A Saturn moon with echoes of Earth

Scientists may have rediscovered the Earth - only it's 826 million miles away.

The Cassini spacecraft, in a dramatic flyby of the Saturn moon Titan, has sent back images of an alien expanse that has many of the hallmarks of a young Earth before the rise of organic life some 3 billion years ago.

The initial evidence: an exotic surface enriched with organic goo and an atmosphere that is largely nitrogen with generous helpings of methane tossed in. Now planetary scientists are sifting through the first gigabytes of what promises to be a mother lode of data on Titan following the Cassini mission's brush with the solar system's second-largest moon Tuesday.

The encounter was the first of 45 planned during the mission's four-year run at Saturn, marking a significant moment in solar-system exploration.

"Titan is the last great expanse of unexplored terrain in the solar system," says Carolyn Porco, who heads the mission's imaging team and has actively participated in solar-system exploration missions since the Voyager program in the mid-1970s. Scientists have been trying for decades to peer through the planet's smoggy curtain from millions of miles away, but now "the window's finally been opened" to a clearer, more richly detailed view.

During its inaugural close pass, Cassini has given planetary scientists virtually everything they had hoped for - intriguing images and head-scratching puzzles.

One relatively firm discovery announced during a post-flyby briefing Wednesday: Titan's atmosphere rotates faster than the moon itself. Thus Titan joins the planet Venus as one of the solar system's rare bodies with a super-rotating atmosphere.

The close-up images often look more like patchworks of light and dark areas than they do recognizable terrain. Some hold wispy hints of what might be hills or valleys. Others display sharp boundaries between vast areas of light and dark surfaces.

Such boundaries could represent shorelines or the edge of lava flows, Dr. Porco suggests. Whatever they turn out to be, "people get very excited about sharp boundaries because it means something's going on at the surface," she says.

The full details may take months to tease from multiple exposures taken at various wavelengths of light.

"We can't make heads or tails out of what we're seeing," acknowledges Kevin Blaines, a planetary scientist at NASA's Jet Propulsion Laboratory in Pasadena and member of one of the mission's instrument teams.

So far, the images are fascinating as much for what they don't show as for what they display, researchers say.

For example, no one has spotted craters, suggesting that the moon's surface could be relatively young or that impact structures could have been covered by material falling from the atmosphere.

And at least on Cassini's side of the moon, optical and infrared images so far have failed to turn up lakes or seas of liquid methane, widely held to exist on the surface. One of the key questions surrounding Titan is why it has an atmosphere at all. One thought is that the methane in its atmosphere that gets broken up by sunlight and disappears is replenished from large liquid reservoirs on its surface.

For now, imaging-team members have dubbed the broad, dark region they see as a "mares," the Latin word for seas, because they resemble the mares on the moon and Mars, which are darker than their surroundings.

Some regions within the mares host white patches "that look like islands - they have a kind of Mediterranean feel," Dr. Blaines says.

Yet Jonathan Lunine, a Cassini team member and planetary scientist at the University of Arizona, notes that close-ups of these white spots show that some are stretched out in similar directions, perhaps suggesting wind-shaped features or a shifting crust.

At least initially, it may depend upon radar data to reveal any fluids on the surface. Meanwhile, chemical analyses of the atmosphere from flyby data suggest larger quantities of carbon monoxide and carbon dioxide than researchers expected, giving scientists yet another unusual piece of Titan's puzzle. If the moon does have high concentrations, they may show up in the moon's ices as well.

For all the interest in what the flyby reveals, the main purpose for skirting to within 1,174 kilometers (729 miles) of the surface was to scoop up samples of the outer atmosphere to better understand the atmosphere's density. The information is critical to the success of the Huygens probe, a European Space Agency capsule that will parachute through Titan's atmosphere to the surface. Cassini will release Huygens Dec. 24, and the probe will make its trip through Titan's atmosphere in mid-January.

Density information is particularly vital because atmospheric density helps determine how quickly Huygens takes the plunge and the kind of heating it will experience as it falls.

The probe's instruments are triggered by timers whose settings are based on its "fall" through computer simulations of Titan's atmosphere. If Huygens parachutes too quickly or too slowly, it won't be gathering information at the altitudes of most interest to scientists.

"We must verify with observations that the assumptions we made are correct and that Huygens will enter the atmosphere we know," says Jean-Pierre Lebreton, the Huygens project scientist.