NASA

NASA rover Curiosity is beavering away up on Mars, examining rocks, drilling holes, checking out the weather -- but it's not just up there to look at the planet's hospitability for humans. It's also looking for conditions favourable for life; not now, but in the past, when Mars may have been home to extraterrestrial microbes.

But maybe the answer is right here on Earth, after all -- in the form of a meteorite.

Tissint landed in the desert of Guelmim-Es Semara, Morocco, on July 18, 2011. It was thrown from the surface of Mars by an asteroid collision some 700,000 years ago -- and there is no other meteorite quite like it. The 7-11 kilogram grey rock -- seared glassy black on the outside by the heat of entry, called a fusion crust -- showed evidence of water. It was riddled with tiny fissures, into which water had deposited material.

This material, on analysis, turned out to be an organic carbon compound -- one that was biological in origin. It is not the only meteorite in which organic carbon has been found, but the debate has always centered on whether the carbon was deposited before or after the meteorite in question landed on Earth -- to wit, whether it is terrestrial or extraterrestrial in origin.

A team of researchers studied the organic carbon found in the fissures of Tissint and determined that it is not of this world.

There are several points of evidence put forward by the team. First, there was a relatively short timeframe between when the meteorite was observed falling to Earth and when it was collected.

The second is that the microscopic fissures in the rock would have had to have been produced by a sudden high heat -- such as, for example, the heat of atmospheric entry. This shock, and the temperatures required to open the fissures, could not have come from the Moroccan desert.

Thirdly, some of the carbon grains inside Tissint had hardened into diamond. There are no known conditions under which this could have occurred on the surface of the Moroccan desert -- and certainly not in the time it took between the meteorite's fall and discovery.

Alain Herzog/École polytechnique fédérale de Lausanne

Fourthly, the carbon contains a high amount of deuterium, heavy hydrogen with one proton and one neutron in its nucleus -- consistent with the composition of Mars geology. "Such an enormous concentration of deuterium is the typical 'finger print' of Martian rocks as we know already from previous measurements," study co-author Professor Ahmed El Goresy of the University of Bayreuth, Germany, said.

These points are supported by the nanoscale secondary ion mass spectroscopy data. This reveals that the material was significantly depleted of carbon isotope 13C, compared to the level of 13C in the carbon dioxide of Mars' atmosphere as measured by Phoenix and Curiosity. This difference was consistent with the levels found on Earth between the atmosphere and a piece of coal -- which is biological in origin.

While the case looks strong, though, it would be a mistake to consider the evidence conclusive just yet, cautioned Yangting Lin, the study's senior author and professor at the Institute of Geology and Geophysics of the Chinese Academy of Sciences in Beijing.

"We cannot and do not want to entirely exclude the possibility that organic carbon within Tissint may be of abiotic origin," Lin wrote, meaning the carbon maybe physical in origin rather than organic -- devoid of life.

"It could be possible that the organic carbon originated from impacts of carbonaceous chondrite meteorites. However, it is not easy to conceive by which processes chondritic carbon could have been selectively extracted from the impacting carbonaceous chondrites, selectively removed from the soil and later impregnated in the extremely fine rock veins."

The full study, "NanoSIMS analysis of organic carbon from the Tissint Martian meteorite: Evidence for the past existence of subsurface organic-bearing fluids on Mars", can be found online in the journal Meteoritics & Planetary Science.