- By Ryan Whitwam on July 2, 2015
Astronomers have long observed comets from afar, wondering what sort of processes on their surface result in the formation of the halo of dust and gas. Now that the European Space Agency’s (ESA) Rosetta probe has settled into orbit of comet 67P/Churyumov–Gerasimenko, we can get a closer look. The newest set of images released by the ESA depict 18 semi-circular pits or “sinkholes” where much of the comet’s tail is released in jets of dust and gas.
Rosetta spent a decade in space lining up for its rendezvous with 67P. After becoming the first spacecraft to enter orbit of a comet, the Philae lander became the first vehicle to make a soft landing on a comet, although the landing wasn’t as soft as everyone had hoped. In between looking for signs of life from Philae (which recently woke up), Rosetta has been surveying the surface of the comet with its high-resolution OSIRIS camera.
When Rosetta was still a few hundred kilometers away, there was no way to tell where all the gas was coming from, but now at just 10-30 kilometers (less than 20 miles) there is enough detail to spot the sinkholes. These structures vary in size from the tens to hundreds of meters in diameter (several hundred to several thousand feet). Studying the nature of the pits (which you can see an example of in the middle of the above image) has given scientists a good idea of how they form.
The most likely mechanism involves a void inside the comet covered with a thin ceiling. As that covering gets thinner, it eventually loses strength and collapses inward. This exposes the fractured interior of the comet to sunlight and allows more material to be released as jets. Researchers are confident this process happens abruptly, but the original cavities could have formed over a much longer period of time. There’s speculation the voids might have been there since the comet formed. Alternatively, they could have developed over time as deposits of ice sublimated out into space or by expanding and contracting pockets of gas heated by the sun.
Not all of the pits appear to be actively producing jets of material, and they are unevenly distributed mostly in the northern hemisphere. This indicates the internal makeup of 67P is much less consistent than many had thought. The Rosetta team is hoping they’ll be able to witness the formation of a new pit, which should come with a massive burst of dust and gas. Rosetta’s mission has been extended to September of 2015, so they’ve got some time left.