Wednesday, February 22, 2012

Titan's Great Dune Seas Rival Science Fiction Worlds

From Discovery News: Titan's Great Dune Seas Rival Science Fiction Worlds
The desert planets Arrakis in Frank Herbert’s 1965 novel Dune, and the planet Tatooine in the Star Wars trilogy are legendary arid worlds covered in sand dune "seas." And a mere one billion miles away orbiting Saturn the planet-sized moon Titan has all the trimmings of the real deal.

However, with surface temperatures of minus 300 degrees Fahrenheit it's hard to imagine Titan as a sun-parched desert world. Radar imaging from the NASA/ESA Cassini mission reveals that there is nothing in the solar system quite like Titan's immense dune fields.

The dunes make the moon look like God took a giant rake to it. The parallel linear features stretch hundreds of miles across the equatorial regions of Titan, covering a surface area the size of the United States. The photo at the top compares radar images of Titan's dunes to those found in the Namibian desert in southern Africa.

Dunes on planets yield insights to weather and climatic changes and surface geological processes. The Martian dune fields are only at far northern latitudes. Venus has just a few dunes, possibly due to lack of strong surface winds and sand particles. Titan's dunes are gargantuan by comparison. Under Titan's weak gravity -- slightly less than that of our moon -- the dunes are half the height of the St Louis Arch, and a mile across.

Confounding their mystery is that Titan's dunes aren't even made of sand. When ultraviolet light from the sun breaks apart methane high in Titan's atmosphere, it produces ethane and hydrogen. When these chemicals coalesce into particles, they settle out as a tar-like rain. Therefore, dune material literally falls out of Titan’s skies as solid hydrocarbon grains the size of coarse sand.

Don't imagine Dune's giant sand worms or Star Wars' elephant-like Banthas among the dunes. Cryo-life on Titan would be strictly microbial, and undergo metabolism at a snail’s pace.

The hydrocarbon sand is blown northward from the dry southern hemisphere where it builds up dunes along an equatorial belt. Saturn's slightly elliptical orbit means that Titan's southern hemisphere has short intense summers. This makes the southern regions drier. The arid sand is transported by the winds to make dunes.

Given this washboard terrain, engineers imagine future robotic explorers to Titan being balloons that survey the great dune fields, volcanoes and methane seas and lakes.

My favorite drawing board design is called the TALE (Titan Airship Latitude Excursion). The nuclear-powered buoyant gas airship would have enough propulsion to travel at different latitudes, hence survey the Titan landscape from polar lakes to equatorial dunes. It would have a steerable antenna for data relay, the ability to pull up surface samples via a tether, and have an onboard organic analysis laboratory.

An equally ambitions scheme calls for building a small winged drone much like those military drones that fly over Afghanistan. However this drone would cost $715 million and be nuclear-powered. Called the Aviatr, it would take 3D photos of Titan's surface. At the end of its mission the 260 pound aircraft would descend to Titan's surface and attempt a landing on the dunes. Proponents say that a heavier-than-air craft is the best way to freely navigate in Titan's the thick nitrogen atmosphere.

In the far future Earth could have a dune sea like that found on Titan. Despite our worries about global warming, carbon dioxide will eventually be depleted in the atmosphere as it is increasingly locked away in the crust. Without carbon dioxide Earth will no longer be able to regulate its temperature, and will begin to warm as the evolving sun grows brighter. Over one billion years from now from now Earth’s surface temperature will reach 120 degrees Fahrenheit, sterilizing much of our planet.

As Earth dehydrates as the oceans evaporate away, the atmosphere will be full of steam. Without water for lubrication, plate tectonics will grind to a halt like a rusty old machine. As with Titan, the last remaining pools of liquid will be at polar latitudes. Wind swept dust should build vast sand dunes along our parched equator.

Ironically, Titan will grow warm enough in the sun's final days to have seas of liquid water. In our restless solar system, one man's Armageddon is another man's Eden.

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