It's "Titan or Bust" for Saturn's newest satellite, Cassini, and its 700-pound sidekick probe Huygens.
A weekend's worth of stunning images from the spacecraft as it began to orbit Saturn - including a distant flyby of a moon that one scientist terms "a deranged version of Earth" - have left scientists elated but also confused.
Saturn's rings and its moon Titan may have taken center stage. But scientists also detected an event that may hold clues to the birth and death of the rings. And the new information is whetting researchers' appetites for two more Titan encounters. During the second of these encounters, perhaps in late December, Europe's Huygens probe is slated to parachute through Titan's atmosphere and land on its surface in what promises to be a 2-1/2 hour tour de force.
In the meantime, Cassini-Huygens already has given planetary scientists an eye-opening performance.
"In one weekend, we've just turned everything we've learned from ground-based and Hubble [telescope] observations on its head," says Mark Leese, program manager for the Huygens probe's surface science package.
On Thursday, as it left Saturn's ring system, Cassini buzzed past Titan some 210,600 miles above its surface, yielding the closest look yet at the moon's surface.
Using optical cameras with special filters to pierce Titan's smoggy veil, scientists detected large patches of bright and dark surfaces that hint at its general terrain. At least one circular feature - perhaps an enormous impact crater - emerged in the images, while other features appeared as broad lines.
Referring to similar line-like forms in earlier images, "this is starting to look suspiciously like tectonic features," says Carolyn Porco, Carolyn Porco, a researcher at the Space Science Institute in Boulder, Colo., and head of Cassini's imaging team. In itself, that isn't unusual, she says; other large moons in the solar system also have them. But Titan's linear features also appear to have unique traits that set them apart from tectonic features, seen elsewhere, she adds.
Researchers caution that from a distance of more than 200,000 miles, they can't distinguish mountain ranges, seas or lakes, or other topographic standouts. That level of detail awaits closer flybys. But their cameras are revealing features that cover wide swaths of the moon's hemispheres. "The fact that we don't see a lot of circular features suggests that Titan has been geologically active," says Elizabeth Turtle, a University of Arizona planetary scientist on Cassini's optical imaging team. Such activity would erase from the surface evidence of old impacts.
In addition, the moon's skies appeared to be cloud-free - with one glaring exception. An Arizona-sized patch of clouds hovered near the South Pole, currently basking in 24-hour sunlight during a frigid Saturnian summer. The patch, which morphed its shaped over the course of four or five hours, suggests clouds similar to summertime cumulus clouds on Earth. But at Titan the cloud droplets are methane rather than water.
This is a vivid confirmation of results from ground-based observations, reported in October 2000. A team led by Caitlin Griffith, another planetary scientist at the University of Arizona in Tucson, reported evidence in infrared data suggesting clouds on Titan covering roughly 1 percent if its surface and changing rapidly - suggesting rainfall.
Images taken with a spectrometer that covers a range of wavelengths from visible light down through infrared radiation also cut through the smog to map the distribution of water ice and hydrocarbons across the surface. Oddly, though, researchers noted that regions that gave off hydrocarbon signatures at one wavelength telegraphed water ice at another.
"This is truly a strange place," says Kevin Baines, a planetary scientist at CalTech's Jet Propulsion Laboratory.
Earlier, researchers marveled at Saturn's rings.
Even with all the preparation, "I'm surprised at how surprised I am at the beauty and clarity of the ring images; it's shocking," says Dr. Porco.
Researchers have found that one gap between rings, known as the Cassini division, is laden with dust, as is the planet's outermost ring. The dust appears to have the same chemical signatures as those found at Phoebe, another Saturn moon. Indeed, even the gaps Cassini shot through were laden with dust, causing as many as 680 collisions per second, or 100,000 total impacts.
And scientists detected an enormous pulse of oxygen coming from the rings as Cassini approached Saturn.
The cause remains elusive. "If it was one event, we missed it," laments Donald Shemansky, a member of the science team that gathered the data. It might have been triggered by a collision within the rings of two objects perhaps three or four miles across.
Whatever the cause, "we're seeing a massive amount of oxygen" being pumped out of the rings, he says.
The process may have a bearing on a debate over the age of the rings. A rough calculation showed that if the rings lost this amount of mass (roughly the equal to the total mass of the tiniest particles in the E ring) on a regular basis without it being replenished, one of the planet's rings - known as the E ring - would vanish in 100 million years.
"We've just had a taste of what's to come," Dr. Griffith says of the weekend's astronomical feeding frenzy.