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All Of These Worlds Are Yours
On July 1 2004, seven years after its launch, the Cassini spacecraft crossed the plane of Saturn's ring system. Its chunky body, wrapped in gold-colored Kapton insulation and crowned by the dish of its high-gain antennae, bristled with instrumentation; an independent instrument package, the Huygens probe, clung to it like a limpet. After falling through the gap between the F and G rings, it fired up its engines for ninety-six minutes, skimming just 100,000 kilometers above Saturn's cloud tops as it ended its interplanetary trajectory and inserted itself into an elliptical orbit.
I had some small personal interest in Cassini's success. In the year it was launched, 1997, I published a short story, "Second Skin", set on Proteus, a tiny moon of Neptune: it described an attempt to assassinate an enigmatic but fearsomely accomplished gene wizard, and was the overture to a long love affair with the outer regions of the Solar System. I wrote eight more stories that shared the same future history, and began to plan a pair of novels, The Quiet War and Gardens of the Sun, about life in the outer regions of the solar system. I drew a great deal of inspiration from maps and best guesses at the geological histories based on images of the many and incredibly varied moons of Jupiter, Saturn, Uranus and Neptune snatched by Pioneer 11 and the two Viking probes; from high-resolution images returned by the Galileo mission; and most of all, because large portions of both novels are set on and around the moons of Saturn, from the images and data returned by Cassini-Huygens.
I wanted to know what it would be like to hike through the cratered and fractured plains of Dione, to swoop low and close across the plane of the rings, to fly above the chaotic landscape inside Herschel Crater on Mimas, the remnant of an impact that nearly shattered the small moon and gives it the appearance of Darth Vader's Death Star. I wanted detail. I wanted to turn maps into landscapes. Cassini-Huygens did not disappoint.
Take Titan, one of its primary targets. First observed by the Dutch astronomer Christian Huygens in 1655, Titan is the biggest moon in the Saturn System: larger than Mercury, 50% bigger than our own Moon, only a fraction smaller than the largest moon in the Solar System, Ganymede. But until Cassini-Huygen's arrival, almost nothing was known about Titan's surface because it possesses a thick and frigid nitrogen-methane atmosphere filled from top to bottom with an opaque haze of hydrocarbon particles. The two Voyager missions failed to penetrate this shroud; ground-based surveys and the Hubble telescope revealed little more than enigmatic dark and light patches. But Cassini was not only armed with radar-mapping equipment and cameras able to peer through the haze and radar-mapping equipment, it was also carrying the Huygens probe, designed to fall through Titan's atmosphere and land on and observe and taste its surface.
The saucer-shaped probe separated from the Cassini orbiter on Christmas Eve 2004, midway through the third orbit around Saturn, and both craft swung out close to Iapetus and fell back towards rendezvous with Titan. Although a software glitch meant that one of the orbiter's two receivers failed to lock onto Huygens's signals, leading to loss of some relayed data, the probe performed exactly as expected, decelerating through Titan's atmosphere in a fiery arc, jettisoning its heat shield, deploying its parachute, and drifting down to surface. Instruments analysed the atmospheric gases and measured wind speed; cameras captured a vista that looked like a dark shoreline backed by bright hills cut by the lightning forks of rivers. The probe impacted east of the hills, and transmitted pictures of a flat landscape littered with cobbles. It looked a little like the rocky plains of Mars, but the surface was organic material with the consistency of crème brûlée, and the cobbles were water ice and rounded like pebbles on a beach from which the waters had withdrawn. Perhaps that's exactly what it was: a beach, or the surface of a dry lake or river.
Huygens stopped transmitting soon after impact, but Cassini continued to swing past Titan at regular intervals, discovering that the moon's poles possess lakes replenished by summer rains, mapping riverine valleys that cut through regions of rugged terrain, and huge tracts of giant sand dunes marching northward from the equator. Very few craters have been spotted, suggesting that traces of most impacts had been eroded by rain and wind, just as on Earth. There may even be volcanoes. But although Titan is stunningly Earthlike in appearance, its surface isn't rock but granitic water ice at 103 kelvins (pure water ice melts at 273 K), rain is methane and ethane, the dunes are composed of gritty grains of frozen petroleum, and if Titan possesses active volcanoes, they'll belch lavas of plastic water ice. There are also indications that, just like Earth's tectonic plates, Titan's surface floats on a deep layer of fluid.
