Discovery rules out many theories of comet formation by demonstrating that comets have substantial variations in their internal structures

Cameras on the Rosetta spacecraft have spotted a series of enormous pits on comet 67P/Churyumov-Gerasimenko that plunge hundreds of metres down into the body’s cold interior.

Scientists on the mission believe the pits formed in the same way as sinkholes on Earth, which appear out of the blue when the natural ceilings above underground caverns suddenly collapse under their own weight.

The comet pits are typically 200 metres across and reach down about the same distance into the comet’s body. Some of the holes may have formed in the distant past, but the walls of others are jagged and show no signs of erosion, suggesting they formed very recently.

The discovery of sinkholes shows that comets have substantial variations in their internal structure, a finding that rules out some theories of how the bodies form in the first place, from rubble left over from the birth of the solar system.

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“Many comet formation models predict that under the surface they should be homogenous, the same throughout, but we show that this cannot be the case,” said Jean-Baptiste Vincent at the Max Planck Institute for Solar System Research in Göttingen.

Vincent and others on the European Space Agency mission studied 18 circular sinkholes in comet 67P. All are in what is designated the north of the comet, which could be pocked by more than 50 similar deep pits.

There are no obvious sinkholes in the south, because that region heats up more as the comet swings around the sun. This speeds up surface erosion and leaves the terrain quite flat.

The sinkholes came in two distinct forms. The first were deep with steep sides from which gas and dust billowed. The second type were shallow and inactive - similar to those seen on other comets, such as comet 9P/Tempel 1 and 81P/Wild.

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At first, the scientists thought the pits formed by explosions on the comet as ice turned into steam. But observations of an explosive event revealed that it ejected far too little material.

Instead they believe that ices made from water, carbon monoxide and carbon dioxide are heated into vapour beneath the surface, creating huge voids as the gas escapes through pores in the comet. Eventually, the cavities grow large enough that the material above collapses to create a sinkhole.

When a sinkhole first appears, it exposes fresh material which steadily vaporises in the sun’s rays, causing the hole to grow. And so the pits transform, from deep, steep-sided holes to shallower ones that have been partially filled in with dust. Details of the study are reported in Nature.



Rosetta will fly around comet 67P as it swings past the sun in August and starts to head back out into the deepest reaches of the solar system. Last week, space agency officials extended the Rosetta mission by nine months, allowing the spacecraft to observe the comet until September 2016.