The most active pit is Seth_01, though other sinkholes have been spotted. Credit: Vincent et al., Nature Publishing Group
Sinkholes have been spotted across the surface of comet 67P/Churyumov-Gerasimenko by ESA’s Rosetta spacecraft.
The craters in the surface were spotted shortly after the probe began to orbit the comet in August 2014 but their cause has remained illusive.
A new study suggests that the culprit could be subsurface ices boiling off, causing the surface above to collapse.
The pits were seen in some of the first images Rosetta sent back upon arriving at the comet.
They are circular, measuring from tens to several hundred metres across, and are as deep as they are wide. Immediately the question was raised of how these unusual formations occurred.
Originally it was suggested that the pits had an explosive origin as underground ice heated and ejected material, but such an outburst had been observed by Rosetta during approach, 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," says Dennis Bodewits, an assistant research scientist in astronomy at the University of Maryland.
"The pits we see are much larger.
It seems that outbursts aren't driving the process, but instead are one of the consequences."
Instead researchers suggested a different method of formation.
"We propose that they are sinkholes, formed by a surface collapse process very similar to the way sinkholes form here on Earth,” says Bodewits.
A heat source within the comet causes the water, carbon dioxide and carbon monoxide ices to heat up and turn into a gas and escape, leaving a void behind.
Eventually the weight of the surface above is too much and it collapses into the hole.
The larger the crater hole the younger it is, as over time the walls get eroded and the bowl fills with dust and ice chunks.
The newest sinkholes could give researchers a way to look beneath the surface at the pristine core of the comet.
"In some sense, these deep sinkholes remind me of the crater excavated on comet Tempel I by the Deep Impact mission," said Michael A'Hearn, a Distinguished University Professor Emeritus of astronomy at University of Maryland.
"The process is completely different, of course, but both allow us to achieve the same broad goal of being able to see deeper into a comet."