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.
See
also...
The Rosetta Spacecraft
moved into position alongside Comet 67P/Churyumov–Gerasimenko on
Wednesday 6 August 2014, the first spacecraft to reach a cometary
target, and ten years after the mission was launched. It will now spend
six weeks making a series of...
Emissions from Comet C/2002 VQ94 (LINEAR). C/2002 VQ94 (LINEAR) was discovered by the Lincoln Near-Earth Asteroid Research (LINEAR) team at the Massachusetts Institute of Technology
on 11... 
Observing Comet 17P/Holmes with the WISE Space Telescope. Comet 17P/Holmes is a Jupiter Family Comet with a 6.89 year orbit that
takes it from 2.06 AU from the Sun (2.06 times the average distance at
which the Earth orbits the Sun, considerably outside the orbit of Mars)...
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