Data collected by NASA’s Alice instrument aboard the European Space Agency’s Rosetta spacecraft reveal that electrons close to the surface of comet 67P/Churyumov-Gerasimenko — not photons from the Sun, as had been believed — cause the rapid breakup of water and carbon dioxide molecules spewing from the comet’s surface.
“The discovery we’re reporting is quite unexpected,” said Alan Stern, principal investigator for the Alice instrument at the Southwest Research Institute (SwRI) in Boulder, Colorado. “It shows us the value of going to comets to observe them up close, since this discovery simply could not have been made from Earth or Earth orbit with any existing or planned observatory. And, it is fundamentally transforming our knowledge of comets.”
A report of the findings has been accepted for publication by the journal Astronomy and Astrophysics.
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 about six-tenths of a mile (one kilometer) from the comet’s nucleus. The carbon dioxide and water are being released from the comet’s nucleus and affected by electrons near the nucleus.
Since last August, Rosetta has orbited within 100 miles (160 kilometers) of comet 67P. The Alice spectrograph on board Rosetta specializes in sensing the far-ultraviolet wavelength band. Alice examines light the comet is emitting to understand the chemistry of the comet’s atmosphere, or coma. A spectrograph is a tool astronomers use to split light into its various colors. Scientists can identify the chemical composition of gases by examining their light spectrum. Alice is the first such far-ultraviolet spectrograph to operate at a comet.
Alice data indicate much of the water and carbon dioxide in the comet’s coma originate from plumes erupting from its surface.