MARS is alive. For decades, we thought the Red Planet was dead, littered with extinct volcanoes. But now it is looking likely that they will erupt again sometime in the distant future.
When Martian volcanoes were first seen by NASA鈥檚 Viking probes in the 1970s, they caused great excitement among planetary scientists. But it was assumed the volcanoes were long extinct. 鈥淓verybody was happy with the idea that Mars was over the hill,鈥� says Lionel Wilson of Lancaster University, UK, who studied the data from these early missions.
Mars is a small planet, a tenth the size of Earth, and models suggested that it should have cooled into an inert, cold lump of rock during the 4.6 billion years since it formed. But earlier this year new data showed that the core of Mars is still molten (快猫短视频, 15 March, p 26). Now the idea that the planet ceased being active hundreds of millions of years ago is also being overturned as evidence emerges that eruptions happened within the past 10 million years.
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There are clear signs, for instance, that the Martian volcano Olympus Mons erupted recently, according to Karl Mitchell, who organised a conference on planetary volcanism at the Royal Astronomical Society in London this month. At more than 500 kilometres wide and 25 kilometres high, Olympus Mons is the biggest known volcano in the solar system 鈥� 100 times the volume of Earth鈥檚 largest, Mauna Loa in Hawaii. Its shallow sides are covered with thread-like rivers of frozen lava up to 200 kilometres long, but only a few tens of metres wide.
Clear images of these lava flows were taken for the first time by Mars Global Surveyor, which started orbiting the planet in 1997. The probe carries a camera that can resolve features on a scale of 1.5 metres, finer than the 8-metre resolution of the best images taken by the Viking missions.
Mitchell鈥檚 analysis of these pictures suggests the lava flows are much younger than anyone thought. The top lava layers lack the impact craters that would be expected if they were hundreds of millions of years old, and the lava flows are much sharper than those smoothed by time on other volcanoes, he told the conference.
He concludes that the lava could have emerged from Olympus Mons less than 10 million years ago. 鈥淭he realisation that there has been recent volcanic activity is slowly seeping into the scientific community,鈥� says John Bridges, an expert in Martian meteorites at the Open University in Milton Keynes, UK.
Wilson thinks the volcanoes are dormant rather than dead, and that they follow cyclical patterns of activity. The 10 volcanoes on Mars are so large that magma must flow into their chambers extremely quickly 鈥� any slower and the molten rock would solidify before the chamber was full. Given the rate that magma is produced within the planet, this could happen only once every 100 million years, Wilson says.
He estimates that the Martian volcanoes鈥� active periods last just 1 million years, meaning that they spend 99 per cent of their time dormant. That would explain why we have not seen any activity in the past few decades of observing the planet, he says.