Nitrogen-rich world near Saturn unveiled
Delta channels beneath an orange haze support a theory that this moon may be Earth's nearest kin in solar system.
When the Huygens spacecraft dipped its golden nose beneath the clouds of a distant moon Friday, it at last drew back the curtain on perhaps the most mysterious and exotic object in the solar system.
For more than 20 years, scientists have peered at pictures of Titan and untethered their imaginations. Here was an object unlike any other known to astronomy: A planet-size satellite with a thick atmosphere rich in the chemicals that once made up Earth's primordial ooze. Yet the very object of intrigue - the orange smog that cloaks the moon - made it impossible to see what was going on at the surface.
Now, Huygens's snapshots have begun to sketch fantasy into reality. In the half-light of a veiled world 886 million miles from the sun and 290 degrees F. below zero, Huygens has left little doubt that Titan was once - and could still be - covered in rivers and lakes of liquid or organic goo.
The pictures are so graphic they bewilder scientists, who see compelling evidence of shorelines and drainage channels where fluids once flowed, reshaping the landscape as water molds terrain on Earth.
Titan is too cold for any known form of life. But in a solar system where most of the solid objects fell dormant long ago - their complexions now changed only by the odd asteroid impact - Titan offers an intoxicating view of a world that might still be alive with processes at once Earth-like and incomprehensible.
"It would have been hard to expect too much more - except perhaps splashing into a lake," says Bruce Betts, a scientist at the Planetary Society in Pasadena, Calif. "But that was made up for by the fact that no one expected these landforms to be so fascinating."
In fact, no one was sure if the mission was going to work. No program had ever attempted to land on a planet or moon beyond Mars, and it was up to European Space Agency (ESA), a relative novice, to do it. While NASA's Cassini probe carried Huygens to the Saturn system, ESA had to manage the 2-1/2 hour descent to the surface - an anxious time for an organization that recently lost a Mars lander. They succeeded with few glitches. Huygens continued sending data for more than an hour after landing. "It's one of the harder missions one could conceive," says Dr. Betts. "It really shows that they're a major player."
By Friday evening, their efforts had begun to reveal the biggest piece of unexplored territory in the solar system. On a world where the atmosphere is thicker than the Earth's but has gravity similar to that on the Earth's moon, Huygens parachuted into Titan's deep orange pall.
At 10 miles above the surface, Huygens imaged light areas etched with dark threads snaking toward dark and featureless plains - strongly suggestive of drainage channels for liquids flowing to a reservoir.
At five miles up, it snapped pictures of the light highlands ending sharply at the dark areas, indicating a coastline with a necklace of islands. And when it eventually touched down, Huygens was surrounded by spherical rocks - again suggesting that they were smoothed by a liquid. "We're seeing a lot of evidence for the role of liquids," says Jonathan Lunine, a scientist with the Cassini mission who was at ESA mission control in Darmstadt, Germany.
Beyond that, however, the conclusions become much less certain. The new Titan now emerging is not the stuff of scientists' imagination, but rather a strange vision impressed on them pixel by pixel.
The liquid could have fallen as rain, but several scientists look at the photos and lean toward the idea that it seeped out of the hills. As to the question of where the liquid is now, some suggest it could still be there in dark seas. Others speak of seas of organic sludge or sodden flood plains with the consistency of crème brûlée.
According to Huygens's calculations, the probe sank six inches into the soil when it landed, suggesting a mushy mix - though scientists were not yet sure Monday if it landed in a light or a dark region.
Part of the reason for caution is that scientists have never before seen a world like this. At minus 290 degrees F., hydrocarbons like methane and ethane can act they way water does on Earth, while water is frozen solid, as a rock. Calculations suggest that as much as half of Titan might be water ice, meaning that the light-toned Titan highlands could well be hills of ice.
It makes Titan an odd analogue of Earth. Although Titan is less than half the size of Earth, its atmosphere is some 10 times as high; the lowest clouds on Titan are higher than the highest clouds on Earth. Titan and Earth are also the only objects in the solar system with nitrogen-based atmospheres. It was of the reasons for Titan's allure - the sense that it is a colder version Earth before life formed. And Huygens has done nothing to dispel that. "It is living up to its billing," says Dr. Lunine. "Titan is an exciting place."