Drifting sand is steadily filling the craters of Saturn's largest moon, Titan, making it look much younger than it is.
Dunes of hydrocarbon sand are slowly but steadily filling in its craters, observations from NASA's Cassini spacecraft show.
"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," says Catherine Neish, a Cassini radar team associate.
"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."
Titan 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. But the resemblances to Earth end there, with the moon featuring a surface temperature of around minus 290 degrees Fahrenheit, and its rain consisting of liquid methane and ethane.
The team compared the surface of Titan to Ganymede: a giant moon with a water ice crust, similar to Titan, but with almost no atmosphere and thus no wind or rain to erode its surface.
"We found that craters on Titan were on average hundreds of yards shallower than similarly sized craters on Ganymede, suggesting that some process on Titan is filling its craters," says Neish.
Titan's atmosphere is mostly nitrogen with a trace of methane and other, more complex hydrocarbons. Methane in the atmosphere is broken down by sunlight, but recombines into more complex hydrocarbons in the upper atmosphere. Some of the larger particles eventually rain out on to the surface, where they appear to get bound together to form sand.
If liquid erosion were primarily responsible for the infill, then the team would expect to see a lot of partially filled craters on Titan. "However, this is not the case," says Neish.
"Instead we see craters at all stages; some just beginning to be filled in, some halfway, and some that are almost completely full. This suggests a process like windblown sand, which fills craters and other features at a steady rate."