Saturn’s largest moon Titan is one of the most Earth-like places in the Solar System, as least in terms of appearances, with its seas, lakes, and rivers (of liquid methane/ethane). But it is similar in another way as well, with vast stretches of huge wind-blown dunes in its equatorial regions. Only Earth, Venus, and Mars are known to have such dunes. Now, scientists think they have figured out how Titan’s dunes can become so immense in size: fast-blowing winds.
That may sound like a simple and reasonable answer, but previously scientists thought that Titan’s winds were too weak to adequately explain how the dunes form. The new studies were conducted using the high-pressure wind tunnel at Arizona State University’s Planetary Aeolian Laboratory, and they showed that previous estimates as to wind speeds necessary to move sand-sized particles around were about 40 percent too low.
The results were published Dec. 8, 2014, in a new paper published in the journal Nature.
As James K. Smith, engineer and manager of ASU’s Planetary Aeolian Laboratory, explained, “We refurbished the high-pressure wind tunnel previously used to study conditions on Venus.”
The team had to factor in Titan’s unique surface conditions, including a temperature of -290 degrees Fahrenheit, atmospheric pressure 1.4 times that of Earth, and gravity about one-seventh that of Earth. The particles themselves, made of hydrocarbons and similar to soot, have a weight of only 4 percent that of terrestrial sand, about the same as freeze-dried coffee grains. Their density is also only about one-third that of terrestrial sand.
According to team leader Devon Burr from the University of Tennessee, Knoxville, “This simulation reproduces the fundamental physics governing particle motion thresholds on Titan.”
It turned out that previous calculations for wind speeds necessary to transport the Titanian sand grains were about 40 to 50 percent too slow. The most easily moved particles would require wind speeds of at least 3.2 miles per hour (1.4 meters per second).
“That doesn’t sound like much,” according to Nathan Bridges of the Johns Hopkins University Applied Physics Laboratory, “but it makes more sense when you realize this is a dense atmosphere blowing against particles that are very light.” Bridges is also another co-author of the new paper.
The usual, everyday winds on Titan (from the east) are not strong enough to do this, but less frequent, stronger winds (from the west) could.
There were some possibilities not taken into account yet in the tests, such as whether the particles are sticky. If they are, then stronger winds would be needed to explain the dune formation process.
The dunes cover about 13 percent of Titan’s surface, stretching over an area of 4 million square miles (10 million square kilometers). That’s about the same surface area as the United States. On average, they are 0.6 to 1.2 miles (1 to 2 kilometers) wide, hundreds of miles (kilometers) long, and around 300 feet (100 meters) high. Their size and spacing can vary, however, according to location. They are similar in shape to the linear dunes found on Earth in Namibia or the Arabian Peninsula, but on larger scales, and confined to the equatorial region, in a band between 30 degrees south latitude and 30 degrees north latitude.
“Understanding how the dunes form as well as explaining their shape, size and distribution on Titan’s surface is of great importance to understanding Titan’s climate and geology because the dunes are a significant atmosphere-surface exchange interface,” said Nicolas Altobelli, ESA’s Cassini-Huygens project scientist. “In particular, as their material is made out of frozen atmospheric hydrocarbon, the dunes might provide us with important clues on the still puzzling methane/ethane cycle on Titan, comparable in many aspects with the water cycle on Earth.”
Titan’s dunes may also record the history of ancient climate change on the Moon. The winds are thought to change direction as Titan’s orbit wobbles relative to the Sun, so the shape of the dunes reflects the resulting changes in weather patterns. At that rate, it may take 3,000 Saturn years (90,000 Earth years) for a single dune to change direction. This discovery was also published in the Dec. 8, 2014, issue of Nature Geoscience.
As noted by Ralph Lorenz, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory, “This work opens dune morphology as a window into palaeoclimate studies on Titan.”
If another lander or a rover is ever sent to Titan, those dunes would be an impressive sight indeed.