A new study published in the journal Nature Geoscience explains why the sand dunes of Titan run in parallel lines from the west to the east.
The moons of our Solar System are brimming with unusual landscapes. However, sometimes they look a little more familiar, as in this new radar image from the Cassini orbiter. The image shows dark streaks carved into dunes reminiscent of those we might find on a beach on Earth, or raked with flowing lines in a Japanese Zen garden — but this scene is actually taking place on Saturn’s moon Titan. While our sand is composed of silicates, the ‘sand’ of these alien dunes is formed from grains of organic materials about the same size as particles of our beach sand. The small size and smoothness of these grains means that the flowing lines carved into the dunes show up as dark to the human eye. While previous images have spotted these eerily familiar patterns on Titan’s dunes, this new image (colorized) shows them in greater detail. The image was obtained by Cassini’s radar mapper on July 10, 2013. The vertical seam near the center is an artifact of radar image data processing. Image credit: NASA / JPL-Caltech / Sci-News.com.
Titan is one of the few bodies in the Solar System known to have fields of wind-blown dunes on its surface.
Though similar in shape to the sand dunes found in the deserts of Namibia or southern Arabia, Titan’s dunes are gigantic by Earthly standards. They are on average 1–2 km wide, hundreds of kilometers long and around 100 m high.
They cover about 13 percent of the moon, stretching over 10 million sq.km – roughly equivalent to the area of Canada.
The direction of these dunes has at times been attributed to the effects of Saturn’s gravitational tides or various land features or wind dynamics, but none quite explained their eastward slant.
“Violent methane storms high in Titan’s dense atmosphere, where winds do blow toward the east, might be the answer,” said Dr Benjamin Charnay of the University of Washington’s Virtual Planetary Laboratory, who is the first author on the study.
Using computer models, Dr Charnay and his colleagues hypothesize that the attitude of Titan’s sand dunes results from rare methane storms that produce eastward gusts much stronger than the usual westward surface winds.
“These fast eastward gusts dominate the sand transport, and thus dunes propagate eastward,” Dr Charnay said.
The storm winds reach up to 10 m a second (22 mph), about ten times faster than the moon’s gentler near-surface winds. And though the storms happen only when the moon is in equinox and its days and nights are of equal length, they are of sufficient power to realign the dunes.
It probably helps that Titan’s atmosphere is in ‘super-rotation above about 5 miles, meaning that it rotates a lot faster than the surface itself.
“The model suggests that these methane storms produce strong downdrafts, flowing eastward when they reach the surface, thus rearranging the dunes,” Dr Charnay said.
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Benjamin Charnay et al. Methane storms as a driver of Titan’s dune orientation. Nature Geoscience, published online April 13, 2015; doi: 10.1038/ngeo2406
