A day on Saturn's moon Titan is nearly 16 Earth-days long, and every one of them is worse than a Stage 3 smog alert in Los Angeles. Beyond that, scientists struggle to get more information about the mysterious moon enshrouded in a global haze.
Titan's high-flying global smog shell is known to change with the seasons, and the moon's atmosphere must circulate continuously, theorists say, yet astronomers have not been able to penetrate the shell and pin down what goes on.
Firm answers should come in two years when the Cassini spacecraft explores Titan in detail.
Meanwhile, a new study to be published in the Aug. 22 issue of the journal Nature helps confirm some of what researchers have suspected.
Unique moon
Titan is the only moon in the solar system with a thick atmosphere.
Observations are limited by the moon's great distance from Earth and the fact that its smog layer completely hides the surface and even clouds, which exist at mid-altitudes. Some of the best data on Titan is two decades old, collected by the Voyager probes.
Titan is 3,200 miles (5,150 kilometers) in diameter, larger than Mercury, nearly half as big as Earth, and among moons second in size only to Jupiter's Ganymede.
Importantly, Titan is not big enough to manage the sort of complex, large-scale wind patterns observed on Earth, scientists suspect. Huge rotating terrestrial structures like hurricanes, key to transporting excess heat from the equator toward the poles, would be too big to fit practically on Titan.
Yet astronomers know Titan must, like any planet or moon, receive more intense sunlight at its equator than near the poles, setting up an unevenly heated world and forcing some sort of circulation. The suspicion has been that warmed air rises near Titan's equatorial region, flows at high altitude toward the poles, cools and sinks to the surface, and then scoots back toward the equator.
The process, called a meridional circulation pattern, is thought to operate slowly compared with terrestrial circulation.
Titan's atmosphere
Using complex circulation models based on Earth dynamics, a team of theoreticians led by Pascal Rannou at the University of Paris traced the formation, evolution and travels of smog particles through Titan's atmosphere.
The researchers found support for the meridional circulation pattern idea, as well as other interesting atmospheric twists.
Titan's atmosphere is mostly nitrogen with a dash of methane and an intriguing batch of so-called "organic molecules" that are vital to life. Though this does not mean Titan supports life, the moon is viewed by some astrobiologists as a place that might be a lot like early Earth and therefore might harbor clues about how life forms.
Titan's gravity is only about one-seventh that of Earth, so it might seem surprising there's much of an atmosphere at all.
However, it is cold -- minus 289 degrees Fahrenheit (-178 degrees Celsius) at the surface -- and cold things are less eager to escape the relatively weak gravity. The atmospheric pressure at the surface of Titan, in fact, is about 50 percent greater than on Earth.
High in the Titan sky, solar radiation helps fuel chemical reactions that break the nitrogen and methane down into other substances. Eventually, lower down, some of these molecules serve as seeds for clouds. Methane condenses on the seeds to form rain or hailstones that fall to the surface. In general, Titan's clouds are rare and the precipitation is probably sparse,
has shown.Tracking smog
All the while, at about 250 miles (400 kilometer) up, large molecules form a detached layer of hazy smog -- mostly methane and ethane -- says Robert Samuelson, an astronomer at the University of Maryland who was not involved in the new study but writes an analysis of it for Nature.
"The smog and organic gases on Titan share a common characteristic," explains Samuelson, who also works at NASA's Goddard Space Flight Center. "Both are born at altitudes higher than where they die."
Rannou and his colleagues used these smog molecules as tracers for the entire atmosphere's movements, providing numerical support -- a model -- for what the theorists and some limited previous observations had supposed.
The smog particles stay intact as they move from the equatorial region toward the poles, building the high, global shell. At the poles, the particles descend, creating a hood or ring that was observed by Voyager 2.
Next up, Cassini
Samuelson writes, however, that some aspects of Rannou's model are inconsistent with actual observations.
A more complete and accurate understanding of Titan should come when Cassini arrives at Saturn in 2004. The joint mission between NASA and the European Space Agency will drop a probe, called Huygens, into Titan's atmosphere to explore wind, temperature and, with some luck, surface conditions for a short period. Meanwhile, Cassini will study the moon from above with instruments far more sophisticated than what Voyager packed.
Samuelson and others expect the mission to provide new data that will help unravel Titan's weather mysteries.
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