(Image: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA)
Even comets have weather. The Rosetta probe has spotted a square-kilometre field of solid ice in the neck region of comet 67P/Churyumov-Gerasimenko. The comet鈥檚 day-night cycle drives its small weather system using the ice field, sublimating the ice into vapour when the sun rises.
Advertisement
The ice was spotted in data gathered last August using Rosetta鈥檚 VIRTIS instrument 鈥� a spectrometer designed to map the comet鈥檚 chemical composition. The water signal was stronger when the neck was in shadow and weaker during the comet鈥檚 day, suggesting the ice patch appeared and vanished as the comet rotated.
鈥淚t鈥檚 not a surprise that the comet contains ice, but direct evidence of water on the surface is a real novelty,鈥� says of the Max Planck Institute for Solar System Research in G枚ttingen, Germany.
Lost in space
The team thinks that as the sun rises over the deathly cold 67P, its peculiar weather system kicks off. The sun鈥檚 heat sublimates the ice, kicking it directly from a solid into a cloud of water vapour above the surface. As the comet rotates into darkness, most of this vapour escapes into space and doesn鈥檛 make it back to the surface.
Instead, it is likely that the surface ice is replenished from beneath. The sharp contrast between day and night time temperatures on the comet may be enough to continually crack its surface, allowing water to escape from the comet鈥檚 interior.
Maria Cristina De Sanctis of the Institute for Space Astrophysics and Planetology in Rome, Italy, first author on a paper detailing the observation, points out that the ice fields they found only account for 3聽per cent of the vapour coming off the comet. But she is confident that there are other regions of ice on the surface of the comet that haven鈥檛 been seen yet.
Violent jets
Sierks says comets like 67P are sublimating away all the time through cycles like this, using up their water. 鈥淲hat we see today is the remnant from the many orbits before,鈥� he says. 鈥淚n about 1000 orbits it will be gone.鈥�
Understanding how these water cycles work may lead to insights about where Earth鈥檚 water came from during a phase when the planet was less peaceful. 鈥淭here were big, big cycles going on in the early days,鈥� says Sierks, 鈥渙ne impact boiling away the upper 3000聽metres of ocean water. This quiet, peaceful environment is not typical.鈥�
There鈥檚 still a lot about 67P that isn鈥檛 understood, including cave-ins at the surface and violent outbursts of material. In fact, because it is currently near the sun, the comet is sending out jets of ice and dust, so the team is holding the probe back to protect it.
Luckily, the mission has just been extended to stay with the comet until September 2016. 鈥淲e will get back close to the surface as soon as we dare,鈥� says Sierks.
Journal reference:
