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Craters on Titan Offer Glimpse Into Saturn Moon's Past

A new study in the journal Icarus provides the latestround-up of the number of impact craters found on Saturn's moon Titan.?

Between 2004 and December 2007, Cassini had surveyed 22percent of Titan'ssurface. Scientists analyzed images taken by the spacecraft'shigh-resolution Radar Mapper instrument, and found 49 impact craters.

"Impact craters are created on every planet because ofasteroids, comets and other debris that collide with their surfaces," saidCharles Wood, senior scientist at the Planetary Science Institute in Tucson, Az, and lead author of the study.?"Analyzing impact craters is a standardtechnique to tell you the history of the world."

Titan has been described by NASA as "one of the most Earth-likeworlds" found to date. But it's no secret that Saturn's largest moon is avery unfriendly place for life.

The main reason Titan seems to be unfit for life is becauseof its temperature. Temperatures on Titan, located approximately 800 millionmiles away from the sun, hover around minus 290 F (minus 179 C). When it isthat cold, water on the surface becomes as hard as rock, eliminating the liquidwater that scientists believe is a precondition for the existence of life as weknow it.

Why then are scientists still so preoccupied with studyingthis planetary body, requesting more missions and thus more funding? The answerlies in Titan's similarity to early Earth.

"Titan is really remarkable because it is such an Earth-likeplanet," said Wood.

Titan is giving scientists like Wood an opportunity to lookback in time. Other than the absence of liquid water on the surface, theatmospheric conditions that exist on Titan today resemble the environment thatprobably existed on early Earth. Titan is the only planetary body in the solarsystem, other than Earth and Venus, to have a solid surface and a denseatmosphere. And like Earth, nitrogen makes up much of that thick, hazyatmosphere, with small amounts of methane.

Methane exists as a liquid, gas and solid at Titan'sfreezing temperatures, and scientists believe it could be playing the role thatwater does on Earth.

"Some people say Titan is like a cold version of earlyEarth," said Wood. "If it were warmer, you'd definitely think lifeexisted there."

Understanding the chemical processes on Titanmay help scientists understand how life began on Earth billions of years ago.

But many questions still linger, and that is the reasonscientists are so committed to studying this satellite of Saturn. The Cassinispacecraft's RADAR instrument is uncovering some of the mysteries that liebeneath the thick hydrocarbon atmosphere that hides our view of Titan'ssurface.

Cassini's Count

Young impact craters are easy to identify because of theirnear-circular outlines, low rims, deep interiors and flat floors. But volcanoescan also create circular depressions that look very much like eroded impactcraters.

"When you see a round (structure) in a new world, youhave to think there are two likely origins: impact cratering or volcanism,"Wood said.

Scientists have fairly fool-proof methods for distinguishingcraters created by volcanoes from those created by an impact. While no one knowsmuch about the type of volcanic activity on Titan, scientists think volcanoeson Titan wouldn't expel hot-rock lava like on Earth. Instead, liquid watermixed with ammonia or other chemicals would probably ooze out of the volcano,and then freeze solid on the surface. Volcanoes also may be a source of themethane in Titan's atmosphere.

Wood says there is strong evidence that most of the cratersseen in these Cassini images were created by impacts.

"On about 22 percent of Titan, five of them looked verymuch like impact craters," Wood said. "I don't think there would beany question."

In this analysis, scientists identified the five"certain impact craters" to be the Ksa, Sinlap, Menrva, Afekan andSelk craters. The remaining 44 craters were classified as Class 2 (nearlycertain impact craters) and Class 3 (probable impact craters).

"I think they did a good job given the nature of thedata," said Alfred McEwen, professor of planetary sciences and director ofthe Planetary Image Research Lab at the University of Arizona.

So far, Cassini images have revealed only about one fifth ofTitan's surface, and that is inadequate to make any definitive conclusionsabout the potential for any kind of life forms to exist on Titan. Wood says ifthey find that the surface of Titan is "very ancient," that probablymeans chances of finding life would be less likely.

If a planetary body is heavily cratered, like Jupiter's moonCallisto, it's because the surface hasn't been altered much by volcanicactivity or atmospheric weathering. An asteroid or comet impact on such aninactive world leaves a crater that lasts for eons. But for an active planetsuch as Earth, impact craters tend to get buried or erased over time. Thegeological or atmospheric activity of a world may be related to its prospectsfor life. Since Titan has very few impact craters,scientists predict it may be an active moon with a very fresh surface.

In another recent study, Wood and a team of scientistslooked at impact craters on an area of Titan known as Xanadu - what theydescribe as "the most unusual region on Titan." Xanadu, about thesize of Australia, is a large bright area that scientists suspect may be madeof water ice.? Xanadu has more impact craters than any other region surveyed byCassini so far, but the distribution of these craters appears to be veryirregular.? This means that the age of Xanadu varies depending on thelocation.?

Right now, scientists don't have enough data to make final conclusionsabout the age or activity of Titan's surface, and that is why there is a needfor a more complete look at the enigmatic moon.

"In the next seven years, the extended mission fundedby NASA will continue to allow the radar instrument to reveal more of thesurface of Titan," Wood said. "Hopefully by the end of the mission,we will have seen about 50 percent of Titan's surface, finding more impactcraters and clues to Titan's history."

McEwen hopes for an even better option.

"A new mission toTitan would be very nice, of course," McEwen said. "We'd like toget much more complete remote sensing data. Now that we can understand Titan,we can design better instruments. There's much more information we'd like tohave from the surface."

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