The Cassini spacecraft completed three close flybys of Saturn’s enigmatic moon Enceladus between
February and July 2005. On the third and closest flyby, on 14 July 2005, multiple Cassini
instruments detected evidence for ongoing endogenic activity in a region centered on Enceladus’
south pole. The polar region is the source of a plume of gas and dust, which probably emanates
from prominent warm troughs seen on the surface. Cassini’s Composite Infrared Spectrometer
(CIRS) detected 3 to 7 gigawatts of thermal emission from the south polar troughs at temperatures
up to 145 kelvin or higher, making Enceladus only the third known solid planetary body—after
Earth and Io—that is sufficiently geologically active for its internal heat to be detected by remote
sensing. If the plume is generated by the sublimation of water ice and if the sublimation source is
visible to CIRS, then sublimation temperatures of at least 180 kelvin are required