The European Space Agency's Rosetta spacecraft first began orbiting comet 67P/Churyumov-Gerasimenko in August 2014. Almost immediately, scientists began to wonder about several surprisingly deep, almost perfectly circular pits on the comet's surface. Now, a new study based on close-up imagery taken by Rosetta suggests that these pits are sinkholes, formed when ices beneath the comet's surface sublimate, or turn directly to gas. The study, which appears in the July 2, 2015 issue of the journal Nature, reveals that the surface of 67P/Churyumov-Gerasimenko is variable and dynamic, undergoing rapid structural changes as it approaches the sun. Far from simple balls of ice and dust, comets have their own life cycles. The latest findings are among the first to show, in detail, how comets change over time. 

"These strange, circular pits are just as deep as they are wide. Rosetta can peer right into them," said Dennis Bodewits, an assistant research scientist in astronomy at the University of Maryland who is a co-author on the study. The pits are large, ranging from tens of meters in diameter up to several hundred meters across. 

"We propose that they are sinkholes, formed by a surface collapse process very similar to the way sinkholes form here on Earth," Bodewits added. Sinkholes occur on Earth when subsurface erosion removes a large amount of material beneath the surface, creating a cavern. Eventually the ceiling of the cavern will collapse under its own weight, leaving a sinkhole behind. "So we already have a library of information to help us understand how this process works, which allows us to use these pits to study what lies under the comet's surface," Bodewits said.

Bodewits and his co-authors analyzed images from Rosetta's Optical, Spectroscopic and Infrared Remote Imaging System (OSIRIS) narrow angle camera, which is designed to image the surface of the comet's nucleus. The team noted two distinct types of pits: deep ones with steep sides and shallower pits that more closely resemble those seen on other comets, such as 9P/Tempel 1 and 81P/Wild. The team also observed that jets of gas and dust streamed from the sides of the deep, steep-sided pits --a phenomenon they did not see in the shallower pits. 

Initially, the Rosetta team suspected that discrete, explosive events might be responsible for creating the deeper pits. Rosetta observed one such outburst during its approach to the comet, on April 30, 2014. Catching this event in the act allowed the team to quantify how much material had been ejected, and it quickly became obvious that the numbers just didn't stack up. Explosive outbursts alone could not explain the formation of these giant pits.

"The amount of material from the outburst was large--about 100,000 kilograms--but this is small compared to the size of the comet and could only explain a hole a couple of meters in diameter," Bodewits explained. "The pits we see are much larger. It seems that outbursts aren't driving the process, but instead are one of the consequences."