Mini martians: Some believe these microscopic marks, found inside the Martian meteorite ALH84001, are fossil evidence of primitive life there more than 3.6 billion years ago.
Credit: NASA
Rescue came, however, in August that year, when then U.S. president Bill Clinton faced the world's media on the White House lawn and announced that NASA had evidence for life on Mars in the form of putative micro-fossils in a Martian meteorite found in Antarctica.
Though few scientists today believe the marks really are fossilised microbes, the episode raised awareness of the possibility that fossil organisms could make the journey from Mars to Earth. So maybe live ones could too.
Nevertheless, my theory still lacked any solid evidence, until fresh support came from a team of Russian scientists. Microbiologists have long been baffled by a group of bacteria that displays astonishing resistance to radiation.
One of these, Deinococcus radiodurans, was found living in the waste pools of nuclear reactors. So hardy are these microbes that no natural radiation environment on Earth is harsh enough to kill them. Yet they are clearly terrestrial organisms closely related to other forms of microbial life. So how did they acquire their remarkable resilience?
Many years ago the maverick British cosmologist Fred Hoyle and his co-worker Chandra Wickramasinghe suggested that Deinococus radiodurans had evolved in the high radiation environment of outer space.
But the Russian group proposed a more credible explanation. They subjected the common bacteria E. coli to repeated cycles of gamma radiation, and as they reported in the journal Astrobiology, they found that the organisms quickly developed some resistance.
The Russians pointed out that Mars offers a natural mechanism to mimic this experiment. Its thin atmosphere and lack of magnetic field exposes the surface to a high radiation flux.
Bacteria lying dormant in the permafrost would acquire a large accumulated dose of radiation, but could revive and evolve repair mechanisms during the brief episodes of warming brought about by the planet's fluctuating tilt.
Significantly, radiation-resistant bacteria can also tolerate desiccation and oxidising chemicals – mirroring precisely the conditions on the Martian surface.
The Russians conjectured that these traits were 'learned' on Mars – a theory that works equally well whether life started there and came to Earth in one or multiple inoculations, or whether terrestrial bacteria first journeyed to Mars and acquired their radiation resistance before a few of these were dispatched home.
Exciting though these developments are, they cloud the bigger philosophical issue of whether we are alone in the universe. If life could be shown to have started independently on both Earth and Mars, it would demonstrate that life emerges readily from non-life.
We might then expect the universe to be teeming with organisms. But if terrestrial and Martian biology has become thoroughly intermingled, we still won't know whether life is a freakish chemical accident that happened only once, or the almost inevitable product of a universe possessing inherently bio-friendly laws.
Paul Davies is the director of BEYOND, a 'cosmic think tank' at Arizona State University and a member of the Cosmos Editorial Advisory Board.
ALH84001
Is there anyone credible that still believes ALH84001 illustrates biological fossils?
I thought that puppy had been put to bed for all but the "Coast to Coast" people...
Alh84001 lives
If the detractors of the life-on-Mars hypothesis would bother to read the many papers written in support of the original Alh84001 hypothesis with an open mind (isn't that what real scientists are supposed to do?), and compare the arguments in favor of the hypothesis with those against it, they might just see that the evidence in favor of the hypothesis is strongly supportive. The thermal and shock decomposition theory for the origin of the magnetites (the only credible counter argument for these distinctive magnetites) has now been completely discredited (see a new paper on the topic online and soon to be published in Geochemica Cosmochemica Acta). The hypothesis by Paul Davis is a reasonable one and should be given careful consideration.