When the Rosetta spacecraft arrived at 67P/C-G last spring, it saw the famous comet kick out around 100 kilograms (220 pounds) of dust. On the surface, the probe could see strange, circular holes. It seemed as though these holes were responsible for the plumes of dust through the collapse of surface material, but Rosetta researcher Dennis Bodewits, from the University of Maryland, explains that something else seems to be at work. 

"When we do the math, it turns out that these holes are too big. If you calculate the amount of material that comes out of such a hole, that would be a much bigger outburst than what we saw," he told IFLScience.

So, Bodewits and colleagues put two and two together and instead predicted that the pits were the result of something going on beneath the surface: sinkholes similar to those on Earth.

What the team predicts could be occurring is that solid water, trapped under the surface of the comet, warms up (maybe as it approaches the sun) and turns straight to vapor. This forms a hole under the surface of the comet, and when it gets big enough, the surface collapses, forming these circular features.

When the water turns from ice to vapor, it skips the liquid stage: "Comets are extremely cold: they have a lot of ice in their interior. And these comets are also in the vacuum of space, so the water goes directly from ice to vapor: It's a process called sublimation," Bodewits explains. 

Fortunately for the team, Rosetta can get a good peek at the sides and bottoms of these holes, leading to some interesting observations. For example, some of the sinkholes were as deep as they were wide, whereas others were shallower. The team even spotted gas coming from the sides of the deeper ones. 

"The shallower ones are probably an older version of the deeper ones," Bodewits said. Over time, they've been partially filled in with dust and debris.

Your thoughts might turn to the Philae lander that is now sitting on the comet's surface. Being swallowed by a sinkhole, especially after its harrowing landing, would just be too tragic. Fortunately, there is little chance of Philae falling into a surprise sinkhole. "You only find sinkholes in specific places on the comet, and that was an important result by itself because it means that the interior is not homogenous. For whatever reason, some parts of the surface are different that others. Luckily for Philae, there are no sink holes where he is now," said Bodewits.

There is the possibility that it isn't water that's causing these sinkholes to emerge. "We don't really know what comets are made of. We have a good clue: There's CO (carbon monoxide) and CO₂ (carbon dioxide); there's water. But in the end, we can only see what comes out of the comet," he explains. 

This is what is so great about these cavities: They give scientists an otherwise impossible window into the surface of the comet. And if we could somehow peer further into these holes and examine what's at the bottom, we may be able to learn about the comet's subsurface structure. Bodewits thinks that we could tie this research together with analysis of other comets to help us predict the formation of our solar system.

The whole team is ecstatic that the Rosetta project has been extended into the summer of next year. "To really follow a comet into an extended part of its orbit and see how it changes, is just a treasure trove," he said. 

[Via Nature, University of Maryland]