Scientists Find Never-Before-Seen Dry Ice on a Comet

The Rosetta mission may have already ended, but the spacecraft continues to share new information about its target, Comet 67P/Churyumov-Gerasimenko.

Just recently, scientists have discovered carbon dioxide ice or dry ice on the surface of Rosetta's comet. This is the first time the compound was spotted on a comet, the European Space Agency (ESA) said.

The carbon dioxide ice layer was spotted by the spacecraft as Comet 67P approached its most active period last year. The probe also found two unusually large patches of water ice.

According to the scientists, all three icy layers had been found in the comet's southern hemisphere. The dry ice layer covered an area about the size of a football pitch, while the two water ice patches were each bigger than an Olympic-sized swimming pool - the largest of any signs of water ice on a comet previously discovered.

"We know comets contain carbon dioxide, which is one of the most abundant species in cometary atmospheres after water, but it's extremely difficult to observe it in solid form on the surface," Gianrico Filacchione from Italy's INAF-IAPS Instituto di Astofisica e Planetologia Spaziali and research lead for the first study said in a statement.

The research team studied infrared spectral images of Comet 67P's nucleus (comet's solid core) taken by Rosetta's VIRTIS instrument (visible, infrared and thermal imaging spectrometer). They focused on a patch of carbon dioxide ice in the Anhur region approximately 60 by 80 meters.

The discovery hints at extreme seasonal changes that can occur on a comet's surface. According to the scientists, Comet 67P's climate is not only determined by its orbit and distance from the sun. It is also affected by the irregular surface of the nucleus, where certain parts of the comet's topography were hidden from the sun for extended periods and vaporized only as the local temperature rose again in April 2015.

Another group of scientists led by Sonia Fornasier of the LESIA Observatory at Paris Diderot University in France studied the seasonal and daily color changes on the comet's surface.

Researchers of the second study used comet images to track its movement from its coldest period in early 2015 to perihelion (April 2015) - a period when the comet is closer to the sun or the season of highest temperatures. They found that the comet's surface reflected a blue light as it moved from shadow to sunlight, which hints at the presence of ice. As the comet moved away from the sun, the surface color turned red again.

"The distribution of water ice beneath the dusty surface of the comet seems widely but not uniformly spread, with small patches punctuating the nucleus, appearing and disappearing as a result of the comet's activity," the researchers said.

According to ESA Rosetta project scientist Matt Taylor, the findings reveal new details about the composition and history of the comet's nucleus.