Titan is one of the few bodies in the Solar System known to have fields of wind-blown dunes on its surface. A new study published in the journal Nature has found that previous estimates of how fast winds need to blow to move sand-size particles around on Titan are about 50 percent too low.
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.
Dune fields are common on Titan, the largest moon of Saturn. They cover about 13 percent of the moon, stretching over 10 million sq km, roughly equivalent to the area of Canada. Thus they offer a large-scale insight into the Titan’s environment.
Though similar in shape to the linear 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.
Dunes begin to form when the wind picks up loose particles from the ground and drives them to hop, or saltate, downwind. A key part of understanding dunes is to identify the threshold wind speed that causes dune particles to start to move.
Now, U.S. scientists led by Dr Devon Burr from the University of Tennessee-Knoxville’s Earth and Planetary Sciences Department has shown that winds on Titan must blow 50 percent faster than previously thought in order to move that sand.
To solve the mystery, Dr Burr and her team dedicated 6 years to refurbishing and modifying a defunct NASA high-pressure wind tunnel to reproduce the Titan environment at Earth temperatures.
The scientists pressurized the wind tunnel to 12.5 atmospheres that correctly simulated wind physics at Titan’s pressure of 1.4 bars.
To account for the very low gravity and density of sand on Titan, and given the uncertainties in the actual materials on the moon, they used 24 different substances, including very low weight particles such as hollow glass spheres and walnut shells.
Two years were spent running the experiments, modeling the results, and calibrating the models to match the observations. These adjustments were made to find the best simulation of Titan’s dense atmosphere.
The experiments show that near the surface of Titan, the most easily moved sand-size particles need winds of at least 1.4 meters per second to start moving.
“That doesn’t sound like much, but it makes more sense when you realize this is a dense atmosphere blowing against particles that are very light,” said co-author Dr Nathan Bridges of the Johns Hopkins University.
A higher threshold wind speed for making particles move creates an either/or situation in which weak, everyday winds do little or nothing to surface particles, but occasional strong ones readily blow them around and reshape the dunes.
The pattern of dunes on Titan shows that despite prevailing winds blowing from the east, the dunes appear shaped by winds from the west, which occur more rarely.
Thus, the new study indicates that Titan’s dunes are seldom stirred into motion – only whenever conditions produce strong westerly winds.
“Titan is a strange place indeed,” Dr Bridges concluded.
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Devon M. Burr et al. Higher-than-predicted saltation threshold wind speeds on Titan. Nature, published online December 8, 2014; doi: 10.1038/nature14088
