Observations of the inner coma of Comet 67P/Churyumov-Gerasimenko with the Alice far-ultraviolet spectrograph onboard ESA’s Rosetta spacecraft reveal that electrons close to the comet’s surface – not solar photons, as had been previously thought – cause the breakup of carbon dioxide and water molecules spewing from the surface of the comet.
Rosetta’s continued study of 67P/Churyumov-Gerasimenko has revealed an unexpected process at work close to the comet nucleus. Image credit: ESA / ATG medialab / Rosetta / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA / Feldman et al / NavCam / CC BY-SA IGO 3.0.
A team of scientists led by Prof Paul Feldman of the Johns Hopkins University in Baltimore reports the detections made by Rosetta’s Alice instrument, when the spacecraft was between 10 km and 80 km from the center of the comet nucleus.
Prof Feldman and co-authors focused on the nature of ‘plumes’ of water and carbon dioxide gas erupting from the comet’s surface, triggered by the warmth of the Sun.
To do so, they looked at the emission from hydrogen and oxygen atoms resulting from broken water molecules, and similarly carbon atoms from carbon dioxide molecules, close to the comet nucleus.
They discovered that the molecules seem to be broken up in a two-step process.
First, an ultraviolet photon from the Sun hits a water molecule in the comet’s coma and ionizes it, knocking out an energetic electron. This electron then hits another water molecule in the coma, breaking it apart into one oxygen and two hydrogen atoms, and energizing them in the process. These atoms then emit UV light that is detected at characteristic wavelengths by NASA’s Alice spectrograph.
Similarly, it is the impact of an electron with a carbon dioxide molecule that results in its break-up into atoms and the observed carbon emissions.
“Analysis of the relative intensities of observed atomic emissions allows us to determine that we are directly observing the ‘parent’ molecules that are being broken up by electrons in the immediate vicinity, about 1 km, of the comet’s nucleus where they are being produced,” explained Prof Feldman, lead author of paper about the results available online in the journal Astronomy and Astrophysics.
“The discovery we’re reporting is quite unexpected,” said co-author Dr Alan Stern of the Southwest Research Institute 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 our planet or its orbit with any existing or planned observatory. And, it is fundamentally transforming our knowledge of comets.”
From Earth or from Earth-orbiting space observatories such as the Hubble Space Telescope, the atomic constituents of comets can only be seen after their parent molecules, such as water and carbon dioxide, have been broken up by sunlight, hundreds to thousands of kilometers away from the nucleus of the comet.
Rosetta’s Alice instrument has also studied the surface of the comet and will be used in further studies of its atmosphere as the comet approaches the Sun and its plumes become more active due to solar heating.
The comet observations will help researchers learn more about the origin and evolution of the Solar System and the role comets may have played in providing our planet with water.
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P.D. Feldman et al. Measurements of the near-nucleus coma of comet 67P/Churyumov-Gerasimenko with the Alice far-ultraviolet spectrograph on Rosetta. A&A, published online June 02, 2015; doi: 10.1051/0004-6361/201525925
