During its examination of comet 67P/Churyumov-Gerasimenko, ESA's Rosetta has detected changes in the sunlit parts of the comet using its Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS). These changes are the result of the sun's heat stripping away the older surface of the comet in order to reveal newer, fresher material, causing the comet to appear much bluer than it did at the start of the mission.
A team of researchers has released a new study on this phenomenon, reporting on data from the spacecraft's arrival at the comet in August 2014 all the way up to November 2014, during which the Rosetta spacecraft was between 100 kilometers to 10 kilometers of the comet's nucleus. During this period, the comet continued on its orbit, making its way closer to the sun.
The VIRTIS monitored the changes in the amount of light that was reflected from comet 67P's surface over a wide range of visible and infrared wavelengths, measurements that indicate changes in the composition of the outer layer of the comet.
Upon arriving at the comet, Rosetta data showed a very dark body that reflected about 6 percent of visible light falling on it due to a surface composed mostly of dark, dry dust.
As time went by, ices deep within Rosetta's surface became warmed, turning them into gases and leading to their escape onto the surface. As these old dust layers escaped the comet, new material became exposed, leading to a more reflective surface and subsequently making the comet richer in ice and, in turn, bluer colors.
"The overall trend seems to be that there is an increasing water-ice abundance in the comet's surface layers that results in a change in the observed spectral signatures. In that respect, it's like the comet is changing color in front of our eyes," said Gianrico Filacchione, lead author of the study and a member of the American Astronomical Society.
"This evolution is a direct consequence of the activity occurring on and immediately beneath the comet's surface," he added. "The partial removal of the dust layer caused by the start of gaseous activity is the probable cause of the increasing abundance of water ice at the surface."
"The evolution of surface properties with activity has never been observed by a cometary mission before and is a major science objective of the Rosetta mission," said Matt Taylor, ESA's Rosetta project scientist. "It is great to see science papers being published directly addressing this topic and we're looking forward to seeing how things have changed over the entire mission."
The findings were published in the March 16 issue of Icarus.
