Many comets have been observed to have pitted surfaces. Initially these pits were thought to be the result of collisions with smaller bodies, as with craters on planets and moons, but they have been shown to be far to numerous for this to be the case, as collisions between bodies are thought to be rare in the Outer Solar System and comets in the Inner Solar System have surfaces which are regularly resurfaced by the heat of the Sun. Alternatively it has been suggested that these pits could be the result of sublimation of certain ices on the surface of the comets (comets are made up of different ices, including water, carbon dioxide and carbon monoxide, which sublimate – pass directly from a solid to a gaseous state – at different temperatures). However this should produce wide, shallow pits, whereas many of the of those seen on comets appear to be deep and narrow.
In a paper published in the journal Nature on 2 July 2015, a team of scientists led by Jean-Baptiste Vincent of the Max Planck-Institut fürSonnensystemforchung discuss observations made of comet 67P/Churyumov-Gerasimenko by the OSIRIS camera system on the Rosetta space probe between July and December 2014, comparing these observations to previous observations of comets 9P/Tempel 1 and 81P/Wild 2, and suggest a theoretical explanation for these phenomena.
Comet 67P/Churyumov-Gerasimenko had a close encounter with Jupiter in 1959, which altered its orbit from one with a perihelion of 2.7 AU (i.e. an orbit which at its closest approach to the Sun was 2.7 times as distant as the Earth) to one with a perihelion of 1.2 AU. This means that the surface of the comet is likely to be relatively 'fresh', with features reflecting a periodic exposure to a new highest temperature regime, rather than one developed from regular exposure to the same conditions over a very long time interval.
Rosetta paid particular attention to an area in the northern hemisphere of 67P/Churyumov-Gerasimenko with eighteen approximately circular pits, clustered in small groups and ranging from tens to hundreds of meters in diameter, with the deepest being a few hundred meters deep.
Image of Comet 67P/Churyumov/Gerasimenko taken on 19 September 2014 by the Rosetta space probe. ESA/Wikipedia.
The walls of these pits have a non-uniform texture, with smooth areas, fractured areas, areas showing terraces and alcoves, and in the deeper parts of the pits, areas with a globular texture. This globular texture extends at least 200 m below the surface, and may reflect the nature of the cometary interior. Jets of material could be seen emerging from the walls of some of these pits, primarily in areas with fractured or globular surfaces. These are thought to be the product of active sublimation of ices on the walls of the pits. However this sublimation cannot account for the formation of the pits, as it would take thousands of years of sublimation at the observed rate to form some of the larger pits.
Comet 9P/Tempel 1, which was visited by the Deep Impact spacecraft in July 2005, had a surface covered by much wider pits than those on 67P/Churyumov-Gerasimenko, with many of these pits having merged together and frequent violent emissions seen on its surface. 9P/Tempel 1 has actually been in its current orbit only slightly longer than 67P/Churyumov-Gerasimenko (it was shifted onto its current orbit by a close encounter with Jupiter in 1953), but this orbit brings it closer to the Sun (at its closest only 1.5 AU from the Sun, or times as far from the Sun as the Earth is) and its surface is therefore considered to be more processed. Comet 81P/Wild 2 also has a more processed surface with larger pits, also having an orbit which brings it much closer to the Sun (1.59 AU), which it switched to after a close encounter with Jupiter in September 1974.
Clearly, therefore, pit formation is a process occurring on comets with 'young' orbital paths, i.e. orbits which they have been switched to fairly recently and which result in more heating than their previous orbits, but cannot be accounted for by surface sublimation.
Vincent et al. suggest that these pits could be formed in a similar way to sinkholes on Earth, with voids within the comets being formed or enlarged by solar heating until material above the voids collapses inwards, leaving a circular or cylindrical pit at the surface. The walls of these pits are then exposed to direct solar heating, leading to further sublimation, which widens the pits, and eventually causes them to merge.
Vincent et al. propose three different (but not mutually exclusive) mechanisms by which such voids might form.
Firstly, they may have formed along with the comet. Comets (and other large bodies in the Solar System) are thought to have formed by the accretion of smaller bodies, and several large protocomets (or cometesimals) coming together in a low gravity environment could conceivably leave voids inside the final structure.
Secondly the voids could be formed by the sublimation pockets of low-temperature ices, such as carbon monoxide or carbon dioxide, within a largely water-ice body. This would require heating of the outer body of the comet combined with sufficient heat conductivity to sublimate these low temperature ices below the surface – conditions which are thought likely to occur in comets.
Finally a subsurface source of heating could trigger the formation of voids. The most likely source of such heat would be the re-crystallization of amorphous water-ice (which only exists at very low temperatures) to crystalline ice. This process requires heat input to begin, but once initiated releases further heat as a bi-product of the re-crystallization process.
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