PLANETARY SCIENCE The Restless World of E RING Tethys Janus Enceladus ENCELWrinkled landscapes and spouting ADUS Dione jets on Saturn’s sixth-largest moon Prometheus hint at underground waters Rhea Epimetheus By Carolyn Porco Mimas Pandora Titan hen the Voyager 2 spacecraft sped dus a cardinal goal of the Cassini mission to Sat­ KEY CONCEPTS through the Saturnian system urn. Launched in 1997, Cassini spent seven long more than a quarter of a century years crossing interplanetary space carrying the n On the Saturnian moon W ago, it came within 90,000 kilometers of the most sophisticated suite of instruments ever tak­ Enceladus, jets of powdery moon Enceladus. Over the course of a few hours, en into the outer solar system. It finally pulled snow and water vapor, its cameras returned a handful of images that into port in the summer of 2004 [see “Saturn at laden with organic com- pounds, vent from the confounded planetary scientists for years. Even Last!” by Jonathan I. Lunine; Scientific Amer- “tiger stripes,” warm gash- by the diverse standards of Saturn’s satellites, ican, June 2004]. In December of that year it es in the surface. How can Enceladus was an outlier. Its icy surface was as dropped a probe into the atmosphere of Titan, a body just over 500 kilo- white and bright as fresh snow, and whereas the Saturn’s largest moon, and then commenced its meters across sustain such other airless moons were heavily pocked with tour of the rest of the Saturnian system—not vigorous activity? craters, Enceladus was mantled in places with ex­ least Enceladus, which it has examined more n The answer may be the tensive plains of smooth, uncratered terrain, a closely than ever over the past several months. presence of underground clear sign of past internally driven geologic activ­ What it found on this tectonically wracked lit­ fluids, perhaps a sea, ity. At just over 500 kilometers across, Enceladus tle world has been a planetary explorer’s dream, which would increase the seemed far too small to generate much heat on its and now this tiny outpost tucked deep within a efficiency of heating by own. Yet something unusual had clearly happened magnificent planetary system clear across the so­ tidal effects. Support for to this body to erase vast tracts of its cratering rec­ lar system has taken on a significance that belies this idea has come from ord so completely. its diminutive size. Enceladus not only has recent flybys. Voyager’s brief encounter allowed no more enough heat to drive surface­altering geologic ac­ n If Enceladus has liquid than a cursory look, and, in hindsight, its imag­ tivity but also is endowed with organic com­ water, it joins Mars and ing coverage of Enceladus was terribly unfortu­ pounds and possibly underground channels or Jupiter’s moon Europa as nate: a few medium­resolution images of the even seas of liquid water. Energy, organics, liquid one of the prime places in northern hemisphere, some low­resolution cov­ water: these are the three requisites for life as we the solar system to look for erage in the south, and none of the south pole. know it. In our explorations of this alien and far­ extraterrestrial life. We had no idea what we had missed. away place, we have come face to face with an en­ R —The Editors The interest generated by Voyager’s visit vironment potentially suitable for living organ­ made a comprehensive examination of Encela­ isms. It does not get much better than this. MILLE RON 52 SCIENTIFIC AMERICAN December 2008 The Restless World of ENCEL ADUS JETS of steam and icy grains erupt from deep fractures in the south polar terrain of Enceladus, making this tiny body one of only four places in the solar system known to have geologic activity in the present day. This artist’s conception includes astronauts for scale. 100 kilometers 1,000 km FIRST FLYBY of Enceladus, by the Voyager 2 ENCELADUS (left of center) is a tenth the size of spacecraft in 1981, produced images of Saturn’s largest moon, Titan. Bodies of its size limited coverage and mediocre resolution. lose their internal heat quickly; apart from The smooth areas indicated geologic activity Enceladus, they are all geologically dead. in the recent past. What keeps Enceladus active? The Slow Reveal of Enceladus how embarrassing it would be to announce a The first hint, not unanimously appreciated at discovery of a plume of material leaping off the the time, that we were in for something very big surface of a moon that was supposed to be geo­ emerged even before Cassini’s first close encoun­ logically dead, only to have to admit soon there­ ter with Enceladus. In January 2005 our cam­ after it was a smudge. Fortunately, we did not eras took the