The Rosetta orbiter, currently studying comet 67P/Churyumov-Gerasimenko as it orbits the Sun, has been watching how the comet has changed as it moves toward its perihelion (point of closest approach to the Sun) in August of 2015. As the comet heats up from the increasing intensity of the Sun’s rays, it releases surface gases and dust into what eventually becomes the atmosphere-like coma surrounding the comet. As the comet moves the material produces the visible tail that we usually associate with a comet. With 67P specifically, Rosetta has recently made a surprising discovery: That the water and carbon dioxide streaming from the surface is broken up by surface electrons.
The measurements were made by the NASA funded ALICE instrument (for once its actually not an acronym), which is a spectrograph that focuses on the far-UV wavelength band. The small flashes during the breakup of the molecules were spotted by the instrument very close to the comet’s surface. Until now the molecular breakup was thought to result from the solar radiation falling on the comet.
“Analysis of the relative intensities of observed atomic emissions allowed the Alice science team to determine the instrument was directly observing the “parent” molecules of water and carbon dioxide that were being broken up by electrons in the immediate vicinity, about six-tenths of a mile (one kilometer) from the comet’s nucleus.” – Alan Stern, principal investigator for the Alice instrument at the Southwest Research Institute (SwRI) in Boulder, Colorado.
As the comet continues to move closer to the Sun, with the Rosetta spacecraft orbiting it, there will be an increase in the amount of material liberated from the surface, giving us real-time data of the evolution of a comet as it is heated by the intense radiation of the Sun.