p l a n e ta r y sc i enc e 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 surface-altering 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 medium-resolution images of the even seas of liquid water. Energy, organics, liquid one of the prime places in northern hemisphere, some low-resolution 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 ) hard-to-see 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 ( 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 mediately reminded of the volcanic plumes ris­ scars to prove it. cience I the Voyager imaging team and ing above Jupiter’s volcanic moon Io and the The February flyby produced yet another ace S Sp /

from 2001 to 2003 served as vice L gossamer hazes in the atmosphere of Neptune’s

high-solar-phase image showing a flare bigger /JP chairperson of the Solar System Exploration Decadal Survey Com­ moon Triton. Some on the imaging team were and more dramatic than before. In addition, the mittee of the National Academy of convinced the flare was hard evidence that ma­ magnetometer noticed that Saturn’s magnetic ); NASA 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 sunward-facing 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 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 C 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 mid-Atlantic 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 si­ni had passed through a tenuous cloud. to drive Enceladus’s activity. The outer part Inclination to Saturn’s equator plane: What is more, the thermal infrared imager degrees of Saturn’s rings is in the foreground. 0.0083 sensed elevated temperatures along the frac­ tures—possibly as high as 180 kelvins, well flyby from 1,000 kilometers to 168 kilometers. above the 70 kelvins that would be expected They agreed. On July 14, Cassini flew under the from simple heating by sunlight. These locales moon and over its high southern midlatitudes, pump out an extraordinary 60 watts per square giving us for the first time a clear view of the meter, many times more than the 2.5 watts per south pole, where lies a landscape as astonish­ square meter of heat arising from Yellowstone’s ing and geologically distinct as any seen any­ geothermal area. And smaller patches of sur­ where in our solar system. face, beyond the resolving power of the infrared The terrain capping the south pole is a rough­ instrument, could be even hotter. ly circular region, completely crater-free and By now we could not believe our good fortune prominently etched by a handful of deep, paral­ to have stumbled on such a fascinating place. In lel cracks we dubbed the “tiger stripes.” Nearly quick response, the imaging team planned a spe­ evenly spaced, they run for 130 kilometers and cial series of images to be taken in four months’ terminate in hook-shaped bends. In between the time—late November 2005—looking over the stripes are brighter than average plains of finely south pole at high resolution and very grooved terrain, and the entire region is sharply high solar phase. In the meantime, a demarcated at 55 degrees south latitude by a sufficient number of images of contiguous and meandering circumpolar bound­ other moons, seen at similar­ ary of concentric mountains and valleys. The ly high phase, had accu­ boundary’s meanders are spaced roughly every mulated, and with the 45 degrees in longitude, with long cracks ex­ help of image analysts tending from some toward the equator into in my group, I proved largely uncratered provinces. to the skeptics on nstitute

I The structure and placement of the moun­ our team that tains and valleys suggested to imaging team as­ these had no flares cience cience S sociate Paul Helfenstein of Cornell University whatsoever, and ace ace

Sp that the boundary formed when the surface what we had on / L

/JP buckled as it was compressed horizontally along Enceladus was

NASA the north-south direction, like a convergent tec­ decidedly not an

SOUTHERN HEMISPHERE of Enceladus was revealed by Cassini for the first time in 2005. This mosaic of images shows the surface as it would appear if our vision extended into the infrared and ultraviolet. (To the unaided eye, the surface looks uniformly white.) The “tiger stripes” near the bottom are warm, geologically active gashes across the south polar terrain. Their bluish colors arise because they are coated with larger than average ice grains that absorb infrared light.

www.SciAm.com SCIENTIFIC AMERICAN 55 [CASSINI SURFACE OBSERVATIONS] The Many Faces of Enceladus Few planetary scientists expected Enceladus to have the diversity of terrains it displays. Large parts of the northern hemisphere, imaged by Cassini (above) with six times the resolution of Voyager 2, are covered with craters and therefore must be older than the uncratered south polar terrain. But fractures, folds, ridges and troughs indicate that both hemispheres have been worked and reworked.

