Three years ago, a report in the journal Science, hinting at possible past life on Mars, made headlines and stimulated a debate about the nature and existence of life elsewhere in the Universe. But what would such life be like, were we to meet it? For Mars, at least, it is assumed that the most likely fossils will be of organisms resembling bacteria found on Earth. In a review of the subject in the Journal of Geophysical Research [25 July], Frances Westall of the NASA Johnson Space Center in Houston, Texas takes us on a comprehensive tour of ancient bacteria and the conditions in which they are known to fossilize on Earth. Her account should be a handy guide to would-be alien-spotters.
The fossil record of bacteria on Earth is abundant. Modern bacterial cells - which range in size from the order of 100 micrometres (thousands of a millimetre) down to as low as 100 nanometres (millionths of a millimetre) or - controversially - even smaller, and come in a wide variety of shapes, are preserved in rocks of all ages, almost back to the origins of the Earth itself. The oldest fossils are more than 3,400 million years old, and chemical traces of life have been picked up in rocks as old as 3,800 million: this compares with 4,500 million for the age of the Earth itself. Many researchers think that Mars was much warmer and wetter in its first few hundred million years, and life could have evolved there, too, as it did on Earth. But springtime on Mars was not met by glorious summer - sometime early in its history, Mars lost all but a wisp of its atmosphere and all its surface water, turning into a deep and sustained winter. Any life that would have evolved on Mars would have been simple but resilient - like the bacteria found today on Earth, in challenging environments such as hot springs, or buried deep in the crust.
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