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Earth AT THE SHORE of a Titan lake, liquid methane laps the coast as a blimp-like “aerobot” drops a tethered device to collect lakeshore mud for analysis. In the distance, clouds rain methane onto the surface, where it collects in low-lying areas of Titan’s northerly “lake district.” MICHAEL CARROLL FOR ASTRONOMY © 2010 Kalmbach Publishing Co. This material may not be reproduced in any form astronomy ⁄⁄⁄ novemberwithout permission from the publisher. www.Astronomy.com Titan unveiled of the outer solar system Each Cassini flyby fills in details of this frostbitten saturnian moon shaped by forces identical to those that sculpted Earth’s surface. ⁄⁄⁄ BY MICHAEL CARROLL crescent Saturn slices an arc through a crystalline blue sky. Snow-dusted rocks tower above icy plains. This was the Titan of the 1940s, a world immortalized in the paintings of pioneering space artist Chesley Bonestell. In fact, his paint- ing of Titan’s surface, which graced the cover of Willy Ley’s Aand Bonestell’s 1949 bestselling book The Conquest of Space, has been called the best-known work of space art ever created. Bonestell’s vision reflected the scientific thinking of the time. But research soon showed that Titan was not a clear-skied desert world. Instead, its ruddy hue traces to a thick shroud of orange smog. Discovery of methane in Titan’s atmosphere raised the possibility of hydrocarbon seas. But the smog and Titan’s great distance kept its secrets hidden. After the Hubble Space Telescope provided tantalizing peeks under Titan’s shroud in the 1990s, the smog lifted January 14, 2005. That was the day the European Space Agency (ESA) Huygens probe dropped into Titan’s atmosphere and parachuted to the surface. And what did it see? Earth. “We now have the key to understanding what shapes Titan’s landscape,” mission scientist Martin Tomasko told reporters at ESA’s head- quarters in Paris. “Geological evidence for precipitation, erosion, mechanical abrasion, and other fluvial [liquid-driven] activity say that the physical processes shaping Titan are much the same as those shaping www.astronomy.com Summit Cryolava flows AN INFRARED VIEW of Titan’s surface MISSION SCIENTISTS initially thought this feature, Ganesa Macula, might be a meteor reveals dark surface features but no fine impact crater. But it turned out to be a wide, gently sloping “cryovolcano” that has erupted details. Radar images ultimately unlocked a slushy mix of water and ammonia from Titan’s icy crust. NASA/JPL Titan’s secrets by peeling away the moon’s obscuring clouds. NASA/JPL/SPACE SCIENCE INSTITUTE Earth.” And these processes would be utterly familiar to any first-year college geology student. “We see dendritic channels, which are the result of liquid erosion,” says Cassini imaging-team leader Carolyn Porco of the Space Science Institute in Boulder, Colo- rado. “We see dark hydrocarbons washed from the highlands, pooling in low-lying areas. We see aeolian [wind-driven] effects. We see cloud systems. In short, we are see- ing earthlike processes, but they are occur- ring in unearthlike materials.” With every new flyby of Titan, Cassini paints a richer and more colorful portrait of Titan than even Chesley Bonestell could envision. We are finally beginning to GRAINY, UNFOCUSED, and distorted yet utterly remarkable, this mosaic of images from understand Saturn’s largest moon as a the descending Huygens probe offers a close-up glimpse of Titan’s surface. Highland ter- world in its own right, even though it seems rain laced with drainage channels slope to darker lowland areas. ESA/NASA/JPL/UNIVERSITY OF ARIZONA just like home in some fundamental ways. molecules — potential building blocks for predicted hydrocarbon oceans? Or were Early glimpses organic life. Despite Voyagers’ batteries of they just solid surface features? To understand the scientific journey to instruments, neither craft penetrated Titan’s Titan, we must look back in time. Voyager clouds to glimpse the landscape below. The Cassini revolution 1 flew past Titan in 1980, followed by Voy- The only way to unveil Saturn’s mystery The answers would finally come with the ager 2 in 1981. The craft radioed back por- moon would be to observe it in wave- Cassini-Huygens mission to Saturn. The traits of an orange-tinted orb with an lengths of light that pass through the Cassini orbiter carries synthetic aperture atmosphere 1.5 times denser than Earth’s. obscuring cloud cover. In October 1994, radar, similar to the radar instrument the Voyager flyby data fueled earlier suspi- flight engineers commanded the Hubble Magellan spacecraft used in 1990–1994 to cions that Titan’s surface might harbor Space Telescope’s Wide Field and Planetary peer through Venus’ cloud cover and map lakes or seas of the hydrocarbons methane Camera to observe