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Rosetta catches jet erupting from comet's dark side

Paul Sutherland, Feature writer
Apr 21, 2015, 21:53 UTC

Sen—Europe’s Rosetta space probe has caught the moment a new jet of dust was ejected from the comet it is accompanying into the inner Solar System. But the jet has presented scientists with something of a puzzle.

The spacecraft’s OSIRIS wide-angle camera was monitoring Comet 67P/Churyumov-Gerasimenko when the outburst occurred. A first image taken on March 12 shows nothing, but two minutes later it is spurting into space.

The puzzle is that the new jet appeared on a side of the comet that was in shade and so not getting warmed directly by the Sun. It is the general warming as a comet approaches the Sun that produces its atmospheric coma and tails of gas and dust.

Rosetta has been observing Comet 67P’s nucleus of rock and ice becoming more active since it first made its rendezvous with it much further out in space in August 2014. The warming of the nucleus causes ice to turn to gas—a process called sublimation because, unlike evaporation, there is no intermediate liquid stage.


Two frames, taken two minutes apart by Rosetta’s OSIRIS wide-angle camera, capture the moment a jet bursts into action from the shadowed underside. Image credits: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

As the gas jets from the nucleus, it carries dust with it, feeding the surrounding coma. The jet caught erupting from 67P emerged from a region named Imhotep on the comet’s underside, ejecting dust at a speed of at least eight metres per second. Previous activity has been seen on the illuminated side.

Sen spoke to one of the world’s leading comet experts, Professor Alan Fitzsimmons, of Queen’s University Belfast, who pointed out that active jets have been observed from the shadowed sides of comets before.

He told us: “There has been evidence of nightside emission of material in previous flybys of comet nuclei. Jets from the unilluminated surface were seen at comet 9P/Tempel 1 by the NASA Deep Impact mission (in 2005) and subsequently also on comet 103P/Hartley 2 (in 2010).”

Professor Fitzsimmons added: “Not only has Rosetta seen this occurring on comet 67P/Churyumov-Gerasimenko, it has shown it can switch on within a minute or less. The problem for cometary scientists is to explain this behaviour.

“The expulsion of material from the comet is driven by sublimation of ices, which in turn needs heating from the Sun. But measurements of the subsurface temperature by the MIRO microwave instrument on Rosetta demonstrate very good insulation of subsurface ices by the surface dust layers, and these temperatures seem to change relatively slowly. So what caused it?

“One suggestion is this was a pocket of more volatile ice such as carbon monoxide (CO) or carbon dioxide (CO2) producing a small outburst. Perhaps this is the start of more exciting times—Deep Impact saw many small outbursts on comet 9P as it approached. Is the rising temperature of the nucleus starting to warm subsurface pockets of CO and CO2?”

The Rosetta scientists have remarked that the outburst happened as it was close to dawn at the Imhotep site, and they speculate that rays of sunlight might have hit cliffs or outcrops hidden from Rosetta’s view.

Professor Fitzsimmons told Sen: “It is interesting that the new jet seen on comet 67P switched on very close to local sunrise. The ROSINA mass spectrometer has shown that poorly illuminated areas can release more CO and CO2 than water.

“But how can the jet source ‘know’ sunrise is approaching? Could it be due to material condensing on a nightside peak or boulder and then being ejected as soon as it is touched by sunlight? One thing is certain—the exquisite data from Rosetta provides the first good opportunity to understand why comet activity can turn to the dark side.”

OSIRIS principal investigator Holger Sierks, of the Max Planck Institute for Solar System Research (MPS) in Germany, said in a statement: “This was a chance discovery. No one has ever witnessed the wake-up of a dust jet before. It is impossible to plan such an image.”