Rosetta's comet has been seen changing colour and brightness in front of the ESA orbiter's eyes, as the Sun's heat strips away the older surface to reveal fresher material.
Rosetta's Visible and InfraRed Thermal Imaging Spectrometer, VIRTIS, began to detect these changes in the sunlit parts of Comet 67P/Churyumov-Gerasimenko – mostly the northern hemisphere and equatorial regions – in the months immediately following the spacecraft's arrival in August 2014.
A new paper, published in the journal Icarus, reports on the early findings of this study, up to November 2014, during which time Rosetta was operating between 100 km to within 10 km of the comet nucleus. At the same time, the comet itself moved along its orbit closer to the Sun, from about 542 million km to 438 million km.
VIRTIS monitored the changes in light reflected from the surface over a wide range of visible and infrared wavelengths, as an indicator of subtle changes in the composition of the comet's outermost layer.
When it arrived, Rosetta found an extremely dark body, reflecting about 6% of the visible light falling on it. This is because the majority of the surface is covered with a layer of dark, dry dust made out of a mixture of minerals and organics.
Some surfaces are slightly brighter, some slightly darker, indicating differences in composition. Most of the surface is slightly reddened by organic-rich material, while the occasional ice-rich material shows up as somewhat bluer.Even when Rosetta first rendezvoused with the comet far from the Sun, ices hidden below the surface were being gently warmed, sublimating into gas, and escaping, lifting some of the surface dust away and contributing to the comet's coma and tail.
VIRTIS shows that as the 'old' dust layers were slowly ejected, fresher material was gradually exposed. This new surface was both more reflective, making the comet brighter, and richer in ice, resulting in bluer measurements.
On average, the comet's brightness changed by about 34%. In the Imhotep region, it increased from 6.4% to 9.7% over the three months of observations.
