“The really cool thing about these sinkholes is that you can stare right into that comet. It’s just crazy. You can see a lot of features in the walls,” University of Maryland planetary scientist Dennis Bodewits told Discovery News.
The pits have near-circular openings, cylindrical shapes and steep walls. At least one of the 18 holes seems to be on a steep angle. Some are active, spewing out jets of dust from their walls or floors.
“Finding the pits was a total surprise,” said space physicist Paul Weissman, with NASA’s Jet Propulsion Laboratory in California.
Scientists suspect the pits are sinkholes that formed when material near the comet’s surface collapsed. The comet’s nucleus is only about half as dense as solid water ice, with an interior that is believed to be mostly empty space.
The comet, known as 67P/Churyumov-Gerasimenko, is is thought to be a "rubble pile" of boulder-sized chunks of silicates and organics that came together to form the comet’s body.
Once the pits form, newly exposed material escapes to space, causing the walls to slowly widen, scientists theorize.
“A fresh cometary surface will have a ragged structure with many pits, while an evolved surface will look smoother,” Jean-Baptiste Vincent, with Germany’s Max Planck Institute for Solar System Research, and colleagues write in this week’s Nature.
“I think they are sinkholes, but I don’t know that for certain,” Weissman, who was not involved in the research, told Discovery News.
“The mass we’ve seen in outbursts from this comet so far is not enough to empty one of these sinkholes,” he said. “Also, also if you look at the surface around the pits, there’s no evidence of debris having been thrown out and then falling back on to the comet. So that leaves us sinkholes. I don’t know that anyone has come up with any other explanation of how they may have formed. I think it’s a good explanation, but we don’t know for certain.”
Similar circular features were found on comets Wild-2 (pronounced “Vilt 2”) and Tempel-1, which were visited by NASA’s Stardust and Deep Impact space probes, respectively. Those depressions, however, were not nearly as deep as the cavities found on 67P, which Europe’s Rosetta spacecraft has been circling since August 2014.
“That suggested that these were older surfaces, older features that had been eroded and filled in,” Bodewits said.
Based in the size and location of 67P’s pits, scientists suspect some variation in materials or structure a few hundred yards beneath the surface.
Read more at Discovery News
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