The probe usually orbits 67P at a distance of a few hundred kilometers. Footage received from Rosetta over the last year showed a number of dust jets coming from the comet, which we expected to see. But, after analyzing high-fidelity images from the lander's OSIRIS instruments, taken just ten to 30 km from the comet's center, scientists saw that at least some of the dust jets come from specific locations on the comet's surface, being projected from huge sinkholes.
The scientists have picked out 18 quasi-circular pits in the northern hemisphere of the comet, some of which are still active now. Each sinkhole is anywhere from a few tens of metres to hundreds of metres in diameter and go below the surface by up to 210m to a smooth dust-covered floor.
“We see jets arising from the fractured areas of the walls inside the pits. These fractures mean that volatiles trapped under the surface can be warmed more easily and subsequently escape into space,” says Jean-Baptiste Vincent from the Max Planck Institute for Solar System Research, lead author of the study.
Similar to the ones on Earth, these sinkholes form when a cavity opens up under the surface. As it widens and deepens, the loss of material makes the ceiling too thin to support its own weight, and collapses. After the collapse, the volatile materials can evaporate or be eroded more easily, and the sinkhole enlarges over time.
“Although we think the collapse that produces a pit is sudden, the cavity in the porous subsurface could have growing over much longer timescales,” says co-author Sebastien Besse, of ESA’s ESTEC technical centre in the Netherlands.
So, what caused these cavities to form in the first place? The team has three theories that they are pursuing.
The first one is that they are artifacts of the comet's weak gravitational field. When it formed, material accreted by means of low-velocity impacts, leaving behind void areas due to the imperfect fit between primordial building blocks. Over time, seismic events or space impacts cause the surface to weaken enough to cause it to collapse.
Another possibility is that the pits are full of volatile ices like carbon dioxide and carbon monoxide, sitting just beneath a layer of dust. These ices could be melted by the warmth of the Sun as the comet draws closer in its orbit every year.
Or it could be that the ice manages to melt itself away by transforming from amorphous ice made up of irregularly packed molecules to crystallised ice, a process that would release heat which could be sufficient to cause evaporation.
“Regardless of the processes creating the cavities, these features show us that there are large structural and/or compositional differences within the first few hundred metres of the comet’s surface and the cavities are revealing relatively unprocessed materials that might not otherwise be visible,” says Besse.
Researchers analyzing the interior structure of the sinkholes found that their interiors differ quite significantly, with some showing fractured material and terraces, others showing horizontal layers and vertical striations and others also showing globular structures nicknamed “goosebumps”.
“We think that we might be able to use the pits to characterise the relative ages of the comet’s surface: the more pits there are in a region, the younger and less processed the surface there is,” explains Vincent. “This is confirmed by recent observations of the southern hemisphere: this is more highly processed because it receives significantly more energy than the northern hemisphere, and does not seem to display similar pit structures.”
Rosetta scientists are hopeful that the spacecraft might yet get to see the formation of a sinkhole in action. The probe did see one outburst during its approach to the comet back in April 2014, which generated between 1,000kg and 100,000kg of material. But although a pit collapse could have been responsible for this, it was much smaller than the researchers expect.
With the collapse of a typical large pit of 140m wide and 140m deep, the team would expect to see the release of around a billion kilograms of material.
“Being able to observe changes in the comet, in particular linking activity to features on the surface, is a key capability of Rosetta and will help us to understand how the comet’s interior and surface have evolved since its formation. And with the extension of the mission until September 2016, we can do the best job possible at unravelling how comets work" says Matt Taylor.