July 1, 2015

A number of the dust jets emerging from Rosetta's comet can be traced back to active pits that were likely formed by a sudden collapse of the surface. These 'sinkholes' are providing a glimpse at the chaotic and diverse interior of the comet.
Rosetta has been monitoring Comet 67P/Churyumov–Gerasimenko's activity for over a year, watching how its halo of dust and gas grows as the comet moves closer to the Sun along its orbit.
From a distance of a few hundred kilometres, Rosetta observes an intricate pattern of the dust jets emitted from the nucleus as they stream out into space. But now, thanks to high-resolution images from the OSIRIS camera from distances of just 10–30 km from the comet centre last year, at least some of these dust jets can be traced back to specific locations on the surface, the first time this has ever been seen.
In a study reported today in the science journal Nature, 18 quasi-circular pits have been identified in the northern hemisphere of the comet, some of which are the source of continuing activity.
The pits are a few tens to a few hundreds of metres in diameter and extend up to 210 m below the surface to a smooth dust-covered floor. Material is seen to be streaming from the most active pits.
"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.
Scientists analysing the images think that the pits are formed when the ceiling of a subsurface cavity becomes too thin to support its own weight and collapses as a sinkhole. This exposes the fractured interior of the comet, allowing otherwise hidden material to sublimate, thus continuing to erode the pit 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.
The authors suggest three possible ways the voids are formed.
One idea is that they have existed since the comet itself formed, as a result of very low-speed collisions between primordial building blocks tens to hundreds of metres in size. The collapse of the roof above such a void could then be triggered through weakening of the surface, perhaps by sublimation or via seismic shaking or impact from boulders ejected from elsewhere on the comet.
Another possibility is the direct sublimation of pockets of volatile ices like carbon dioxide and carbon monoxide below the surface, heated by the warmth of sunlight penetrating an insulating top layer of dust.
Alternatively, sublimation could be driven by the energy liberated by water ice changing its physical state from amorphous to crystalline then sublimating the more volatile surrounding carbon dioxide and carbon monoxide ices.

Image: (Above) Active pits on comet. (Bottom-Left) Active regions in Seth. Image: Active pits.