Gassy Dust

This image, covering about 300 kilometers from top to bottom, reveals dark linear features that join and separate. Credit: NASA/JPL-Caltech/ASI. Click to enlarge.

Sep 26, 2018

Methane dust on Titan?

NASA launched the Cassini-Huygens mission on October 15, 1997. The six ton orbiter was the largest satellite ever deployed, entering orbit around Saturn on June 30, 2004. Its name was changed twice during its run: Cassini Equinox Mission on July 1, 2008, following the completion of its Prime Mission and then to the Cassini-Solstice Mission, named for the summer solstice on Saturn that took place in May 2017.

One of Titan’s more intriguing formations are long, massive dunes that cover an area about half the size of Australia. Titan’s dunes are well-defined, almost solid-looking waves that pass over craters and around what are referred to as “yardangs.” They appear to follow the prevailing wind patterns on Titan that blow a mere eight kilometers per hour, but there are some unusual characteristics that might mean they are not wind-generated in the conventional sense.

According to a recent press release, “Data from NASA’s Cassini spacecraft has revealed what appear to be giant dust storms in equatorial regions of Saturn’s moon Titan.”

The surface temperature on Titan averages minus 180 Celsius. It is so cold that if water were present it would be rock-like and would not contribute to any chemistry. So, it might be asked, what force can create regular, evenly spaced piles, some hundreds of meters high, despite the cold environment?

Titan’s dunes are now thought to be organic molecules precipitated out of its atmosphere when sunlight reacts with methane. This is said to cause molecular particles to clump together until they are heavy enough to fall to the surface. Planetary scientists think that the extreme cold causes them to act like sand, accumulating into the vast dune fields. This speculation is due to the detection of giant dust storms in Titan’s equatorial region.

As previously written, dunes are found only in the driest places. Sand and dust must slide freely over one another for dunes to accumulate and move across the landscape. Strong winds are usually required to move the sand. Sand dunes in Australia’s Simpson Desert are fossilized and immobile, although they cover thousands of square kilometers. Whatever deposited the sand there did not move it again, so they crusted over, plants took root, and gullies formed.

Many of the dunes on Titan look like fingerprint patterns, with whorls and arches that are crisscrossed by other ripples in a perpendicular arrangement, looking almost exactly like the dune fields found along the coast of Namibia.

The popular press published articles in the past, saying that Titan is “wet” with hydrocarbons so many times it is now stated as a fact, although when the Huygens probe landed on Titan, it sank slightly into a friable surface that NASA described as “like wet snow,” or “loose clay,” or “dry sand.” Huygens detected methane in the area but it quickly dissipated.

If Titan were wet, the winds that blow a mere eight kilometers per hour would not be able to accumulate material into long dunes: moisture would cause the dust to become sticky, while any larger grains would be mired in slush. With such minimal wind speeds recorded by the Huygens lander, it stands to reason that Titan is dry, despite claims that there are “lakes” of liquid ethane and “methane storm” speculations.

Since Titan is dry, with folded-rim craters that have wide, flat bottoms (often with more than one tier or concentric basin), parallel fractures, large domes similar to those on Venus, and Lichtenberg figures (called river channels by the Cassini team), then a better explanation for what happened to Saturn’s planet-sized moon might be found by considering Mars.

Stephen Smith

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