There’s more than meets the (naked) eye on Rosetta’s comet. OSIRIS, one of the probe's instruments, was able to spot a subtle color difference on Comet 67P/Churyumov-Gerasimenko by looking at how different colors of light were reflected by the comet.
67P appears a dull charcoal color when observed by us, but the Rosetta analysis has highlighted three different types of terrains on the comet associated with different reflectivities. They are found on both lobes of the comet, and they often (but not always) coincide with the 25 different regions identified on the surface. The discovery was reported in Astronomy & Astrophysics.
OSIRIS, which stands for Optical, Spectroscopic, and Infrared Remote Imaging System, has 20 color filters that allow the camera to capture different types of light coming from the comet. Since different materials absorb different types of light, using filters can tell us about the composition of the ground on the comet.
Blue regions suppress red light, green has moderate red light reflectivity, and red has a lot. ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
The researchers were also able to identify a feature, a peak in ultraviolet (UV) light, by observing 67P using OSIRIS. “This feature lies in the ultraviolet range where instrument calibrations tend to be tricky and need still to be confirmed,” said Dr. Sonia Fornasier, lead author of the study, in a statement. “If the feature proves to be real, it is compatible with the presence of frozen sulfur dioxide on the comet’s surface,” she adds. Sulfur dioxide has been detected around several comets, including 67P.
OSIRIS has an incredible resolution of one meter (3.3 feet) per pixel, and this allowed the scientists to investigate the finer structure of the surface material. The comet is covered in a layer of loose rock, soil, and dust (known as regolith) with an incredibly high porosity (87 percent). These findings are consistent with the previous observation of comets like Wild 2 and Tempel 1.
Although there’s a variation in terrain, this doesn’t provide indication of a diversified core deep in the comet. “The three groups of terrain we identified are not correlated to a particular morphology that may expose material from deeper inside the nucleus,” added Dr. Fornasier. “Therefore, the reflectivity variations of the surface do not show evidence of vertical diversity in the nucleus composition, at least for the first tens of meters.”
Rosetta will continue to operate for most of 2016 until it will be sent crashing down on 67P.