By peeking through Titan's hydrocarbon shroud, Cassini has discovered what may be two large impact craters.

Impact craters are the birthday candles of a planet's surface. A heavily cratered body like our Moon indicates a long-dead world, a place where not much happens to disturb the surface.

An active planet that is constantly changing, such as the Earth, has far fewer impact craters visible on its face.

The geological or atmospheric activity of a world may be related to its prospects for life. The geologically inert, airless Moon is not thought to be capable of sustaining life as we know it.

Whereas the Earth, with its weather, erosion, volcanism, and tectonic burial and uplift, has energy to spare, and life thrives in part because of this energy.

Titan may be another place in the solar system where life could gain a foothold. Although Titan is very cold - the average surface temperature is about 94 degrees Kelvin (minus 291 Fahrenheit) - it does have a thick atmosphere full of churning chemical reactions.

Titan also may have ice volcanoes erupting on the surface, places that could potentially provide energetic havens for at least single-celled life.

Robert Nelson of NASA's Jet Propulsion Laboratory says that Titan was long thought to be the fresh-faced ing�nue of the solar system, with a dynamic weather system that kept the surface even younger than Earth's.

"The Titan model that we had been working with had thunderstorms of methane and ethane, with oceans and winds all over," says Nelson. "But Titan seems to be a lot more mild and benign then we had previously thought."

The proposed oceans of liquid hydrocarbons have not been found. And while images taken by the Huygens probe show river channels that may have formed as a result of methane rain showers, it is not known how often such storms occur, or even if they occur at all.

Scientists do think that chemical reactions in Titan's atmosphere result in a nearly continuous snowfall of dark organic sludge.

Ralph Lorenz of the Lunar and Planetary Lab at the University of Arizona notes that, if this sludge has been falling over the age of the solar system, it could be several hundred meters deep by now.

Such an accumulation could hide many surface features, although craters deeper than a kilometer (or mountains taller than a kilometer) should still be visible.

Small impact craters aren't seen in the Cassini images, but their absence is probably not due to burial. Instead, Titan's thick atmosphere causes smaller meteorites to burn up before they can reach the moon's surface.

Larger meteorites should have hit Titan on a regular basis. Mimas, Tethys and other satellites of Saturn have many large impact craters, and it doesn't seem likely that Titan should have escaped the fate of its neighbors.

But because only two large impact craters on Titan have been discovered so far, this suggests Titan's surface is extremely young and changeable, with the missing craters having been buried or otherwise obliterated by weather or geologic processes.

However, Nelson thinks the two craters could point to a more ancient surface. Both impact craters are quite large - one is 80 kilometers across, the second is 440 kilometers across - and Nelson says that such large craters tend to be extremely old.

Billions of years ago, when the solar system was still forming, everything was essentially an asteroid swirling around the sun. These "planetesimals" grew larger by slamming into each other, and eventually formed planetary cores. As time went on, there were fewer big asteroids remaining in this celestial collision course to make large impact craters.

Nelson says that because large, presumably old craters have been preserved on Titan's surface, then the surface can't have changed very much over time.

Of course, the equation "big crater = old crater" does not always hold true. The dinosaurs on Earth were privy to that fact when a meteorite carved out a huge 180-kilometer wide crater a mere 65 million years ago.

Does Titan have an anti-greenhouse atmosphere? Image Credit: ESA.

There are currently over a thousand near-Earth asteroids bigger than a kilometer in size, and some of them could potentially carve out large craters on our planet on some future day.

Charles Wood of the Planetary Science Institute notes that the craters on Titan look remarkably fresh, with finely defined edges. Older craters tend to look fuzzy as gravity or atmospheric processes wear them down.

Either the craters are fairly recent, says Wood, or the surface of Titan changes so slowly that even ancient craters look newly-made.

The two impact craters are deep, with substantial topographic relief. Nelson, a member of the Cassini Visual and Infrared Mapping Spectrometer (VIMS) team, says they have spotted circular features that could be the flattened remains of other craters.

