October 7, 2003
Delving Into the Liquid Intrigue of Saturn's Biggest Moon
omething on Titan, Saturn's largest moon, is very flat, possibly as flat as the surface of an ocean.
Through an experiment involving radar on an interplanetary scale, astronomers have made the first observations that support a long-held suspicion that liquid oceans cover much of Titan.
With the temperature of Titan estimated at minus 290 degrees Fahrenheit, the liquid cannot be water. Rather, scientists suspect hydrocarbons, a class of molecules that consist entirely of hydrogen and carbon. The gas methane is the lightest hydrocarbon; heavier hydrocarbons are the main components of smog.
Researchers from Cornell, the University of Virginia and NASA's Jet Propulsion Laboratory reported their findings in an article that the journal Science published on its Web site last week. Titan intrigues planetary scientists. At a diameter of 3,200 miles, it is larger than Mercury and Pluto, and its atmosphere, which consists mostly of nitrogen, is thicker than Earth's.
"It's the largest area of real estate that we don't know much about in the solar system," said Dr. Donald B. Campbell, a professor of astronomy at Cornell, who led the research.
Some astronomers suspect that Titan may even preserve conditions similar to those that existed on the early Earth. Although few expect life on Titan, "it could be a natural laboratory for the chemistry leading to life," said Dr. Jonathan I. Lunine, a professor at the University of Arizona's Lunar and Planetary Laboratory.
"Titan is more like a museum," Dr. Lunine said.
Several decades ago, the author and cosmologist Carl Sagan suggested that methane in Titan's atmosphere could condense, forming a global ocean.
When NASA's two Voyager spacecraft flew by Saturn in November 1980 and August 1981, their cameras were unable to peer through the orange haze of Titan's atmosphere to look at the surface. Their instruments, however, did not measure as much methane as would be expected to be evaporating from an ocean of pure methane.
In 1983, Dr. Lunine and other researchers suggested that sunlight might generate chemical reactions similar to those that create smog over cities and that some methane would turn into heavier hydrocarbons like ethane. "At the time, we were trying to understand the Voyager data," Dr. Lunine said.
In 1994, the Hubble Space Telescope took photographs of Titan in infrared light, which can penetrate the clouds. They showed splotchy dark and light regions, ruling out a global ocean. The light areas are probably ice, but the dark regions may be seas of hydrocarbons.
In November 2001, when Titan came within aim of the Arecibo radiotelescope in Puerto Rico, the telescope fired the first in a series of radio pulses. After traveling 750 million miles, the pulse bounced off Titan's surface and traveled the 750 million miles back to the 1,000-foot-wide dish in Arecibo, a 2-hour 15-minute round trip. A radio telescope in West Virginia was also used to detect the faint echo.
Arecibo fired 25 pulses at Titan. In three-quarters of the echoes, the astronomers detected sharp reflections like the blinding glints seen when sunlight bounces off a mirror or the ocean surface. The most likely explanation is that the radio waves had bounced off pools of liquid hydrocarbons.
"It's evidence they may be there," Dr. Campbell said. "It's not conclusive evidence."
The scientists said the data did not tell the size of the pools, whether they were ponds, lakes or seas.
Dr. Campbell said that it was also possible that the reflections were produced by smooth solid surfaces, but that he doubted that much of Titan could be that smooth.
"You would have ice-skating rinks over much of Titan," he said.
NASA's Cassini spacecraft, which will arrive at Saturn in July, could provide more definitive answers with radar that will better map the moon's surface and instruments that can detect what it is made of.
Cassini is also carrying a probe that will parachute onto Titan. Instruments on the probe will be able to tell a splash landing into liquid from a hard crash into ice.