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.
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.