The recent evidence from the Martian meteorite found in Antarctica that life once existed on the Red Planet took the world by storm. On-going investigation has challenged the original interpretation of the data, claiming that the "biological" features seen in the meteorite were produced by non-living physical and chemical processes.
But the whole debate about life on Mars has received a new boost with the recent discovery that micro-organisms can flourish on Earth under extremely inhospitable conditions indeed. This has vastly extended our conceptions of the range of conditions on alien worlds capable of harbouring life.
First let me recap on the Martian meteorite story. The rock was recovered from Antarctic ice in 1984. Analysis of gas bubbles in the rock strongly indicates that it came from Mars. It was dislodged from its parent planet about 16 million years ago, wandered in space and, eventually, crashed to Earth 13,000 years ago on to the Alan Hills region of Antarctica.
But the really interesting part of the story is that further analysis of the meteorite apparently indicated clear fingerprints of life.
The Viking missions to Mars in the 1970s showed reasonably clearly that life as we know it does not and cannot live in the surface soil of that planet. But Martian surface and atmospheric conditions were much more hospitable for life in the distant past. There is good evidence the surface once carried running water.
So the explanation proposed to explain the meteorite evidence was that life arose on Mars billions of years ago and left fossilised evidence in the Martian rock, a piece of which found its way to Earth.
David McKay led the research team that announced the evidence for life in the Martian rock in 1996.
The principal evidence was that the meteorite contained: (a) carbonate minerals similar to deposits associated with biological life on Earth; (b) grains of the mineral magnetite similar to those seen in some terrestrial bacteria; (c) organic molecules that may be the breakdown products of biological molecules; (d) tiny rod-like structures interpreted as fossilised bacteria.
No single piece of evidence in itself is proof of biological activity but, taken together, they constitute strong circumstantial evidence that the rock once harboured life.
Carbonate minerals can form on Earth as a result of bacterial activity. But carbonate minerals can also form under circumstances where no biological activity is involved, e.g. inside the Earth at temperatures of hundreds of degrees Fahrenheit.
Magnetite crystals are found in a variety of life on Earth, from whales to homing pigeons to bacteria. In the two former cases the magnetite is believed to act as a compass, allowing the animals to navigate along the Earth's magnetic field. When magnetite-containing bacteria die, the crystals can accumulate in sediments.
But recent studies have shown that some of the magnetite crystals in the Martian meteorite contain features that form only at temperatures of hundreds of degrees, a temperature completely incompatible with life.
The complex organic molecules in the rock are called polycyclic aromatic hydrocarbons (PAHs). Small amounts of such molecules are found all over the Earth. They are often formed by breakdown of biological molecules, e.g. burning or decomposition of plant matter. Coal and oil are formed from plant remains and naturally contain PAHs. When coal and oil is burned these PAHs are released.
But PAHs are also found in carbonaceous meteorites formed in the outer solar system and nobody has ever suggested that they were produced by biological activity. Also, recent analysis of another Martian meteorite found in Antarctica discovered PAHs similar to those found in the Alan Hills meteorite.
This second Martian meteorite is younger than the Alan Hills meteorite and was formed long after liquid water flowed on Mars and therefore long after life was possible on the surface of the planet. PAHs are therefore not a very reliable indicator of biological activity.
What about the apparent fossilised bacteria in the Alan Hills rock visualised using the scanning electron microscope? If these are images of bacteria, they are tiny (1,000 times smaller) compared to any known bacteria on Earth today.
Perhaps the structures in the meteorite are fossilised remains of an early form of life when the cells were very small. Or perhaps such small bacteria are present today in rocks on Earth but have yet to be discovered. However, recent work has shown magnetite crystals similar in size and shape to McKay's "microfossils".
It may well turn out that all the evidence for life brought forward by McKay's team will soon be shown to be in serious error. If so, this would not be the first time that evidence of life in a meteorite failed to stand up.
Over 100 years ago a meteorite fell in Orgueil, France. In 1961 scientists announced that the rock contained complex hydrocarbons and fossilised bacteria. After further investigation, however, the organic chemicals and fossils turned out to be ragweed pollen and furnace ash.
But, even if the evidence for life in the Martian meteorite is entirely overturned, interest in the possibility of life on Mars will continue unabated. In my last article I described how science has recently discovered that bacterial life exists deep beneath the surface of the Earth, living at high pressure and temperatures up to 110C and with no direct dependence on sunlight.
Therefore, if life did manage to begin on Mars billions of years ago, although conditions on that planet now preclude the possibility of life in surface soil, bacterial life may still persist deep beneath the surface. Plans are being prepared to investigate this possibility.