Active pits spotted on Rosetta's comet

Some of the dust jets emitted from Rosetta's comet can be traced back to active pits on its surface. They could be the remnants of collapsed cavities.

Cavities measuring up to a few hundred meters in diameter can be found under the surface of Rosetta's comet 67P/Churyumov-Gerasimenko. They can be instable and collapse in a kind of sinkhole process.

This is the result of a new study led by researchers from the Max Planck Institute for Solar System Research (MPS) in Germany, which analyzes images of the comet's surface.

The images show peculiar, pit-like recesses that are unlike ordinary craters and that emit dust and gas into space. In their study the researchers argue that these pits arise when cavities beneath the surface cave in.

Researchers under the lead of Jean-Baptiste Vincent from the MPS have studied 18 peculiar pit-like depressions all occurring in the northern hemisphere of Rosetta's comet 67P/Churyumov-Gerasimenko.

The pits' diameters vary between ten and a few hundreds of meters. They exhibit nearly vertical sidewalls and are exceptionally deep with the largest ones extending up to two hundred meters into the comet's interior. The walls of these depressions are characterized by layers and terraces, their bottoms are mostly flat.

Because of their unusual morphology, these pits can be clearly distinguished from impact craters, added Vincent, noting that they seem to be a typical characteristic of comets.

Some of the pits are also active: fine jets of dust are emitted from the inside walls. The scientists reached this conclusion by studying images showing the same jet from different perspectives.

Already, the active pits prove useful for estimating the age of cometary surfaces. Since the pits are active, they change with time, says Vincent. By and by the pits expand: the edges retreat forming extensive terraces. A cometary surface exhibiting deep holes is therefore rather young. Older areas present themselves as smooth plateaus.

The results appear in the journal Nature.