Solving the puzzle of Enceladus’s active south pole

Abstract

Enceladus is a small moon of Saturn that has active geysers at its south pole. Why this activity is confined to one small region of the surface has been a puzzle for 15 y. Now, in PNAS, Kang and Flierl (1) provide a possible answer: Localization of activity can arise spontaneously via a feedback process in the ice shell. While their model is a highly abstracted version of reality, it may also be applicable elsewhere, including Jupiter’s moon Europa and even our own Moon. The discovery of geysers and excess heat at the south pole of Enceladus was one of the biggest surprises of the Cassini spacecraft mission (2). Because Enceladus is so small, the only plausible source of this energy is tidal heating: Enceladus gets squeezed and stretched by Saturn’s gravity as it follows a slightly elliptical path around Saturn. A fundamental characteristic of tidal heating is that it is symmetrical: Both poles are expected to experience the same degree of heating (3). It was therefore very puzzling that the north pole of Enceladus appears ancient and heavily cratered, while the south pole is geologically young, active, and warm. Further spacecraft investigation revealed that Enceladus consists of an ice shell of variable thickness sitting atop a salty ocean and a low-density rocky core (4). Although the ice shell is only ∼6 km thick at the south pole, it is also notably thinned at the north pole (∼14 km) … [↵][1]1Email: fnimmo{at}ucsc.edu. [1]: #xref-corresp-1-1