Clinton stood in front of the cameras for a 1.9 kg rock from Antarctica. Within five years every line of evidence for Martian microbes had a non-biological answer.
ALH84001 was collected from the Allan Hills ice field in Antarctica on 27 December 1984: a 1.93-kilogram lump of greenish rock, misfiled as an ordinary asteroid fragment and left in a Houston curation cabinet. Reclassification came in 1993, when its oxygen isotopes and mineral chemistry marked it as Martian. It crystallised roughly 4.09 billion years ago, older than every other Martian meteorite but the breccia NWA 7034, whose clasts date to about 4.4 billion years.
On 7 August 1996 a NASA team led by David McKay at Johnson Space Center announced four lines of evidence for past biological activity: carbonate globules of the kind microbial colonies leave behind; polycyclic aromatic hydrocarbons on the carbonate-rich fracture surfaces; magnetite and iron sulfide grains resembling the crystals magnetotactic bacteria make; and segmented ovoid and rod shapes 20 to 100 nanometres long. President Clinton spoke from the South Lawn that day; the paper ran in Science nine days later.
The ovoids are an order of magnitude smaller than any known free-living cell, the smallest no bigger than a ribosome, and in 1997 Bradley, Harvey and McSween argued in Nature that most of the forms were growth steps on pyroxene and carbonate crystals, segmented only by the metal coating sputtered on to make the sample conduct under the electron beam. PAHs occur in carbonaceous meteorites with no biology near them, and in Antarctic meltwater; the carbonates could precipitate from warm fluid circulating through hot rock.
The magnetite argument held longest. Kathie Thomas-Keprta's group maintained that about a quarter of the crystals were chemically pure, defect-free and truncated hexa-octahedral, a form they held only biology produced. Peter Buseck's group at Arizona State answered in 2001 that crystal shape could not be mapped reliably at 40 to 100 nanometres with the instruments then available, and that bacterial crystal shapes vary far more than assumed.
The announcement led to the NASA Astrobiology Institute, founded in 1998. McKay defended the biogenic reading until his death in 2013.