Comet 67P changed color like a cosmic chameleon as it cruised around our Solar System. Thanks to a fresh look at the data produced by the Rosetta space probe, a team of European scientists has detailed how the comet’s nucleus appeared to shift from red to blue as it came into close orbit with the Sun before turning reddish again as it voyaged into deep space.
According to their analysis, reported in the journal Nature, this was all down to how much water ice or dust was presented on and around the comet. The colors appear redder when there are more organic molecules rich in carbon, but appear bluer if there are higher levels of frozen water ice that’s rich in magnesium silicate.
Comet 67P/Churyumov-Gerasimenko was closely watched for around two years by the European Space Agency’s (ESA) Rosetta probe, which managed to reach and start orbiting the comet in August 2014. During the mission’s early days, the comet's nucleus was relatively far away from the Sun and covered in layers of dust and a little bit of ice. This made the comet appear red surrounded with an icy blue haze (the coma) when analyzed with the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument.
As it approached the Sun, the comet crossed a boundary called the frostline, heating the ice and turning it into gas, a process called sublimation. However, sublimation of the water ice also pushed out the surface dust grains too, revealing layers of fresh pristine ice. This made the nucleus turn bluer in color when viewed by VIRTIS.
After the comet looped around the Sun and started its journey back into deep space, its previous appearance resumed with the nucleus becoming redder and the coma bluer.
“Spectral analysis indicates an enrichment of submicrometre grains made of organic material and amorphous carbon in the coma, causing reddening during the passage. At the same time, the progressive removal of dust from the nucleus causes the exposure of more pristine and bluish icy layers on the surface,” the study authors summarise in their paper.
There are still many gaps in our knowledge of comets, despite the valiant work of Rosetta and its lander module Philae. For example, we still don’t understand much about the dusty organic molecules rich in carbon that appear red.
Until researchers can get their hands on these molecules from a piece of the comet, the best bet for understanding these organics is using data gathered by VIRTIS.
"There are definitely more exciting results to come," Matt Taylor, ESA project scientist for Rosetta, said in a statement. "The data collection may be over, but the analysis and the results will continue for years yet, adding to the rich legacy of cometary knowledge provided by Rosetta."