The European Space Agency's Rosetta spacecraft detected relatively large grains of water ice in two different places on the surface of Comet 67P/Churyumov-Gerasimenko, which the probe has been orbiting since August 2014.
These big grains may have formed after heat from the sun sublimated (or vaporized) buried water ice, which then recondensed and was redeposited in subsurface layers, without ever leaving Comet 67P, researchers said.
"If the thin ice-rich layers that we see exposed close to the surface are the result of the comet's activity, then they represent its evolution, and it does not necessarily require global layering to have occurred early in the comet's formation history," study lead author Gianrico Filacchione, of the Institute for Space Astrophysics and Planetology at the National Institute for Astrophysics in Rome, told Space.com via email.
Comets are made primarily of water ice, but the stuff is rarely observed on their frigid surfaces. Indeed, the 2.5-mile-wide (4 kilometers) Comet 67P appears to be covered by a nearly uniform layer of dark dust, Filacchione said.
"We have measured that the surface reflects only a few percent of solar light," he said. "Ices are not stable for a long time on the surface of the nucleus because, during the perihelion passage [closest approach to the sun], they sublimate, originating the gaseous coma."
Filacchione and his colleagues studied observations of Comet 67P made by Rosetta's Visual and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument. VIRTIS detected surface water ice in two separate, 3.3-foot-wide (1 meter) areas within a region of the comet dubbed Imhotep, the researchers report in a study published online today (Jan. 13) in the journal Nature.
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