ESA’s Rosetta gave Evidence for Daily Water-Ice Cycle on Surface of Comets

This week, VIRITS revealed evidence of a daily water-ice cycle on the surface of comments.

But as Comet 67P hurtles closer to the sun, the Rosetta spacecraft is keeping its distance from its target, which means future findings will have to wait.

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. This gas then flows away from the comet, carrying dust particles along with it. Together, gas and dust build up the bright halo and tails that are characteristic of comets. On August 13, 2015, the comet reached the closest point to the Sun along its 6.5-year orbit, and is now returning towards the outer Solar System.Rosetta’s scientists are investigating how activity on the comet and the associated “outgassing” work.Using a Rosetta instrument called the Visible, InfraRed and Thermal Imaging Spectrometer (VIRTIS), they identified an area on the comet’s surface where water ice appears and disappears in sync with its spinning.

“We found a mechanism that replenishes the surface of the comet with fresh ice at every rotation: this keeps the comet alive”, said lead author Dr Maria Cristina De Sanctis of INAF-IAPS in Rome, Italy. When the sun rises, the molecules in the newest ice layer sublimate, and the whole outgassing cycle starts over. “This indicates a cyclical behavior of water ice during each comet rotation”. Rather than raining out onto the comet’s surface, 67P’s ephemeral cloud zips away into space when night falls.

Even comets have weather, it turns out, a discovery made by the Rosetta spacecraft orbiting the comet 67P/Churyumov-Gerasimenko. The scientists focused on a one square km region located on the comet’s ‘neck.’ This was one of the most active areas on the comet at the time.

Some of the water vapor sublimating from the warmer near-surface layers of Comet 67P (and, presumably, other comets as well) recondenses on the frigid surface, De Sanctis and her team said. According to research published this week in Nature, Comet 67P has a small weather system driven by an intense day-night cycle.

“We say that this cycle is in a position to fill the surface levels of ice packs from the originial and internal…”

“By this mechanism, the surface layer becomes enriched in water ice”, the scientists wrote. Down to a few cm deep over the region of the portion of the comet nucleus that was surveyed, water ice accounts for 10-15 per cent of the material and appears to be well-mixed with the other constituents.

Rosetta first spotted water vapor ejections on Comet 67P-G when it was approaching the sun and its ice was warming up, melting, and breaking through like a geyser. The daily exposure to sun also drives the outbursts of water.