270º 270º

Like Earth’s moon, Enceladus always presents the same Fractured terrain

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90º 90º Northern hemisphere Southern hemisphere

artifact. Our fence-sitting days were over as we current internal activity on an otherwise cold all realized we were looking at a plume of tiny little moon. I could not help but feel an immedi­ particles that was absolutely gargantuan, ex­ ate kinship with those long ago who first set eyes tending at least several hundred kilometers on the geysering turmoil of Yellowstone. above the moon’s south pole. On November 27 our series of striking black- Turning Up the Heat and-white images of a crescent Enceladus finally The first Cassini science papers on Enceladus arrived and showed, clear as day, a dozen or were published in early March 2006, and the more distinct and narrow fountains of fine, icy craze began. Everyone began talking Enceladus. particles jetting into space and feeding a faint Cassini has since made a number of flybys of but giant, flame-shaped plume towering over the Enceladus, penetrating deeper into denser south polar region. Later analysis by imaging regions of the plume, reaching as low as 25 kilo­ team associate Joseph Spitale of the Space Sci­ meters’ altitude. During one very low flyby this ence Institute and me showed that the jet sources past March, Cassini refined its measurements of coincide with the hottest locations on the tiger the water vapor, nitrogen, carbon dioxide and stripes—the first definitive evidence of a connec­ methane, and, in addition, it discovered a smat­ tion between warmth and active venting. Most tering of other carbon-bearing compounds, such nstitute

of the particles fall back to the surface, but some as acetylene and hydrogen cyanide, as well as I have sufficient velocity to go into orbit around trace amounts of ethane, propane, benzene, cience cience Saturn and are in fact responsible for creating its formaldehyde and other organics. S ace ace

outermost ring, known as the E ring. During another very close flyby in August, Sp / L

By anyone’s measure, these images were a our cameras focused on the surface sources of /JP

dramatic find: an incontrovertible indicium of the jets. The spacecraft’s flyover was so fast that NASA

58 SCIENTIFIC AMERICAN December 2008 10 km

On close inspection by Cassi­ ni, large regions on Encela­ dus that looked smooth to Voyager are actually highly textured (below). Deep 1 km chasms extend northward into fractured terrains. Even craters are fractured, sliced and often degraded (left).

100 km

Close-up of the tiger-stripe region shows blocks of ice the size of houses (top). 100 km Surrounding the stripes is a sinuous circumpolar chain of mountains that may be the Enceladan tectonic equivalent of the Himalaya (above).

a special camera-panning technique, akin to also yielded precise measurements of Encela­ skeet shooting, had to be devised to avoid mo­ dus’s size and shape. Together with the moon’s tion smear. Perfectly executed, the sequence re­ mass, derived from its gravitational perturba­ vealed the tiger stripes to be as deep as 300 me­ tions to Cassini’s trajectory during their close en­ ters, with V-shaped walls and blocks of ice the counters, this information has revealed that size of houses strewn across their flanks and be­ Enceladus is the rockiest of Saturn’s major yond. Areas along the flanks appear smoother moons. Its average density of 1.6 grams per cu­ than average—probably blankets of freshly fall­ bic centimeter implies that rock is 60 percent of en snow. the moon’s mass, and chances are good that the Unexpectedly, the immediate vicinity of each rock is concentrated in a rocky core surrounded vent is not obviously distinguishable from other by a mantle of water ice several tens of kilome­ places along the fractures. We preliminarily con­ ters thick. cluded that no one vent stays active very long. Ice On Earth, rock contains radioactive sub­ plugs grow from condensing vapor and choke off stances that produce heat. The same is undoubt­ the vent before it can significantly alter the edly true on Enceladus. But even all that rock is ground around it. At that point, pressure forces insufficient to produce the observed heat. The open a new vent somewhere else along the frac­ only other plausible source of warmth is tidal ture, it, too, becomes choked off, and on it goes. heating. Just as the gravity exerted by the sun A long time-lapse movie might show migration and Earth’s moon deforms our planet slightly, of jets up and down long linear extents of the creating the ocean tides, Saturn’s gravity de­ fracture. forms Enceladus. Enceladus has a noncircular Apart from giving us a window on a breath­ orbit; its distance from Saturn varies. The closer taking geologic phenomenon, the images have it gets, the more it is deformed. This daily varia­

www.SciAm.com SCIENTIFIC AMERICAN 59 [CASSINI GEYSER OBSERVATIONS] Hot Springs on a Cold Moon Shooting out of the south pole are jets of material, which feed a broad, towering plume.

