The Restless World of Enceladus

Abstract

When the Voyager 2 spacecraft sped through the Saturnian system more than a quarter of a century ago, it came within 90,000 kilometers of the moon Enceladus. Over the course of a few hours, its cameras returned a handful of images that confounded planetary scientists for years. Even by the diverse standards of Saturn’s satellites, Enceladus was an outlier. Its icy surface was as white and bright as fresh snow, and whereas the other airless moons were heavily pocked with craters, Enceladus was mantled in places with ex­ tensive plains of smooth, uncratered terrain, a clear sign of past internally driven geologic activ­ ity. At just over 500 kilometers across, Enceladus seemed far too small to generate much heat on its own. Yet something unusual had clearly happened to this body to erase vast tracts of its cratering rec­ ord so completely. Voyager’s brief encounter allowed no more than a cursory look, and, in hindsight, its imag­ ing coverage of Enceladus was terribly unfortu­ nate: a few medium­resolution images of the northern hemisphere, some low­resolution cov­ erage in the south, and none of the south pole. We had no idea what we had missed. The interest generated by Voyager’s visit made a comprehensive examination of Encela­ dus a cardinal goal of the Cassini mission to Sat­ urn. Launched in 1997, Cassini spent seven long years crossing interplanetary space carrying the most sophisticated suite of instruments ever tak­ en into the outer solar system. It finally pulled into port in the summer of 2004 [see “Saturn at Last!” by Jonathan I. Lunine; Scientific American, June 2004]. In December of that year it dropped a probe into the atmosphere of Titan, Saturn’s largest moon, and then commenced its tour of the rest of the Saturnian system—not least Enceladus, which it has examined more closely than ever over the past several months. What it found on this tectonically wracked lit­ tle world has been a planetary explorer’s dream, and now this tiny outpost tucked deep within a magnificent planetary system clear across the so­ lar system has taken on a significance that belies its diminutive size. Enceladus not only has enough heat to drive surface­altering geologic ac­ tivity but also is endowed with organic com­ pounds and possibly underground channels or even seas of liquid water. Energy, organics, liquid water: these are the three requisites for life as we know it. In our explorations of this alien and far­ away place, we have come face to face with an en­ vironment potentially suitable for living organ­ isms. It does not get much better than this. ENCEL ADUS The Restless World of