When the European Space Agency’s Rosetta spacecraft slotted into orbit around Comet 67P/Churyumov-Gerasimenko in August 2014, nobody was sure what to expect. After all, it was the first time we’d had a chance to check out a comet from such close range.
But no one expected dunes.
Dunes are formed by wind but the gases jetting from comets are so tenuous, nobody thought they’d be strong enough to move surface particles around – let alone pile them into dunes.
But dunes there were, says Philippe Claudin, of the Institute of Industrial Physics and Chemistry, Paris, France.
The first were spotted in the comet’s “neck” region – the narrow zone between the two lobes that form its duck-shaped body. But soon they were seen in other places too.
What’s more, the ripples moved, meaning they were active features, not relics of the distant past frozen in time.
“The question is how on Earth – or should I say, ‘How on comet’ – is it possible to have ripples or dunes on a comet?” Claudin asked last week at a meeting of the American Physical Society’s Division of Fluid Dynamics in Portland, Oregon.
Dunes generally form under the influence of winds that cause sand grains to skip across the surface in a process known as saltation.
But comets have no atmosphere as we know it, and Comet 67P’s gravity is so low – only about 1/50,000 that of Earth’s – that anything that gets moving more than about a metre per second will escape into space.
But comets have gas jets escaping their sun-warmed daytime sides – jets that, in the case of Comet 67P, may graze the surface at speeds of 100 metres or more per second.
This is fast enough that in the comet’s low gravity they have the power, Claudin says, to move not just dust, but rocks up to a metre wide.
The physics of tenuous, high-speed gas flows means they don’t launch these rocks into space, but instead move them sedately over the surface until they pile up into dunes.
“The conditions on the comet are so unusual that it’s difficult to have intuition of what’s going on,” he says.
Findings such as this show how much we still have to learn about comets, says Matt Taylor, the Rosetta mission’s project scientist.
And it won’t be the last surprise the mission serves up, he predicts. “Rosetta’s operations are over, but we have only touched about 5% of the data. We have decades of science to do.”