The jets feed Saturn’s E ring, as Cassini saw from a distance of about two million kilometers. Some of the long, tendril-like structures seen in the vicinity of Enceladus are the extensions of the jets. Others are cre- ated by Enceladus’s gravitational perturbations 10,000 km of passing particles in the E ring.

10 km 10 km

Source I Source III Source II

100 km

The tiny, icy grains sprayed into space are easiest to see when backlit by the sun. Mountains The jets can be traced to specific locations on the tiger stripes. Oddly, these and valleys are visible along the edge of this silhouette. locations do not obviously differ from other parts of the stripes.

tion gives rise to flexure and internal heating. however, remains in an elliptical orbit because Gravity may also play a role in forming the sur­ of an orbital resonance with its bigger sibling face features. The tiger stripes are angled 45 de­ moon Dione. For every two orbits Enceladus grees from the direction to Saturn—an orienta­ makes, Dione makes one. This synchrony al­ tion that tidal forces would naturally explain. lows periodic gravitational kicks from Dione to The magnitude of heating depends not only act coherently over time and keep Enceladus’s on the orbital eccentricity—the degree to which orbit out-of-round. the orbit departs from a perfectly circular shape— Yet even these special circumstances are not but also on the consistency of the moon’s interi­ enough. Jennifer Meyer and Jack Wisdom of the or. A very rigid body would resist deformation. Massachusetts Institute of Technology have ex­ A perfectly elastic body would deform but not amined the orbital configuration of Enceladus dissipate energy as heat. A pliable moon consist­ and found that the amount of tidal energy being ing of viscous material would yield and heat up, injected into the body falls short of the energy as would a moon that is partly rigid but shot coming out of Enceladus’s south pole by a fac­ through with cracks, so that slabs of ice rub tor of five. This result is completely independent LIFE ON against one another and generate heat by fric­ of how tidal energy is internally dissipated. ENCELADUS? tion. The heating need not occur uniformly Enceladus, in its present orbit, simply lacks throughout the moon. It can be concentrated in enough energy to explain its heat output. Enceladus has three of the essential the outer shell of ice or in limited regions within requirements for life: liquid water, the outer shell such as cracks. The Silly Putty Moon organic compounds and energy. Have these putative habitable conditions Typically tidal heating tends to shut itself off. The paradox arises only if you assume that persisted long enough for life to The material in a moon takes time to deform, so Enceladus’s present-day tidal heating should nstitute

develop there? No one knows how the distortion is always out of alignment with precisely match its present-day heat output. I long it would take. Judging from the the forces that produce it. The result is a gravi­ What if Enceladus is still giving off heat from an cience cience geologic record, microbes emerged tational torque that alters the moon’s orbital earlier heating episode? One possible scenario, S

on Earth very quickly in geologic ace

motion and gradually circularizes the orbit. The first examined in 1986 for Io by Greg Ojakangas Sp /

terms: within a few hundred million L years (or even sooner) after the tidal stresses stop varying, the moon settles into and David Stevenson, both then at the Califor­ /JP

planet’s final throes of formation. a fixed shape, and heating ceases. Enceladus, nia Institute of Technology, is that the orbit of a NASA

