The Rosetta spacecraft appears to have discovered the presence of a series of sinkholes on the surface of comet 67P/Churyumov–Gerasimenko (67P). They're responsible for some of the increasingly active gas streams that are being observed by the orbiter as the comet speeds towards perihelion.
The new discovery wasmade 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 werediscovered pockmarking the irregular northern hemisphere of 67P.
The pits vary in sizebetween a few tens of meters to hundreds of meters in diameter, withsome extending 210 m (689 ft) below the comet's surface.The scientists who made the discovery note that the internal featuresof the walls vary greatly from pit to pit, and that a concentrationof the sinkholes on one area of the comet compared to another couldhint at an uneven distribution of ice on 67P.
"We see jets arisingfrom the fractured areas of the walls inside the pits," statesJean-Baptiste Vincent of the Max Planck Institute for Solar SystemResearch and lead author of the study. "Thesefractures mean that volatiles trapped under the surface can be warmedmore easily and subsequently escape into space."
The characteristics ofthe sinkholes may aid scientists in gauging the age of terrain on thecomet. For example, active and therefore younger pits have steeperwalls, while the walls of older pits tend to be sloped and featuredegraded rims.
"We are continuing to analyze our observations to see if this theory holds true, and ifthis 'time series' is related to the internal thermal evolutionof the comet, for example," explains Sebastien Besse of the ESTECtechnical center, Netherlands. "But we think thatmost of the active pits must have been present for several orbitsaround the Sun already, or else we would have expected to see anumber of outbursts as their collapses were triggered this timearound."
There are a number oftheories being proffered to explain the creation of the sinkholes – first, that they were created as the comet came to form. Collisionswith other rocky bodies may have created a void, and the roof abovethis void could then have collapsed by a deterioration of the surfacethrough seismic activity or sublimation of the material beneath.
Another theory holdsthat sublimation is directly responsible for the sinkholes, assubsurface ice deposits are thawed by the heat of the Sun, releasingcarbon dioxide and carbon monoxide that cause a cavity, andeventually directly penetrate the surface, causing the roof tocollapse.
Whatever the cause ofthe sinkholes, their appearance has allowed an abundance ofsubsurface material to sublimate tothe surface, contributing to the increasingly active coma of comet67P 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 furtherrevolutionize our knowledge of the enigmatic celestial wanderer.
A paper regardingRosetta's findings has been published in the online journal Nature.
Source:ESA