With its thick, hazy atmosphere and surface rivers, mountains, lakes and dunes, Titan, Saturn's largest moon, is one of the most Earthlike places in the solar system.
As the Cassini-Huygens spacecraft examines Titan over many years, its discoveries bring new mysteries. One of those involves the seemingly wind-created sand dunes spotted by Cassini near the moon's equator, and the contrary winds just above.
Here's the problem: Climate simulations indicate that Titan's near-surface winds -- like Earth's trade winds -- blow toward the west. So why do the surface dunes, reaching a hundred yards high and many miles long, point to the east?
The direction of the 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, according to new research by University of Washington astronomer Benjamin Charnay and co-authors in a paper published today in the journal Nature Geoscience.
Using computer models, Charnay, a UW post-doctoral researcher, and co-authors 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," Charnay said.
The storm winds reach up to 10 meters a second (22 mph), about 10 times faster than Titan's gentler near-surface winds. And though the storms happen only when Titan is in equinox and its days and nights are of equal length -- about every 14.75 years -- they are of sufficient power to realign Titan's dunes. Titan was last in equinox in August 2009.
It probably helps that, according to Cassini's observations, Titan's atmosphere is in "super-rotation" above about 5 miles, meaning that it rotates a lot faster than the surface itself. Their model, Charnay said, suggests that these methane storms "produce strong downdrafts, flowing eastward when they reach the surface," thus rearranging the dunes.
Charnay said he tried first, without success, to solve the problem with a global climate model that didn't factor in methane clouds, then realized that it was impossible, hinting that methane could be part of the solution.
"It was a kind of detective game, as often is the case in planetary sciences, where we have many mysteries and a few clues to solve them," he said.
The dunes in question, which are linear and run parallel to Titan's equator, are probably not composed of silicates like Earth sand, Charnay said, but of hydrocarbon polymers -- a kind of soot resulting from the decomposition of methane in the atmosphere.
Charnay noted a December study reported in Nature showing that it would take winds of at least 3.2 mph to lift and transport sand across Titan's surface -- that's 40 percent to 50 percent stronger wind than previous estimates.
The measurement of such a high wind speed threshold was a pleasant surprise, Charnay said: "That means that only fast winds transport Titan's sand, compatible with our hypothesis of strong storm gusts controlling the orientation and propagation of dunes."
Titan, discovered in 1655 by Christiaan Huygens, has long intrigued astronomers. Its atmosphere is 98.4 percent nitrogen and most of the rest is methane, and a bit of hydrogen. Its gravity is one-sixth that of Earth's and its air density is four- to five-times higher, meaning that flight will be relatively easy for visiting spacecraft. The European Space Agency's Huygens probe, which rode along on Cassini, successfully landed on Titan in 2005 and sent back the first photo of the moon's stone-strewn surface.
Charnay said direct observation by Cassini would be the way to confirm his hypothesis. Unfortunately, the Cassini mission will end in 2017 and Titan's next equinox is not until 2023.
"But there will be other missions," he said. "There are still a lot of mysteries about Titan. We still don't know how a thick nitrogen atmosphere formed, where the methane comes from nor how Titan's sand forms.
"And it is not completely excluded that life can be there, perhaps in its methane seas or lakes. So Titan really is a fascinating and evolving world, which has to be understood as a whole."
Charnay's co-authors are Erika Barth and Scot Rafkin of the Southwest Research Institute in Boulder, Colorado; Sébastien Lebonnois of the Laboratory of Dynamic Meteorology; and Sylvain Courrech du Pont, Clément Narteau, Sebastian Rodriguez and Antoine Lucas of Paris Diderot University.
The research was done in part through the Virtual Planetary Laboratory, a UW-based interdisciplinary research group, and funded by the NASA Postdoctoral Program and the French National Research Agency.
Peter Kelley | EurekAlert!
Magnetic nano-imaging on a table top
20.04.2018 | Georg-August-Universität Göttingen
New record on squeezing light to one atom: Atomic Lego guides light below one nanometer
20.04.2018 | ICFO-The Institute of Photonic Sciences
University of Connecticut researchers have created a biodegradable composite made of silk fibers that can be used to repair broken load-bearing bones without the complications sometimes presented by other materials.
Repairing major load-bearing bones such as those in the leg can be a long and uncomfortable process.
Study published in the journal ACS Applied Materials & Interfaces is the outcome of an international effort that included teams from Dresden and Berlin in Germany, and the US.
Scientists at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) together with colleagues from the Helmholtz-Zentrum Berlin (HZB) and the University of Virginia...
Novel highly efficient and brilliant gamma-ray source: Based on model calculations, physicists of the Max PIanck Institute for Nuclear Physics in Heidelberg propose a novel method for an efficient high-brilliance gamma-ray source. A giant collimated gamma-ray pulse is generated from the interaction of a dense ultra-relativistic electron beam with a thin solid conductor. Energetic gamma-rays are copiously produced as the electron beam splits into filaments while propagating across the conductor. The resulting gamma-ray energy and flux enable novel experiments in nuclear and fundamental physics.
The typical wavelength of light interacting with an object of the microcosm scales with the size of this object. For atoms, this ranges from visible light to...
Stable joint cartilage can be produced from adult stem cells originating from bone marrow. This is made possible by inducing specific molecular processes occurring during embryonic cartilage formation, as researchers from the University and University Hospital of Basel report in the scientific journal PNAS.
Certain mesenchymal stem/stromal cells from the bone marrow of adults are considered extremely promising for skeletal tissue regeneration. These adult stem...
In the fight against cancer, scientists are developing new drugs to hit tumor cells at so far unused weak points. Such a “sore spot” is the protein complex...
13.04.2018 | Event News
12.04.2018 | Event News
09.04.2018 | Event News
20.04.2018 | Physics and Astronomy
20.04.2018 | Interdisciplinary Research
20.04.2018 | Physics and Astronomy