Echoes From Saturn’s Largest Moon : Radar Images Tell of Distant Ethane Ocean
Saturn’s largest moon, Titan, is so far away that takes light 2 1/2 hours to travel there and back, but scientists have managed to bounce a radar beam from California off its surface and capture its echo in New Mexico to learn more about a moon that lies hidden beneath a dense atmosphere.
What they learned is that Titan has an ocean of liquid ethane, which many scientists had expected because of the chemical composition of its atmosphere. But they also learned it is not entirely covered by the ocean, which few scientists had anticipated.
“The radar echo from Titan is the weakest such echo that has ever been measured, yet we were able to obtain a great deal of information,” Duane O. Muhleman, a Caltech planetary scientist, said in announcing the finding Monday.
The echo was weak, but it varied considerably, indicating that Titan has an ocean with beaches and maybe even mountains, though hardly the type to lure tourists. The beaches may well be ice, since the surface temperature is 290 degrees below zero.
Surface Composition
“It is reasonable to assume that the solid surface is made up of water ice, and perhaps solid carbon dioxide, silicates and tars,” Muhleman said. “If the surface can support hills and mountains, they could stand above the liquid hydrocarbons and the surface may resemble oceans and continents, but this is pure speculation. Our experiment was designed to narrow the range of possibilities.”
Scientists for years have used radar to study distant bodies, because the strength of the echo tells them much about the texture of the surface. It also penetrates clouds, thus offering a glimpse at what lies below.
In the case of Titan, that process was carried to new limits.
The scientists used a 230-foot antenna at Goldstone, in California’s Mojave Desert, to transmit a 360,000-watt signal for three 5 1/2-hour sessions on June 3, 4 and 5. The antenna is part of the Deep Space Network that the National Aeronautics and Space Administration uses to track spacecraft speeding through the solar system.
After traveling 2 1/2 hours to Titan and back, the signal--traveling at the speed of light--was received at the Very Large Array, a huge radio telescope consisting of 27 large antennas near Socorro, N.M.
Roughness Measured
The strength of the echo depends on the electrical reflectivity and the roughness of the surface of the target. A deep ethane ocean would have very low reflectivity, which is exactly what the scientists found most of the time.
But on June 4 an echo was received that was far too strong to have been reflected by an ocean, indicating it came instead from a highly reflective surface and was similar to echoes from Venus. On two other occasions the signal was much weaker than normal, suggesting the surface of Titan is very irregular.
Since Titan rotates about 23 degrees per day, the scientists concluded that it was presenting different faces to Earth, and those faces were quite variable.
The full portrait of Titan, which has nearly twice the mass of Earth’s moon, will have to wait awhile. NASA hopes to eventually send an unmanned probe to Saturn and its moons, but that proposal is still in the design stage.
Joining Muhleman in the Titan radar project were Martin Slade, a planetary scientist at JPL, and Caltech graduate students Arie Grossman and Bryan Butler.