60 SCIENTIFIC AMERICAN December 2008 of millions of years, then begins anew. The idea WHY THE illustrates how we might happen on a moon at SOUTH POLE? a time when its heat input and output are not in How did geologic activity on Enceladus a steady state. In such an oscillatory scheme, the come to be concentrated at the south energy input and output balance only over the pole? Chance may be the answer. full cycle. At any given instant, the moon’s heat Perhaps an event in the moon’s output might be found to be above or below av­ past—a meteor impact into the moon, like those that created the huge basins erage—and above or below its instantaneous on other moons of Saturn, such as heating rate. Iapetus and Tethys—initially cracked Ojakangas and Stevenson have shown that a or otherwise weakened its outer ice cycle mediated by the temperature dependence shell, focusing tidal energy there. The of the viscosity of ice may work for Io, which, ice shell might then have slid around like Enceladus, has a mismatch of heat input over time in a process known as true polar wander, in which case and output. Unfortunately, it cannot work for centrifugal forces would have shifted Enceladus: Meyer and Wisdom have concluded low-density regions to the poles. Paul that the moon is not sufficiently massive. The Helfenstein of Cornell University has crack-mediated cycle may be possible but has found evidence for such movement: a yet to be fully investigated. region called Sarandib Planitia near the equator looks like an eroded version of the south polar terrain. It What Lies Beneath? might once have been at the pole and Gabriel Tobie of the University of Nantes in since moved to its present location. 50 km France and his collaborators have examined another possible solution: that a zone of weak­ A thermal map shows temperatures up to 180 kelvins along the ness at the south pole can focus tidal energy stripes, much too hot to be explained by solar heating. The jets there and sustain itself through time. They sim­ originate in the very hottest regions (white circles). ulated the response of Enceladus to tidal forcing assuming that underlying the south pole is a sec­ moon and the consistency of its interior can play tor of low viscosity, making this part of En­cela­ off each other, leading to a cyclic variation of dus more pliable than the rest. This model can both orbital eccentricity and heat output. reproduce the observed heat output, but only Imagine starting with a cold, mostly rigid under two conditions that revolutionize our Enceladus in a nearly circular orbit. The rate of view of Enceladus. tidal heating is relatively low. Dione forces the First, the ice in the zone must be warm —near orbital eccentricity to increase, which leads to its melting point—and, second, there must be a greater flexure and viscous heating in the ice liquid layer wedged between the overlying ice shell. The eccentricity and heating continue to shell and the rocky core. This layer must under­ increase until, at some point, the heating rate ex­ lie nearly the entire southern hemisphere. With­ ceeds the ability of the moon to lose its heat. The out it, not only would flexure and hence viscous internal temperature then starts to rise, and the heating be insufficient, it would tend to occur at interior material becomes softer and less rigid, the equator rather than the pole. which leads to yet more tidal heating. In another The idea of a subsurface sea becomes all the possible oscillatory scheme, the moon becomes more compelling when one considers that less rigid not because it becomes more pliable but Enceladus’s south polar cap is actually a half- because cracks develop. Tidal stresses fracture kilometer-deep basin carved into the moon’s the ice and cause shear motions. Friction be­ overall figure. According to work by Geoffrey tween the crack surfaces leads to tidal energy Collins of Wheaton College and Jason Good­ dissipation and heating along the cracks. man of the Woods Hole Oceanographic Institu­ In either case, the additional dissipation tion, this basin could be the surface expression causes the moon’s orbit to become more circu­ of a subsurface sea. Liquid water is denser than lar, and eventually the trend reverses; tidal heat­ ice, so the total volume of water in this region is ing begins to decrease and eventually falls be­ lower. In essence, the entire south polar region is low the rate of heat loss from the surface. The a giant sinkhole. moon begins to cool off and either the ice re­ In fact, a sea could indirectly account for turns to being stiff or, in the second scenario, much of Enceladus’s geologic diversity. Isamu the cracks heal. The cycle, which can take tens Matsuyama of the Carnegie Institution of Wash­

