Rosetta Discovery of Surprise Molecular Breakup Mechanism in Comet Coma Alters Perceptions

A NASA science instrument flying aboard the European Space Agency’s (ESA) Rosetta spacecraft has made a very surprising discovery – namely that the molecular breakup mechanism of “water and carbon dioxide molecules spewing from the comet’s surface” into the atmosphere of comet 67P/Churyumov-Gerasimenko is caused by “electrons close to the surface.”

The surprising results relating to the emission of the comet coma came from measurements gathered by the probes NASA funded Alice instrument and is causing scientists to completely rethink what we know about the wandering bodies, according to the instruments science team.

“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, in a statement.

“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 paper reporting the Alice findings has been accepted for publication by the journal Astronomy and Astrophysics, according to statements from NASA and ESA.

Alice is a spectrograph that focuses on sensing the far-ultraviolet wavelength band and is the first instrument of its kind to operate at a comet.

Until now it had been thought that photons from the sun were responsible for causing the molecular breakup, said the team.

The carbon dioxide and water are being released from the nucleus and the excitation breakup occurs barely half a mile above the comet’s nucleus.

“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.”

The excitation mechanism is detailed in the graphic below.

Rosetta’s continued close study of Comet 67P/Churyumov-Gerasimenko has revealed an unexpected process at work close to the comet nucleus that causes the rapid breakup of water and carbon dioxide molecules. Credits: ESA/ATG medialab; ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; ESA/Rosetta/NavCam – CC BY-SA IGO 3.0

“The spatial variation of the emissions along the slit indicates that the excitation occurs within a few hundred meters of the surface and the gas and dust production are correlated,” according to the Astronomy and Astrophysics journal paper.

The data shows that the water and CO2 molecules break up via a two-step process.

“First, an ultraviolet photon from the Sun hits a water molecule in the comet’s coma and ionises it, knocking out an energetic electron. This electron then hits another water molecule in the coma, breaking it apart into two hydrogen atoms and one oxygen, and energising them in the process. These atoms then emit ultraviolet light that is detected at characteristic wavelengths by Alice.”

“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.”

After a decade long chase of over 6.4 billion kilometers (4 Billion miles), ESA’s Rosetta spacecraft arrived at the pockmarked Comet 67P/Churyumov-Gerasimenko on Aug. 6, 2014 for history’s first ever attempt to orbit a comet for long term study.

Since then, Rosetta deployed the Philae landing craft to accomplish history’s first ever touchdown on a comets nucleus. It has also orbited the comet for over 10 months of up close observation, coming at times to as close as 8 kilometers. It is equipped with a suite 11 instruments to analyze every facet of the comet’s nature and environment.

Comet 67P is still becoming more and more active as it orbits closer and closer to the sun over the next two months. The pair reach perihelion on August 13, 2015 at a distance of 186 million km from the Sun, between the orbits of Earth and Mars.

Alice works by examining light emitted from the comet to understand the chemistry of the comet’s atmosphere, or coma and determine the chemical composition with the far-ultraviolet spectrograph.

According to the measurements from Alice, the water and carbon dioxide in the comet’s atmospheric coma originate from plumes erupting from its surface.

“It is similar to those that the Hubble Space Telescope discovered on Jupiter’s moon Europa, with the exception that the electrons at the comet are produced by solar radiation, while the electrons at Europa come from Jupiter’s magnetosphere,” said Paul Feldman, an Alice co-investigator from the Johns Hopkins University in Baltimore, Maryland, in a statement.

Rosetta discovered an unexpected process at comet nucleus that causes the rapid breakup of water and carbon dioxide molecules. Jets of gas and dust are blasting from the active neck of comet 67P/Churyumov-Gerasimenko in this photo mosaic assembled from four images taken on 26 September 2014 by the European Space Agency’s Rosetta spacecraft at a distance of 26.3 kilometers (16 miles) from the center of the comet. Credit: ESA/Rosetta/NAVCAM/Marco Di Lorenzo/Ken Kremer/

Other instruments aboard Rosetta including MIRO, ROSINA and VIRTIS, which study relative abundances of coma constituents, corroborate the Alice findings.

“These early results from Alice demonstrate how important it is to study a comet at different wavelengths and with different techniques, in order to probe various aspects of the comet environment,” says ESA’s Rosetta project scientist Matt Taylor, in a statement.

“We’re actively watching how the comet evolves as it moves closer to the Sun along its orbit towards perihelion in August, seeing how the plumes become more active due to solar heating, and studying the effects of the comet’s interaction with the solar wind.”

Stay tuned here for Ken’s continuing Earth and planetary science and human spaceflight news.

Ken Kremer

Ken Kremer

Dr. Ken Kremer is a speaker, research scientist, freelance science journalist (KSC area,FL) and photographer whose articles, space exploration images and Mars mosaics have appeared in magazines, books, websites and calendars including Astronomy Picture of the Day, NBC, FOX, BBC,, Spaceflight Now, Science and the covers of Aviation Week & Space Technology, Spaceflight and the Explorers Club magazines. Ken has presented at numerous educational institutions, civic & religious organizations, museums and astronomy clubs. Ken has reported first hand from the Kennedy Space Center, Cape Canaveral, NASA Wallops, NASA Michoud/Stennis/Langley and on over 80 launches including 8 shuttle launches. He lectures on both Human and Robotic spaceflight - Follow Ken on Facebook and Twitter

View Comments

  • When were the conclusive data obtained? Did they see this activity immediately upon arrival or is it due to increasing electron density as the comet approaches Sol? It would be interesting to note when the electron density began to dissociate the molecules. Inside Jupiter or Mars' orbit?

  • I think that this runaway electrons at the surface induced by photo-energy are well know in material science. Take for example a semiconductor which at certain photon energy, the electrons at the surface of a semiconductor ( immersed in water) become de-localized and the surface of the semiconductor can only achieve stable equilibrium by crossing of these de-localized electrons from the surface and reacting with water to produce hydrogen through a process called catalytic photosynthesis. I am waiting for Philae to wake up from its deep sleep when the comet moves closer to the Sun along its orbit towards perihelion in August. I believe that if Philae walks up, the 500 million US lottery will be mine.

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