Friday, January 27, 2012

Space - Astrophile: Picture yourself on a sandboard on Titan

Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

Object type: Sand dune
Location: Saturn's moon Titan

Standing atop a huge mound of black, hydrocarbon sand, your sandboard tucked under your arm, you take in the view. Row after row of black dunes march into the distance as far as the eye can see, until everything disappears behind an orange curtain of smog.
This is no Earthly vista: you're on Saturn's largest moon, Titan. You strap your feet onto the board and slip off down the dune. Titan's low gravity means it takes a while to build up speed, but also keeps friction to a minimum, so it's a long glide down before you come to a halt.
Sandboarding on Titan still, sadly, only happens in our imagination, but the moon's amazing dunes are real – and lie in a trippy landscape worthy of a late Beatles song. They were discovered in 2006 in radar images from NASA's Cassini spacecraft (see photo) and could be key to unravelling the climatic history of this eerily Earth-like moon.
Though chilling at -179 °C, Titan has rain and lakes – albeit of liquid methane rather than water – along with mountains and river channels.
"Methane raining out and flowing across the surface leads to landscapes that are so much like Earth," says Jani Radebaugh, a planetary scientist at Brigham Young University in Provo, Utah.

Plastic sand

Perhaps more like Earth than anywhere else in the solar system, in fact. Comparing and contrasting the two worlds could lead to a better understanding of climate and surface features on both, she says.
What makes the similarities so astonishing is the completely different materials of which Titan and Earth are made. Titan's crust and mountains are made of water ice. The sand grains comprising its dunes are thought to be hydrocarbons like benzene, which has been detected in the dunes by the Cassini spacecraft.
On Earth, hydrocarbons tend to exist as liquids or gases in oil deposits. On Titan, though, many are frozen solid. They are thought to form when ultraviolet light drives chemical reactions in Titan's atmosphere, and then to rain down onto the surface.
"The dunes may have a composition that's a little like plastic," says Radebaugh. To visualise standing on a dune on Titan, imagine "standing on huge volumes of plastic sand", she says.

Seasonal sculptures

Despite their unusual composition, Titan's dunes – typically 100 metres tall, a kilometre wide, and up to hundreds of kilometres long – are very similar in shape and size to long, skinny dunes in the Sahara desert called linear dunes.
As on Earth, Titan's dunes can tell us about climate. Last year, simulations of the dunes suggested the winds on Titan change seasonally, reversing direction and getting much faster twice a year. This solved a mystery of why Titan's dunes look as though they have been sculpted by winds blowing from west to east, even though the moon's winds were thought to blow in the opposite direction.
Now Alice Le Gall of the Space Atmospheres, Environments and Observations Laboratory (LATMOS-UVSQ) in Paris, France and colleagues have discovered more tantalising climate clues from measurements of the dunes.
They lie in a band 30 degrees both north and south of Titan's equator. Le Gall's team have shown that the dunes get smaller and more widely spaced towards the northern end of this range.

Egg-shaped orbit

The team conclude that this happens because the ground gets wetter with liquid methane towards the north, making the sand stick together and less prone to forming dunes.
This latitudinal variation in weather is likely to be due to Saturn's egg-shaped orbit, the team conclude, which produces more intense, drier summers in Titan's southern hemisphere compared with the north.
The discovery of dunes on Titan was a stroke of luck, says Radebaugh, who worked with Le Gall's team on the latest analysis. "We had no idea that these things would be there," she says. "We were surprised to find such a close analogue to Earth in something so far away."
And if there were only some way to hop over to Titan, she would love to try sandboarding there. "I think it would be possible and probably would be really fun," she says.

Journal reference: Icarus, DOI: 10.1016/j.icarus.2011.10.024

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