Even comets have weather, it turns out, a discovery made by the Rosetta spacecraft orbiting the comet 67P/Churyumov-Gerasimenko.

The European Space Agency spacecraft has uncovered evidence of a daily water-ice cycle on the comet's surface, tied to its rotation that exposes areas of the comet to regularly alternating sunlight and shadow, scientists say.

That cycle drives a small weather system, in which the ice sublimates into vapor during the comet's "sunrise," they explain.

A square-kilometer field of solid ice in the "neck" region of the dumbbell-shaped comet appears and then vanishes with each rotation, the researchers say.

The discovery was made using Rosetta's Visible, InfraRed and Thermal Imaging Spectrometer, or Virtis, the researchers report in the journal Nature.

"We observed this cycle for several comet rotations," says planetary scientist Maria Cristina De Sanctis from the Institute for Space Astrophysics and Planetology in Rome.

"We were surprised to see so clearly the appearance and disappearance of the ice due to temperature and illumination conditions," she says.

Heat from sunlight kicks off the sublimation, where the ice goes directly from its frozen state to a vapor without an intermediate liquid stage.

When the region of the comet rotates back into darkness, most of that vapor escapes into space rather than settling back onto the surface as ice, the scientist says.

So, how does the icy patch reappear?

The researchers say they suspect it is being replenished from beneath, from within the comet's interior, as the extreme difference between sunlight and shadow temperatures cracks the surface, creating a path for water to rise from inside.

"It's not a surprise that the comet contains ice, but direct evidence of water on the surface is a real novelty," says Holger Sierks of the Max Planck Institute for Solar System Research in Göttingen, Germany.

Data gathered by Rosetta suggests water-ice accounts for 10 to 15 percent of the material on the comet's surface, well-mixed with other constituents, the researchers say.

Sublimation is happening all the time on comets such as 67P, Sierks says, and will eventually use up all the comet's water.

"What we see today is the remnant from the many orbits before," he says. "In about 1,000 orbits it will be gone."

The researchers will also present their results at the European Planetary Science Congress, which starts Sept. 27 in Nantes, France.

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