Exobiologists and SF writers have long speculated that sluggish methanogenic creatures might inhabit Titan, but perhaps a more familiar form of life exists in a chthonic ocean of liquid water beneath the moon's frozen crust. And while many SF novels and stories set in the Saturn system have assumed that its biggest moon was the best candidate for colonisation, and a vast resource of hydrocarbons and nitrogen, in reality, it is, despite its Earthlike landscapes, about as hospitable as the outer suburbs of Hell. The haze which extends all the way to the surface reduces visibility and incident sunlight; its gravity well as almost as deep as that of Mars, and because of its thick atmosphere (which is explosive when mixed with breathable air), descent from orbit requires the use of heat shields. No, if you want to colonise the Saturn system, there are much better places to live.
Enceladus, for instance. In 2005, images from Cassini's third close encounter with this little moon showed plumes of water ice rising from its south pole. Tremendously exciting, but not completely unexpected. At just 500 kilometers in diameter Enceladus should have frozen solid long ago, but its icy surface is differentiated into smooth plains and cratered regions and shows signs of recent resurfacing and tectonic deformation, and the broad faint E Ring is brightest in the region in which Enceladus orbits, suggesting that it is the source of ring material. And so it is. Some of the ice dust lofted by the plumes falls back to the moon's surface, giving it the highest albedo of any body in the Solar System (and making it one of the coldest surfaces in the Solar System too, because it reflects almost all incident sunlight); the rest goes into orbit around Saturn, replenishing the E Ring. Later flybys, including one within 52 kilometers of Enceladus's surface, pinpointed the source of the plumes: long fractures or sulci in a wrinkled tiger-stripe terrain at the south pole that are significantly warmer than the rest of the surface.
The mechanism producing the plumes isn't yet fully explained, but there's convincing evidence for liquid water beneath the moon's surface. Particles in the E ring contain sodium salts, suggesting that water has eroded material from the moon's silicate core, while recent data show that the icy plumes contain ammonia, which when dissolved in water acts as an antifreeze, reducing the freezing point to temperatures as low as 176 kelvins. As on Titan, the presence of liquid water suggests that some form of life may exist deep beneath Enceladus's surface. Thirty years ago, Mars was the only likely target in the search for extraterrestrial life; now, the range had been vastly expanded. In addition to Titan and Enceladus, there's extremely strong evidence for oceans beneath the surfaces of Jupiter's moons Europa and Ganymede, and liquid water might also be found beneath the surface of the largest body in the asteroid belt, Ceres, and beneath the surfaces of Triton and Charon, the largest moons of Neptune and Pluto respectively. Our kind of life, found on the surface of rocky planets and ultimately dependent on sunlight-driven photosynthesis, may be less common than chemosynthetic subsurface life.
It might be possible to colonise Enceladus's subsurface sea, although it would be as dark as a coal cellar, and its freezing water would lack free oxygen — a fish would drown in it as quickly as a human being. Still, bubble cities and racks of sunlamps could be strung under the icy ceiling, and perhaps modified kelps could be grown. But if the sea harbors some kind of indigenous life, it might best be left as a kind of nature reserve; and besides, the surfaces of Enceladus and the rest of Saturn's inner moons are more attractive places to live. Essentially, they're frozen oceans of dirty ice wrapped around silicate cores, and ice can be melted to provide water and, by electrolysis, oxygen and hydrogen. As for building materials, there's an abundance of hydrocarbons.
When it first approached Saturn, Cassini-Huygens passed within 2000 kilometers of Phoebe, the largest of Saturn's flock of the small irregular outer moons. Once thought to be a captured asteroid, Phoebe was revealed to be more like an inactive comet or one of the planetesimals of the outer system, with dark carbonaceous material wrapped around a core of water ice. It most likely wandered inwards from the Kuiper Belt, and it is not the only wanderer captured by Saturn. Cassini and ground-based observations have detected almost forty tiny moonlets in large-radius, irregular orbits around Saturn, sorted into the Inuit, Norse (to which Phoebe belongs), and Gallic groups. A swarming bounty of moons, with the chemistry of the Solar System's formation frozen across their surfaces. These primordial hydrocarbons could be turned into every kind of polymer, and habitats constructed from exotic plastics, fullerenes and diamond could be built into bright ice cliffs of Dione or fitted into Rhea's craters, and fractures in the adamantine ice of Tethys and Mimas could be roofed over and linear cities the size of New York or London built on their floors. Gravity on these small moons is low enough to allow all kinds of architectural fantasies — even arboreal cities constructed on platforms and aerial streets strung between giant trees.
