The atmospheric shroud of Saturn's large moon Titan includes methane — lending it its orange hue (picured) and an element of mystery, because sunlight breaks the gas down relatively quickly. If Titan's store of methane were limited to its lakes and rivers, it would have lost the gas from its atmosphere long ago. So where is it coming from?
Olivier Mousis at the University of Arizona in Tucson and his colleagues rule out one idea: that a reaction between subsurface water and rock is continually replenishing the methane. The ratio of deuterium and hydrogen in the gas is not consistent with a geological origin, they find.
Instead, their analysis suggests that Titan scooped up all of its methane 4.5 billion years ago when it was formed — and could have as much as 1,300 times the amount seen in its atmosphere still trapped in its interior.