Titan, Saturn’s largest moon, is one of the most “Earth-like” places in the solar system. It has a thick atmosphere, rivers and lakes, mountains and sand dunes. Unfortunately those rivers and lakes are composed primarily of methane, making it not very hospitable for humans.
The survey of Titan by the Cassini-Huygens spacecraft revealed a great deal of information about the moon. It also raised some perplexing questions. One of the mysteries involves the dunes located near Titan’s equator.
Climate simulations of Titan indicated that it’s “near surface” winds, similar to Earth’s trade winds, blow east to west. However, the dunes which stretch for many miles and reach more than 100 yards in hight point to the east.
A variety of theories have been put forward to explain this. Explanations include Saturn’s gravitational tides as well as speculation about wind dynamics and land features. None of these theories has quite added up to date.
In a new paper published in the journal Nature Geoscience, Benjamin Charnay of the University of Washington and his associates suggest that Titan’s dunes could be explained by strong methane storms in Titan’s atmosphere. These winds, unlike the surface winds, do blow toward the east.
Charley and his co-authors used computer models, tested the idea that these methane storms, despite being rare, could alter the direction of the sand dunes. Although not much by Earth standards, the methane storm winds can reach 22 miles per hour, which is 10 times greater than Titan’s relatively timid near-surface winds.
“These fast eastward gusts dominate the sand transport, and thus dunes propagate eastward,” said Charnay in a statement.
The storms only happen on Titan about every 14.75 years, when Titan is in equinox but, according to the researchers, they are powerful enough to move Titan’s dunes. Titan was last in equinox, with days and nights of equal length, in 2009.
The researchers state that the “super-rotation” of Titan’s atmosphere plays a large role. According to Cassini’s observations, Titan’s atmosphere above 5 miles rotates much faster than the surface does. According to the computer models, this would produce strong, eastward flowing, downdrafts which could rearrange the dunes.
Charley says that he tried initially to solve the problem using climate models that didn’t factor in the methane clouds, but found that it was impossible. This suggested to him that the clouds were very significant and could answer the question.
“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 run parallel to Titan’s equator are likely composed of hydrocarbon polymers, a soot resulting from the decomposition of Methane. Charley noted that, according to prior studies, it would take winds of at least 3.2 miles per hour, to lift and transport such sand and that is about 50 percent stronger than wind estimates for Titan.
“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,” said Charnay.
According to the study, 2023 would provide the next opportunity to observe the phenomenon and the Cassini mission ends in 2017 which means that it is unlikely that any craft will be close enough to Titan to make any observations.
“But there will be other missions. 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,” said Charnay
“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.”
While Titan is certainly not-very hospitable to human life, research published in February suggests that some form of life is, theoretically, chemically possible.