The best candidate for colonisation may be the outermost of Saturn's large moons, Iapetus. In the 17th Century, the Italian-born astronomer Giovanni Cassini first observed that it was divided into a light and dark half (Arthur C. Clarke originally located a giant monolith at the centre of its dark plains, named Cassini Regio after their discoverer). The Cassini orbiter found that the dark material is very similar to the stuff caulking small irregular moons like Phoebe; possibly it was knocked off them by collisions early in the history of the Saturn system, and swept up by Iapetus's leading hemisphere. Cassini also found that Iapetus possesses a huge range of mountains at its equator. Thirteen thousand kilometers long, thirty wide, and with peaks rearing up some thirteen kilometers, more than twice as high as the Himalayas, this equatorial ridge gives the moon the seamed profile of a walnut. The easy availability of carbonaceous material and possibility that activity associated with the formation of this huge mountain range may also have uplifted useful minerals and metals are excellent reasons to settle on Iapetus. And while most of the inner moons orbit in Saturn's equatorial plane, so that the rings would be edge-on and difficult to see from their surfaces, Iapetus's orbit is inclined with respect to Saturn and it is the only large moon from which the rings would be visible in all their glory.
But why should human beings go to the Saturn system when robots like Cassini can explore it for us? In addition to mapping Titan's hidden surface, probing the mystery of Enceladus's icy fountains, and discovering Iapetus's great equatorial ridge, Cassini has produced hundreds of stunning images of Saturn and its rings and moons. It has discovered a bizarre, deep hexagonal feature at Saturn's north pole, some kind of standing wave that's as wide as two Earths and extends deep into the atmosphere, and found a vast hurricane at the south pole. It has confirmed the presence of spokes in the ring system — clouds of dark material lofted above the ring plane by some as yet unknown mechanism — and discovered propeller moonlets within the A Ring and intricate patterns of ripples created by interaction between the rubble and dust of the rings and the gravity of the shepherd moons orbiting in ring gaps.
After successfully completing its original four-year tour, Cassini is now in the middle of its two-year Equinox Mission, due to end in 2010, and was recently granted an extension for a third mission of seven years duration. At the end of that mission, with propellant required for manoeuvring close to exhaustion, it will be deorbited and flung across the ring plane, gathering its last data as it plunges inward and finally flares out across Saturn's atmosphere.
It will leave a tremendous legacy of scientific data and awe-inspiring images. Awe. Now there's a word that's been somewhat devalued; everything is either awesome of full of fail on the internet, these days. But many of Cassini's images — misty Titan hung beyond the arc of Saturn's rings, for instance, or a glistening close-up of the ravaged ancient battlefield of Dione's surface — really are awesome. They evoke the kind of magisterial wonder beyond human plight and experience, unconstrained by calculation or measurement, to which the very best science fiction aspires. And the thought that human beings not only constructed and launched a spacecraft capable of travelling to the sixth planet out from the sun, but are also at this moment piloting it in an intricate and continuous ballet that now cuts it past the very edge of the rings, now sends it swinging outward to very nearly graze the top of Titan's atmosphere, now hurls it through the plumes jetting up from Enceladus's south pole . . . well that's purely awesome, too. The things we can do!
I'm writing this as the fortieth anniversary of the Apollo 11 Moon landing is being celebrated by a plethora of newspaper articles and editorials, books, and documentaries. The three Apollo 11 astronauts were guests of honor at a banquet at the Smithsonian Museum. Google released Google Moon. And so on, and so forth. But the anniversary of the landing of the first men of the Moon is also the anniversary of another great achievement: the touchdown of the Viking 1 lander on Mars, in 1975. The coincidence of the two anniversaries was almost unnoticed, and I doubt there will be the same kind of hoopla about Viking's fortieth anniversary. Unmanned probes like Viking and Voyager and Cassini-Huygens have given us immeasurably valuable insights into the moons and planets of the Solar System, and we are yes, awed, by the images they return. And yet we do not celebrate them in the way that we celebrate the skill and daring and true grit of astronauts who have placed their lives in mortal danger to extend human knowledge and the human frontier. For we are, after all, human, and not entirely governed by logic. Places aren't really real to us until we can plant our boots on the ground. Thanks to the images and data of the Pioneer, Viking, and Cassini missions, the moons of Saturn have been mapped and given shape, but they are not yet — not yet — inhabited. For now, that's still science fiction. For now.
Further Reading and Viewing:
Photos courtesy of Cassini Imaging Central Laboratory for OPerations (CICLOPS).
ABOUT THE AUTHOR
Paul McAuley is the author of more than twenty novels, several collections of short stories, a Doctor Who novella and a BFI Film Classic monograph on Terry Gilliam's film Brazil. His fiction has won the Philip K Dick Memorial Award, the Arthur C. Clarke Award, the John W Campbell Memorial Award, the Sidewise Award, the British Fantasy Award, and the Theodore Sturgeon Memorial Award. His latest novels are Something Coming Through and Into Everywhere.
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