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Clash of Titan's winds shape its sandy dunes

The size and shape of dunes on Titan follow a strikingly similar pattern to those on Earth. Like sands through the hourglass, Dr Karl ponders the dunes on the Saturnian moon.

The first flash of sunlight reflected off a lake on Titan

The first flash of sunlight reflected off a lake on Saturn's moon, Titan, detected by the visual and infrared mapping spectrometer on NASA's Cassini spacecraft on 8 July 2009 (Source: NASA/JPL/University of Arizona/DLR)

Last time, I talked about the remarkable similarities between our lovely planet Earth, and the moon Titan, which orbits the ringed planet, Saturn. And I idly wondered if Titan could carry life.

For life, you need three things — the raw materials, a way to shift them around and a power supply.

On Titan, we do have the right raw materials — hydrocarbons of many types.

It also seems as though we have the right states of matter — solid, liquid, and gas.

So the raw materials are not locked away in the ground, but move freely around the surface of the planet and below the surface and above it.

But what about an energy supply?

The problem is that the Sun is so far away, that the solar energy is weak.

But it turns out that the radiation from Saturn is huge — powerful enough to kill an unprotected human in minutes.

This radiation could be a fine energy source to kick chemical reactions along, and maybe life as well.

Is there life on Titan? We don't know.

But we do know, thanks to the Cassini orbiter and the Huygens lander, that there are sand dunes on Titan.

The sand dunes are located both north and south of the equator of Titan. They run for tens, or hundreds, of kilometres in unbroken stretches.

There's one stretch of 1500 kilometres. The individual sand dunes on Titan are up to 100 metres high and a few kilometres apart.

And would you believe it, these are the same dimensions that you find in the sand dunes in the Namib Sand Sea of Africa, and in the Sahara Desert.

The sand dunes of Titan seem to be formed by the same mechanisms that gave us sand dunes on Earth.

But while the physics of Titan's sand dunes is the same as here on Earth, the raw materials are different, because of the much lower temperature.

So is the 'sand' made of silica, like on Earth? Nope! It's made of various hydrocarbons. Just think of coffee grounds and you'll be pretty close.

But if you want sand dunes, you need wind.

It turns out that the wind on Titan is not very fast, about three to six kilometres per hour.

But that is fast enough to move the 'sand', thanks to Titan's lower gravity and slightly thicker atmosphere.

Theory tells us that the grains of this hydrocarbon sand have to be in the range of 0.18–0.25 millimetres in diameter — about two to three times the diameter of a human hair.

If they were larger and heavier, the wind could not lift them. If they were smaller, they would clump together.

But in that size range of two-or-three human hairs, they can get carried by the wind in a bouncing process called saltation. And that process will give us sand dunes.

Give them enough time, and different winds will give you different shapes of sand dunes.

On Earth, if the winds come onto a sand field from all different directions, you get star dunes, shaped like a star with radiating arms.

But suppose that the wind comes consistently from one single direction (say, the west). In this case, the sand dune that is formed runs at right angles to that direction (say, north–south).

However, suppose that on average the wind comes from one direction (say, the west), but spends most of time varying a little to each side (from north–west, then from south–west, and back again, and so on).

In this case, you'll get a sand dune that will run from the west to the east — it will follow the average wind direction.

You see this pattern in Australia — the wind comes from the Indian Ocean over the bottom half of Australia and heads east.

But once it gets just past the centre of Australia, it chucks its first leftie over the Simpson Desert, and then after a few hundred kilometres another leftie, and heads back out to the Indian Ocean, over the top half of Australia. It's a big horseshoe over the continent of Australia.

If you look at a map of Australia that shows the orientation of the sand dunes, you'll see that they're lined up in this horseshow shape following the average prevailing winds.

According to the scientists, that's why the sand dunes of Titan run in parallel lines from the west to the east. Because, on average, the wind on Titan at the equator runs from the west to the east, with a bit of variation to each side.

But I wonder if it will be our children, or our grand-children, or our great-grand-children, who will be the first to build castles out of the sand?


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Published 13 April 2010

© 2016 Karl S. Kruszelnicki Pty Ltd