Paul Davies: How to find out if we are all Martians

From the Faraday Award Lecture at the Royal Society, given by the Professor of Natural Philosophy at Macquarie University, Australia

To studydeep-living Martian microbes properly, it will be necessary to probe beneath the surface. Rock samples could give clues about past life on Mars; the best evidence would be the discovery of microfossils. Such advances should enable the Martian surface material to be studied in great depth. Nevertheless, not too much hope should be pinned on finding Mars rocks containing 3.5 billion-year-old fossils. On Earth there are only a handful of places where fossils of this age are found. The chances of an unmanned probe grabbing a Mars rock with fossils are slim.

But if we do find life on Mars, the significance of the discovery will hinge on whether Martian life is the same as terrestrial life. This is important, given the possibility that Mars and Earth may have cross-contaminated each other. It is possible that life started on one planet and spread to the other. In this respect, Mars looks the more favourable for life to have begun. Smaller than Earth, it cooled more quickly and may have been ready for life as long ago as 4.4 billion years; Earth may not have been habitable until 3.9 billion years ago.

Astrobiologists are agreed that the chances of finding any type of life in the solar system more advanced than simple bacteria are very slim. Complex life-forms of the sort that occur on Earth would probably require a planet very much like Earth. The search for earth-like planets in other star systems will be a major priority in the next few decades.