In August 2014, ESA made history when its Rosetta spacecraft became the first to orbit a comet. Over the past 10 months, Rosetta and its newly reawakened lander Philae have been supplying us with unprecedented data on comet 67P/Churyumov-Gerasimenko since then. Right away, scientists noticed the presence of several enigmatic deep pits on the comet’s surface. New data suggests the pits are sinkholes caused by the sublimation of subsurface ice and are one source of active dust jets. That and data on the composition of the body hint at a comet that’s riddled with underground caverns.

Comets—the leftover pieces of ice and rock from the formation of the Solar System—are like icy time capsules. By studying them, we can learn a great deal about the conditions in the solar nebula—the cloud of gas and dust in which the Sun and the planets formed over 4 billion years ago. Comets come in multiple varieties and have spent their lives in one of two “cold storage” areas in the outer Solar System—the Kuiper Belt and the Oort Cloud.

The Kuiper Belt, a region beyond the orbit of Neptune, is the source of Jupiter-family comets like 67P. This class typically has an orbital period of less than 20 years and is strongly affected by Jupiter’s gravity. Beyond the Kuiper Belt and extending nearly halfway to the nearest star is a spherical region known as the Oort Cloud. This is where we see long-period comets which have orbital periods that can last up to millions of years.

Comets are more than just chunks of ice speeding through space; they have their own life cycles and can change drastically. As comets approach perihelion, or the closest point to the Sun, they warm up, and the nucleus—the solid central part composed of volatile ices and other material—sublimates. As sublimation occurs, the comet becomes shrouded in an atmosphere or coma.

Comets come in many different shapes, but one of the distinctive characteristics of 67P is its dual lobes. Connected by a small “neck,” the lobes sit on top of each other, resembling a rubber duck. The odd shape of 67P has left many scientists questioning how it formed. Could such a shape have been sculpted by sublimation, or did the two lobes fuse together during a collision inside the solar nebula. Recent computer simulations seem to think the latter is the likely scenario.

Rosetta is equipped with a dual camera system called the Optical Spectroscopic and Infrared Remote Imaging System (OSIRIS), which is composed of both a narrow- and wide-angle digital camera. Scientists are using OSIRIS to map the surface of 67P. During the mapping process, the team discovered 18 surprisingly deep, almost perfectly circular pits on the comet’s surface.

“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.

“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. (Given the very low gravity on 67P—or any comet—sinkhole formation would be a very slow-motion process.) “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.

The paper describing these sinkholes will be released by the journal Nature today. 67P is not the first comet to harbor surface depressions; however, the characteristics of these pits are unlike any previously detected. 67P’s nucleus spans approximately 4km (2.4mi) and features two different types of holes: deep ones measuring 180 meters deep by 200 meters in diameter with steep walls, and shallower pits much like the ones previously detected on other comets such as 81P/Wild. The deeper pits are more intriguing and show clear signs of activity, with jets of gas and dust streaming from the pit walls. This activity was not present in the shallow pits.

How did the pits form? The team first suspected that explosive outgassing events may be behind the creation of the deeper pits. On April 30, 2014, one such event was observed by Rosetta as it approached the comet. Extensive analysis revealed exactly how much material was ejected, and the numbers didn’t add up.

“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.”

The team suggests that the pits are actually sinkholes, forming as an interior heat source sublimates the comet’s surface ice. Data shows the observed outbursts are actually a result of the sinkholes, rather than the cause. The team determined that roughly 80 percent of the nucleus’ mass is attributed to silicates and organics rather than ice. From this they inferred that the interior of the nucleus is approximately 75-percent to 80-percent empty space, with the interior of the nucleus likely harboring cavern-like pockets.

As the surface ice sublimates, depressions or basins are formed. Dust and other material then collects in the basins, which eventually collapse under their own weight, forming deep pits. As the pits grow, subsurface ices are exposed to heat from the Sun for the first time, triggering sublimation and producing outbursts of gas and dust.

Rosetta has already taught us more about comets than we could have predicted and has many more surprises still in store. Activity on the comet is expected to increase dramatically as it approaches perihelion in mid-August. At closest approach, 67P will be only 1.25 AU or 1.25 times the distance of the Earth to the Sun.

The Rosetta mission was scheduled to end this summer; however, on June 23, 2015, ESA announced the mission would be extended until September 2016. The extension will allow Rosetta to monitor how 67P changes as it moves away from the Sun.

Nature, 2015. DOI: 10.1038/nature14564 (About DOIs).