It could land with a thump, a splash, or a squish. However it touches down, a robotic probe's arrival Friday at Saturn's moon Titan marks a significant turning point in the history of solar-system exploration.

If all goes well, the European Space Agency's Huygens probe will be the first mechanical Columbus to settle onto a planetary surface in the frigid realm of the outer solar system. There, beyond what's whimsically called the "snow line," rock gives way to ice and liquid gases as primary building blocks for giant planets and their atmospheres.

Huygens's descent onto Titan will offer the closest look yet at an object from the outer solar system and reveal possible insights into the kind of planetary chemistry that led to life on Earth.

Titan "is unlike any place we've ever explored," says David Grinspoon, a planetary scientist at the Southwest Research Institute in Boulder, Colo. "If everything works, this could be one of the most revelatory moments in nearly five decades of planetary exploration."

For 350 years, since its discovery by Dutch astronomer Christiaan Huygens, Titan has tantalized astronomers. These days, scientists trying to pierce the moon's smoggy shroud catch dim, distant glimpses of a world both familiar and alien. During the probe's 2-1/2- hour descent, scientists expect to rip that shroud for a front-row view.

Huygens is designed to gauge winds and weather in its vicinity and sample the stew of chemicals that form and reform in Titan's atmosphere. Meanwhile, cameras will give scientists an airline passenger's view of the landscape as the probe parachutes to the surface. There it will measure basic properties of its landing spot - whether an icy island, a sea or lake of liquid hydrocarbons, or perhaps a solid outcropping covered with organic goo.

Many planets and moons are "dead worlds, and we're trying to reconstruct the deep past," says Dr. Grinspoon. "But a few - Earth, Venus, to some extent Mars, and now apparently Titan - display ongoing activity today that we can monitor and learn from."

Outside of Saturn itself, Titan is the most highly sought prize for scientists attached to the Cassini-Huygens mission, an international effort spearheaded by the National Aeronautics and Space Administration and which includes the European and Italian space agencies. The Cassini orbiter, Huygens's interplanetary taxi, is slated to orbit Saturn 74 times during its mission, which is scheduled to end in July 2008. Titan, larger than Mercury or Pluto, will attract 45 close flybys. Cassini will also buzz six other moons eight times collectively.

Some of the scientists most eager to see what Huygens turns up are those using Cassini's suite of mapping radar, cameras, and spectrometers. Since Cassini arrived at Saturn last July, the orbiter has swung by Titan twice. But Titan has been a tough nut to crack. Cameras and radar have distinguished between regions of light and dark on the surface, with dark suggesting the presence of hydrocarbons and light suggestive of methane snow or ice. With Titan's surface temperature hovering at about -197 degrees C, or only 94 degrees above absolute zero, the ice would be hard as rock.

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