The journal “Astronomy and Astrophysics” will publish an article that illustrates a discovery about the atmosphere of the comet 67P/Churyumov-Gerasimenko generated by the sublimation of the ice it contains. Water, but also carbon dioxide, are turned into steam but thanks to the instrument Alice of the space probe Rosetta it was possible to discover that these molecules get broken and that this happens in two stages.
The Alice instrument is a spectrograph provided by NASA among its contributions to the Rosetta mission. ESA is using it to examine the chemical composition of the coma of the comet 67P/Churyumov-Gerasimenko, the atmosphere very faint but in a growing phase as the comet approaches the Sun and a greater amount of ice sublimates.
The spectrograph Alice conducts its exams at the far ultraviolet wavelengths. This allows to identify some elements very abundant in the universe: hydrogen, oxygen, carbon and nitrogen. Alice splits the light coming from the comet in its various colors from which scientists can identify the chemical composition of the gases of the coma.
In this study, the researchers observed the emissions of the hydrogen and oxygen atoms generated by broken water molecules and carbon atoms generated by broken carbon dioxide molecules. By examining these atoms close to the comet’s nucleus, they discovered that the molecules get broken in a two-stage process.
In the first stage, photons that make ultraviolet light from the Sun hit a molecule of water in the comet’s coma and ionize it, ejecting an energetic electron. This electron hits another water molecule in the coma, breaking it into its two hydrogen atoms and one oxygen atom energizing them. These atoms emit ultraviolet light that is detected at characteristic wavelengths by the Alice spectrograph.
In a similar manner, the impact of an electron with a molecule of carbon dioxide breaks it into its two oxygen atoms and one carbon atom. Again, they emit ultraviolet light that is detected by the Alice spectrograph.
The analysis of the relative intensities of the emission of atoms observed allowed to determin that the “parent” molecules that get broken by the electrons are close to the comet’s nucleus, about a kilometer from it.
This finding is surprising and was only possible thanks to the presence of the Alice spectrograph on the Rosetta space probe with its ability to detect far ultraviolet wavelengths. They get absorbed by Earth’s atmosphere so it would be impossible to detect them using ground-based instruments. Even the Hubble Space Telescope couldn’t make those observations because it could detect the atoms only when they’re very far away from the comet.
In essence, this discovery shows that it’s necessary to send a space probe near a comet to really understand what physical and chemical processes are taking place. It also demonstrates that it’s necessary for the probe to be equipped with instruments that can examine it at various wavelengths and with various techniques to get a complete picture of those processes.