Titan, the largest moon of Saturn was discovered by Dutch astronomer Christian Huygens in 1655 and certainly deserves its name. With a diameter of no less than 5,150 km, it is larger than Mercury and twice as large as Pluto. It is unique in having a hazy atmosphere of nitrogen, methane and oily hydrocarbons. Titan's atmospheric pressure near its surface is 60 percent greater than on Earth at sea level.

Although it was explored in some detail by the NASA Voyager missions, many aspects of the atmosphere and surface still remain unknown. Thus, the existence of seasonal or diurnal phenomena, the presence of clouds, the surface composition and topography are still under debate. There have even been speculations that some kind of primitive life (now possibly extinct) may be found on Titan.

Titan is the main target of the NASA/ESA Cassini/Huygens mission, launched in 1997 and scheduled to arrive at Saturn on July 1, 2004. The ESA Huygens probe is designed to enter the atmosphere of Titan, and to descend by parachute to the surface.

To prepare cartographic views of the moon, ground-based telescopes like the European Southern Observatory's Very Large Telescope (VLT) at the Paranal Observatory in Chile now is studying the resolved disc of Titan with high sensitivity and increased spatial resolution.

Scientists would like to know the origin of the atmospheric patches imaged on Saturn's moon.

These new extraordinary images show details of the order of 200 km (120 miles) on the surface of Titan (a nominal resolution of 1/30th arcsec ).

By changing a filter, the telescope can effectively zoom down beneath Titan's heavy cloud cover to various depths from the surface. Images of Titan were obtained through 9 narrow-band filters, sampling near-infrared wavelengths with large variations in methane opacity. Titan has a large amount of hydrocarbons, both on the surface in what is predicted to be lakes of sludge and also in the atmosphere as methane.

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Primarily clouded by nitrogen laced with such poisonous substances as methane and ethane, Titan is thickly veiled by this dense hydrocarbon haze that forms in the high stratosphere as atmospheric methane is destroyed by sunlight. The haze is much thicker than Earth's worst city smog. It was impenetrable to cameras aboard the Pioneer and Voyager spacecraft that flew by the Saturn system in the late 1970s and early 1980s.

When viewed at various depths, Titan harbors a "southern smile" at smaller average filter widths (at 1.24 and 2.12 micrometers), that has a north-south asymmetry, while the opposite situation is observed with filters probing higher altitudes, (such as 1.64, 1.75 and 2.17 micrometers). Changing near-infrared filters permits sounding of different altitudes which range from the stratosphere to the surface. Smaller average filters will probe deeper beneath the clouds.

A high-contrast bright feature is observed at the South Pole and is apparently caused by a phenomenon in the atmosphere, at an altitude below 140 km or so. This feature was found to change its location on the images from one side of the south polar axis to the other during the week of observations.

Titan, with a diameter about two-fifths that of Earth, ranks second largest of all the solar system's moons after Jupiter's Ganymede. This Mercury-sized world is comprised of a 50-50 mix of ices and rock.

The chemical composition of its environment resembles that of early Earth but it is far colder and lacks liquid water. Scientists think Titan may have carbon- and nitrogen-containing molecules accumulated on its surface. And these primitive precursors to life might be brought even further towards life's door if liquid water makes an occasional appearance.

Studies of Titan so far have indicated enough evidence for both temporal and spatial variability, two signatures required for the presence of organic molecules.

Although Titan's underlying surface is thought to be water ice, the complex chemistry in the upper atmosphere might have resulted in the icy surface being at least partly covered in liquid ethane and methane and solid hydrocarbons. One class of the solid hydrocarbons, often referred to as Titan tholins (from the Greek word, muddy), was artificially created in a laboratory by a team led by the late Cornell astronomer Carl Sagan.

When scientists analyze the building blocks of tholins by burning them (pyrolysis), splitting up the tholins using plasma, scientists find a rich array of biomolecular building blocks such as pyrroles, pyrazines, pyridines and pyrimidines.

All of these molecules have played an important role in the evolution of terrestrial life. Recent reports of methane on Mars have sparked interest in their volcanic or biological origins, because in the thinner martian atmosphere, ultraviolet radiation quickly destroys methane without an underlying source to replenish it.

What's Next
An additional series of observations of Titan is foreseen later this month (April 2004). These will be a great asset in helping optimize the return of the Cassini/Huygens mission. Several of the instruments aboard the spacecraft depend on such ground-based data to better infer the properties of Titan's surface and lower atmosphere. The astronomers look next to model and interpret the physical and geophysical phenomena now observed and to produce a full cartography of the surface.

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