Object type: Sand dune
Location: Saturn鈥檚 moon Titan
Standing atop a huge mound of black, hydrocarbon sand, your 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.
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This is no Earthly vista: you鈥檙e on Saturn鈥檚 largest moon, Titan. You strap your feet onto the board and slip off down the dune. Titan鈥檚 low gravity means it takes a while to build up speed, but also keeps friction to a minimum, so it鈥檚 a long glide down before you come to a halt.
Sandboarding on Titan still, sadly, only happens in our imagination, but the moon鈥檚 amazing dunes are real 鈥� and lie in a trippy landscape worthy of a late Beatles song. They were discovered in 2006 in radar images from NASA鈥檚 (see photo) and could be key to unravelling the climatic history of this eerily Earth-like moon.
Though chilling at -179聽掳C, Titan has rain and lakes 鈥� albeit of liquid methane rather than water 鈥� along with mountains and river channels.
鈥淢ethane raining out and flowing across the surface leads to landscapes that are so much like Earth,鈥� says , 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鈥檚 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鈥檚 atmosphere, and then to rain down onto the surface.
鈥淭he dunes may have a composition that鈥檚 a little like plastic,鈥� says Radebaugh. To visualise standing on a dune on Titan, imagine 鈥渟tanding on huge volumes of plastic sand鈥�, she says.
Seasonal sculptures
Despite their unusual composition, Titan鈥檚 dunes 鈥� typically 100 metres tall, a kilometre wide, and up to hundreds of kilometres long 鈥� are very similar in shape and size to long, skinny dunes in the Sahara desert called .
As on Earth, Titan鈥檚 dunes can tell us about climate. Last year, simulations of the dunes , reversing direction and getting much faster twice a year. This solved a mystery of why Titan鈥檚 dunes look as though they have been sculpted by winds blowing from west to east, even though the moon鈥檚 winds were thought to blow in the opposite direction.
Now Alice Le Gall of the 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鈥檚 equator. Le Gall鈥檚 team have shown that the dunes .
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鈥檚 egg-shaped orbit, the team conclude, which produces more intense, drier summers in Titan鈥檚 southern hemisphere compared with the north.
The discovery of dunes on Titan was a stroke of luck, says Radebaugh, who worked with Le Gall鈥檚 team on the latest analysis. 鈥淲e had no idea that these things would be there,鈥� she says. 鈥淲e were surprised to find such a close analogue to Earth in something so far away.鈥�
And if there were only some way to hop over to Titan, she would love to try sandboarding there. 鈥淚 think it would be possible and probably would be really fun,鈥� she says.
Journal reference:
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