Sublime Surprise: Rosetta's Comet Cycles its Ice

Scientists have discovered an unexpectedly regular cycle of ice formation and depletion on the surface of a comet, a pattern tied to an orbital dance of shadow and sunlight.

Scientists have discovered an unexpectedly regular cycle of ice formation and depletion on the surface of a comet, a pattern tied to an orbital dance of shadow and sunlight.

Measurements taken by the European Space Agency's Rosetta spacecraft, currently orbiting comet 67P/Churyumov-Gerasimenko, show that ice builds up when a particular region of the comet is in shadow. The ice then transitions, or sublimates, to gaseous water vapor when that region shifts into sunlight.

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"We observed this cycle for several comet rotations ... We were surprised to see so clearly the appearance and disappearance of the ice due to temperature and illumination conditions," planetary scientist Maria Cristina De Sanctis, with the Institute for Space Astrophysics and Planetology in Rome, wrote in an email to Discovery News.

The finding helps resolve a puzzle about why a comet's surface can be relatively free of ice, such as what has been observed on 67P and other comets, even though the bodies are outgassing water. The cycle of condensation and sublimation shows how water ice can be transported from the interior of the comet to the surface.

"This water cycle appears to be an important process in the evolution of the comet," researchers wrote in an article published in this week's Nature.

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The cycle also helps explain why comets stay active. "In some way, it can prolong the comet's life," De Sanctis added.

Scientists are not yet sure if the water cycle accounts for 67Ps' odd twin-lobed, duck shape. One theory is that 67P originally was two comets that melded together over time. The other idea is that the region between the comet's lobes, informally referred to as "the neck", has been especially active over time, causing a gradual reshaping of what was once a rounder (and single) body.

"There is a strong debate about this issue," De Sanctis said. "Personally, I think that the neck region can be the result of an evolution of the comet that experienced different thermal regimes at different distance from the sun."

"Rosetta sees that at relatively large distances from the sun, the neck region is the most active and thus it is also the one that is most largely affected by the condensation and sublimation phenomena," she said.

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Scientists will have a chance to explore this theory, among others, over the next month. On Wednesday, Rosetta is due to begin a three-week excursion that will bring it to a point more than 932 miles from the comet's nucleus, the farthest since its arrival in August 2014. The spacecraft currently orbits about 280 miles from the nucleus.

Scientists are hoping to get a broad picture view of the comet, which reached its closest point to the sun, known as perihelion, last month. They also want to study how the envelope of ionized gas around 67P interacts with the solar wind.

"While it may appear odd to depart from the nucleus at this time, these measurements are also key to understanding the comet's behavior ... and must be performed not too long after perihelion so that the comet is still appreciably active," Rosetta scientist Claire Vallat said in a blog post on the project's website.

This single frame Rosetta navigation camera image of Comet 67P/Churyumov-Gerasimenko was taken on Sept. 11, 2015, from a distance of 319 kilometers (193 miles) from the comet center.

At 5:29 a.m. EDT (9:29 a.m. GMT) on Aug. 6, 2014, the European Space Agency's comet-chasing spacecraft Rosetta was a comet-chaser no more.

After 10 years and 3.7 billion miles, Rosetta reached its target, Comet 67P/Churyumov-Gerasimenko, to begin an unprecedented mission in cometary orbit -- the first mission to ever do so. Rosetta will remain in orbit as the comet swings past close approach of the sun, observing changes in the icy body's structure as they travel with one another. Comets possess some of the most pristine material in the solar system, having been in deep freeze since before the formation of the planets. So through Rosetta's instruments we'll not only be studying a fascinating celestial body, we'll be probing billions of years into the past. To help achieve this goal, in November Rosetta will even drop a small probe, called Philae, to attempt the first ever landing on a comet's surface.

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But for now, ESA has released the most detailed, and stunning, photos of 67P/Churyumov-Gerasimenko to date as Rosetta settles into orbit around the 2.5 mile-wide 'dirty snowball.' Shown in this image is 67P/Churyumov-Gerasimenko in its entirety taken by Rosetta's OSIRIS narrow-angle camera on Aug. 3 from a distance of 177 miles.

The comet is composed to two very distinct "lobes" -- a configuration that made the cometary nucleus look like a "rubber ducky" during Rosetta's approach to 67P/Churyumov-Gerasimenko. Astronomers believe that the object may be a "contract binary," where two separate objects collided and fused together as one. Shown here is an extreme closeup of a smooth region of the "body" (or the larger lobe) of the comet. The photo was captured on the day of rendezvous (Aug. 6) by Rosetta’s OSIRIS narrow-angle camera and it shows small boulders, cliffs and other, as-yet to be explained features. The resolution of this observation is 2.4 meters per pixel. Rosetta was a mere 81 miles from the comet.

Looking down on the comet's "body" from a distance of 177 miles on Aug. 3, 2014. Beautiful layering of material can be seen with big boulders breaking up large expanses of smooth terrain. This is a truly alien landscape and Rosetta has only just begun its mission to try to understand 67P/Churyumov-Gerasimenko's composition and dynamics.

Where the two lobes of 67P/Churyumov-Gerasimenko's mass meet, is a clearly defined "neck." As Rosetta was approaching the comet, a bright feature at the neck became clear, something that the mission will study to help us understand its nature. This photo was taken on Aug. 6, 2014, when the spacecraft was just 75 miles from the comet's surface, acquiring a resolution of 2.2 meters per pixel.

This earlier observation taken by the Rosetta probe as it was chasing the comet on Aug. 2, 2014, shows how active its nucleus is. The rays of light emanating from the top are jets of vapor forming the comet's coma, within which Rosetta is now orbiting. This cometary activity will increase as Comet 67P/Churyumov-Gerasimenko continues to approach the sun. And thanks to Rosetta, for the first time ever, we are going to have a ringside seat for the voyage.

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