Extraterrestrial life in the solar system

The possibility that life began on Mars as well as Earth has long been entertained. Evidence that there was once liquid water on Mars, that the atmosphere was once denser and the climate warmer, has made many scientists take the idea seriously. Mars is almost certainly dead now - its tenuous atmosphere shows chemical equilibrium, no Gaia effect - but there might have been life there long ago.

In 1996, a meteorite found in Antarctica was heralded as evidence that life had existed on Mars at around the time it began on Earth. The meteorite, weighing less than two kilograms, had cracks containing carbonate deposits, within which were embedded tiny hair-like structures rich in complicated hydrocarbons. These hydrocarbon-rich structures had apparently formed before the rock left Mars. Were they bacteria?

The meteorite bore the name ALH84001, indicating that it was the first such meteorite (001) to have been found in the Allan Hills area of Antarctica (ALH) in 1984. It was known to have originated on a planet because it was mainly igneous; planets have volcanoes but asteroids and comets do not. There was compelling evidence that it came from Mars rather than any other planet. For example, the nitrogen, argon and carbon dioxide contents of gas bubbles trapped in the rock closely resembled those in the Martian atmosphere, and the rock itself contained iron disulphide, quite a hallmark of

Martian origin. However, some media reports in 1996 gave confused accounts of dating, obscuring the arguments about the meteorite.

Three dates are relevant: the formation of the rock; the formation of the carbonate deposits that caused all the excitement; and the ejection of the rock from Mars in meteorite form. Rubidium isotope decay evidence showed that the rock itself dated from some 4.5 thousand million years ago, when the Solar System planets were forming. So ALH84001 was a piece of the original stuff of Mars. Potassium-argon dating showed that the cracks in the meteorite, possibly consequences of a comet impact on the young Mars, were around four thousand million years old. The carbonate deposits in these cracks were formed considerably later, no more than 3.5 thousand million years ago. (This figure was imprecise; the carbonate might be only half or even a third of that age.) To determine the ejection date, investigators took advantage of the fact that cosmic radiation would have produced carbon-14 while the rock was in interplanetary space. When the rock reached the Earth this production would have stopped and the carbon isotope would have begun to decay. Carbon dating indicated that ALH84001 left Mars only about 13-14,000 years ago.

The hair-like formations in the carbonate deposits appeared in the electron microscope as strings of tiny beads about 25 nanometres across. They were surrounded by iron sulphide and magnetite deposits, which are characteristic of the activities of some prokaryotes on Earth. They contained organic material, mainly polyaromatic hydrocarbons. These components were not the results of terrestrial contamination: their concentrations were greater in the middle of the meteorite than at the periphery. (If they had been contaminants, the higher concentrations would have been on the outside not the inside.) The suggestion that the 25-nanometre particles might be the remains of tiny bacteria, evidence of ancient life on Mars, made media headlines around the world.

They were not fossil bacteria, because they were well below the minimum size of a living cell - around 100 times smaller than a prokaryote (unless, of course, life on Mars was organised quite differently from ours). Moreover, the carbonate deposition seems to have taken place at a temperature around 650oC, incompatible with the survival of even the toughest archaea. This might seem disappointing, but the organic deposits are still interesting. The presence of complicated hydrocarbons on primitive Mars is exciting in itself, particularly when we recall Gold's idea about the origins of oil and gas on Earth (see chapter 14). But the rumours were premature. There might once have been life on Mars, but ALH84001 was not evidence of it.

The search for signs of ancient life on Mars is still worth pursuing. It might throw light on the origins of the ALH84001 hydrocarbons, and that would be informative. There are also plans to send probes to Europa, Jupiter's largest moon, which might contain liquid water - the surface is covered with thick sheets of water ice rich in organic materials, and the lower depths might be heated and melted by volcanoes. The presence of water and organic material suggests that life with much the same chemistry as ours might have originated on Europa.

If either the Mars or the Europa quest is successful then some of our judgements about the origin of life on Earth might need to be revised. For example, in chapter 14 we considered the possibility that terrestrial life was imported by meteorite, having begun on another planet. We dismissed this idea is implausible - and as unhelpful, because it fails to address the "origin" problem. However, some authors have argued in favour of the idea, and optimistic interpretations of the ALH84001 evidence were recruited in their support. If clear evidence of life on Mars, Europa or elsewhere is ever found, we might be obliged to reconsider our position on this topic; but at present, we are not35.

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