Photographs of the nucleus of comet 67P/Churyumov-Gerasimenko show its unexpected shape: two ellipsoidal blocks called the Head and Body, connected by a “neck.” On the surface are numerous depressions, pits, and basins of different shapes and sizes. The largest among them are Hatmehit and Imhotep, both roughly oval, relatively shallow, and about half a kilometer in radius. One of them is located on the Head, and the second on the Body. The explanation of their origin through numerical simulations is a demanding task.
The surface erosion of comets is related to the sublimation of ice buried under porous granular material historically called a “dust mantle,” but it contains more than just dust. The efficiency of this process strongly depends on the temperature, i.e. on the distance to the Sun. Unfortunately, the orbital history of comet 67P/Churyumov-Gerasimenko more than a few hundred years ago is unknown. The erosion during known history is not sufficient to cause formation of a basin more than a few tens of meters deep, while the depth of Hatmehit is about 300 meters. What other processes can be responsible for the formation of this basin?
The nuclei of comets contain not only water ice, but also highly volatile carbon monoxide. To make the situation even more complex, water ice can be in amorphous form. In such case, the diffusion of heat from the surface warmed by the Sun may trigger crystallization and the release of heat of the phase change. This may cause sublimation of carbon monoxide and the increase of the gas pressure, possibly several meters beneath the surface. The rising pressure may cause an explosion, but may not. It is possible that pressure rises beneath a thick layer of material that has high tensile strength, while the pristine material in the interior of the nucleus has very low compression strength. In such case, the formation of an empty space by compression of the underlying material may happen.
Is the formation of caves a common phenomenon? Probably not. The tensile strength of ice is an order of magnitude lower than the compression strength, while the favorable relation would be just the opposite. However, comets are composed of granular material and porous granular ice may undergo metamorphism, also without any temperature gradient. The rate of the temperature-driven sintering of ice grains is an exponential function of the temperature and leads to the formation of a strengthened layer of material just beneath the dust mantle. When the granulation of ice is very fine, the strengthening may be sufficiently effective to cause the formation of the needed thick layer of resistant material, and a cave may form.
The next question is what happens when a cave already exists beneath the surface of a comet. The surface is still warmed by the Sun, ice sublimes, and the layer covering the cave erodes. After some time, it becomes too thin to remain stable and should be either ejected or collapse. The delay depends on the properties of the dust mantle and also on the presence of organic admixtures in ice. The list of chemical species present in comets is long and their influence on the sublimation of ice can be very different.
One more interesting problem is the fate of material once composing the ceiling of a cave. It depends on the speed and direction of ejection. The speed of escape from comet 67P/Churyumov-Gerasimenko is small (it is on the order of 1 m s-1), but the velocity of ejected material can actually be smaller than the speed of escape from the comet. This material (slow ejecta) returns to the comet, creating sedimentary layers. They should be very porous, depleted in ice and unstable. On comet 67P/Churyumov-Gerasimenko an impact of 1 kg meteorite with the velocity of 20 km s-1 may be sufficient to cause a landslide.
These findings are described in the article entitled Comet 67p/Churyumov–Gerasimenko, possible origin of the depression Hatmehit, recently published in the journal Icarus. This work was conducted by Konrad J. Kossacki and Leszek Czechowski from the University of Warsaw.