Study of Saturn's moon finds Titan's liquid oceans are likely solid seas of sand

COURTESY OF NASA/JPL Saturn’s moon, Titan

Radar images taken when the Cassini satellite flew by Saturn’s moon, Titan, last October reveal 330-foot-tall dunes running parallel for hundreds of miles along its equator.

Until a couple of years ago, scientists thought that the dark equatorial regions of Saturn's moon Titan might be liquid oceans. But according to new satellite radar data published in the journal Science, Titan's wet seas may actually be solid dunes of sand similar in appearance to the vast Namib Desert in southwest Africa.

"It's bizarre," said University of Arizona scientist Ralph Lorenz, lead author of the May 5 Science study. "These images from a moon of Saturn look just like radar images of Namibia or Arabia. Titan's atmosphere is thicker than Earth's, its gravity is lower, its sand is certainly different—everything is different except for the physical process that forms the dunes and resulting landscape."

The new images of Titan were obtained by a radar instrument aboard the Cassini satellite, which has been orbiting Saturn since June 2004. The Cassini mission—a cooperative project of NASA, the European Space Agency and the Italian Space Agency—is designed to explore Saturn and its 34 significant moons, including Titan, which is larger than the planets Mercury and Pluto.

Titan is of particular interest, NASA scientists say, because it is one of the few moons in the solar system with its own atmosphere—a thick haze that may be similar to that of early Earth's. Cassini's radar instrument uses microwaves to penetrate Titan's dense atmosphere and produce clear images of the landscape below.

Two Stanford researchers—Howard Zebker, professor of electrical engineering and of geophysics, and graduate student Lauren Wye—are among 40 co-authors of the Science study. "Our role at Stanford is to analyze the radar and radiometer data acquired by the Cassini radar instrument," Zebker said. "Our science investigation is to try to infer the composition and structure of Titan's surface from these data."

Windy landscapeThe Science study is based on radar images taken when Cassini flew by Titan in October 2005. The images revealed dunes 330 feet high that run parallel for hundreds of miles at Titan's equator. One dune field is more than 930 miles long, according to the study.

A decade ago, many scientists believed that Titan was too far from the sun to have solar-driven surface winds strong enough to sculpt sand dunes. They speculated that the dark regions at Titan's equator might be liquid ethane oceans that would trap sand. But researchers since have learned that Saturn's powerful gravity creates significant tides in Titan's atmosphere. This tidal effect is roughly 400 times greater than our moon's tidal pull on Earth.

"Tides apparently dominate the near-surface winds because they're so strong throughout the atmosphere, top to bottom," Lorenz said. "Solar-driven winds are strong only high up."

According to the authors, Titan's dunes are a longitudinal type that is characteristic of dunes formed by winds blowing from different directions. The tides cause wind to change direction toward the equator, Lorenz explained, and when the tidal wind combines with Titan's west-to-east wind, they create dunes that are aligned roughly west-to-east—except near mountains that influence local wind direction.

"When we saw these dunes in radar it started to make sense," Lorenz said. "If you look at the dunes, you see tidal winds might be blowing sand around the moon several times and working it into dunes at the equator. It's possible that tidal winds are carrying dark sediments from higher latitudes to the equator, forming Titan's dark belt."

The researchers' model of Titan suggests that tides can create surface winds that reach speeds of about 1 mile per hour. "Even though this is a very gentle wind, this is enough to blow grains along the ground in Titan's thick atmosphere and low gravity," Lorenz said.

Coffee grounds and snowflakesTitan's sand appears to be a little coarser but less dense than typical sand on Earth or Mars, Lorenz said. "These grains might resemble coffee grounds," he added.

Whether the grains are made of organic solids, water ice or a mixture of both is a mystery.

"I think it's possible that the dunes are snow rather than sand, but we don't have any real hard data to back that up yet," Zebker said. "We are analyzing the radar and radiometer properties of the dunes to see if they are consistent with piles of snowflakes. These snowflakes could be made of many materials, from ice as on Earth to solid methane, for example."

According to the authors, sand may be created when rainfall in the form of liquid methane erodes particles of ice bedrock, or by photochemical reactions in Titan's atmosphere that produce organic solids. "We need more work to see if alternate explanations for the accumulation of dust exist," Zebker noted.

"It's exciting that the radar, which is mainly to study the surface of Titan, is telling us so much about how winds on Titan work," Lorenz added. "This will be important information for when we return to Titan in the future, perhaps with a balloon."

The Jet Propulsion Laboratory (JPL), a division of the California Institute of Technology, manages the Cassini mission for NASA's Science Mission Directorate, Washington, D.C. The Cassini orbiter was designed, developed and assembled at JPL.

An international team of researchers contributed to the Science study from Stanford, University of Arizona, JPL, California Institute of Technology, Proxemy Research Inc., U.S. Geological Survey, Planetary Science Institute, Wheeling Jesuit College, NASA Goddard Institute for Space Studies, Observatoire de Paris, Università d'Annunzio, Università di Napoli, Università La Sapienza, Politecnico di Bari and the Italian Space Agency.

This article is based on a press release written by Lori Stiles, senior science writer and editor at the University of Arizona communications office.

Namibia