Earth has a surprising sibling in the solar system

WASHINGTON — Scientists say there's an alien world floating almost 900 million miles away in deep space that bears an eerie resemblance to Earth.

Recent observations have discovered astonishing similarities between Titan, Saturn's largest moon, and our own planet.

Bigger than our moon and the planet Mercury, Titan has rain, lakes, rivers, deserts, dunes, clouds and a thick hydrocarbon atmosphere that puts Los Angeles' notorious smog to shame.

On the other hand, Titan lacks oceans and oxygen and is far too cold, at minus 300 degrees Fahrenheit, to support life as we know it.

This distant moon is a sibling of Earth, but not a twin.

Radar images of Titan that NASA's Cassini space satellite took in July show hills, valleys, river networks and Appalachian-size mountains crisscrossing an Australia-size region called Xanadu.

"Surprisingly, this cold, faraway region has geological features remarkably like Earth," Jonathan Lunine, a planetary scientist at the University of Arizona in Tucson, reported in the journal Nature last month.

The first close-up pictures of Titan were taken by the two Voyager spaceships as they flew by Saturn in 1980-81. They could see no details because of a thick orange haze covering the moon's surface.

New instruments aboard the Cassini satellite, which has been orbiting Saturn since July 2004, have made a series of scientific breakthroughs. A European robotic probe, called Huygens, landed on the surface in January 2005, taking measurements all the way down. Since then, infrared radar beams have penetrated the smog repeatedly as Cassini flies by the moon.

A key discovery is that Titan, unlike Mars or our moon, is wet. Instead of water, however, the moisture consists of liquid methane, a compound of carbon and hydrogen and the chief constituent of natural gas on Earth.

"Titan and Earth are the only worlds in the solar system where rain reaches the surface," Ricardo Hueso, a scientist at the University of the Basque Country in Bilbao, Spain, wrote in Nature.

About 2 inches of liquid methane falls on Titan's surface each year, about as much rain as California's Death Valley gets, according to Christopher McKay, a planetary scientist at NASA's Ames Research Center in Mountain View, Calif.

The drizzle is interrupted occasionally by torrential rainstorms that cause flash floods and carve out riverbeds, according to Caitlin Griffith, a scientist at the Lunar and Planetary Laboratory in Tucson.

Flying by Titan on July 22, Cassini's radar spotted dark patches on the surface that scientists identified as lakes, up to 20 miles wide and 62 miles long.

"We've now seen a place other than Earth where lakes are present," Wall said.

Radar images taken last October revealed widespread regions, up to 1,000 miles long and 150 miles wide, of parallel ridges that resemble sand dunes in the Namib desert, along the Atlantic coast of Africa.

Ralph Lorenz, a scientist at the Lunar and Planetary Laboratory, said the dunes indicated that part of Titan must be covered with a "sand sea" of small, wind-blown particles.

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