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Saturn Moon's Bizarre Geography Revealed by Spacecraft

Brian Handwerk
for National Geographic News
May 12, 2005
 
Scientists have speculated for years that Titan, Saturn's largest moon, may give clues to what Earth's chemistry was like before life formed on Earth.

They have also wondered what lies under the moon's thick cloud cover—perhaps large lakes and rivers filled by methane rain?

Now, thanks to the Cassini spacecraft and the Huygens probe, they're getting some answers.

Analysis of the first flybys of Titan reveals that this second largest moon in the solar system (larger even than the planet Mercury) is perhaps even more mysterious than scientists previously believed: Titan is an icy planet-like rock, where it rains methane and where perhaps ice volcanoes exude frozen "cryomagma."

NASA's Cassini first flew past Titan in October last year, providing close-up radar images of the moon's cloud- shrouded surface.

On January 14 the European Space Agency's Huygens probe (which was deployed from from the Cassini oribiter) landed successfully on Titan. (Read the story and see the first photo.)

Since then scientists have been crunching the data gathered during the first flybys and the Huygens landing. Their findings are published today in a special issue of the journal Science.

"We realized that Titan is a very complex place, really a planet in its own right that happens to be in orbit around another planet," said Candice Hansen, a Cassini Scientist at NASA's California Institute of Technology-based Jet Propulsion Laboratory (JPL). "From a complexity point of view, it's as interesting as the Earth."

The nitrogen and methane in Titan's atmosphere combine into long chains of hydrocarbons. For nearly two decades scientists had theorized that these hydrocarbons would condense and rain onto Titan's surface, perhaps creating methane lakes or liquid-filled craters.

Cassini has altered such theories. "We haven't seen liquid lakes, and we've been looking," Hansen said. "We're now thinking that regions of liquid will be fairly confined if we find them at all."

A new hypothesis likens Titan's rain cycle to that of a frigid Arizona, where occasional methane precipitation carves riverbeds that are dry during most of the year.

Ice Volcanoes

"This is the first large moon we've explored that bears any resemblance at all to earthly processes," said Jonathan Lunine of the University of Arizona's Lunar and Planetary Laboratory in Tucson.

Lunine co-authored one of the Titan research papers appearing in Science.

Jupiter's moon "Europa may be a better candidate for finding life, but it's a very unearthly situation. On Titan, processes we see on Earth, like windswept surfaces and streams carved by liquid, are present but are played out with different materials."

Researchers expected the moon to be dotted with craters, but only two have been spotted so far. "That tells us immediately that there active geological processes which are erasing surface features," Hansen said.

Volcanism is one such process that may well be at work. But on frigid Titan it involves not lava but water-ice-based "cryolava."

Radar images of Titan's surface reveal the remnants of lavalike flows. "We're all familiar with the fact that liquid water is more dense than water ice. That's why we can have icebergs," Lunine explained.

"If you add something like ammonia to water you get a much colder melting point, a molasses-like texture, and a more buoyant liquid that will periodically come out [of cracks or other fissures in the icy surface]."

These "cryovolcanic" processes may be at working shaping and reshaping Titan's surface, in much the same way that hot volcanic activity does on Earth.

"It's telling us that Titan has been active internally, it has a geology. It's not just a cratered, dead moon with an atmosphere around it," Lunine said.

Ozone Holes and the Emergence of Life

Specific atmospheric traits are also emerging from the Cassini data.

Titan's winter polar atmosphere may have interesting parallels with Earth's Antarctic ozone hole—albeit with different chemistry.

Infrared images reveal frigid temperatures (-290 degrees Fahrenheit, -180 degrees Celsius) circumpolar winds up to 358 miles per hour (576 kilometers per hour), and polar concentrations of atmospheric organic compounds that are different from those of other latitudes.

"Different concentrations suggest to us that strong winds my be isolating air at the high northern latitudes and inhibiting global mixing of air masses. We see this happen on the Earth," said F. Michael Flasar, an astrophysicist at NASA's Goddard Space Flight Center in Maryland.

On Earth these seasonal processes help to create the infamous Antarctic ozone hole.

"It's a complex process and one we wouldn't have thought of [on Earth] until we got data that there was a hole in the ozone layer," Flasar added.

Titan may offer a chance to learn about an analogous process—one involving the chemistry of organic molecules, not ozone—by observing it in a second environment.

Comparison With Pre-Life Earth

Some atmospheric scientists believe that Titan may offer a glimpse at what Earth's atmosphere was like before life arose and changed it forever.

"It's life on Earth that gives us the oxygen in our atmosphere," JPL's Hansen said. "In Titan we have kind of a case study of an atmosphere before that happened. For those people who look at evolution of atmospheres, it's a very interesting starting point."

If the atmosphere suggests a pre-life Earth, might other surface conditions offer some clues to the basic building blocks of life on Earth?

"We've been speculating for a long time that the surface might offer a glimpse of what the chemistry was like at the time that life started on Earth," said Steve Wall of Cassini's Radar Science Team.

"There are a lot of big differences, most notably a very big temperature difference, but we know there are some of the same basic [chemical] structures that were there before life began. We're hoping it will give us a second example of just what that chemistry does."

Cassini arrived in Saturn's orbit on July 1, 2004, after seven years of space travel. It will make 74 Saturn orbits, including 44 flybys of Titan, by June of 2008.

The craft packs an impressive array of equipment, including radar, visible- and ultraviolet-wavelength optics, and myriad remote-sensing instruments. Current Titan data comes from the Huygens landing and just five flybys.

"It's only the first glimpse," JPL's Wall said. "We've seen just a percent and a half or maybe 2 percent of Titan. We expect to see 25 percent or more, so we're going to get a good global picture of what this moon is like. The bad news is we have to be patient, because it dribbles in a bit at a time."

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency, and the Italian Space Agency. NASA's Jet Propulsion Laboratory at the California Institute of Technology in Pasadena manages the Cassini-Huygens mission for NASA's Science Mission Directorate in Washington, D.C.

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