The new discovery was made using Rosetta's OSIRIS camera at a distance of 10 - 30 km (6 - 19 miles) between August - October 2014. In all, 18 of the pits were discovered pockmarking the irregular northern hemisphere of 67P.
The pits vary in size between a few tens of meters to hundreds of meters in diameter, with some extending 210 m (689 ft) below the comet's surface. The scientists who made the discovery note that the internal features of the walls vary greatly from pit to pit, and that a concentration of the sinkholes on one area of the comet compared to another could hint at an uneven distribution of ice on 67P.
"We see jets arising from the fractured areas of the walls inside the pits," states Jean-Baptiste Vincent of the Max Planck Institute for Solar System Research and lead author of the study. "These fractures mean that volatiles trapped under the surface can be warmed more easily and subsequently escape into space."
The characteristics of the sinkholes may aid scientists in gauging the age of terrain on the comet. For example, active and therefore younger pits have steeper walls, while the walls of older pits tend to be sloped and feature degraded rims.
"We are continuing to analyze our observations to see if this theory holds true, and if this 'time series' is related to the internal thermal evolution of the comet, for example," explains Sebastien Besse of the ESTEC technical center, Netherlands. "But we think that most of the active pits must have been present for several orbits around the Sun already, or else we would have expected to see a number of outbursts as their collapses were triggered this time around."
There are a number of theories being proffered to explain the creation of the sinkholes – first, that they were created as the comet came to form. Collisions with other rocky bodies may have created a void, and the roof above this void could then have collapsed by a deterioration of the surface through seismic activity or sublimation of the material beneath.
Another theory holds that sublimation is directly responsible for the sinkholes, as subsurface ice deposits are thawed by the heat of the Sun, releasing carbon dioxide and carbon monoxide that cause a cavity, and eventually directly penetrate the surface, causing the roof to collapse.
Whatever the cause of the sinkholes, their appearance has allowed an abundance of subsurface material to sublimate to the surface, contributing to the increasingly active coma of comet 67P whilst further excavating the holes.
With a nine-month mission extension and 67P moving swiftly towards perihelion, Rosetta and its newly awakened sidekick Philae are sure to further revolutionize our knowledge of the enigmatic celestial wanderer.
A paper regarding Rosetta's findings has been published in the online journal Nature.