In the Stars: Titan's critical secret

Nov. 2, 2004 at 11:21 AM
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WASHINGTON, Oct. 28 (UPI) -- NASA's Cassini spacecraft just flew closer to Saturn's giant moon Titan than any other device built by humans. The mission, in one of the far corners of the solar system, is attempting to answer a very large, critical question -- does life exist anywhere else in the universe?

In some ways, the discovery of life on Titan would be an even bigger story than finding life on Mars.

Hidden behind a thick veil of haze, mysterious Titan is the solar system's only known moon with an atmosphere. Not only that, its mostly nitrogen atmosphere also contains methane and possibly ammonia, two of the building blocks of organic molecules.

Bigger than the planets Mercury and Pluto, Titan was discovered in 1655 by Dutch astronomer Christiaan Huygens -- whose name was bestowed on the European Space Agency's probe that will attempt a soft landing on the moon next Jan. 14. It is the second-largest moon in the solar system, smaller only than Jupiter's moon Ganymede.

Last Monday, Cassini flew a mere 750 miles (1,200 kilometers) above Titan. It is the second of 45 planned flybys during the spacecraft's four-year mission, some of which will take the spacecraft even closer to the moon.

"Cassini will see Titan as it has never been seen before," said Charles Elachi, director of NASA's Jet Propulsion Laboratory in Pasadena, Calif., which controls the mission. "We expect the onboard instruments will pierce the moon's dense atmosphere and reveal a whole new world."

Probably the most important goal of this close encounter is to confirm predictions about Titan's atmospheric composition to help scientists prepare for the Huygens probe's descent.

"Titan has been lying still, waiting," said Jean-Pierre Lebreton, project manager for Huygens and project scientist for the European Space Research and Technology Center in Noordwijk, Netherlands. "Cassini may finally show us if what we thought of this moon is true, and whether the Huygens probe touchdown will be a splash."

A sophisticated radar aboard Cassini will peer through Titan's atmosphere to determine whether or how much of Titan's surface is liquid.

A water ocean is probably out of the question, given the low surface temperatures -- estimated at nearly minus 290 degrees Fahrenheit (about minus 180 degrees Celsius) -- but an ocean of ethane or methane is possible.

"We know our instrument will see through the haze to Titan's surface," said Robert H. Brown of the University of Arizona in Tucson and team leader for the spacecraft's visual and infrared mapping spectrometer, and instrument that can produce images of a planetary body's chemical composition. "This encounter is about digging down below the atmosphere and getting our first glimpse of Titan geology."

The flyby will not neglect the atmosphere, however -- Cassini's ion and neutral mass spectrometer actually will taste it.

"Our instrument will scoop up a breath of Titan's puffy atmosphere during the flyby," said Roger Yelle, instrument team member, also with the University of Arizona. It will measure relative molecular amounts of different gases, possibly yielding new insights about the moon's atmospheric components and chemistry.

The key question about Titan's atmosphere is how is its methane being generated. It is the same question currently intriguing scientists about Mars, where atmospheric methane also was discovered recently.

Methane poses a potentially exciting problem. The gas cannot exist indefinitely in a planet's atmosphere because sunlight breaks it down. That means any atmospheric methane must have a source of replenishment.

The most common source of methane is living organisms, which produce it as a byproduct of their metabolism. Cattle, for example, digest the plant material they eat and excrete -- through their burps and other means -- methane. In fact, cattle excretions account for nearly one-fifth of the methane in Earth's atmosphere -- the second-biggest greenhouse gas.

Does the methane on Titan mean living organisms are at work somewhere on the surface? Not necessarily. There is that aforementioned possibility of a methane ocean, which could be supplying the gas to the atmosphere through evaporation -- and creating the truly alien specter of methane raining back down to the surface.

It would be difficult to imagine the kind of organism that could survive on such a cold, turbulent, noxious world -- difficult, but not impossible.

Titan orbits a miniature solar system -- comprising Saturn, 33 known moons and the planet's immense rings -- that is full of activity. There are plenty of gravitational forces at work and plenty of rocky fragments -- such as Saturnian ring leftovers -- careening around the neighborhood looking for a planetoid to impact.

Those two factors -- gravitational tides and impacts -- could provide enough geothermal heat to keep parts of the surface warm enough to liquefy water, at least periodically. Given the thickness of the atmosphere, some scientists think an impact from a good-sized meteorite could keep ice melted for perhaps hundreds of years at a time.

One interesting, early surface characteristic discovered by Cassini: Titan seem to have very few visible impact craters, meaning its topography is relatively young and dynamic. That is another feature suggesting there might be niches for organisms to have gained a foothold.

The twin Martian rovers, Spirit and Opportunity, continue to discover more evidence that Mars once was a wet and warm planet like Earth, and it seems to have been wet for a very long time. Steadily, the odds are growing that life -- past and/or present -- will be discovered on Mars.

Yet even that major revelation will not necessarily resolve the bigger issue of whether life is common in the universe. The reason is Earth and Mars are close, relatively speaking, and have shared material from time to time. Plenty of Martian meteorites have been found in Antarctica, for example, so it is quite possible Terran and Martian life shared a common origin.

Titan is another story. Located more than 800 million miles away, it is far less likely that life, if it exists there, evolved from the same source that gave rise to life on Earth. The same probably could be said about Jupiter's four large icy moons -- Io, Callisto, Ganymede and Europa -- of which Europa might have the best chance of all of harboring life in the liquid oceans underlying its ice-covered surface.

Considering the extremely harsh conditions on Titan's surface, the discovery of living organisms would testify to the adaptability and persistence of biology. It would offer a strong, perhaps overwhelming argument, that many if not most of the approximately 10,000,000,000,000,000,000,000 Earth-like planets in the universe are inhabited.

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Editors: UPI photos WAX2004101701, WAX2004101705, WAX2004101706 and WAX2004101707 are available

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