QUESTION:
What was it about the Martian meteorites that made scientists think to look
for tiny cells in them?

ANSWER from Mary Urquhart on November 6, 1997:
The possible "fossils" of bacteria similar to the tiny "nanobacteria" some
biologists believe exist on the Earth is just one of the pieces of evidence
that life may once have existed in the martian meteorite ALH84001.  The
other evidence discovered by the team led by David McKay in their original
announcement includes:

* The presence of polycyclic aromatic hydrocarbons (PAHs) which can be
produced by the decay of life,

* globules of carbonates with a layered structure similar to stuctures
produced by bacteria on the Earth, and

* grains of minerals usually not found togther (magnetite and pyrrhotite)
unless deposited by living organisms.

I don't know in what order the discoveries of each piece of evidence were
made.  The other pieces of evidence for possible ancient life may have
caused the team to look more closely at ALH84001 than they would otherwise
have, or the detailed examination necessary to find the "microfossils" may
have been planned all along.  I don't know which is correct. I am sure,
however, that the discovery of such tiny "microfossils" was unexpected.

You can find out more about the topic of "Life on Mars?" at:
http://nssdc.gsfc.nasa.gov/planetary/marslife.html