Titan in near-infrared its surface. Cassini’s imaging system also or ethane, and that Titan’s skies may drench light. Infrared’s long wavelengths bounced peers through the clouds in near-infrared, the surface in a rain of hydrocarbon-based off surface features and then passed back just like Hubble did. The European Huy- through Titan’s cloud cover out to space, gens probe would complete the reconnais- Michael Carroll is a space artist and science where Hubble imaged them. The new sance by parachuting to Titan’s surface journalist in Littleton, Colorado. His work Hubble snapshots revealed dark and light while sniffing its atmosphere and taking appears frequently in Astronomy. features on the surface. Could these be the snapshots of the terrain below. astronomy ⁄⁄⁄ november Visible and infrared imaging from Cassini’s first Titan flyby, in October 2004, Surface ice crust Rocky core Inside Titan 56 miles 2,345 miles revealed a surface with dark and bright TITAN’S INTERIOR structure (90 km) thick (3,750 km) thick regions in complex patterns. No features remains a mystery. But plane- were immediately recognizable as moun- tary scientist Giuseppe Mitri tains, rivers, or meteor impact craters. of the Jet Propulsion Labora- “At first the images were very ambigu- tory has developed a theo- ous,” Porco recalls. “We were looking retical model of Titan’s through layers of haze. We saw no shadows. interior that describes one We couldn’t say what was up or down.” possibility. In Mitri’s model, Cassini’s radar helped clarify things. Titan has a warm rocky core Microwaves beamed from Cassini’s main surrounded by a layer of antenna pierced Titan’s atmospheric haze, compressed ice about 1.4 bounced off the surface, and returned to times denser than common ice. (The high-pressure layer the craft. Patterns in the returning radar formed as Titan was cooling “echoes” provided clues to Titan’s topogra- and contracting in the early phy and surface composition, such as solar system.) An ocean of whether the dark patches were liquid. water and ammonia under- lies Titan’s frozen outer crust. High-pressure Water-ammonia ocean Titanic volcanoes ASTRONOMY: JAY SMITH ice layer 250 miles The first sharp radar images came in a few Illustration not to scale 130 miles (210 km) thick (400 km) deep days after Cassini’s inaugural flyby of Titan. A radar scan along a strip nearly 5,000 miles (8,000 kilometers) long showed Some cryomagma flows on Titan are Fluctus covers at least 9,150 square miles smooth plains and rugged highlands. But massive. About 1,375 miles (2,200 km) (23,700 square kilometers), or slightly less mission scientists also saw a prominent, from Ganesa lies a flow, named Rohe Fluc- than the area of Vermont. Sub-zero cryo- 110-mile-wide (180 km) circular feature tus, estimated to be 1,000 feet (300 meters) volcanism may have substantially shaped spanning the strip from top to bottom. thick. To pile up that high, cryo magma Titan’s icy surface, just as molten-hot volca- They named the structure Ganesa, after the would need to be thicker than a water- nism has transformed Earth’s rocky surface, elephant-headed Hindu god. ammonia slurry. Lopes and several col- over billions of years. And it may still. Mis- The scientists wondered if Ganesa were leagues proposed in the February 2007 sion scientists are on the lookout for signs a scar from a past meteor impact. A closer issue of Icarus that a dash of methanol of recent eruptions. look showed otherwise. Ganesa appeared to would make the flows viscous enough to be a volcano. “We realized it wasn’t an account for their thickness. Huygens away! impact feature, and then we saw similarities Cryovolcanism is not simply an occa- On December 24, 2004, the Cassini orbiter to pancake domes on Venus,” explains sional gurgle of slush; it’s a major force for dispatched its hitchhiker, the European Rosaly Lopes, a Cassini radar-team mem- change on Titan. The flow dubbed Winia Space Agency’s Huygens probe. Three ber at NASA’s Jet Propulsion Laboratory weeks later, Huygens entered Titan’s atmo- (JPL) in Pasadena, California. sphere and released a series of parachutes Pancake domes are flat-topped volcanic to slow its 2½-hour descent to the surface. mountains on Venus built by eruptions of The craft gathered data on atmospheric molten rock, or magma — just like volca- conditions and composition on its way nism on Earth. But Titan’s surface, although down — data to feed computer models of hard as rock, is mainly frozen water. Titan’s atmosphere. Instead of molten rock, Ganesa erupts a After passing through the cloud deck, slushy “cryomagma” composed of melted Huygens snapped aerial photos of river-cut water, perhaps mixed with ammonia. hills and flat regions that may be dry lake- “We now think that Ganesa may be a beds.

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