www.SciAm.com SCIENTIFIC AMERICAN 61 ington and Francis Nimmo of the University of polar terrain. For the ice shell to move in this California, Santa Cruz, have shown that the po­ way requires a liquid layer to decouple the ice sition and orientation of the moon’s major geo­ from the deeper interior. logic features—in particular, the north-south The full cause of Enceladus’s activity may be trending cracks and the circumpolar moun­ a combination of these effects. If Enceladus un­ tains—are a sign that the moon’s ice shell has dergoes a crack-mediated heating cycle and if the slipped relative to its spin axis. The moon acts rate of tidal deformation of the moon’s outer ice Rock core like a giant gyroscope whose outer shell is free shell is fast enough, cracks may propagate into to pivot around. the underlying ductile warm zone and perhaps Ice mantle This idea would explain why the geologically all the way down to the sea. Frictional heating active region is exactly at the south pole: a warm within these fractures would contribute to the region, with lower than average density, will overall viscous heating under the south pole. Ice naturally drift toward the axis of rotation. Fur­ might melt along the deep cracks, and the melt­ thermore, a warmer zone under the south pole water would substantially enhance the heating would rise in convective motion under the up­ rate. In this way, an underground sea might be permost brittle layer of the ice shell, explaining self-sustaining, with the liquid water in the over­ the spreading-center characteristics of the south lying shell supplying the sea below. As long as the sea never completely freezes during the cool­ ing phase of the cycle, the whole process would [INTERIOR OF ENCELADUS] continue as long as Enceladus remains in orbital synchrony with Dione. Waterworld To top it off, liquid water could naturally ac­ The jets and warm temperatures strongly suggest that Enceladus has count for the observed eruptions. Michael Man­ underground liquid water. Not only do the jets spout water ice particles ga of the University of California, Berkeley, has and vapor, but the intense heating that drives them seems to require shown that partial freezing of an underground liquid water to facilitate tidal energy deposition by Saturn. Underground sea would increase its pressure and force liquid sea Enceladus may consist of a rock core surrounded by a thick layer of up. As the pressure is released on ascent, dis­ water ice. The entire south polar area is a half-kilometer-deep depression solved gases such as carbon dioxide would come in the moon’s figure, perhaps the result of a subsurface sea. Circumpolar out of solution and form bubbles, which, like mountains extend a kilometer above the surface of the depression. shaking a bottle of champagne, can assist the ris­ ing liquid. If liquid is indeed making it all the way to the surface, it provides a ready answer to Reference the question of how heat gets from where it is altitude Geyser produced, deep within the moon, to the surface: flowing water is very efficient in carrying heat. It also implies that the jets in fact are geysers and Tiger stripe originate in subterranean liquid reservoirs.

Circumpolar Enceladus as the Abode of Life mountains We are still testing and refining our notions Brittle ice about how Enceladus has come to be the way it

Convection is. But in all, it is almost unavoidable that liquid water is present somewhere below its surface. If Ductile ice so, we face the thrilling possibility that within this little moon is an environment where life, or at least its precursor steps, may be stirring. Liquid sea Everything life needs appears to be available: liq­ uid water, the requisite chemical elements and excess energy. The best analogues for an Encela­ Rock core dan ecosystem are terrestrial subsurface volca­ nic strata where liquid water circulates around hot rocks, in the complete absence of sunlight From a liquid sea at the base of the ice, water under pressure, helped by dissolved gases, and anything produced by sunlight. Here are r could flow up through deep cracks and vent at the surface. Friction within the cracks might create enough heat to melt ice. Convection currents below the south polar terrain found organisms that consume either hydrogen

could cause the surface to spread outward and create the circumpolar mountains. and carbon dioxide, creating methane, or hydro­ Mille Ron

