A lump of rock from Mars is back in the news, with fresh claims that it contains proof of past life. If true, this unimpressive chunk of stone will become the most important in history, providing the first evidence of extra-terrestrial life.
Researchers say that the two kilogram rock, which reached Antarctica 13,000 years ago as a meteorite, contains chains of tiny magnetic crystals that could only have come from magnetotactic bacteria.
The claims, by a team of researchers from the US, Spain and Germany, are published this week in the Proceedings of the National Academy of Sciences.
Funding for the study came from the US National Science Foundation and the National Aeronautics and Space Administration.
This is not the first time NASA has made claims about life on Mars. About four years ago it announced research findings that "proved" that the Martian meteorite, known as ALH84001, contained fossil evidence for past bacterial life on Mars.
Subsequent research shot down those initial findings however, a negative view that was repeated by several independent research groups. Clearly NASA was not willing to give up the dream and this week presented its latest findings about ALH84001.
No one disputes the origins of the rock. It is ancient, at least 3.9 billion years old.
It was blasted off the face of Mars after a cataclysmic cometary or asteroid impact 16 million years ago and spent millions of years drifting in space. It eventually survived a plunge through our atmosphere to reach Antarctica's Allen Hills about 13,000 years ago. It was recovered from the ice during a 1984 NSF expedition.
One of the claims for life made by NASA in 1996 related to the presence in the rock of magnetite crystals, tiny natural magnets.
These crystals are found in modern magnetotactic bacteria, which use the magnetite like a miniature compass to navigate their environment.
This new claim also focuses on magnetite crystals. Dr Imre Friedmann of NASA's Ames Research Centre in California, who leads the research group, specialises in "seeing" into solid rock.
He found chains of magnetite crystals which he argues could only have been formed by once living organisms.
"The chains we discovered are of biological origin," he said. "Such a chain of magnets outside an organism would immediately collapse into a clump due to magnetic forces." Finding the crystals was itself an achievement, given they are no more than two millionths of a centimetre across.
The team also found that the chain-like formations are of similar size and shape and are "flexible", which they also interpret as evidence of biological origin.
A number of issues arise immediately, if the finding does indeed indicate ancient life. The two-kilo lump of rock contains large numbers of these chains, which suggest that such bacteria were widespread on the Martian surface.
This form of bacteria also required low levels of oxygen and needed a source for this to survive.
This would indicate that photosynthesising organisms of some form, green algae perhaps, must have been present and active on Mars 3.9 billion years ago to provide the oxygen.
No researcher rules out the possibility of the same type of life on Mars as existed on the Earth all those billions of years ago, given data coming from the Mars Global Surveyor satellite.
There seems little doubt that water flowed in plentiful supply on the Red Planet and where there is water there is the possibility of life as we know it.
This opens the intriguing idea that the magnetotatic bacteria we see today on Earth actually originated on Mars. The powerful impacts common in the solar system three and four billion years ago would have caused a "sharing" of planetary material between the two. Scientists have speculated that perhaps life started on Mars but continued on Earth, where conditions remained much more conducive to life.