What mechanism is responsible for the creation of these dunes on Comet 67P/Churyumov-Gerasimenko?

Question

What mechanism is responsible for the creation of these dunes on Comet 67P/Churyumov-Gerasimenko?

The following high resolution picture from ESA's Rosetta mission shows the dunes:

At a distance of 7.8 km from the surface, the image scale is about 66.5 cm/pixel, so each 1024 x 1024 pixel frame is about 680 m across (although if we assume the furthest point away is an additional ~1 km further from the centre, the image scale is about 92 cm/pixel).

So the scale of the dunes is on the order of tens of meters along the crest. And each crest are probably just a few meters apart.

Answer 1

As the comet approaches the Sun, it starts to melt. It means that what was once rock and ice is exposed to very high temperatures and forms a liquid which flows over and behind the comet.

The above is a picture of fluid boundary layer on a sphere.

Notice how the fluid motion is visible, and my guess is that the fluid drags along with it, some of the solid, but easily movable part of the comet (dust maybe), thereby displacing it.

However once the comet moves out of the zone where it was melting, the dust has already been displaced, and now there is no more fluid to drag it anymore, and so it stays put. I would say that the dunes would likely all be in the same direction and point to what was once the tail of the comet. And to what I can see they seem to be in the same direction.

Answer 2

Wild unsubstantiated guess: Could it be flow of the sand down an incline, more like a glacier than sand dunes. Maybe the flow gets a boost from tidal forces flexing the comet whenever it passes near a massive body. Some of the patterns in the sand in the lower left corner look like what happens when sand slips down a critical incline like on an over ambitious sand castle. Of course it is hard to tell which way is actually down in this picture.

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Did a little internet search. Comet 67P is kind of dumbell shaped. Seems like the valley we looking down into is the "neck" around the middle of the comet, so the may be no down on the valley floor where the dunes are. However, being the thin neck between two massive bulges would make the valley floor extra prone to stresses from tidal forces acting differentially on the 2 bulges. Could the dunes be the result of the core of the comet just flexing under the sand in place? The patterns on the side of the valley in the lower left would be consistent with sandslides towards the valley floor.

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Another possibility is that because of it's highly irregular shape, the rotation of 67P is chaotic like Hyeprion's. If that is the case then sands on the surface would experience tangential forces as the the rotation of the core changes direction. This could produce interesting patterns on the surface of the sands. However, 67P's rotation looks regular in this video. The Wikipedia article does mention changes in rotation from "sublimation induced torque" though.

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It is also possible that the gases the comet gives of when near the sun do flow along the surface. So the cause of the patterns could be more like earth dunes after all.

Answer 3

Since there is effectively no atmosphere and thus little chance for any type of erosion, the individual "dirty sand" particles are most likely very jagged like the lunar regolith. Thus, I doubt that small distortions in the shape of the comet are to blame as these particles do not slip past each other as easily as, say, sand on Earth.

So my best guess is that the dunes are due to vibrations of the comet, either from variations in sublimation during its orbit or collisions with other orbiting bodies. If you shook the comet once and produced such dunes, I see no reason why they should change over time. Perhaps I am wrong, but those look just like what happens to partially dried mud that has been shook/vibrated. If the individual grains are jagged like lunar regolith, then it would take a significant energy input to move them. Thus, I think these dune-like features are probably very old.

Answer 4

There seems to be no direct evidence that the comet has any ice at all. The statement that there is ice below 8 to 10 inches of dust was based on an assumed hardness of the substrate based on how high the lander bounced after initial impact. It could be ice or it could be another hard material like rock under that dust. The other significant "evidence" given for the comet being composed of ice is the overall density of the comet. However the figure of something around 420 gm. per 1000 cu.cm. is less than half that of water ice. So while there may be a little ice, the comet seems to be much less dense than a solid object of water ice would be. This may give support to the "heretical" theory that the comet has almost no water and is composed of cosmic dust and debris which has accumulated in a low density object with some hard rocky parts and a lot of internal voids. If so, the "jets" which are now being observed are not sublimating ice but dust particles which are being attracted by the solar wind, (a stream of charged particles travelling out from the sun through space). The comet and solar wind have opposite electrical charges causing the dust to be "attracted" by the solar wind off the surface. If this is true, the comet, being a very large and dumbell shaped object may have differential charge on the "leading" lobe to the "trailing" lobe. This would mean that there is an electrical difference between the two ends of the comet, and field lines along the surface where the dust is aligning itself with the field, at the narrowest part, the "neck". The fact that the jets seem to "neck" further from the comet also suggests that the internal gravitational attraction of particles within each jet is causing them to move towards each other as the jet leaves the comet. The "aura" of bright particles all around the horizon (surface) of the comet when seen against space, suggests that there are many particles that have left the surface but are still bound to it gravitationally. Lets see what the close up images show on February 14th.