Saturn's largest moon, Titan, is one of the few solar system bodies – and the only planetary moon – known to have fields of wind-blown dunes on its surface. (The others are Venus, Earth and Mars.)
New research, using experimental results from the high-pressure wind tunnel at Arizona State University's Planetary Aeolian Laboratory, has found that previous estimates of how fast winds need to blow to move sand-size particles around on Titan are about 40 percent too low.
Lines of dunes crawl across the surface of Titan, Saturn's largest moon, in a radar image showing dunes as dark. Experiments at ASU's wind tunnel indicate the dune particles move only under winds that blow stronger than scientists previously thought.
Photo by: NASA/JPL-Caltech/ASI
A team of scientists led by Devon Burr of the University of Tennessee, Knoxville reported the findings Dec. 8 in a paper published in the journal Nature. James K. Smith, engineer and manager of ASU's Planetary Aeolian Laboratory, is one of the paper's co-authors.
Saturn and Titan orbit about ten times farther from the sun than Earth. Scientists got their first detailed information about Titan when the Cassini/Huygens orbiter and lander arrived in 2004. The short-lived Huygens lander took photos when it reached the surface and as it was descending through Titan's dense, smoggy atmosphere, which has 1.4 times greater pressure than Earth's. These images, plus studies using instruments on the Cassini orbiter, revealed that Titan's geological features include mountains, craters, river channels, lakes of ethane, methane and propane – and dunes.
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. Geologists have found threshold speeds for sand and dust under various conditions on Earth, Mars and Venus. But for Titan, with its bizarre conditions, this remained unknown.
Particles of 'sand' as light as freeze-dried coffee
On Titan, where the surface temperature is negative 290 degrees Fahrenheit, even "sand" is probably unlike sand on Earth, Mars or Venus. From the Cassini observations and other data, scientists think it is composed of small particles of solid hydrocarbons (or ice wrapped in hydrocarbons), with a density about one-third that of terrestrial sand. In addition, Titan's gravity is low, roughly one-seventh that on Earth. Combined with the particles' low density, this gives them a weight of only about four percent that of terrestrial sand, or roughly as light as freeze-dried coffee grains.
The scientists led by Burr began their study with carefully designed wind tunnel experiments. "We refurbished the high-pressure wind tunnel previously used to study conditions on Venus," Smith explains. To recreate in the tunnel on Earth the wind conditions on Titan, the scientists had to increase the air pressure in the wind tunnel to about 12 times the surface pressure of Earth. And they compensated for the low density of Titan "sand" and the moon's reduced gravity through numerical modeling.
In the end, the Burr team explains, "This simulation reproduces the fundamental physics governing particle motion thresholds on Titan." They add that previous studies, which had extrapolated from wind tunnel experiments designed to mimic conditions on Earth and Mars, produced results that were questionable under Titan's conditions.
The outcome of the wind tunnel experiments show that the previous calculations for wind speeds necessary to lift particles were about 40 to 50 percent too slow. The new experiments show that near the surface of Titan, the most easily moved sand-size particles need winds of at least 3.2 miles per hour (1.4 meters per second) to start moving.
That doesn't sound like much, says Nathan Bridges of the Johns Hopkins University Applied Physics Laboratory, one of the co-authors, "but it makes more sense when you realize this is a dense atmosphere blowing against particles that are very light."
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 work indicates that Titan's dunes are seldom stirred into motion – only whenever conditions produce strong westerly winds.
For simplicity, the wind-tunnel modeling ignored some factors, among them whether Titan dune particles are sticky. If they are, the paper's scientists note, then it will take yet-stronger winds to get the particles moving, and the contrasts will be even greater between the normal east wind pattern and the stronger west winds that shape the dunes.
Bridges says, "Titan is a strange place indeed."
The facility that has grown to become ASU's Planetary Aeolian Laboratory was founded in the mid-1970s by the late Ronald Greeley of ASU. The laboratory, located at NASA's Ames Research Center in Mountain View, California, has been used for many studies of how wind interacts with particles of sand, dust and rock. Scientists have also used it to investigate what blowing sand and dust do to Mars spacecraft, such as NASA's Opportunity and Curiosity rovers. ASU operates the laboratory through an agreement with NASA.
The School of Earth and Space Exploration is an academic unit of ASU's College of Liberal Arts and Sciences.
Robert Burnham, email@example.com
Mars Space Flight Facility
Robert Burnham | EurekAlert!
Researchers create artificial materials atom-by-atom
28.03.2017 | Aalto University
Astronomers probe swirling particles in halo of starburst galaxy
28.03.2017 | International Centre for Radio Astronomy Research
The Institute of Semiconductor Technology and the Institute of Physical and Theoretical Chemistry, both members of the Laboratory for Emerging Nanometrology (LENA), at Technische Universität Braunschweig are partners in a new European research project entitled ChipScope, which aims to develop a completely new and extremely small optical microscope capable of observing the interior of living cells in real time. A consortium of 7 partners from 5 countries will tackle this issue with very ambitious objectives during a four-year research program.
To demonstrate the usefulness of this new scientific tool, at the end of the project the developed chip-sized microscope will be used to observe in real-time...
Astronomers from Bonn and Tautenburg in Thuringia (Germany) used the 100-m radio telescope at Effelsberg to observe several galaxy clusters. At the edges of these large accumulations of dark matter, stellar systems (galaxies), hot gas, and charged particles, they found magnetic fields that are exceptionally ordered over distances of many million light years. This makes them the most extended magnetic fields in the universe known so far.
The results will be published on March 22 in the journal „Astronomy & Astrophysics“.
Galaxy clusters are the largest gravitationally bound structures in the universe. With a typical extent of about 10 million light years, i.e. 100 times the...
Researchers at the Goethe University Frankfurt, together with partners from the University of Tübingen in Germany and Queen Mary University as well as Francis Crick Institute from London (UK) have developed a novel technology to decipher the secret ubiquitin code.
Ubiquitin is a small protein that can be linked to other cellular proteins, thereby controlling and modulating their functions. The attachment occurs in many...
In the eternal search for next generation high-efficiency solar cells and LEDs, scientists at Los Alamos National Laboratory and their partners are creating...
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are less stable. Now researchers at the Technical University of Munich (TUM) have, for the first time ever, produced a composite material combining silicon nanosheets and a polymer that is both UV-resistant and easy to process. This brings the scientists a significant step closer to industrial applications like flexible displays and photosensors.
Silicon nanosheets are thin, two-dimensional layers with exceptional optoelectronic properties very similar to those of graphene. Albeit, the nanosheets are...
20.03.2017 | Event News
14.03.2017 | Event News
07.03.2017 | Event News
28.03.2017 | Physics and Astronomy
28.03.2017 | Health and Medicine
28.03.2017 | Life Sciences