As the sun’s heat strips away the older surface of Rosetta’s comet, it has been observed changing color and brightness as fresher material is revealed.
The observations were made in the months immediately following the spacecraft’s arrival at Comet 67P/Churyumov–Gerasimenko in August 2014. Rosetta’s Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS) began to detect these changes in the sunlit parts, mainly in the northern hemisphere and equatorial regions of the comet.
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, according to ESA.
To see the subtle changes in the composition of the comet’s outermost layer, researchers used VIRTIS to monitor changes in the light that was reflected from the surface over a wide range of visible and infrared wavelengths.
When Rosetta first arrived, the comet was an exceptionally dark body reflecting only around 6 percent of all visible light that fell on it. It is believed this was due to its surface being mostly covered in a dark, dry, dust that was made of a mixture of minerals and organics.
Now on the comet surface there are areas that are slightly brighter and some that are slightly darker, which indicates the differences in composition. However, most of the surface is slightly reddened by organic-rich material, while there is the occasional ice-rich material that shows up as somewhat bluer.
According to ESA, 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 is able to show as the “old” dust layers were gradually ejected, fresher material was gradually reviled. The new surface was both more reflective, which made the comet brighter, and was richer in ice, resulting in the bluer measurements.
ESA wrote that on average the comet’s brightness had changed by about 34 percent. In the Imhotep region it had increased from 6.4 percent to 9.7 percent over the three months of observations. Gianrico Filacchione, lead author of the study, said in a statement:
“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.
“This evolution is a direct consequence of the activity occurring on and immediately beneath the comet’s surface. 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.”
With Fabrizio Capaccioni, VIRTIS principal investigator, adding:
“The surface properties are really dynamic, changing with the distance from the sun and with the levels of comet activity.
“We’ve started analyzing the subsequent datasets, and can already see that the trend continues in the observations made beyond November 2014.”
Fabrizio Capaccioni, VIRTIS principal investigator, said that:
“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.
“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.”