In August 1996, a group of scientists published their claim that life may have existed on Mars at one time. (David S. McKay et al. 1996 [Aug. 16]. Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001. Science Vol. 273, pp. 924-930.)
This claim is based on study of a potato-sized rock found in Antarctica (on Earth!), designated ALH84001 (discovered in the Allan Hills area in 1984).
This rock is alleged:
(1) to have been formed 4.5 billion years ago on Mars;
(2) to have been blasted into space by a meteorite impact about 1.5 million years ago;
(3) to have landed on Earth 13,000 years ago;
(4) to contain evidence that life existed on Mars.
(1) tube-like carbonate globules that superficially resemble the shape of bacteria;
(2) the presence of molecules called polycyclic aromatic hydrocarbons (PAHs), which are sometimes produced (on Earth) by the decay of microbes;
(3) the presence of the minerals magnetite and iron sulfide, which are produced (on Earth) by the metabolic activities of certain bacteria.
(1) no trace of cell walls or any remnant of cell parts were found in the rock.
(2) the carbonate globules are much smaller than the smallest known Earth bacteria.
(3) analysis of the sulfur and carbon isotope ratios in the rock has produced no evidence of biological activity.
(4) PAHs are common both among interplanetary dust particles and on Earth (e.g., in coal tar). So the PAHs in the rock could have come from elsewhere to Mars, or from terrestrial contaminants, not necessarily from Mars itself. In any case, PAHs are not proof of life.
(5) it is questionable whether a meteorite impact on Mars could have blasted a rock out of the pull of Mars gravity.
(6) the magnetite crystals in the rock appear unlike those known to be produced by Earth bacteria.
(7) its interesting that in August 1996, NASA desperately needed new funding and the Mars rock announcement brought them new funding immediately!