Saturn's largest moon, Titan, will soon get its first visitor from Earth. A European space probe has split off from its U.S. mothership for a three-week long descent to the mysterious, murky moon, the only moon in the solar system with an atmosphere. Scientists want to learn more about conditions that are believed to be similar to the Earth, just before life arose nearly four-billion years ago.
The European Huygens probe has been dormant for almost seven years, riding piggyback on the U.S. Cassini spacecraft since their 1997 launch. Mission controllers have awakened it briefly only every six months to check its systems.
Now the time has come for it to spring into action - literally. Its separation from Cassini, initiated by the push of springs, lets it drift in a free-fall toward Titan. Then on January 15, Huygens is to plunge through Titan's smoggy, obscure atmosphere. During a two-and-a-half-hour parachute descent, it is to relay data about the atmosphere's chemical makeup and turbulence back to the Cassini orbiter before landing on the surface of Titan.
Scientists hope to learn things about this hidden moon that Cassini cannot in its 45 planned flybys over four years. They are fascinated by this satellite because it is thought to harbor an ocean and the organic compounds necessary for life, although U.S. Cassini mission official Charles Elachi says no one thinks there is life on Titan. "It is a pre-biotic environment, so we do not expect biology to be active now. So it is really more of an organic chemistry kind of mission," he says.
As the Huygens probe falls toward Titan, its camera and other sensors will measure the surface around its landing site. Mission officials do not know if it will hit ground or liquid. If it survives a hard impact of 25 kilometers per hour, the craft will be able to transmit for up to two hours. But if it lands in liquid, frigid temperatures of about minus 180-degrees Celsius would kill the batteries after only 30 minutes. But at least it would be able to measure the properties of the liquid.
Scientists are seeking answers to questions about Titan that have evaded them despite flybys of Saturn by two U.S. spacecraft in the 1980s, three close encounters this year by Cassini, and intense scrutiny by the Hubble Space Telescope and powerful ground telescopes.
U.S. space agency planetary scientist Torrence Johnson says we know as little about Titan as old time astronomers knew about Mars. "This is sort of like we were in the early 19th century looking at maps drawn of Mars from telescopes, where you have got smudgy areas on the surface. You do not know what forms those smudgy areas," he says.
The reason for the obscurity is Titan's unique atmosphere. It is the only other one in our solar system besides Earth composed mainly of the gas nitrogen.
But unlike Earth it is also about five-percent methane, which sunlight breaks down into its component elements, carbon and hydrogen. Cassini scientist Candice Hansen says these elements then recombine into dark hydrocarbons that form a nearly opaque mist. "It is a phenomenon too familiar to anyone who lives in a smoggy city," he says.
Because of Titan's atmospheric hydrocarbons, it may be a land of a thousand oily lakes. Scientists say the reason it has not burst into flame is because the oxygen needed for combustion is locked up in surface ice.
This lack of atmospheric oxygen and the presence of nitrogen and methane remind Cassini researcher Toby Owen of the University of Hawaii of what Earth's primitive atmosphere was like billions of years ago. "We think that studying Titan can help us understand the origin of our own atmosphere. It is a natural laboratory where chemistry is constantly taking place. The nitrogen and methane molecules are being broken apart, the fragments recombined, making more complex molecules. We are very interested to see what these reactions have produced because this, we think, is like what happened on the early Earth before life began," he says.
In recent flybys of Titan, the Cassini mothership has detected the first evidence of changing weather patterns and moving cloud formations. It also saw traces of flowing ice, suggesting the existence of an ice volcano.