captured by the Rosetta spacecraft's navigation camera,
showing two patches of exposed water ice
(which are seen close
up in images C and D).
A European spacecraft has spotted water ice on the surface of a comet, shedding new light on the formation and evolution of the icy object.
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.
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, Gianrico Filacchione said.
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.
Both patches are associated with cliff walls and recent debris falls, which likely explains why the ice did not quickly boil away into space.
The instrument's observations suggest that the water ice is present in two different grain sizes, he added - those on the micrometer (one-millionth of a meter) scale, and those with an average size of about 2 millimeters (0.08 inches).
The 2-millimeter grains are particularly intriguing, Filacchione said, because they can be explained by the growth of secondary ice crystals. These secondary crystals can form via "sintering" (the compaction of smaller grains) or via the sublimation process outlined above.
Lab work suggests that sublimation could well be involved - and, therefore, that layers of water ice have been deposited beneath 67P's surface over the course of the comet's history.
The Rosetta mission blasted off in March 2004 and, a decade later, in August 2014, became the first spacecraft ever to orbit a comet. In November 2014, Rosetta scored another spaceflight first when it dropped a lander called Philae onto the surface of 67P.
The comet made its closest approach to the sun in August 2015 and is now zooming back out toward the outer solar system.
Rosetta will continue making measurements from orbit until Sept. 30, when it is scheduled to end its historic mission with a slow-motion crash landing onto the comet's surface.