Heat from friction could power hydrothermal activity on Saturn's moon Enceladus for billions of years if the moon has a highly porous core, according to a new modeling study by European and U.S. researchers working on NASA's Cassini mission.The study, published today in the journal Nature Astronomy, helps resolve a question scientist have grappled with for a decade:  Where does the energy to power the extraordinary geologic activity on Enceladus come from?