Jets of gas and dust being spat out from Rosetta’s comet have been found to be coming from sinkholes on its surface. The active pits, which are not too dissimilar from those found on Earth, offer a glimpse into the heart of comet 67P/Churyumov-Gerasimenko where there seems to be a great deal of turmoil.
If you could stand on comet 67P, you would find cavities, measuring a few hundred metres in diameter, under the comet’s surface. These ‘chambers’ are thought to be so unstable that they collapse in a sinkhole process – that’s according to research that was kickstarted by a team of comet scientists at the Max Planck Institute, who poured over images of the rugged surface returned by the Rosetta spacecraft. Comet 67P has been revealed to be covered in ice patches, which appear as metre-sized ice boulders peaking through a skin of dark dust and steaming pits.
“Apparently, these underground voids grow larger with time until the top layer becomes unstable and caves in,” says Holger Sierks from the Max Planck Institute and Principal Investigator of OSIRIS – the scientific imaging system on board Rosetta – which has been snapping the comet’s every facet ever since it began orbiting 67P in August 2014. The pits are exceptionally deep with the largest extending up to two hundred metres into the comet’s interior. Layers and terraces characterise the walls of these depressions, which reach up from flat floors. The fine jets of dust are emitted from the inside of these walls – a conclusion reached by looking closely at images of a single jet from varying angles. “In this way we obtain information on the jet’s three dimensional structure and can determine their origin on the surface,” says Jean-Baptiste Vincent, an OSIRIS scientist.
It’s easy to think that these indentations in the comet’s surface could be impact craters. However, Vincent puts this speculation to bed since similar structures have been discovered on other comets such as the likes of 9P/Tempel 1 and 81P/Wild 2, which have been visited by NASA’s Deep Impact and Stardust in the past. “Because of their unusual morphology, these pits can be clearly distinguished from impact craters,” says Vincent. “They seem to be a typical characteristic of comets.”
However, the dust that’s fired from the walls hasn’t made these structures either. The frozen gases that evaporate from comet 67P’s surface – under the influence of the Sun’s heat – are unable to carry enough dust with them to make holes of such sizes. Even sudden bursts of activity on the comet’s surface, such as the one that occurred in April 2014, are unable to shift enough cometary material to make the pits. The plausible explanation is that the pits are collapsed cavities, with evidence of fresh material exposed at the edges of the depression. It is from here that the gases are able to vaporise, in turn charging the jets.
The pits are useful in estimating the age of cometary surfaces. “Since the pits are active, they change with time,” says Vincent. Comet 67P’s pits expand, with edges that retreat to form extensive terraces. A cometary surface exhibiting deep holes is therefore rather young, while older areas present themselves as smooth plateaus.
There are several possibilities as to how the cavities came to exist. The holes could date all of the way back to the comet’s birth, when smaller chunks of rock – known as planetesimals – collided at low speeds, which made the dents. Another suggestion is that they are the result of frozen carbon dioxide and monoxide evaporating from within the comet’s heart.
At present, the team at the Max Planck Institute are keeping an open mind. “We do not favour any options. Maybe many effects work together,” adds Sierks. “But we hope that the mission will bring clarity in [the future].”