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The penetrometer was the first thing to hit. The stick-like probe on the bottom of the Huygens lander punched aside a hard pebble made of water ice on the surface of Saturn’s largest moon, Titan, and sliced down through five inches of soft, muddy material. Scientists watching from Earth were ecstatic–the probe was not expected to survive the landing–but at the same time puzzled: If Titan really was, as they suspected, much like a young Earth, where were the liquid oceans predicted to cover the surface? Where was the methane rain that served as analogue to Earth’s H2O? Suddenly, minutes after landing, the first clue emerged, as the craft began to sink into the soil as if perched on quicksand.

Huygens‘s landing on Titan and what it revealed about that unknown world is the most dramatic success of the CassiniHuygens mission, now finishing its first year of touring Saturn and its 34 known moons. But surprises like this have been the norm, not the exception. In its travels, the Cassini spacecraft has discovered an atmosphere blanketing a moon that shouldn’t be able to hold one, mountain ridges that dwarf the Himalayas, and evidence for short-lived rivers of methane on Titan. For every mystery Cassini solves, it finds 10 more puzzles for scientists to explore.

No one could be sure, for instance, what Huygens–a lander built by the European Space Agency and ferried to Titan by NASA’s much larger Cassini–would land on. Titan’s thick, nitrogen-based atmosphere is opaque, impermeable to all but radar and infrared light. It is also saturated with methane, the principal component of natural gas. Scientists believe that the methane on Titan plays the role that water does on Earth: raining out of clouds; creating rivers, lakes and oceans; then evaporating to form more clouds. The concentration of methane in the atmosphere is so high that many scientists expected there to be enormous seas covering the moon’s surface.

Although Huygens‘s first radar images of Titan showed what appeared to be dark rivers flowing into a sea, images from the moon’s surface showed the “sea” to be more like a barren desert, empty and dry save for the occasional ice pebble. But when the craft started to sink, scientists realized that the surface is actually marshlike–suffused with liquid methane that has been absorbed by the soil. The heat of Huygens‘s batteries boiled off some of the liquid methane beneath the spacecraft.

Yet the methane absorbed into the surface isn’t enough to account for all the rain that has poured down onto Titan’s surface over millions of years. Where did the
rest of the methane go? “That, perhaps, is one of the biggest puzzles,” says Linda Spilker, deputy project scientist for the Cassini mission. Maybe, she ventures, the amount of methane on Titan isn’t constant–something is pumping additional methane into the atmosphere.

Cassini has also found surprises on several of Saturn’s other moons (four of which the spacecraft discovered). Images of icy Iapetus show a moon composed of materials that are among the brightest and darkest in the solar
system. The bright regions are highly reflective water ice, but researchers don’t know what covers the areas that are, as Spilker describes them, “as dark as Xerox toner.” And nothing on Iapetus is so puzzling as the thin, sharp mountain ridge dividing the moon in two. This “belly band” juts almost 12 miles above the moon’s darkened plains and runs for 500 miles along its equator. If Iapetus were scaled to Earth size, aircraft would not be able to fly over the ridge, which would stand more than 110 miles tall.

A March flyby of Enceladus, another icy moon, revealed a thin atmosphere–strange for a moon that’s less than a quarter the diameter of Earth’s moon and thus without enough gravity to hold an atmosphere in place. Scientists theorize that Enceladus’s atmosphere steadily drifts away but is continually replenished by ice volcanoes or geysers.

Cassini will scrutinize these and other moons, follow the changing of the seasons, and watch the rings from high above the planet’s poles. If the spacecraft holds together,
its mission may be extended to 2010 or beyond. Until then, you can get updates at saturn.jpl.nasa.gov.

Ten Minutes to Titan

1.View from the Top
During its descent, Huygens shot this radar mosaic of Titan´s surface. The dark areas look like rivers emptying into a sea. The image seemed to confirm that Titan is a moon covered with flowing liquid methane. 2.Touchdown on Titan
Once Huygens landed, though, it didn´t spot any liquid methane. Instead it found chunks of water ice. Scientists speculate that the ice forms during the eruption of “ice volcanoes,” geyser-like fissures shooting water into the -290

Storms and Waves

1.Smoking Gun
As the moon Pan orbits around Saturn, its gravitational pull creates waves in Saturn´s rings. This image provided the first confirmation of the effect, seen at left. 2.Shadow Planet
Saturn is seen here in the shadow cast by its rings. Blue streaks appear where the rings are thin and absorb only red light. The moon Mimas floats at right.

FLy Me to 34 Moons

Lost Comet
Cassini first flew by Phoebe, Saturn´s most distant moon, last June. Researchers think that Phoebe´s strange orbit (it rotates in the opposite direction of Saturn´s other moons) suggests that it could be a comet captured by Saturn´s gravity many millions of years ago.

Storms and Waves

1. Better Than Evian
In this UV image of the rings, blue regions are water ice; red, empty space. Scientists can´t explain how the ice stays 99 percent pure though bombarded by meteorites. 2.Dragon Storm
Scientists think that this storm, which flares up and recedes every few months, creates electrical eruptions that may cause powerful radio bursts.

by Courtesy of NASA

Cassini and Huygens, the piggyback probe For seven years and two billion miles, the Cassini space-craft carried the Huygens lander up to Saturn. Last December, six months after Cassini began its four-year study of Saturn, it released Huygens–a probe the size of a Volkswagen Beetle–on a trajectory toward Saturn´s largest moon, Titan. When Huygens entered Titan´s atmosphere 21 days later, it relayed pictures and data back to Earth through Cassini.

Fly Me to 34 Moons

Dione and the Storms
This true-color image shows the moon Dione with Saturn in the background. Look closely and you can see several oval-shaped storms in Saturn´s atmosphere.

Fly Me to 34 Moons

The Ring-Stealer
Tiny Prometheus occasionally passes close enough to Saturn´s multistranded F ring to steal some of its icy material–creating gaps or kinks in the strands. Near the top of this image, you can see traces of Prometheus´s last pass.

Fly Me to 34 Moons

The Ring-Stealer
Tiny Prometheus occasionally passes close enough to Saturn´s multistranded F ring to steal some of its icy material–creating gaps or kinks in the strands. Near the top of this image, you can see traces of Prometheus´s last pass.

Fly Me to 34 Moons

Yin-Yang Moon
Iapetus is one of the oddest objects in the solar system. The moon is made of ice, its white surface visible in the top half of this image and ringing many of the craters. One hemisphere of the moon, though, is covered with a fluffy, porous, almost black material that scientists can´t identify. Because it occurs on only one side of the moon, the material was probably deposited there from elsewhere in space, but no one knows by what. A huge mountain ridge encircles the planet, making it look like a giant walnut.

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