Voyage to Titan

Earth craft to probe Saturn's big, murky, tantalizing moon

Science

December 24, 2004|By Michael Stroh | Michael Stroh,Sun Staff

It's considered the most mysterious corner of the solar system: a secretive, smog-shrouded world nearly a billion miles from the sun, where volcanoes spew frozen ammonia slush and liquid natural gas rains down from the skies.

At least that's what scientists think might be going on beneath the amber-hued haze of Saturn's largest moon, Titan. For nearly four centuries, the moon's thick atmosphere has stymied every attempt to get a clear picture of the its surface. But Titan's secret life may be coming to an end.

Just after 11 tonight, NASA's Saturn-orbiting Cassini spacecraft is scheduled to spring loose a piggyback probe called Huygens. The 705-pound saucer, built by the European Space Agency and named for the Dutch astronomer who discovered Titan in 1655, will spend the next three weeks coasting to the moon before plunging into its dense atmosphere for a landing Jan. 14.

If it survives, Huygens' three cameras and other instruments could provide the first glimpses of the mysterious world lurking beneath the smog.

"I'm not sure the Cat in the Hat is there," says Jonathan Lunine, a planetary scientist at the University of Arizona and member of the Huygens team. "But it could be a Dr. Seuss kind of place."

Ever since Cassini pulled into orbit around Saturn in July, anticipation of the encounter has been building. One reason Titan attracts such attention is that it's bigger than Mercury and Pluto. "We can call it a planet and not feel guilty," says Lunine. Even more intriguing: It's one of only four bodies in the solar system with a substantial atmosphere. The others are Venus, Mars and Earth.

Few details emerge

Scientists have known about Titan's atmosphere since the 1940s. But attempts to pierce its thick curtain of smog have turned into one disappointment after another. In 1980, NASA's Voyager 1 swept past the moon but saw little detail. Since then, smog-penetrating radar and infrared cameras on the Hubble Space Telescope have provided a few nuggets. But the data remain skimpy.

Scientists do know that the moon's surface temperature dips to minus 290 degrees Fahrenheit. That's too cold for liquid water, although the moon may have frozen water in its crust. Researchers also know Titan's thick atmosphere is composed mostly of nitrogen, laced with traces of methane, acetylene and propane.

It's a toxic mixture only a welder could love, but it has made Titan an even more tantalizing target. Scientists say these carbon-based chemicals are some of the raw ingredients that may have given rise to life on Earth.

"If you take this stuff and add water, it instantly forms amino acids," says planetary scientist Ralph Lorenz, author of Lifting Titan's Veil and a member of the Huygens team.

Amino acids, he explains, are the building blocks of proteins and the chemical foundation for life on Earth. As a result, astronomers of no less renown than the late Carl Sagan have mused that Titan could be a frozen time capsule of sorts -- a glimpse of the primordial Earth nearly 4 billion years ago, before biology took hold.

Using Hubble, scientists have also identified vast swaths of light and dark patches on the moon's surface -- including masses as big as Australia. Some of the edges of these areas look distinctly like shorelines. But whether these are continents and oceans -- or something else entirely -- is a matter of heated debate.

In the months since its arrival, Cassini has already swung by the moon twice, on one occasion buzzing within 750 miles of its surface. Rather than clearing up mysteries, however, Cassini has only added to the puzzle.

Cassini's radar images show a smooth surface nearly free of craters -- a sign that the moon may still be geologically active. Most of the solar system's rock orbs are routinely peppered with meteors and other cosmic buckshot, leaving telltale craters visible from afar.

The absence of pockmarks on Titan's surface -- or at least the 1 percent of it that scientists have observed -- means that some Zamboni-like force is covering them up or smoothing them over.

Scientists have theorized that Titan might be home to exotic "cryovolcanoes," formations that ooze not scorching magma, but icy ammonia slush onto the surface. Another possibility is that photochemical smog particles have rained down onto Titan's surface over thousands or even millions of years, all but burying impact craters.

Cassini's flybys have turned up clouds, the first solid signs of weather activity on the moon.

But the two encounters also put an early damper on speculation that the moon's surface is awash with giant lakes or oceans of liquid methane. Despite the oddly shaped light and dark patches, scientists saw none of the telltale glints -- known as "specular reflections" -- that they would expect from a liquid-filled body.

With so much of the moon's surface left to explore, it's still possible, of course, that one may exist. And that would help resolve another mystery: why the Titanian air contains about 6 percent methane.

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