Discovery of Probable Endogenic Hot Spots on Enceladus by the Cassini Composite Infrared Spectrometer

During the Cassini orbiter's July 14th 2005 flyby of Saturn's moon Enceladus, the Composite Infrared Spectrometer (CIRS) mapped the thermal emission from from the moon, obtaining useful signal at wavelengths between 12 microns and several hundred microns, with spatial resolution as good as 25 km for global mapping and local coverage at resolutions of a few km. Daytime and nighttime equatorial temperatures are consistent with a surface illuminated by sunlight, with subsolar temperatures between 75 and 80 K and a bolometric albedo near 0.80. However, south polar temperatures, poleward of about 65 S, are much warmer than the value of about 70 K expected from thermal equilibrium with sunlight. 16 micron brightness temperature peaks near 85 K at the pole, and the wavelength distribution of the thermal emission indicates that a few percent of the polar region is at temperatures near 125 K, perhaps with smaller areas at even higher temperatures. Such temperatures are very difficult to reconcile with passive heating by sunlight, though extreme heating of the subsurface by solid-state greenhouse effects is perhaps not completely ruled out. Local heat flow, averaged over the south polar region, is 0.25 W m-2, about 10% of the value for Io. If the heat was generated at depth and heat flow was uniformly distributed over the pole, the implied thermal gradient would produce melting of pure ice at a depth of only 8 km: a localized near-surface heat source is perhaps more likely. Total radiated endogenic power from the south polar region is about 8 GW. Io's tidally-generated global heat flow is 105 GW, 30 times larger per unit volume, but Enceladus' power output is still remarkable for such a small body in a low-eccentricity orbit. Sublimation of the warm ice is a likely source for the H2O vapor cloud seen over Enceladus' south polar region by other Cassini instruments.