The spacecraft Rosetta keeps surprising everyone with amazing new photos taken in pursuit of comet 67P/Churyumov-Gerasimenko, taken just 7.4 kilometres from its surface. These images reveal dunes just like those you can find on Earth. Scientists have also found that it really stinks.
According to principal investigator for ROSINA, the instrument that is analysing its composition, it really stinks:
The perfume of 67P/C-G is quite strong, with the odour of rotten eggs (hydrogen sulphide), horse stable (ammonia), and the pungent, suffocating odour of formaldehyde. This is mixed with the faint, bitter, almond-like aroma of hydrogen cyanide. Add some whiff of alcohol (methanol) to this mixture, paired with the vinegar-like aroma of sulphur dioxide and a hint of the sweet aromatic scent of carbon disulphide, and you arrive at the 'perfume' of our comet.
Really disgusting odour.
As for the dunes, one of the commenters at the ESA site has a good analysis:
I think I've decided they are deposited by the gas plumes from the surrounding cliffs firing across the already Laktritz covered floor of the "crater" Logan. Just as the mounds and dunes near cliff edges are created. No need to find a different "agent". The gas plumes don't have to go straight up do they.
I spotted this some time ago at the base of a cliff in the neck area. The demarcation between exposed subsurface and dust blanket, is really sharp. Now we can see why and how. The rubble strewn area to the right of image 4, the surface blanket looks like snow melting, all patchy, where thicker drifts and mounds take longer to melt. These dust mounds must be where all the dust has been "blown" and collected into quiet zones, so these little pockets take longer to be dispersed.
The chimneys I think are the "smoking guns" for the body penetrating impacts we discussed. The molten ice stuff coming out would build a wall as it immediately freezes on reaching the surface. Frozen and semi frozen ice would slowly fall back down. By the time it reaches the surface the comet has rotated so one side of the caldera has a lot more ice deposited on it. The liquid in the caldera will soon level and freeze once the gas pressure is released and the "ice lava" tube becomes blocked. This core and crater floor will have far fewer volatile ices and gases in it making it a lot less active than the surrounding surface, hence the build up of Laktritz. As the higher side of the icy crater rim takes longer to erode we are left with lots of semicircular cliffs beside flat areas.
The overturned cups are more recent eruptions where less of the crater rim has eroded. The huge flat Star Wars Landing Zone near site C. Is perched on top of huge steep cliffs, the wall of frozen ices that built up as the cryovolcano was erupting. Then we see the partially eroded rim as the cliffs round the flat area. The little cup volcanos show that an overhang actually builds up on the taller side of the rim, so this overhang eventually collapses into the crater, hence the pile of rubble only on one side of the crater. Something that is seen in nearly all the craters.
If the refrozen ices form an amorphous solid like glass, as lava does to make basalt on Earth, we get the smooth solid material full of cracks and fissures we s most commonly on the tops of the lobes. A different composition and slower flow rate would give lumpy, rubble "ice lava, (top left Image 2).. Ices with more gases in them would give a more pumice type material when it freezes, full of holes and tunnels, (Philae landing site image). A different composite of more dense volatile ices give a material like "pillow lava", which can also be seen in image 2 as the flat topped bulges top middle.
How far this analogy can be used in actuality, I have no idea. No one else does either, since no one has seen cryovolcanism up close before, only on flyby pictures from thousands of kilometres away. The laws of physics don't change and the behaviour of molten fluids on freezing is a pretty well understood phenomenon. The low gravity is the big difference, though it seems to make little difference on the Moon and Mars.
Of course, that's just speculation at this point, but it's a fascinating idea.