The Rosetta mission reached the end of its observation phase in late September when the orbiter touched down spectacularly on the surface of Comet 67P/Churyumov-Gersimenko. By then, it had spent 4595 days in space, and had travelled 7.9 billion kilometres, performed a total of six fly-bys past Earth, Mars and two asteroids and accompanied Comet 67P during its journey through the Solar System on a mission lasting more than two years. If that were not enough, it also conducted 11 scientific experiments and dispatched the robot lander Philae in 2014 to investigate the comet. The journal Science has now published one of the mission's discoveries with contributions by 10 scientists from the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR). Based on data acquired from the Visible and Infrared Imaging Spectrometer (VIRTIS), the article reports on the discovery of brief appearances of dry ice (carbon dioxide ice) on certain areas of the comet's surface, followed by the emergence of two unusually large outcroppings of water ice. "This was the first time that the presence of carbon dioxide ice has been found on the surface of a comet. The data was validated based on unequivocal spectral properties in a larger expanse of roughly 80 by 60 metres in the Anhur region," reports DLR planetary researcher Gabriele Arnold, who is coordinating the work on VIRTIS in Germany.
This phenomenon was observed on two successive days at the end of March 2015, when the comet was close to the Sun and was therefore extremely active. At the time, sunlight was falling on the comet's southern hemisphere, which due to the cometary seasons had been shrouded in darkness during the early phase of the observation campaign. The measurements show that the observed patch comprises just a small percentage of dry ice, mixed with a perennial crust made of dark organic material with non-volatile components.
Carbon dioxide in the passage of the seasons
"This kind of dry ice is a frequent component of a comet's nucleus, but it had never been observed on the surface," explains Arnold. "This is due to its low evaporation temperature, which is substantially below that of water ice and which causes it to sublimate, or evaporate, immediately upon exposure." An analysis of the same region three weeks later therefore revealed that the dry ice had disappeared completely. Model calculations show that the identified area must have contained 57 kilograms of carbon dioxide in a roughly nine-centimetre layer of dry ice. After the dry ice disappeared, the OSIRIS camera on board Rosetta picked up a stronger presence of blue components in the reflected light in April 2015, leading it to detect two large patches with water ice at precisely the location where the carbon dioxide had previously evaporated. Most likely, this water ice came from layers deposited beneath the dry ice.
"It is conceivable that these deposits originated when the comet passed its perihelion for the last time in 2009," says Arnold. "In this case, it would have been deposited by the evaporated dry ice that condensed on the surface at the time. It would then have frozen there as the darker seasons spread over the southern hemisphere and the comet travelled further away from the Sun." This describes a complete seasonal cycle of carbon dioxide that would match the comet's orbital period of 6.5 years. In contrast, the water ice cycle is more likely to be determined by daily fluctuations.
Inside the Solar System's oldest objects
In total, the Rosetta mission transmitted approximately 220 GB of scientific data to Earth, which is being analysed. This data will enable a significantly better understanding of the origins, nature, properties and role of comets in the nascent Solar System. Here, processes relating to cometary activity and dynamics reveal important information on the inner workings of the comet and hence on the development and origins of these small celestial bodies, which are among the oldest objects to populate the early Solar System.
"For instance, the variable and dynamic properties of the ice on the comet's surface are indicative of complex cometary activity. They will lead to further investigations into the origins and history of comet 67P," affirms Arnold.