Researchers using the Visible, InfraRed and Thermal Imaging Spectrometer aboard ESA’s Rosetta spacecraft have identified a region on the surface of 67P/Churyumov-Gerasimenko where water ice appears and disappears in sync with its rotation period. The scientists report on this discovery in the current issue of the journal Nature.
Water-ice cycle of Comet 67P/Churyumov-Gerasimenko. Left, top: 67P/Churyumov–Gerasimenko based on four images taken by Rosetta’s navigation camera on September 2, 2014. Left, bottom: images of the comet taken with Rosetta’s VIRTIS spectrometer (left), and maps of water ice abundance (middle) and surface temperature (right). The images were taken on 12 (top), 13 (middle) and 14 September (bottom) and focus on Hapi, a region on the comet’s neck, one of the most active spots on the nucleus at the time. Right: the daily water ice cycle; during the local day, water ice on and a few centimetres below the surface sublimates and escapes; during the local night, the surface rapidly cools while the underlying layers are still warm, so subsurface water ice continues sublimating and finding its way to the surface, where it freezes again. On the next comet day, sublimation starts again, beginning from water ice in the newly formed surface layer. Image credit: data – ESA / Rosetta / VIRTIS / INAF-IAPS / OBS DE PARIS-LESIA / DLR / M.C. De Sanctis et al.; comet – ESA / Rosetta / NavCam / CC BY-SA IGO 3.0.
Comets are impressive phenomena in the night sky. As their orbit brings them into the inner Solar System, their icy cores heat up, setting gas and dust free.
The escaping gases, primarily derived from water ice, can also carry dust particles that form the coma and cometary tail.
“How and where exactly the sources of cometary activity arise has been a largely unsolved mystery in comet research,” said co-author Dr Gabriele Arnold of the German Aerospace Center.
“We found a mechanism that replenishes the surface of the comet with fresh ice at every rotation: this keeps the comet alive,” said lead author Dr Maria Cristina De Sanctis of INAF-IAPS in Rome, Italy.
The scientists studied a set of data taken in September 2014, concentrating on a one square km region on the comet’s neck. At the time, 67P/Churyumov-Gerasimenko was about 500 million km from the Sun and the neck was one of the most active areas.
As the comet rotates, taking just over 12 hours to complete a full revolution, the various regions undergo different illumination.
“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,” Dr Cristina said. “Conversely, when the Sun was shining on these regions, the ice was gone. This indicates a cyclical behavior of water ice during each comet rotation.”
The data suggest that water ice on and a few centimeters below the surface ‘sublimates’ when illuminated by sunlight, turning it into gas that then flows away from the comet. Then, as the comet rotates and the same region falls into darkness, the surface rapidly cools again.
However, the underlying layers remain warm owing to the sunlight they received in the previous hours, and, as a result, subsurface water ice keeps sublimating and finding its way to the surface through the comet’s interior.
But as soon as this ‘underground’ water vapor reaches the cold surface, it freezes again, blanketing that patch of comet surface with a thin layer of fresh ice.
Eventually, as the Sun rises again over this part of the surface on the next comet day, the molecules in the newly formed ice layer are the first to sublimate and flow away from the comet, restarting the cycle.
The comets 9P/Tempel 1 and 103P/Hartley 2 have also exhibited local water ice patterns, which could be explained by a similar day-night cycle. With their discovery, the researchers assume that this process is also found on other comets.
“We suspected such a water ice cycle might be at play at comets, on the basis of theoretical models and previous observations of other comets but now, thanks to Rosetta’s extensive monitoring at 67P/Churyumov’Gerasimenko, we finally have observational proof,” said Dr Fabrizio Capaccioni of INAF-IAPS.
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M.C. De Sanctis et al. 2015. The diurnal cycle of water ice on comet 67P/Churyumov-Gerasimenko. Nature 525, 500-503; doi: 10.1038/nature14869
