A Martian Meteorite and the Search for Life on Mars

G. Jeffrey Taylor
Director, Hawaii Space Grant College
Hawaii Institute of Geophysics and Planetology

In August 1996, a group of scientists led by Dave McKay (Johnson Space
Center) announced that they had found evidence for microscopic fossil
life in a martian meteorite, ALH84001. The announcement stunned the
world and brought instant fame to the group. Scientists around the world
embraced the idea with as much enthusiasm as they had for the face on
Mars, and set out to test it ("test," of course, means "to prove it
wrong"). This talk tells the story of the discovery and the evidence
assembled by McKay and his colleagues, and how that evidence has been
whittled away by subsequent investigations. The evidence and current
status: (1) The bacteria-like shapes appear to be mostly
crystallographic features, not tiny fossils. (2) The organic compounds
in the rock appear to be contaminants, though some of that contamination
occurred on Mars, showing that at least some organic compounds exist
there. However, the organic compounds do not demonstrate the existence
of life in the rock. (3) The fossil-like objects reside in carbonate
globules. McKay and colleagues argued that the carbonates formed at low
temperature, suitable for the origin of life. There is no consensus on
the precise temperature, though most interpretations point to a
low-temperature origin originally. This does not mean that life existed
in the rock, however, as some of the low-temperature values are too hot
for life to have existed, >150oC. In addition, one or more impacts have
blasted the rock; these events have redistributed the carbonate, further
obscuring the record. (4) The final line of evidence was the presence of
small grains of magnetite (Fe3O4), which are used as biomarkers on
Earth. About a quarter of the grains are similar to those produced by
magnetotactic bacterial on Earth: right size and aspect ratio,
apparently pure Fe3O4, free of crystallographic defects. On the other
hand, we do not know that such magnetite can form only by biological
processes, and some experiments hint at a non-biological origin. In
addition, recent evidence indicates that tiny grains of MgO formed
inside Mg-rich carbonate regions. This gives credence to the idea that
magnetite formed by thermal decomposition of the iron-rich carbonates.
Thus, of the four lines of evidence, only the magnetite argument holds
up at all, and it's a bit shakey. McKay et al. stated in their famous
paper in Science, "Although there are alternative explanations for each
of these phenomena taken individually, when they are considered
collectively...we conclude that they are evidence for primitive life on
early Mars." Since three of the four lines of evidence are now defunct,
their interpretation is weakened considerably. Nevertheless, they are
not deterred and continue to defend their interpretation