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
The dramatic pictures released by NASA in December 2006, which suggested that water may have flowed on Mars during the last few years, have rekindled hope that the Red Planet harbours life.
They are also likely to spark controversy over whether Martian life would be the same as life on Earth, or whether it's radically different. The issue goes right to the heart of some of the biggest questions in science: what is life and how did it begin?
What we do know is life established itself on Earth surprisingly rapidly. Our planet was mercilessly pounded by giant asteroids until about 3.8 billion years ago, yet rocks from Western Australia as old as 3.5 billion years contain tantalising fossil evidence of sophisticated microbes (see "Life on Earth", Cosmos 14, p60). Many scientists have interpreted this speedy appearance as evidence that life came here from space.
About 20 years ago I began toying with the idea that life started on Mars and somehow found its way to Earth. Being a smaller planet, Mars would have cooled more quickly than Earth, and consequently, could have been ready for life sooner.
Today the Martian surface is a freeze-dried desert, but four billion years ago it would have been a very different story. Space probes suggest Mars was once warm and wet, with a thick atmosphere. Its low-density porous rocks, percolated by circulating fluids driven by volcanism, would have made an ideal biological incubator.
However, my theory that life originated on Mars contained a big hole. How could Martian organisms have possibly been conveyed to Earth? In 1987, while working at the University of Newcastle upon Tyne, England, I found the answer.
At the time, the university was hosting the 50th meeting of the Meteoritical Society, which conducts research on meteorites and other extraterrestrial materials. There I learned that a handful of weird meteorites may have come to Earth from Mars.
Jay Melosh of the University of Arizona, USA, convinced fellow scientists that large asteroid impacts would have had enough force to splatter Martian rocks around the Solar System. And some would be bound to fall on Earth. It was then a simple step to postulate that Martian microbes might have hitched a ride on this ejected material and thereby seeded our planet with life.
Cocooned inside rock, micro-organisms would be comfortably shielded from the harsh environment of interplanetary space. The cold, dry conditions could preserve bacteria for millions of years. And being so tiny, microbes could survive the massive g-forces that hurled them from the Martian surface.
Of course the same scenario works in reverse. Earth too gets struck by asteroids and comets. A fraction of ejected rocks will hit Mars, though in less abundance because of our planet's deeper gravity well.
Nevertheless, this two-way trade in material scotches the widespread belief that the planets are quarantined. It seems inevitable that viable terrestrial organisms will have reached Mars during the multi-billion year history of planetary bombardment, and vice versa.
When I began discussing these ideas in the 1990s they were greeted with scepticism and derision. At a scientific meeting in London in early 1996, a notable geologist even saw fit to ridicule me in his after-dinner speech.
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.
