In the two-year course of the Rosetta mission, a data review shows the color changes in the comet 67P or Churyumov-Gerasimenko. The comet sometimes appeared reddish while it became bluish in hue at some instances. Weird as it may sound, scientists came up with an explanation for the phenomenon that does not involve aliens.
According to research published by G.Filacchione, et.al of the Institute for Space Astrophysics and Planetology in Italy, INAF-IAPS, the color changes in comet 67P depends in its orbital location. When facing the sun, the comet's nucleus becomes blue while when the nucleus turns away from the sun, it's color changes to red. Whilst, the bubble of gas and dust surrounding the comet, also known as its coma, shifts color the other way around. Thus when near the sun, it appears red and emit a bluish hue away from the sun. The authors of the study have also linked the amount of water in the surface of the comet and area around the spectral variability.
During the course of the two-year Rosetta mission, the European Space Agency took measurements of the comet countless times. The mission started in July of 2014 and ended in September 2016. The collected data included almost 4,000 images of the comet which were taken using the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) has revealed the comet's continuous color shifting behavior.
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The comet, Churyumov-Gerasimenko, has an elliptical orbit that takes is closer to the center of the solar system - the sun, every 6.4 years and past Jupiter. When Mission Rosetta began in 2014, comet 67P was still far from the sun. In the initial period of the mission, observations on the comet revealed traces of visible ice and dust on its surface. During that time, the images from VIRTIS revealed a distinct red color on the comet's nucleus.
However, as the comet moved past the solar system's frost line- a boundary where exposed ice water undergoes sublimation and has ventured much closer to the sun images from the VIRTIS revealed the comet emitting a new bluish hue and less red.
According to the researchers, the sublimation process displaced grains of dust on the surface of the comet exposing the more bluish icy and pristine layers on the surface. They further added that the blue colors are produced fro,e the frozen water ice-rich in magnesium silicate whilst carbon-rich organic molecules cause the reddish color.
The phenomenon, however, was the other way around in the coma of the comet, before passing the frost line, the coma was blue but turned red when it came closer to the sun. According to the authors, this is because the ice in the grains of dust inside the coma were frozen away from the sun, hence the blue color. However, it turned red closer to the sun due to the sublimation of the ice, which dehydrated and turned the dust grains red. After passing the frost line, the coma turned back to its original blue color.
This study shows how the comet's nucleus and coma change during an orbit and how it undergoes seasonal changed dubbing it as an "orbital water-ice cycle."
