As Saturn’s south pole slips into its long winter, so does the active southern region of Enceladus, nicknamed the Tiger Stripes, bid farewell to the Sun for the next 15 years. The latest flyby of Enceladus by Cassini – the E-8 flyby – provided the most dramatic and perhaps final views of such clarity of the water vapor geysers emanating from the Tiger Stripes.
The vapor is visible in geometries when we look back toward the Sun. In the image below, the Tiger Stripes are seen in relief making use of detailed images and a topographic map created by Paul Schenk at the Lunar and Planetary Institute.
Researchers are still working on models to explain how such a small moon, just a few hundred miles across and therefore an object that would cool off and freeze solid shortly after formation, manages to have a reservoir of liquid water – or at least very warm ice – near its surface. If it is like the active moons of Jupiter, then flexing of the moon by tidal forces from Saturn explain the melting in Enceladus. To maintain tidal heating, Enceladus must be pushed around by gravitational interactions with nearby moons. The problem is that the tidal heating scenario for Enceladus is far less clear than it was for Io, the volcanically hyperactive moon of Jupiter. Stan Peale, a professor at UC Santa Barbara and lead author of the paper that predicted Io’s volcanoes, presented an alternative hypothesis for Enceladus at this year’s DPS meeting. Some of Saturn’s moons have co-orbital satellites: small satellite shards that share an orbit with their larger lunar siblings. Peale and co-author Rick Greenberg suggested that a collision between Enceladus and a co-orbital moon within the relatively recent past (less than 200,000 years ago) could have supplied the necessary heating to drive the geysers to the present epoch.