How a water cycle keeps a comet 'alive'

A comet's water-ice cycle follows a daily routine, according to data collected by European space probe Rosetta.

Launched by the European Space Agency in 2004, the Rosetta spacecraft arrived at at Comet 67P/Churyumov–Gerasimenko in August 2014. Observations gathered thus far indicate that surface ice and its outgassing activities follow a daily cycle based on illumination by the sun – a cycle that may be shared by other comets. 

"We found a mechanism that replenishes the surface of the comet with fresh ice at every rotation: this keeps the comet 'alive,' " says Maria Cristina De Sanctis from INAF-IAPS, Italy’s National Institute of Astrophysics, in a press release.

Comets are massive, dirty snowballs made up of of rocks, dust, and ices. Left over from the formation the solar system billions of years ago, they zip around the Sun along eccentric orbits.

As a comet moves closer to the Sun, the heat vaporizes the ice on its surface – water ice, carbon monoxide ice, and carbon dioxide "dry" ice – into gas. This is called outgassing. As the ice sublimates, bits of dust or rock that had been frozen to the comet find themselves suddenly freed. Together, the gas and dust flow away from the surface, comprising a comet’s signature bright halo and tails. 

Using Rosetta’s Visible, InfraRed, and Thermal Imaging Spectrometer (VIRTIS), scientists have discovered a region on the Comet 67P’s surface where water-ice appears and disappears in direct correlation to its rotation period – the comet's own day-night cycle.

Their data revealed that water ice on and a few centimeters below the comet’s surface turns into gas when illuminated by sunlight. When the comet rotates and the same region falls into darkness – that is, when night falls – the surface cools but the deeper layers of water ice remain warm enough to continue to sublimate and vaporize to the surface.

However, because the surface is cold from lack of sunlight, the water vapors refreeze at the surface and coat the comet with thin layer of ice. When the sun rises, the molecules in the newest ice layer sublimate, and the whole outgassing cycle starts over.

"We suspected such a water ice cycle might be at play at comets, on the basis of theoretical models and previous observations of other comets, but now, thanks to Rosetta’s extensive monitoring at 67P/Churyumov-Gerasimenko, we finally have observational proof," says Fabrizio Capaccioni, also from INAF-IAPS, in the ESA statement.

The Rosetta mission so far has provided fruitful information on 67P's composition, potentially offering new insights about the early days of the solar system.

Scientists will continue to analyze the VIRTIS data collected in the following months, as Comet 67P keeps thawing from its proximity to the sun. Its moment of closest approach, known as "perihelion," was Aug. 13, but the longer it stays close to the sun, the more active the comet becomes.