"There has to be a source of methane on the surface, otherwise all the methane in the atmosphere would be destroyed in about 10 thousand years. It cannot be solid methane ice, because it would not give off much methane gas.

"So it is probably some source of liquid methane, and liquid methane is stable on the surface if it's mixed with ethene, which also exists on Titan. The boiling point of the combination of the two is very close to the surface temperature. You would then have a source of liquid, although it would be probably mushy, gunky, and very dark," Smith says.

Indeed, the map shows a very large, dark feature on the opposite side to the bright feature on Titan. Lunine and Stevenson suggested why Saturn's moon must have a methane ocean.

Methane would rise from the ground and diffuse in the atmosphere, where the sun's ultraviolet radiation would photochemically destroy it, creating organic haze that resembles the notorious smog over Los Angeles.

"These big, organic molecules stick together like tar and slowly sink back to the surface, continually raining down from the moon's atmosphere," Smith says.

Titan's atmosphere is much more diffuse than the Earth's and stretches several hundred kilometers above the surface."When the haze gets to about 80 kilometers (48 miles) above the surface, it rains out, and the atmosphere is crystal-clear with the visibility of hundreds of miles."

About 40 kilometers (24 miles) above the surface, temperatures plummet to a level cold enough to freeze nitrogen. Heavy, millimeter-sized droplets fall very rapidly to the surface and may pond in lakes or an ocean.

Descent Imager Spectral Radiometer (DISR)
When the Cassini spacecraft arrives at Saturn in 2004, it will release the Huygens Probe. The probe will descend to Titan's surface, landing on the western edge of the bright feature that Smith and Lemmon discovered in 1994.

"We're hoping that the wind will blow the probe east, as close to that region as possible," Smith says. But first, Cassini instruments will see the moon's atmosphere.

Obliquity causes a significant difference from summer to winter in the sun's position in the sky, which drives atmospheric dynamics and changes the density of the haze particles on Titan.

The Huygens Probe's Descent Imager Spectral Radiometer (DISR), conceived by LPL Research Professor Martin G. Tomasko and Smith, consists of 13 separate instruments. Three instruments are imagers of low, medium, and high resolution.

"With these 3 cameras and a spinning spacecraft, we'll time our pictures in such a way that we'll take an entire hemispheric panorama from above the horizon down to the surface. We'll take about 50 such panoramas during the descent through the atmosphere. The information we'll get out of these images will be absolutely stupendous," Smith anticipates.

The DISR spectrometers will measure the absorption of various gases and hazes in the atmosphere at near infrared wavelengths, which are absorption bands of organic molecules.

"By watching where the light gets absorbed we will know where methane is," Smith explains. As DISR approaches the surface, it turns on a special lamp. "Although in many wavelengths it is quite bright on Titan, in the methane bands it is pitch-black," he adds.

The LPL scientists tested the landing light on the black asphalt street and took measurements.Then they hosed the street with water and took measurements again.

"The difference between dry asphalt and asphalt with water was obvious, so we could measure the composition of the street, which after all isn't too different from Titan's tar," Smith concludes.