The Alice instrument found that electrons close to the surface of comet 67P/Churyumov-Gerasimenko - and 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 Alice on board Rosetta is probing the origin, composition and workings of comet 67P/Churyumov-Gerasimenko to gather sensitive, high-resolution insights that cannot be obtained by either ground-based or Earth-orbiting observation.
It has more than 1,000 times the data-gathering capability of instruments flown a generation ago, yet it weighs less than four kgs and draws just four watts of power.
"The discovery we are reporting is quite unexpected. 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," said Alan Stern, principal investigator for the Alice instrument at the Southwest Research Institute (SwRI) in Boulder, Colorado.
It is fundamentally transforming our knowledge of comets, he added.
The carbon dioxide and water are being released from the comet's nucleus and affected by electrons near the nucleus.
Alice data indicate much of the water and carbon dioxide in the comet's coma originate from plumes erupting from its surface.
It is similar to those that the Hubble Space Telescope discovered on Jupiter's moon Europa.
By looking at the emission from hydrogen and oxygen atoms broken from the water molecules, Alice scientists can actually trace the location and structure of water plumes from the surface of the comet.
The comet observations will help scientists learn more about the origin and evolution of our solar system and the role comets may have played in providing Earth with water, and perhaps even life.
The Alice instrument is one of two ultra-violet spectrometers named Alice currently flying in space.
The other is on board Nasa's New Horizons spacecraft which is set to make a fly-by of Pluto in July.
A report of the findings has been accepted for publication by the journal Astronomy and Astrophysics.
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