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A space probe scheduled to land on Saturn's largest moon, Titan, in 2004 may carry with it a small disk, slightly larger than a quarter, made of diamond and etched with a time-capsule message. The message would be intended for explorers millennia in the future -- or for communicating with intelligent aliens.

If approved by NASA and the European space agency, joint operators of the Cassini Saturn mission, disks will be attached to the main vehicle, which will orbit Saturn, and to the Huygens probe designed to land on Titan.

The message containing information and diagrams would be etched on the diamond disk, then sealed with diamond vapor to make it an "artificial fossil," according to University of California astronomer Gregory Benford.

Benford, the author of several science-fiction novels, proposed the idea along with artist John Lomberg, a co-designer of the Voyager records attached to the sides of the Voyager 1 and 2 space probes receding from the solar system.

Benford says he is "optimistic" the proposal will get official approval in May so the design of the disk and the message can be completed well before the 1998 launch.

New laser writing techniques would permit microscopic images to be imbedded in the disk. "There will be room to write a lot," Benford said, adding that once the concept is approved, he and Lomberg will be soliciting proposals about the disk's contents. Much of it would center around showing where the disk came from and how it was transported.

Titan is the only moon in the solar system with a substantial atmosphere. The atmosphere is slightly denser than Earth's and largely consists of nitrogen, the same gas that dominates Earth's atmosphere. But the average temperature at Titan's surface is minus-290 degrees Fahrenheit.

Nevertheless, conditions on Titan are closer to those thought to exist on the primordial Earth than any other place in the solar system. In his recent book "Pale Blue Dot," astronomer Carl Sagan describes Titan as a place "where the very stuff of life falls out of the skies onto the unknown surface below." Although this is true in a chemical sense, Sagan hedges his bets about the actual existence of life as we know it on Titan.

In any case, Benford suggests that "the message will be aimed at any life forms that may evolve in the organic soup of Titan.

A more likely scenario is that explorers from Earth's future will collect the artifact. Regardless, Benford thinks he has an idea that will stimulate the imagination.

The Cassini mission itself should provide plenty of excitement for astronomy fans early in the 21st century. If it works as planned, the $2 billion spacecraft will return roughly 200,000 images of Saturn and its rings and moons during its four-year mission.

TERENCE DICKINSON is the author of "Night-Watch" and several other books for amateur astronomers.

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