Titan's Windswept Dunes

Discovered on March 25, 1655 by the Dutch astronomer Christiaan Huygens, Saturn's largest moon, Titan, is one of only a handful of Solar System bodies--as well as the only planetary moon--known to have fields of windswept dunes on its surface (the others are Earth, Venus, and Mars). Heavily veiled by an obscuring, dense blanket of orange smog, this eerie, mysterious moon-world is the second-largest moon in our entire Solar System, after Ganymede of Jupiter. In December 2014, a new study was released by scientists using experimental results derived from the high-pressure wind tunnel at Arizona State University's Planetary Aeolian Laboratory in Tempe, Arizona. The new study suggests that earlier estimates of how fast winds need to blow to move sand-size particles around on this strange, bewitching, and distant moon are about 40 percent too low.

A team of planetary scientists led by Dr. Devon Burr of the University of Tennessee, Knoxville reported the findings December 9, 2014 in a paper published in the journal Nature. Dr. James K. Smith, engineer and manager of Arizona State University's (ASU) Planetary Aeolian Laboratory, is one of the paper's co-authors.

Saturn and its moon Titan orbit about ten times farther from our Star than the Earth. Planetary scientists obtained their first batch of detailed information about Titan when the Cassini/Huygens orbiter and lander arrived there in 2004. The Huygens lander successfully managed to obtain revealing images when it reached the weird surface of Titan, as well as when it was still falling slowly down to the surface through the moon's thick, orange, foggy atmosphere--which has 1.4 times greater pressure than Earth's. These pictures, along with additional studies using instruments on the Cassini orbiter, at long last unveiled, to the prying eyes of curious planetary scientists, that Titan's geological features include river channels and lakes of ethane, methane, and propane. Titan also has mountains, craters, and sand dunes.

Dunes begin to take shape when the wind sweeps up loose particles from the ground and then causes them to do a jitter-bug, or saltate, downwind. Geologists have discovered threshold speeds for sand and dust under many different conditions on our own planet, as well as on Venus and Mars. However, for Titan, with its weird conditions, this behavior has long been unknown.

Bewitching, Bewildering, Bizarre Orange Moon

The surface temperature on Titan is a truly frigid minus 290 degrees Fahrenheit, and even its "sand" is probably not like the sand on Earth, Venus, or Mars. Titan's bizarre and alien "sand" is probably made up of tiny particles of solid hydrocarbons--or, alternatively, ice encased in hydrocarbons--with a density of approximately one-third that of the sand of our own planet. In addition, Titan's gravity is low--it is only about one-seventh that of our own Earth. This basically means that, combined with the low density of Titan's "sand" particles, they sport only the truly puny weight of a mere four percent that of terrestrial sand. Titan's strange "sand" is about as light as freeze-dried coffee grains!

Titan is a strange and tormented moon-world, composed mostly of water ice and rocky material. Its alien surface is slashed by weird seas and rivers of liquid hydrocarbons, and it is pelted mercilessly by heavy showers of large drops of hydrocarbon rain. It is also heavily shrouded by a relentless, dense rust-colored hydrocarbon smog, and displays a marvelous, major continent that scientists have named Xanadu after the "Xanadu" of Samuel Taylor Coleridge's poem Kubla Khan. Titan's Xanadu sparkles as if lit by the brilliant cold fires of trillions of glittering crystals!

Titan is almost as big as the planet Mars! However, its heavily smog-shrouded surface has historically been extraordinarily difficult to observe. This is because the dense, obscuring orange veil, composed of complex hydrocarbons, is extremely hard to penetrate. Titan's alien atmosphere is very dense, composed of numerous thick layers of mist that together knit an intricate, complicated, and almost impenetrable shroud, hiding its surface from the curious eyes of observers.

Because Titan is located in the outer regions of our Solar System, circling the ringed gas-giant planet, Saturn--the sixth planet from our Star, the Sun--it is extremely cold, and its chemical atmosphere is frozen. This intriguing atmosphere is made up of a wonderful mix of marvelous compounds that many planetary scientists believe are similar to those that existed in our Earth's primordial atmosphere. Titan's atmosphere is primarily composed of nitrogen--just like our Earth's--but it also harbors significantly larger quantities of "smoggy" hydrocarbons like methane and ethane. This heavy smog is so extremely dense that it pours "gasoline-like" rain down on the tortured surface of this hydrocarbon-slashed moon from its bizarre sky of orange.

