Titan on Tuesday

True color and surface infrared images show features resembling clouds and a continental area about the size of Australia Image Credit: NASA/JPL

On Tuesday, October 26, the Cassini spacecraft will approach Saturn’s largest moon, Titan. Cassini will fly by Titan at a distance of 1,200 kilometers (745 miles) above the surface, nearly 300 times closer than the first Cassini flyby of Titan on July 3.

"The surface that we saw last July, at the resolutions we saw it, was extremely puzzling," says Torrence Johnson, imaging scientist on the Cassini-Huygens mission. "It’s like nothing that I’ve ever seen before. We didn’t see evidence for large impact structures, large craters or basins, we didn’t see mountains, we didn’t see the types of things we’ve become accustomed to seeing on the surfaces of icy satellites and other worlds. This is really terra incognita."

Titan descent by Huygens probe leaving Cassini storage, Christmas 2004. Image Credit: JPL/Space Science Institute

Cassini will make its closest approach at 9:44 a.m. Pacific Time. Eleven of Cassini’s twelve instruments will be aimed at Titan during this encounter. Cassini will take hundreds of images, but the images won’t be available immediately. The spacecraft will be out of Earth contact for about 30 hours as it records details of the moon’s thick atmosphere and icy surface.

Cassini will only be able to transmit the data after the spacecraft faces back toward Earth. Cassini will playback the recorded data around 18:30 Pacific Time Tuesday evening. It takes about one hour and fourteen minutes for signals sent from Cassini to reach Earth, and project scientists estimate images should be available to the public soon after the first ones arrive.

The haze of an atmospheric layer on Saturn’s moon, Titan. With an atmosphere thicker than Earth’s, and composed of many biochemically interesting molecules (methane, hydrogen and carbon), Titan’s rich chemistry will continue to interest astrobiologists as they look forward to landing a probe on its surface in 2004-5. Credit: Voyager Project, JPL, NASA

Toby Owen of the University of Hawaii at Manoa, interdisciplinary scientist on the Cassini-Huygens mission, says they expect to see two possible kinds of surfaces on Titan. "One would be a rugged surface such as we have on the Earth which has been shaped by erosion, in this case, the erosion of liquid hydrocarbons instead of liquid water. Or at the other extreme, the surface of Titan might look like a melted and refrozen chocolate sundae, a very smooth surface perhaps marked by a few impact craters."

Since the spacecraft actually will be traveling through the outermost portion of Titan’s atmosphere, it will be able to take direct readings of the atmospheric gases. Titan is the only other solar system object besides the Earth to have a nitrogen-dominated atmosphere. But while the second most abundant atmospheric gas on Earth is oxygen, on Titan the second-most abundant gas is methane.

"On Titan, we have a flammable world," says Owen. "We find deposits of organic aerosols that have accumulated on the surface. There may be lakes of liquid hydrocarbons. There is methane in the atmosphere. The only reason Titan hasn’t gone up in flames long ago is that there is no free oxygen such as we have on the Earth."

The temperature at Titan’s surface is minus 178 degrees C (minus 289 degrees F). Because Titan is so cold, water is trapped at the surface and unable to interact with the methane to produce carbon dioxide.

Although Titan’s atmosphere is often compared to an early Earth-like primordial soup, where organic chemical reactions eventually led to the origin of life, Owen says they’re not expecting to find life on Titan.

Titan’s changing face as dark and light patches rotate in circulation. Image Credit: JPL/Space Science Institute

"It’s much too cold for that. But we are expecting to find prebiotic chemistry, which may tell us something interesting about what happened on the early Earth," says Owen. "In other words, Titan gives us a chance for a kind of cosmic time travel, as if we found an island somewhere on the Earth with dinosaurs on it. But in this case we’re going much further back in time than the dinosaurs. We going back to the earliest days of the Earth, when it had at least local environments that must have been similar to what we find on Titan."

"So no primordial soup as we had on the Earth, but maybe some primordial ice cream," Owen adds.

This is one of 45 planned flybys of Titan during Cassini’s four-year tour. Subsequent flybys will bring the spacecraft even closer to the moon’s surface.

On Christmas Eve, the Huygens probe will be released from the Cassini spacecraft. It will travel down through Titan’s atmosphere, taking photos and transmitting data during the descent. If it survives the landing, the probe will take photos and readings of the surface as well.

Related Web Pages

Saturn Edition, Astrobiology Magaz.
Saturn’s Rings in UV
Cassini Closes In on Saturn

Saturn– JPL Cassini Main Page
Lord of the Rings
Space Science Institute, Imaging Team Boulder, Colorado
Saturn: The Closest Pass
Prebiotic Laboratory
Planet Wannabe
Where is Cassini Now?