MARS, it seems, has been bitterly cold for most of its 4.6-billion-year history. This means that liquid water could never have existed for long on the surface, lowering the chances of life taking hold on the Red Planet.
Mars has channels and rocks that appear to have been eroded by liquid water, so most planetary scientists think that there must have been times when the planet鈥檚 surface temperature climbed above freezing. To find out just how long these periods lasted, David Shuster of the California Institute of Technology in Pasadena and Ben Weiss of the Massachusetts Institute of Technology examined previously published data on the ratio of two particular isotopes in meteorites from Mars found on Earth. Radioactive potassium-40 is a solid that decays into the gas argon-40, which then diffuses very slowly out of the rock. But it does so in a way that depends on temperature: the higher the temperature, the faster it leaks out.
They deduced that the oldest meteorite studied could not have been warmer than 0 掳C for more than a million years in total since it formed 3.5 billion years ago (Science, vol 309, p 594). So while surface water could have existed in liquid form for short periods, 鈥淢ars may have just cooled off too quickly [for life to evolve]鈥�, says Weiss.
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