...The presence of CO2 suggests radiolysis reactions between surface oxidants and organics, or sputtering and/or outgassing of CO2 endogenic to Rhea’s ice. Observations of outflowing positive and negative ions give evidence for pickup ionization as a major atmospheric loss mechanism...
Sadly to say in the corresponding NASA-PR the possible cause "sputtering" turns to "complex chemistry", so again trying to avoid the E-word.
This although could turn out to be another confirmation of the EU-predictions, if we remember Thornhill's theorizing about the origin of CO2 on Venus at:
http://www.holoscience.com/news.php?article=9aqt6cz5
..."When performing comparisons, we must allow for the fact that the Venusian atmosphere is being modified continually by electric discharge activity on the surface of that planet. It has increased the carbon dioxide content of the Venusian atmosphere at the expense of nitrogen and water vapor. Scientists think that most of Venus' water must have split into hydrogen and oxygen and all the hydrogen was lost to space. But if so, where is the oxygen that was left behind? The four Pioneer probe craft didn't find it in the atmosphere. The answer is that it has combined with carbon monoxide to form a heavy atmosphere of carbon dioxide. The process I envisage is this:
"Venus probably began with an atmosphere more like Titan's and the Earth's, where nitrogen dominates, and with more water. It suggests that Saturn must have considerable nitrogen at depth in its atmosphere. The icy rings and satellites of Saturn and abundant water on Earth also point to water on Saturn. On the Venusian surface, nitrogen molecules are converted to carbon monoxide molecules by a catalytic surface nuclear reaction in the presence of red-hot iron. The brilliant French chemist, Louis Kervran, when investigating carbon monoxide poisoning of welders, discovered this surprising nuclear transformation. The carbon monoxide reacts at the hot surface of Venus with water vapor to form carbon dioxide and hydrogen. It is a well-known industrial process. The hydrogen produced escapes from Venus...
This can turn out to be interesting...
FS3