first images of the moon backlit by have long to wait. the sun, a viewing geometry that planetary The first two close flybys of Enceladus, in astronomers call high solar phase. Just as the February and March, took the spacecraft sail­ [THE AUTHOR] dust that coats your car’s windshield becomes ing above and along the equator of Enceladus. dramatically more visible when you drive into Both returned spectacular results. The smooth the sun, so do the very fine particulates that are plains seen by Voyager are not smooth at all. In­ spread throughout the solar system when you stead they are extensively and finely fractured at look through them toward the sun. These view­ subkilometer scales, in places crisscrossed by ing circumstances had proved very successful multiple generations of fractures and grooves, throughout the Voyager mission in revealing some linear, some curved. In other places, the ) hard­to­see structures in rings and atmospheres surface is deeply scored with chasms half a ki­ of the outer planets and their moons, and they lometer deep. On an even finer scale, a spidery were key to the investigation of Enceladus. network of roughly parallel narrow cracks slic­ Carolyn Porco is leader of the Enceladus/Titan The January images showed a flare protrud­ es topographic forms into slabs. Enceladus has Cassini imaging team and director ing from the moon’s south polar limb. No one obviously seen multiple and distinct episodes of of the Cassini Imaging Central NSTITUTE ( I Laboratory for Operations needed to say it; we Voyager veterans were im­ severe tectonic activity in its past—and has the (CICLOPS). She was a member of CIENCE mediately reminded of the volcanic plumes ris­ scars to prove it. S the Voyager imaging team and ing above Jupiter’s volcanic moon Io and the The February flyby produced yet another ACE Sp / from 2001 to 2003 served as vice L gossamer hazes in the atmosphere of Neptune’s high­solar­phase image showing a flare bigger /JP chairperson of the Solar System moon Triton. Some on the imaging team were and more dramatic than before. In addition, the Exploration Decadal Survey Com­ NASA mittee of the National Academy of convinced the flare was hard evidence that ma­ magnetometer noticed that Saturn’s magnetic ); Sciences, which set priorities for terial was erupting from the south pole; others field lines were being distorted as the planet’s planetary science. In January the cautioned that the feature was probably one of rotation carried them past Enceladus—a sign author and E.T. American Humanist Association ( those annoying camera artifacts that often turn that the field lines were picking up heavy ions. M awarded her the Isaac Asimov Science Award, and in October up under sunward­facing conditions. The source of the ions appeared to be the moon’s CO AND IL Wired magazine named her one I was on the fence. Unfortunately, we were all south pole. The evidence was mounting: our im­ R of 15 people the next president too busy with planning future observations and aging artifacts were beginning to look like any­ should listen to. Porco served as OLYN PO R writing scientific papers to undertake the kind thing but. A a consultant on the 1997 movie C of detailed analysis that might settle the matter. The Cassini scientists presented the case to Contact and is now advising film With no time for verification, I made the deci­ the project managers to get a better look—spe­ TESY OF director J. J. Abrams on the up­ R coming Star Trek movie. sion to say nothing publicly; I knew too well cifically, to lower the altitude of the July 2005 COU 54 SCIENTIFIC AMERICAN December 2008 tonic boundary resembling the Himalaya, and DATA BANK: that the entire enclosed region is the Enceladus ENCELADUS equivalent of the mid­Atlantic ridge—a spread­ 20 ing center where new surface is formed and Mass: 1.08 10 kilograms pushes outward. Diameter: 504 kilometers There is obviously a tale writ on the counte­ nance of this little moon that tells of dramatic Density: 1.61 grams per cubic events in its past, but its present, we were about centimeter to find out, is more stunning by far. In its excur­ sion over the outskirts of the south polar terrain, Average orbital distance from Saturn: Cassini’s dust analyzer picked up tiny particles, 238,037 km apparently coming from the region of the tiger stripes. Two other instruments detected water Orbital period: 1.37 days AS SEEN FROM CASSINI spacecraft, Enceladus vapor, and one of them delivered the signature Eccentricity: 0.0047 slips in front of Dione, a larger and more of carbon dioxide, nitrogen and methane. Cas­ distant moon whose gravity indirectly helps si ni had passed through a tenuous cloud.