The hills may not be alive, but they are moving. The comet 67P/Churyumov-Gerasimenko has small cliffs that migrate across the landscape for months at a time.
The European Space Agency鈥檚 Rosetta spacecraft arrived at 67P in 2014 and orbited the comet for two years. The pictures it took revealed a changing landscape, especially when the comet聽reached its closest approach to the sun, and聽jets of dust and gas were seen shooting off the surface into space.
The jets form when sunlight hits the surface and turns ice into gas, which floats away along with any dust that was mixed in with the frost. Now Sam Birch at Cornell University in New York has found that Rosetta images show聽a similar process is causing small cliffs on the comet鈥檚 surface to move.
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The cliffs, or scarps, in question are only between 1 and 2 metres tall, but on a comet the size of 67P, which is just聽4 kilometres across at its longest point, they aren鈥檛 negligible 鈥� cliffs of a similar scale on Earth would be about 3 kilometres high. Birch and his colleagues found that when the comet was relatively close to the sun, the scarps moved at a rate of between 3 and 7 centimetres an hour.
This seasonal process means that every time 67P is near the sun聽鈥� for about two months every 6.5 years 鈥� its landscapes dramatically change.
The big surprise was that the cliffs were moving towards, not away from, the sun. 鈥淭he sun is shining down on these features, but they鈥檙e going the wrong way,鈥� said Birch, when he presented this work at the Lunar and Planetary Science conference in Texas on 21 March. This is unlike other landscapes in the solar system, where聽features such as ice or snow dunes move because of sunlight.
The team found that this backwards movement was聽probably because the scarps are聽located next to a much larger cliff face. This feature, called Hathor cliff, absorbs heat from the sun then radiates it out again, warming the聽ice聽in the scarps and pushing them away. 鈥淭he hot surface radiates onto the ice, the ice melts, the scarp retreats,鈥� said Birch.
