Widespread outcrops on Mars of a green rock called olivine suggest the planet has long been too dry and cold for life to fl ourish on its surface. A blend of iron and magnesium silicates, olivine is...

Widespread outcrops on Mars of a green rock called olivine suggest the planet has long been too dry and cold for life to fl ourish on its surface. A blend of iron and magnesium silicates, olivine is found inside some volcanic deposits on Earth, but it does not last long once it has been exposed. In warm, moist environments it starts weathering in months, says Roger Clark of the US Geological Survey in Boulder, Colorado. Yet spectroscopic data shows that olivine covers over 2.5 million square kilometres on Mars, including some ancient regions that are heavily cratered or eroded, he told a meeting of the American Astronomical Society last month in Pasadena, California. That means that Mars cannot have been warm and moist since the olivine was deposited about 1 to 3 billion years ago. (‘Parched planet’, New Scientist, 11 November 2000, p. 31)