Drier future?
Cassini's radar has identified lakes of liquid methane (dark areas) near Titan's poles, a geology that could presage Earth's in a billion years.

Titan's Dark Mirror

SAN FRANCISCO, CALIFORNIA--What can Saturn's largest moon tell us about our own planet's future? Quite a lot, apparently, researchers told the annual meeting of the American Association for the Advancement of Science (which publishes ScienceNOW) today. In a presentation here, scientists poring over data from the Cassini spacecraft and ground-based observations painted close parallels between the evolution of Earth and the climate and geology of Titan.

For a body more than a billion kilometers away, Titan has a surprisingly lot in common with Earth. Its atmosphere--while roughly four times denser and 10 times thicker than Earth's--is dominated by nitrogen, just like ours is. Instead of water vapor as an atmospheric constituent, Titan has methane, but it behaves just as water vapor does in Earth's air. Titan has weather, too. Planetary scientist Jonathan Lunine of the University of Arizona in Tucson explains that during a cycle taking up to 100 years, methane on Titan's surface slowly evaporates into the atmosphere and migrates toward the polar regions, where it falls in brief but torrential downpours, carving deep channels into the moon's surface and filling lakes as wide as 100 kilometers.

It's possible that Titan once had much more methane on its surface--perhaps oceans of it--as a process called cryovolcanism forced large volumes of liquid methane, ammonia, and water ice onto the surface. But as the methane slowly evaporated, ultraviolet (UV) radiation from the sun began breaking it down, releasing its hydrogen and preventing it from being reconstituted. The same fate awaits Earth's water, Lunine says. He explains that over the next 500 million years or so, as the sun's brightness increases, UV radiation will break down more and more water vapor in the planet's atmosphere. Eventually, Lunine says, the terrestrial landscape will start to look a lot like Titan's, with scattered lakes only at the poles and dry terrain everywhere else. Even sooner, he says--although still on a geologic timescale--Earth will begin experiencing longer and longer droughts punctuated by severe downpours as UV radiation increases.

The findings are ironic, says planetary geologist Larry Soderblom of the U.S. Geological Survey in Flagstaff, Arizona. Scientists expected Titan to reveal a bizarre and alien landscape, he says, but "Titan more resembles Earth than any other body in our solar system." That distinction can be a plus, says research scientist Ralph Lorenz of Johns Hopkins University's Applied Physics Laboratory in Laurel, Maryland. In the 100 years since Titan's atmosphere was discovered, planetary researchers have been able to use the big moon as a laboratory to study atmospheric dynamics, including greenhouse effects, seasonal changes, and the transfer of vapor from the equator to the poles.

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