Comet 67/P covered in sinkholes and caves

Ars Technica : The European Space Agency’s Rosetta spacecraft, which is in orbit around comet 67P/Churyumov-Gerasimenko, has been supplying astronomers with highly detailed pictures of the comet’s surface. Many of those pictures have revealed sinkholes as wide as 200 m and as deep as 180 m. At first it was thought that the sinkholes could be the result of outgassing—as the comet approaches the Sun, frozen gases warm and explode—but the amount of material in outgassing events didn’t match the size of any of the sinkholes. Instead, Dennis Bodewits of the University of Maryland, College Park, and his colleagues say it appears to be the other way around—the outgassing results from the formation of the sinkholes. The holes themselves form as surface ice sublimates and dust fills the depression until it collapses, creating a deeper hole. Bodewits’s team believes that the comet’s nucleus is 75–80% empty space, which is what allows so many sinkholes to form.