NASA's Voyager 1 spacecraft made a heroic but, nevertheless, unsuccessful attempt back in 1980 to obtain close-up images of Titan's secret surface. Unfortunately, it was not able to penetrate the dense orange veil, and the resulting pictures merely revealed some minor color and brightness variations in Titan's weird atmosphere. In 1994, the Hubble Space Telescope (HST) succeeded in obtaining some valuable and revealing pictures of Titan's mysterious surface--revealing the existence of the sparkling continent Xanadu for the first time!

The Cassini-Huygens mission is a collaborative NASA/European Space Agency/Italian Space Agency robotic spacecraft that is currently observing the Saturn system. The spacecraft was originally constructed to carry two main components: the European Space Agency-designed Huygens Probe, named in honor of Christiaan Huygens, and the NASA-designed Cassini Orbiter that was named for the Italian-French astronomer Giovanni Dominico Cassini (1625-1712) who discovered four of Saturn's moons. After an incredible journey through interplanetary space, from our planet to the Saturn system, Cassini-Huygens at last arrived at the realm of the ringed planet on July 1, 2004. On December 25, 2004, the Huygens Probe was deliberately separated from the Cassini Orbiter, and it slowly descended down to the long-shrouded and secretive surface of the strange moon-world--sending back to scientists on Earth precious information about this mystery moon. Titan, at last, was revealing its long-hidden secrets. This mission will continue until 2017.

Titan possesses a geologically youthful surface that is quite smooth; pockmarked by very few impact craters. The climate of this moon-world--including wind and rain--carves out surface features that are similar to those on Earth--such as rivers, lakes, dunes, seas (likely of liquid methane and ethane), and deltas. Indeed, Titan bears an eerie resemblance to Earth, and is thought to be similar to the way Earth once was long ago before the emergence of life (prebiotic).

Titan orbits its planet once every 15 days and 22 hours. Like Earth's Moon, and many of the other moons of the four giant planets of the outer Solar System--Jupiter, Saturn, Uranus, and Neptune--its rotation period is identical to its orbital period. Therefore, Titan is tidally locked in synchronous rotation with Saturn, and always shows one face to its planet.

A Strange Place Indeed!

The team of planetary scientists, led by Dr, Burr, began their study of the strange moon-world that is Titan with meticulously designed wind tunnel experiments. "We refurbished the high-pressure wind tunnel previously used to study conditions on Venus," Dr. Smith noted in a December 6, 2014 ASU Press Release.

In order to recreate in the tunnel on Earth the alien wind conditions on Titan, the team of scientists had to increase the air pressure in the wind tunnel to about 12 times the surface pressure of our own planet. In addition, they compensated for the low density of Titan "sand" and the moon-world's reduced gravity by using numerical modeling.

In summary, the scientists explain that "This simulation reproduces the fundamental physics governing particle motion thresholds on Titan." They further note that earlier studies designed to replicate the conditions on our own planet and Mars, yielded results that were questionable under the conditions that exist on Titan.

The findings of the wind tunnel experiments reveal that the earlier calculations for wind speeds needed to lift particles were about 40 to 50 percent too slow. The new study shows that near the surface of Titan, the most readily moved sand-sized particles require winds of at least 3.2 miles per hour (1.4 meters per second) in order to start moving.

That doesn't sound like much, commented Dr. Nathan Bridges in the December 6, 2014 ASU Press Release, "but it makes more sense when you realize this is a dense atmosphere blowing against particles that are very light." Dr. Bridges, of the Johns Hopkins University Applied Physics Laboratory, is one of the study's co-authors.

A higher threshold wind speed for forcing particles to move creates an "either/or" situation in which weak, everyday winds can exert little or no influence on surface particles. However, occasional strong gusts of wind can readily blow the particles around and also reshape the dunes on this bizarre moon-world. The pattern of dunes on Titan indicates that in spite of prevailing winds blowing in from the east, the dunes appear to be shaped by winds coming from the west. The winds blowing in from the west occur much less frequently and, therefore, the new study suggests that Titan's dunes are only rarely stirred into motion--only whenever conditions create strong westerly winds.

In order to simplify matters, the wind-tunnel modeling did not take into account certain factors, among them whether Titan's dune particles are sticky. If they are sticky, the authors of the study note, then it will require even stronger winds to get the particles moving--and the contrasts will be even greater between the normal, weaker east wind pattern and the much more powerful west winds that shape the dunes.

As Dr. Bridges told the press, "Titan is a strange place indeed."

Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, newspapers, and journals. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, "Wisps, Ashes, and Smoke," will be published soon.

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