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A relatively "fresh" crater called Sinlap (left) and an extremely degraded crater called Soi (right). Image credit: NASA/JPL-Caltech/ASI/GSFC A relatively "fresh" crater called Sinlap (left) and an extremely degraded crater called Soi (right). Image credit: NASA/JPL-Caltech/ASI/GSFC

Titan's disappearing craters

Sen— New reserarch using observations from NASA's Cassini spacecraft suggest that Saturn's largest moon Titan may look much younger than it really is because its craters are getting erased as dunes of exotic, hydrocarbon sand are slowly but steadily filling in the craters.

"Most of the Saturnian satellites, Titan's siblings, have thousands and thousands of craters on their surface. So far on Titan, of the 50 percent of the surface that we've seen in high resolution, we've only found about 60 craters," said Catherine Neish, a Cassini radar team associate based at NASA's Goddard Space Flight Center. "It's possible that there are many more craters on Titan, but they are not visible from space because they are so eroded. We typically estimate the age of a planet's surface by counting the number of craters on it (more craters means an older surface). But if processes like stream erosion or drifting sand dunes are filling them in, it's possible that the surface is much older that it appears."

Neish and her team compared craters on Titan to craters on Jupiter's moon Ganymede. Ganymede is a giant moon believed to have a water ice crust, similar to Titan, so craters on the two moons should have similar shapes. However, Ganymede has almost no atmosphere and thus no wind or rain to erode its surface.

Titan on the other hand is the only moon in the solar system with a thick atmosphere, and the only world besides Earth known to have lakes and seas on its surface. However, with surface temperatures of around minus 290 degrees Fahrenheit (94 kelvins), the rain that falls on Titan is not water but liquid methane and ethane, compounds that are normally gases on Earth.

"This research is the first quantitative estimate of how much the weather on Titan has modified its surface," said Neish.

The source of Titan's methane remains a mystery as methane in the atmosphere is broken down over relatively short timescales by sunlight. Fragments of methane molecules then recombine into more complex hydrocarbons in the upper atmosphere, forming a thick, orange smog that hides the surface from view. Some of the larger particles eventually rain out on to the surface, where they appear to get bound together to form the sand.

"Since the sand appears to be produced from the atmospheric methane, Titan must have had methane in its atmosphere for at least several hundred million years in order to fill craters to the levels we are seeing," says Neish. However, researchers estimate Titan's current supply of methane should be broken down by sunlight within tens of millions of years, so Titan either had a lot more methane in the past, or it is being replenished somehow.

The difference in depth between craters on Titan and Ganymede is best explained by filling from windblown sand, although erosion from liquids and viscous flow might contribute to the modification of Titan's craters. The team thinks these considerations need further investigations.  A paper about this research was published online in the journal Icarus December 3, 2012.

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