Alice Reveals Surprising Discovery about Famed Comet's Atmosphere

Scientists have made a surprising discovery about a comet's atmosphere. A close-up of Comet 67P/Churyumov-Gerasimenko revealed that electrons close to the comet's surface-not photons from the sun-caused the rapid breakup of water and carbon dioxide molecules spewing from the surface.

Since last August, the European Space Agency's Rosetta spacecraft has orbited within a hundred miles of the comet in this historic mission. The spectrograph onboard, named Alice, specializes in the far-ultraviolet wavelength band. It looks at the light that the comet is emitting to understand the chemistry of the comet's atmosphere, or coma.

"The discovery we're reporting is quite unexpected," said Alan Stern, Alice instrument Principal Investigator, in a news release. "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's orbit with any existing or planned observatory. And, it is fundamentally transforming our knowledge of comets."

Much of the water and carbon dioxide originates from "plumes" erupting from the comet's surface, similar to those that the Hubble Space Telescope discovered on Jupiter's moon Europa. Based on the new Alice data, the explanation for the breakup of these molecules is similar to what happens to the plumes on Europa.

"By looking at the emission from hydrogen and oxygen atoms broken from the water molecules, we also can actually trace the location and structure of water plumes from the surface of the comet," said Joel Parker, an assistant director in SWRI's Space Science and Engineering Division.

The findings reveal a bit more about comet atmospheres. This, in turn, will help inform future studies.

The findings are published in the journal Astronomy& Astrophysics.

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