If water exists beneath the Martian surface, why shouldn't life be present today on Mars?

Dr Everett K Gibson, Nasa Johnson Space Center

Their announcement is just the latest in a flurry of statements to come out of labs all supporting the idea that traces of past life can be found in rocks originating on the Red Planet.

On Tuesday, two papers appeared in the Proceedings of the National Academy of Sciences linking crystal structures in a meteorite known as ALH84001 to Martian bacterial activity.

These papers are supported by further work from Everett Gibson and his team which is published in Precambrian Research. This work argues there are traces of Martian microbes in two other meteorites.

However, it has to be said none of the research is likely to convince the sceptics.

Small spheres

Dr Gibson, a geochemist at the Nasa Johnson Space Center in Houston, says his team's new work centres on the Nakhla and Shergotty meteorites, which were blasted off Mars 1.3 billion and 175 million years ago respectively.

He says these rocks show the same evidence of microfossils seen in the 3.9bn-year-old ALH84001 meteorite, which is the subject of the PNAS papers and which formed the basis for the original, dramatic 1996 claims.

The structures seen in bacteria-containing Earth rocks...

"If the features observed in the two younger Martian meteorites are confirmed to have a biogenic origin, life may have existed on Mars from 3.9 billion years ago to as recently as 165 to 175 million years ago," Dr Gibson said.

The Gibson team believe that clusters of very small spheres found in the meteorites are very similar to those seen in bacteria-containing samples retrieved from deep beneath the Earth's surface in the Columbia River Basalts of eastern Washington State.

Whether or not these sphere-like structures are evidence of past life, they must be of Martian origin and not terrestrial contaminants because they are embedded in, or coated by, clays that certainly are of Martian origin.

Martian origin

Using an electron microscope, Gibson and his colleagues have also provided further evidence to back up the specific claims made for ALH84001.

Last year, the team published a paper in which they said tiny magnetite crystals, identical to those used by bacteria on Earth, were also present in ALH84001.

...are similar to those found on the Shergotty meteorite

Magnetite crystals can be produced chemically on Earth without the influence of life. But the team say the crystals produced by so-called magnetotactic bacteria are different: they are chemically pure and defect-free, with a distinct size and shape. What is more, the magnetotactic bacteria arrange these magnetite crystals in chains within their cells. All this has now been seen in ALH84001.

Additional studies of the Martian meteorites show that substantial portions of the hydrocarbons found in the rocks were in them when they left Mars and are not the result of terrestrial contamination.

Life debate

There is also strong evidence, they say, that most of the carbonates in all three meteorites were formed at a time when Mars was warmer and wetter - an environment much more conducive to life than the current surface on the Red Planet.

Dr Gibson says: "If water exists beneath the Martian surface, why shouldn't life be present today on Mars?"

Many scientists dismissed the original claims of past life in Mars rock ALH84001, and continue to express doubt in the face of the latest claims. They argue that evidence is needed to show that the structures seen in the meteorites could not have had a simple chemical origin.

In essence, the critics are asking for the proof of a negative - something which is almost impossible to do.

All Dr Gibson and like-minded researchers can do is to continue to present their evidence. The debate about life on Mars will not go away.