Here's something we never thought we'd see on a comet: shifting dunes

By Nancy Atkinson

Features in the Hapi region show evidence of local gas-driven transport producing dune-like ripples (left) and boulders with ‘wind-tails’ (right) – where the boulder has acted as a natural obstacle to the direction of the gas flow, creating a streak of material ‘downwind’ of it. The images were taken with the OSIRIS narrow-angle camera on 18 September 2014. - ImageCredit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

The Rosetta mission’s close-up views of the curiously-shaped Comet 67P/Churyumov-Gerasimenko have already changed some long-held ideas about comets. But here’s more: there’s a ‘wind’ blowing across the comet’s surface, creating moving shifting dunes.

“The approach to comet 67P/Churyumov–Gerasimenko by the spacecraft Rosetta has revealed the presence of astonishing dune-like patterns,” wrote Philippe Claudin, of the Institute of Industrial Physics and Chemistry, Paris, France, in his new paper, noting the unusual and unexpected conditions found on Comet 67P.

Images from Rosetta’s cameras revealed the dusty covering of the comet may be several meters thick in places, which was surprising. But even more surprising was seeing active dunes that are changing. The dunes were seen on both of the ‘lobes’ of the comet as well as on the neck that connects them. Comparisons images taken 16 months of the same region shows evidence that the dunes moved, and are therefore active.

Left, an image of comet Chury showing outgassing of water vapor, which entrains dust (© ESA/Rosetta/NAVCAM). Right, the neck region, between the comet’s two lobes. Various types of relief can be seen, including the dunes, at bottom left (circled in red), in the sandy region. - Image Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)

Claudin and his team said that the formation of sedimentary dunes requires the presence of grains and of winds that are strong enough to transport them along the ground. However, comets do not have a dense, permanent and active atmosphere like Earth does. Also, Comet 67P’s gravity is so weak – only about 1/50,000 that of Earth’s – that fast moving grains might be ‘launched’ into space.

What could be creating a wind strong enough that not only moved the grains, but also some boulders up to a meter wide?

There is indeed a wind blowing along the comet’s surface, said Claudin, coming from gases that escape from the surface.
Gases escape at ‘sunset’ on the comet, caused by the pressure difference between the sunlit side, where the surface ice can sublimate due to the energy provided by the sunlight, and the night side.

“This transient atmosphere is still extremely tenuous, with a maximum pressure at perihelion, when the comet is closest to the Sun, 100,000 times lower than on Earth,” the team said in a press release. “However, gravity on the comet is also very weak, and an analysis of the forces exerted on the grains at the comet’s surface shows that these thermal winds can transport centimeter-scale grains, whose presence has been confirmed by images of the ground. The conditions required to allow the formation of dunes, namely winds able to transport the grains along the ground, are thus met on Chury’s surface.”

Summary of properties of Comet 67P/Churyumov–Gerasimenko, as determined by Rosetta’s instruments during the first few months of its comet encounter. - Image Credit: ESA.

The transportation of dust has created dune-like ripples, and boulders with ‘wind-tails’ – the boulders act as natural obstacles to the direction of the gas flow, creating streaks of material ‘downwind’ of them.

Claudin said this finding represents a step forward in understanding the various processes at work on cometary surfaces, and also shows the Rosetta mission still has many surprises and discoveries in store.

Paper: Giant ripples on comet 67P/Churyumov–Gerasimenko sculpted by sunset thermal wind

Source: Universe Today