The passage of the comet past the Sun made it either red or bluish, which was caused by the water cycle - the ice turned into gas.
When Comet 67P / Churyumov-Gerasimenko crossed a boundary around the Sun known as the "frost line," the ice on its surface began to turn into gas, lifting a layer of reddish mud and dust into space. As a result, fresh and clear blue ice was exposed.
The researchers who found this out write in their article that these changes took place over a long time - from January 2015 to August 2016 and were observed by the orbiter of the European Space Agency "Rosetta". It reached the comet on August 6, 2014 and was destroyed by a controlled hard landing on the comet on September 30, 2016.
The diagram below shows how the comet's hue changed from red to blue and back to red as it passed the Sun.
Approaching the Sun and crossing the "frost line", the comet "threw off" dust from itself, after which, freezing, it overgrown with it again. The reddish carbon-rich dust did not fly far from the comet and was attracted as soon as the expanding gas stopped throwing it away.
The changes observed over several months with the probe's color-sensitive camera were not visible from Earth - telescopes are not able to properly view the comet's nucleus. A two-year observation of the Churyumov-Gerasimenko comet from close range allowed a more thorough analysis of its composition and life cycle.
Despite the fact that Rosetta's mission is over, scientists still have to process a lot of collected data, which promises new discoveries.