Titan146;s like Earth

It took nine years, hundreds of millions of dollars and a huge amount of effort, but planetary scientists have finally found another place with a topography quite like Earth8217;s.

It took nine years, hundreds of millions of dollars and a huge amount of effort, but planetary scientists have finally found another place with a topography quite like Earth8217;s. The first detailed pictures of Titan, one of the moons of Saturn, were eerily familiar. What the scientists saw looked like dunes, hills, valleys and8212;most unusual8212;rivers running into lakes. If further studies prove the dark, ovoid features on the landscape are indeed lakes, Titan will be the only body in the solar system besides Earth possessing that geological feature.

The differences between the two places, however, are as striking as their similarities. Titan8217;s surface temperature averages 292 degrees below zero Fahrenheit. The landscape, carved by wind and a constant drizzle, is made up largely of ice, not rock. It takes nearly 30 years for Saturn to orbit the sun, so each of Titan8217;s seasons is a little more than seven years long. The liquid that falls from the sky and runs down into the lakes isn8217;t water. It is some form of liquid hydrocarbon8212;possibly methane.

In the journal Nature, scientists reported methane appears to fall on Titan in a constant, year-round slight drizzle. 8216;8216;It is almost a parody of the Earth,8217;8217; said Jonathan Lunine, a professor of planetary science at the University of Arizona. 8216;8216;It is very funny to go to this place and see all these processes being played out, but with different materials.8217;8217;

Elsewhere in the solar system8212;on Mars, for instance8212;there may once have been the cycles of weather and landscape-building that still exist on Earth. They ended billions of years ago, but are still taking place on Titan8212;about one-third the size of Earth but nearly 10 times as far from the sun. 8216;8216;This tells us that we have to go a very long way from Earth to find the processes we have here,8217;8217; Lunine said.

The revelation comes thanks to the Cassini-Huygens spacecraft, which lifted off from Cape Canaveral in October 1997. Despite the dramatic, ringed appearance of Saturn, the environs of that huge gas planet are pretty gloomy. Saturn and its moons get one-tenth the solar radiation Earth does. Titan8217;s atmosphere is four times as dense as Earth8217;s at sea level, further blocking sunlight. To top things off, it has a serious smog problem. Hydrocarbons evaporate off Titan8217;s surface and recondense in clouds. During that process, some molecules react with sunlight, just as happens in Earth8217;s smoggy cities.

Because there is little oxygen on Titan, the compounds are different. 8216;8216;They are ethane, acetylene, benzene, hydrogen cyanide,8217;8217; Lunine said.

The area with the lakes and streams, which is about the size of Australia, was first seen by the Hubble Space Telescope in 1994 through infrared imaging. It was named Xanadu. The images gathered by Cassini this month were made with radar waves, not with visible light. The scientists are not positive the smooth, black areas in the images are liquid. But they have the same appearance that smooth bodies of water have on Earth when photographed with radar-frequency waves. The spacecraft will orbit Saturn at least until late 2008 and, perhaps, a few years longer if all goes well.

8212;David Brown