© NASAIn 1996, researchers suggested that microscopic rod-like structures in a Martian meteorite called ALH84001 were the fossilised remains of tiny bacteria. Although most researchers now doubt the claim, since the structures can be made inorganically, a new study suggests the rock was once bathed in water at the right temperature to support life as we know it.
A 1996 claim of fossilised microbes in a meteorite from Mars has yet to be confirmed, but a new analysis does suggest the rock's Martian environment had the conditions conducive to life.

Researchers led by David McKay of NASA's Johnson Space Center in Houston, Texas, caused a sensation 13 years ago when they proposed that a chunk of Mars rock found in Antarctica, called ALH 84001, contained possible signs of past life on the Red Planet, including complex carbon-based molecules and some microscopic objects shaped like bacteria.

But the claim was never widely accepted. Other scientists countered that the shapes were ambiguous and that the complex carbon-based molecules could have been produced without life, since they are also found in chunks of asteroids that fall to Earth as meteorites, for example.

And some argued that the carbon in the meteorite could have been deposited in very harsh conditions, involving water at more than 150 °C. Even the hardiest known terrestrial microbes die above about 120 °C.

But a new analysis suggests the water involved was cool enough to allow for life, which at least keeps open the possibility of fossilised life in the meteorite. The study was led by Paul Niles of NASA Johnson. Neither he nor any of the other team members were part of the 1996 life claim.

Bubbling up

To explain deposits of minerals containing calcium, magnesium, and iron, in the rock, Niles and his colleagues suggest the rock was sitting at or near the surface of Mars, with water rich in carbon dioxide bubbling up to the surface in the area from deep underground, perhaps as part of a hot spring.

The relative amounts of the three metals deposited from solution depend on the temperature of the water they were dissolved in. The team used previous measurements of these amounts to calculate a water temperature of less than 100 °C. This was not a certainty beforehand, since water can remain liquid above that temperature at the higher pressures underground.

"These minerals were formed in what is very likely to have been a habitable environment," Niles says.

The study shows there is still more to learn from what is "probably the single most examined rock in all of human history," says Marc Fries of NASA's Jet Propulsion Laboratory in Pasadena, California, who has previously examined samples of the meteorite but was not a member of Niles's team.

But there is still no proof of any past Martian life in the rock, he and Niles agree. Determining whether Mars ever harboured life may require a mission to bring back rock samples from the planet, he says.

Journal reference: Earth and Planetary Science Letters (DOI: link)