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.