Saturday, July 31, 2010

Sandy Solutions

Apparently it is a wonderful day for astronomical news. Who knew? I'm one of those people who loves to read about astronomy (particularly planetary science) but never actually goes outside and looks up. Anyway, this next story I found in a couple of places and thought it was pretty interesting.
Way back in 1655 Christian Huygens discovered the moon Titan orbiting around Saturn. In fact, it is Saturn's largest moon and for a long time it was thought to be the largest moon in the solar system. After all, it is bigger than the planet Mercury. Alas, it was then realized we had based that judgement by measuring from the center to the edge of the atmosphere instead of from the center to the surface. But a big ole thick atmosphere on a moon is pretty neat all in itself. The moon runs about -180 degrees Celsius (-290 degrees Fahrenheit), cold enough for liquid methane to rain down and collect into lakes, rivers, and seas. Other parts of the surface, particularly the equatorial regions, are covered in giant sand dunes. These dunes are approximately 1 km (~0.5mi) wide, tens to hundreds of kilometers long, more than 100 m (300ft) high, and composed of organic hydrocarbons.

So lets take a look at a particular paradox that exhists on Titan. What we know about the rotation of planetary atmospheres plus data from the Huygens probe (which landed on Titan's surface) tells us that surface winds should go east to west. Then the Cassini spacecraft sent images of Titan's equatorial dunes which suggested that the winds were moving west to east. A new paper in Aeolian Research, and a related paper in Science, attempt to explain this phenomenon.

These papers propose that there are seasonal changes on Titan that reverse the wind patterns for a short period. The equinoxes appear to be particuarly important. Now it is easy to start thinking of other planets like you think of your own, but Saturn is a large planet to orbit around and it is very far away from the sun. One year on Titan is about 29 Earth years, and during that time the moon will have two equinoxes. These are times when the heat from the sun creates atmospheric upwellings, causing the winds to reverse and accelerate (kinda like monsoons on Earth). Short gusts of west-to-east winds occurring intermittently over two years are so strong that they transport the dunes' sand more effectively than the normal east-to-west winds, causing these reverse striations in the dunes.

Take a look through the articles:
Tokano, Tetsuya. (2010) Relevance of fast westerlies at equinox for the eastward elongation of Titan's dunes. Aeolian Research: In press. (DOI: 10.1016/j.aeolia.2010.04.003)

Lorenz, Ralph D. (2010) Winds of Change on Titan. Science: 329(5991), 519 - 520. (DOI: 10.1126/science.1192840)


(image credit: NASA/JPL)
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