"The surface is colored in a way that it appears to look like a crater, much as if Leonardo da Vinci had painted a crater on a white billiard ball, we'd think it was a crater when we saw it from a distance," says Nelson.

"But it has very little topographic relief. Large, round features that don't have a lot of topographic relief have been seen before throughout the solar system; they're seen on the Moon, Mercury, and the Galilean satellites. Very large impacts create craters, and then the surface relaxes and fills in over time, but it leaves behind an albedo marking - a dark spot."

Nelson says that on Earth, craters tend to be completely obliterated over time, leaving no dark spots behind. So the fact that Titan has dark spots similar to those on more ancient, unaltered surfaces indicates to him that Titan's surface isn't seeing much alteration, either.

Wood wonders if any conclusions can be made about dark spots on Titan, whose origins are unknown. Further studies by Cassini's various instruments may provide more insight into these blemishes on Titan's skin.

For Wood, the paucity of impact craters on Titan suggests a young surface. If Titan's surface was unaltered and ancient, it should be littered with hundreds or even thousands of craters. However, both Wood and Nelson say future Cassini images may turn up more craters.

Nelson adds that impact craters are not the only features on Titan that could betray the age of the surface. There is a large, bright region known as "Xanadu" that dominates one side of the moon, and there are no comparable large landscapes on the opposite side.

Nelson says that if the surface were being continually altered, such bright areas would be more evenly distributed across Titan's entire surface.

"If you look at the eastern and western hemispheres of the Earth in the wintertime, you'll see snow. The snow has about the same brightness in any hemisphere," says Nelson.

"On Titan, we're seeing more bright stuff in one hemisphere than the other, and that's suggesting that the bright stuff may be more permanent, and not getting subjected to weathering patterns."

The bright features on Titan are generally believed to be hydrocarbon ice. Ice on Titan is as hard as rock, although it's not made of silicate like the rocks on Earth. Titan's ice rocks are probably made of water or ammonia, or perhaps methane, ethane, propane, butane, or some chemical combinations.

Even if the bright areas on Titan aren't fluffy snow, but instead represent a hard surface that is has been exposed, Nelson says this sort of exposure should occur uniformly on a surface that is continually being buffeted by wind and weather.

Wood, on the other hand, says we can't make any conclusions yet about the distribution of Titan's surface features. Distribution depends on many things, including how the features are made in the first place.

"Earth is an ocean planet as seen from space above Hawaii," says Wood. "But the other hemisphere has much more land. We have no knowledge of what Xanadu is - high or low [elevation], old or young - we just know it is bright in images."

Nelson calls Titan's surface "old," because he is comparing it to the much-altered surface of the Earth. Wood calls Titan's surface "young," because he is comparing it to heavily cratered bodies like Jupiter's moons Ganymede and Callisto.

But despite their different perspectives and different interpretations of the data, they come to a similar conclusion: Titan's surface age may be several hundred million years old, rather than several billions of years old, and comparable to regions of Mars.

The martian surface has many impact craters, but also gradually evolves due to wind storms. Like Mars, Titan's surface may be changing slowly.

Or, like Mars, Earth, and many other bodies in the solar system, Titan's surface is a mix of young terrain interspersed by older terrain that sees little change over the eons. Winds, liquid run-off, and volcanic eruptions may be altering the surface in ways we don't yet understand.

Scientists are hesitant to make any final conclusions about Titan at this time. Two impact craters allow for some comparative analysis, but not much. Hopefully, such disputes will be settled as more of the moon is imaged by the Cassini spacecraft. Titan is still mostly unexplored: only six flybys have been completed so far, with nearly 40 flybys scheduled over the next few years.

"This is much like watching the election returns on election night," says Nelson.

"The first results coming from New Hampshire tell us something, but they're not the total pattern for the country. While the early returns have the incumbent paradigm shaking, the final tally isn't in yet."

Listen to sounds from the microphone onboard the Huygens during its descent (wav file format, approx. 600 kB each):

- Track One - Track Two - Track Three - Flash animation of Landing - Surface GIF animation, rock shadow

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