62 SCIENTIFIC AMERICAN December 2008 [SOURCE OF ENERGY] Turning the Tide Tidal Energy Input  In the same process that produces the ocean tides on Earth, Saturn’s gravity stretches Enceladus into a slightly oblong shape. The amount of stretching varies as Enceladus revolves around Saturn because its orbit is not perfectly circular. The resulting stresses heat the interior. This process also tends to circularize the orbit, but the gravity of another Saturnian moon, Dione, keeps the orbit noncircular. SATURN

s u Orbital ELONGATION d Cold Satellite a l n n Orbit elongated by Dione interaction e Nearly circular orbit nc n Tidal stresses increase E n Rigid interior n Cracks form n Minimal tidal stressing and heating ione D

 heating cycle The present amount of tidal heating is too weak to pow- er the observed geologic activity. One resolution of this discrepancy is that Enceladus is living off an injection of heat thousands or millions of years ago. The heating Energy Dissipation Hot Satellite could have been stronger in the past if the orbit was even n Heating dissipates orbital energy n Frictional heating occurs along cracks more noncircular. That could happen because the degree n Orbit begins to circularize n Heat input exceeds loss of noncircularity, the amount of interior fracturing and n Stresses diminish and cracks seal up n Possible melting of ice along cracks the strength of heating all depend on one another— allowing for a cycle in which all three vary.

gen and sulfate—all powered not by the sun but vents on Europa, sampling its interior to test for by Earth’s own internal heat. the presence of an ecosystem would require drill­ ➥ m ore to The Enceladus of today is very much the child ing to great depths, an enterprise so challenging explore of yesterday, and we are only just beginning to it will surely not happen in our lifetime. By com­ assemble its past from the precious clues littered parison, to sample the interior of Enceladus, all Cassini Observes the Active South Pole of Enceladus. C. C. Porco et al. on its surface. Many questions remain that even you have to do is fly through the plume or land in Science, Vol. 311, pages Cassini, as capable as it is, cannot answer—ques­ on the south polar terrain, look up and stick 1393–1401; March 10, 2006. tions that are and will remain beyond our reach your tongue out. until we set a specially equipped spacecraft in or­ In addition, because Saturn’s magnetosphere Life on a Tiny Moon: NOVA Voyage bit around the moon or a lander down on its sur­ is anemic compared with Jupiter’s, a spacecraft to the Mystery Moon. Interview with Carolyn Porco, March 2006. face. An orbiter could fully map the moon’s grav­ on or around Enceladus does not have to con­ www.pbs.org/wgbh/nova/titan/ ity field and surface topography, revealing its in­ tend with a severe, mission-limiting radiation porco.html ternal distribution of mass, including any field, as would a Europa orbiter. Finally, a jour­ subsurface liquid layer. A small lander, equipped ney to Enceladus could permit tandem explora­ Cassini: The First One Thousand with a seismometer, could detect the rumblings tion of Titan, another Saturnian destination of­ Days. Carolyn C. Porco in American Scientist, Vol. 95, No. 4, pages of underground liquids. fering a possible look at life’s chemical begin­ 334–341; July/August 2007. The sheer cost and long development time of nings [see “The Mystery of Methane on Mars complex missions mean we have to be very se­ and Titan,” by Sushil K. Atreya; Scientific Solid Tidal Friction above a Liquid lective in where we go. Many scientists are ea­ American, May 2007]. Water Reservoir as the Origin ger to return to Europa, because this moon of For now, we delight in knowing that human­ of the South Pole Hotspot on Enceladus. G. Tobie, O. Cadekˇ and Jupiter is also apparently endowed with a sub­ ity’s first in-depth survey of Saturn and its reti­ C. Sotin in Icarus, Vol. 196, No. 2, terranean ocean that could harbor extraterres­ nue of worlds has uncovered something won­ pages 642–652; August 2008. trial life [see “The Hidden Ocean of Europa,” drous at the southern tip of this small, enigmatic by Robert T. Pappalardo, James W. Head and moon: a fantastic and restless place of deep, icy The Possible Origin and Persis­ Ronald Greeley; Scientific American,Octo ­ chasms and soaring ghostly fountains, a rare tence of Life on Enceladus and as m Detection of Biomarkers in the

ho ber 1999]. To my mind, however, further explo­ realm whose discovery has rewarded both mind

T Plume. C. P. McKay et al. in Astrobiol- ration of Enceladus is so much more promising. and soul. As planetary explorers, we could not ogy, Vol. 8, No. 5, pages 909–922;

Melissa Melissa Because scientists do not know of any active have asked for more. n October 2008.

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