Saturn's mysterious giant moon Titan is expected to give up some of its secrets Wednesday when the U.S. Cassini spacecraft makes its closest encounter yet. Cassini arrived at Saturn in late June and came within 350,000 kilometers of the murky moon a few days later, but the distant images of the smoggy body yield little information about its surface. The coming encounter will be much closer, making scientists hopeful they will finally penetrate Titan's haze.
Titan may be a moon, but at 5100 kilometers across, it is bigger than the planets Mercury and Pluto. Titan is the biggest object in our solar system whose surface remains unexplored. This lack of knowledge is not because the U.S. space agency NASA has not tried to observe the Saturn moon. Long before Cassini first looked at it in July, two U.S. Voyager spacecraft flew by in the early 1980s, and the Hubble Space Telescope and powerful ground telescopes have also peered at it.
Yet, for all the effort, NASA 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've got smudgy areas on the surface," he notes. "You don't know what forms those smudgy areas."
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. Unlike Earth, however, it is also about five percent methane, which sunlight breaks down into its component elements, carbon and hydrogen. These elements then recombine into dark hydrocarbons that form a nearly opaque mist.
"It's a phenomenon to familiar to anyone who lives in a smoggy city," says is Cassini scientist Candice Hansen.
She adds that Titan's thick smog stymied efforts to get good surface pictures during the July approach at 350,000 kilometers distance.
"I think we were expecting things to be easier," she says. "We were expecting to just immediately have great images of the surface, and the surprise was that it's really quite challenging to sort out what is surface and what is haze."
But Cassini imaging experts are not unarmed. The spacecraft's cameras have an array of filters designed to lift Titan's veil. Experiments during the July encounter have taught the Cassini team what combinations are best. In addition, the spacecraft is equipped with radar, which can penetrate any smog or dust.
The scientists will take pictures every 15 minutes as Cassini approaches Titan, getting as close as 1200 kilometers. Their goal is to map the big moon's surface at the highest resolution yet so they can distinguish features down to 50 to 200 meters across.
Project scientist Toby Owen of the University of Hawaii says the images could reveal Titan's surface to be anything from a rugged Earth-like terrain to smooth, perhaps interrupted by occasional craters from ancient impacts. Because of the hydrocarbons in the atmosphere, he notes that it may also be a land of a thousand oily lakes.
"On Titan, we have a flammable world where we'll find deposits of organic aerosols that will have accumulated on the surface," he explains There may be lakes and ponds of liquid hydrocarbons. The only reason Titan hasn't gone up in flames long ago is that there is no free oxygen as we have on the Earth."
That is because Titan is so cold that the oxygen is locked up in surface ice. This lack of oxygen in the atmosphere and the presence nitrogen and methane remind scientists of what Earth's primitive atmosphere was like billions of years ago. Mr. Owen says measuring Titan's atmosphere during this fly-by will be like going back in time to study the early Earth.
"We think that studying Titan can help us understand the origin of our own atmosphere," he explains. "It's 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're very interested to see what these reactions have produced because this, we think, is like what happened on the early Earth before life began."
Titan is of such importance to scientists that Cassini will approach it 44 more times over the next four years and the European Huygens probe will detach from Cassini in late December to plunge through the moon's atmosphere and land on the surface.