The European Space Agency's Rosetta spacecraft has provided the first evidence of water cycles on the surface of comets.
Rosetta arrived at Comet 67P/Churyumov-Gerasimenko in August 2014, and the spacecraft recently observed the object at its closest point to the Sun for the first time. Comets are composed of dust and ice, and when they move closer to the Sun their frozen nucleus turns into gas and escapes into space, carrying the surface dust with it. This phenomenon is what causes comets' characteristic bright halos and tails.
Using Rosetta's Visible, InfraRed and Thermal Imaging Spectrometer (VIRTIS), a team of ESA researchers spotted a region on the comet where water ice appeared to evaporate and reappear in sync with the object's rotation patterns.
"We found a mechanism that replenishes the surface of the comet with fresh ice at every rotation: this keeps the comet 'alive'," said Maria Cristina De Sanctis from INAF-IAPS in Rome, Italy, lead author of the study recently published in the journal Nature.
The comet takes about 12 hours to complete a full revolution, and during this rotation different regions of 67P/Churyumov-Gerasimenko are exposed to various levels of illumination from the Sun. By looking at a data set taken in September 2014, the researchers were able to determine that the neck was one of the most active areas.
"We saw the tell-tale signature of water ice in the spectra of the study region but only when certain portions were cast in shadow," De Sanctis said. "Conversely, when the Sun was shining on these regions, the ice was gone. This indicates a cyclical behaviour of water ice during each comet rotation."
The findings suggest water on and below the comet's surface "sublimates" when exposed to sunlight, but the surface rapidly cools down when the region is rotated into the darkness.
The fascinating water cycle is one of the many findings made by the Rosetta spacecraft. In July, Rosetta spotted sinkholes on the surface of the comet that appear to be as large as the Pyramids of Giza.
