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 Cassini-Huygens 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.