Tell me more about meteorites from Mars.

By Donald Goldsmith

Geologists conclude that this meteorite came from Mars from the fact that the details of its chemical composition, especially the relative amounts of different isotopes of the elements contained in the rock, are unlike those on Earth, but exactly match the measurements of Martian soil made by the Viking spacecraft that landed on Mars in 1976.

ALH 84001 is by far the oldest of the dozen meteorites from Mars that have been identified -- well over 4 billion years in age. After billions of years on Mars, the rock was blasted from the Martian surface by an impact of a much larger meteorite about 16 million years ago. The rock orbited the sun for millions of years, eventually collided with the Earth many thousand years ago, and then remained in Antarctica until its discovery in 1984. Similar meteorites almost certainly await discovery in the Arctic (and in fact all over the world); the trick is to recognize them as extraterrestrial. This is made easier in certain regions of Antarctica that have relatively few Earth rocks, and where the slow flow of ice tends to concentrate the rocks that do exist into areas that can be more easily examined.

The rock from Mars contains chemical and mineral compounds similar to those produced by some bacteria on Earth, as well as tiny cylindrical objects, with sizes less than one one-thousandth the thickness of a human hair, that resemble the shapes of living cells. This evidence that life once existed in a 4-billion-year-old Martian rock is intriguing but far from conclusive. The compounds can be produced by processes that do not involve life, and the cylindrical objects could also arise from non-biological chemical processes.

Scientists are now deeply engaged in further studies of pieces of ALH 84001 in an attempt to discover more evidence for or against ancient life on Mars. In those long-vanished eras, Mars had liquid water on its surface; today it has none whatsoever. If liquid water is essential for life, as some scientists believe, it seems quite possible that Mars once did have life on its surface but no longer does. Martian life might have become completely extinct, or perhaps some of its life forms might have managed to survive beneath the surface of Mars, in soil similar to that of the frozen Arctic tundra, where small amounts of ice in the soil occasionally become liquid.

If a rock from Mars contains ancient fossils--or possibly even tiny living organisms--could it infect living creatures on Earth? This seems highly unlikely, because we would expect differences between Martian life and Earthlife to be so large that organisms from one planet could not affect the other planet's life forms. But even a tiny possibility of such infection should not be ignored: When we do manage to collect samples of the Martian surface, we must be as careful as we can be to analyze them in a way that does not risk contamination, either of the Martian samples with Earthlife (which would make our results useless) or of Earthlife with the samples, which could possibly have dangerous results for life on Earth.

Most experts estimate the chances of infection from organisms that might exist in Martian meteorites as vanishingly small. These rocks have been striking Earth for millions of years without producing any noticeable effect on Earthlife. In addition, it is easy to theorize that since any Martian organisms would not have evolved to succeed in Earthlike conditions, but would still have to compete with our own bacteria, the danger from terrestrial organisms will always be much greater than those from Martian forms of life, if they exist and reach our planet.

Donald Goldsmith is an astronomer and science writer whose most recent books are The Hunt for Life on Mars and Worlds Unnumbered: The Search for Extrasolar Planets.

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