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.
While imaging scientists have seen interesting features on the surface - what could be long fault scars, shorelines, or collections of what look like chains of hydrocarbon lakes - they haven't picked up clear evidence of changing terrain that would help them draw firmer conclusions.
So far, the sun has been behind Cassini when cameras have been trained on the moon. This orientation and the light-scattering haze pervading the thick atmosphere have eliminated surface shadows that might hint at topography. And so far, Cassini's radar-map coverage has not dovetailed with parts of the surface other instruments have observed. So researchers are having a tougher time than they anticipated figuring out what their data mean.
In short, the moon's surface has remained something of a Rorschach test for planetary scientists.
Cassini's first close flyby of Titan Oct. 26 nevertheless yielded several surprises, notes Bonnie Buratti, a planetary scientist at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif. Dr. Buratti, a member of the team using the orbiter's visual and infrared mapping spectrometer, notes that so far, Titan appears to be virtually free of impact craters, suggesting that the surface is geologically young.
In addition, recent radar images show "what looks to geologists like a huge volcano," whose flanks are likewise free of impact craters, she says.
Researchers also have spotted wispy short-lived clouds at mid-latitudes, far from the larger cloud formations that seem to hug the moon's southernmost region at this time in its solar year. Some think the clouds are signs of seasonal change, while others suggest they could be plumes from icy volcanic action on the surface. When Voyager 2 hurtled past Neptune and its moon Triton in 1989, the craft's cameras recorded geysers spewing nitrogen gas and dust high into the moon's atmosphere. The same process could be at work on Titan, she speculates.
Absent from the images so far is evidence of liquid oceans or lakes.
The prospect that portions of Titan's surface are liquid received a boost in 2003 when a team led by Cornell University astronomer Donald Campbell reported that it had detected radar reflections consistent with mirror-like glints from liquid hydrocarbons.
But so far, Cassini has failed to pick up equivalent glints in the wavelengths its instruments are using. That could mean the standing bodies of liquid hydrocarbons aren't there; that they exist but are covered with organic scum built up over millions of years; or that they're trapped in the ground.
"The lack of indications so far doesn't demonstrate a lack of existence," cautions Ralph Lorenz, a University of Arizona planetary scientist working with Cassini and Huygens.
He adds that volcanism on Titan is likely driven by radioactive decay in the moon's rocky core. Water ice is thought to be present in a slushy layer between the surface and the moon's frozen interior. As a result, the heat from radioactive decay could supply the energy to drive some interesting organic chemistry in the presence of water. These are conditions thought to replicate those of an early Earth.
Indeed, Dr. Lorenz says he finds Titan more promising in this regard than Jupiter's moon Europa, which captivated planetary geologists with evidence of an active icy crust overtopping an ocean of liquid or slush.
Grinspoon gingerly takes the line of reasoning one step further, suggesting Titan could host simple life forms.
"The safe way to talk about Titan is that we'll learn about pre-biotic chemical evolution," he says. "The more enticing but more out-on-a-limb way of talking is to wonder if there's some kind of life there today."
Prior to October's flyby, he wasn't a believer. But the early results from Cassini "have made me more open to that possibility" of extant life, he adds.
Huygens could help begin to shed light on many of these issues. The craft carries a suite of instruments to measure Titan's winds, tiny particles in its atmosphere, the amounts of solar radiation ricocheting around within the atmosphere, and the composition of its gases. The probe will also track the atmosphere's density with altitude, and yield basic data on the surface it finds.
Indeed, the projected landing site represents a bonus, JPL's Buratti notes. The probe will be parachuting toward an area that hosts both light and dark features as seen from Cassini. The hope is that the probe will shed enough light on surface composition that the information can be used as "ground truth" for what the orbiter's instruments see.
Scientist expect to receive the probe's full batch of data relayed through the orbiter by 2:07 p.m. Pacific Standard Time Friday.
"We're keeping our fingers crossed," Grinspoon says. "This is rocket science, and anything can happen."
• At least 139 moons orbit seven planets in our solar system. More than half of these satellites have been discovered since 2000 - many of those from images taken by the Voyager 1, Voyager 2, and Cassini spacecrafts.
• Jupiter has the most moons, 62. Mercury and Venus have the fewest, 0. Jupiter also sports the largest and, so far, the smallest moons in the solar system. With a diameter of 3,280 miles, Ganymede is bigger than Mercury; S/2003 J9 and S/2003 J12 are less than a mile across.
• Saturn's Titan, where the Huygens probe is scheduled to touch down Friday, is the second-largest moon in the solar system.
• Neptune's moon Triton is covered with nitrogen and methane frost, which makes it the coldest place in the solar system (-391 degrees F.).
• Pluto's only moon, Charon, is more accurately the other half of a double-planet system. The orbits of the two are so carefully synchronized that from one side of Pluto, Charon never changes position in the sky. It's never seen from the other side.
Source: National Aeronautics and Space Administration