Titan's Oily Lake
The smog-shrouded atmosphere of Titan, Saturn's largest moon, has been parted by Earth-based radar to reveal the first evidence of liquid hydrocarbon lakes on its surface. The observations are reported by a Cornell University-led astronomy team working with the world's largest radio/radar telescope at the National Science Foundation's (NSF) Arecibo Observatory.
|Scientists would like to know the origin of the atmospheric patches imaged on Saturn's moon, Titan, as imaged by Hubble. Image Credit: Hubble Space Telescope/UA Smith
The radar observations, reported in the journal Science on its Science Express Web site (Oct. 2, 2003) [Radar Evidence for Liquid Surfaces on Titan ], detected specular -- or mirrorlike -- glints from Titan with properties that are consistent with liquid hydrocarbon surfaces. Titan's haze was first breached by the Hubble telescope, scanning in the near infrared wavelengths, in 1994. Surface features included a bright spot the size of Australia.
Cornell astronomer Donald Campbell, who led the observation team, does not rule out that the reflections could be from very smooth solid surfaces. "The surface of Titan is one of the last unstudied parcels of real estate in the solar system, and we really know very little about it," he says. Titan, with a diameter about two-fifths that of Earth, ranks second largest of all the solar system's moons. Only Jupiter's moon Ganymede is larger. The atmospheric pressure near its surface is 60 percent greater than on Earth at sea level.
Titan is a Mercury-sized world comprised of a 50-50 mix of ices and rock. The chemical composition of its environment resembles that of early Earth but it is far colder and lacks liquid water. Scientists think Titan may have carbon- and nitrogen-containing molecules accumulated on its surface. And these primitive precursors to life might be brought even further towards life's door if liquid water makes an occasional appearance which Lunine believes it may well do. Studies of Titan so far have indicated enough evidence for both temporal and spatial variability, two signatures required for the presence of organic molecules.
The Arecibo observations were made possible by the 1997 upgrade of the telescope's 305-meter (1,000 feet) diameter dish, which has greatly increased the sensitivity of what was already the world's most powerful radar system. The observatory is managed by the National Astronomy and Ionosphere Center (NAIC), based at Cornell in Ithaca, N.Y., which has been operating the huge telescope for the NSF since 1971.
Campbell, who is associate director of NAIC as well as a Cornell professor of astronomy, notes that for more than two decades astronomers have speculated that the interaction of the sun's ultraviolet radiation with methane in Titan's upper atmosphere -- photochemical reactions similar to those that cause urban smog -- could have resulted in large amounts of liquid and solid hydrocarbons raining onto Titan's frigid surface (minus 290 degrees Fahrenheit, or minus 179 degrees Celsius).
|Huygens parachutes onto Titan. ESA's Huygens probe descends through Titan's mysterious atmosphere to unveil the hidden surface (artist's impression) Credit: ESA
Campbell explains that radar signals would specularly reflect -- or glint -- from liquid surfaces on Titan, similar to sunlight glinting off the ocean. Although Titan's underlying surface is thought to be water ice, the complex chemistry in the upper atmosphere might have resulted in the icy surface being at least partly covered in liquid ethane and methane and solid hydrocarbons, says Campbell.
One class of the solid hydrocarbons, often referred to as Titan tholins (from the Greek word, muddy), was artificially created in a campus laboratory by a team led by the late Cornell astronomer Carl Sagan. When scientists analyze the building blocks of tholins by pyrrolysis, splitting up the tholins using plasma, scientists find a rich array of biomolecular building blocks such as pyrroles, pyrazines, pyridines and pyrimidines. All of these molecules have played an important role in the evolution of life.
Titan, which is about 50 percent larger than the Earth's moon, is the only satellite in the solar system with a dense atmosphere. This atmosphere is transparent to radio/radar waves and partially transparent at short infrared wavelengths but is opaque at visible wavelengths. Titan's atmosphere, ten times as massive as Earth's, is primarily nitrogen laced with such poisonous substances as methane and ethane. Titan is thickly veiled by a dense hydrocarbon haze that forms in the high stratosphere as atmospheric methane is destroyed by sunlight. The haze is much thicker than Earth's worst city smog. It was impenetrable to cameras aboard the Pioneer and Voyager spacecraft that flew by the Saturn system in the late 1970s and early 1980s.
The by-products of methane molecules destroyed in the sun's ultraviolet light react with other molecules in Titan's atmosphere, forming organic droplets and particulates that fall onto the moon's surface, blanketing the icy bedrock and forming lakes and oceans.
The observations were made in November and December of both 2001 and 2002. The radar signal takes 2.25 hours to travel to Titan and back. The Arecibo radar operates at a 13-centimeter wavelength (2,380 megahertz), and the transmitted power is close to one megawatt (the equivalent of about 1,000 microwave ovens). Both the Arecibo telescope and the NSF's new 100-meter Robert C. Byrd Green Bank Telescope were used to receive the extremely weak radar echoes.
Scientists would like a better idea of how optically thick Titan's haze is, and how bright or dark its surface will be, to calculate camera exposure times. In addition, scientists are fine tuning their questions as they plan the Cassini observations.
Next summer, NASA's Cassini spacecraft, launched in 1997, is scheduled to go into orbit around Saturn and its moons for four years. The piggybacking Huygens probe is scheduled to plunge into the hazy Titan atmosphere and land on the moon's surface. The Huygens probe is geared primarily towards sampling the atmosphere. The probe is equipped to take measurements and record images for up to a half an hour on the surface. But the probe has no legs, so when it sets down on Titan's surface its orientation will be random. And its landing may not be by a site bearing organics.
On Campbell's team for the Arecibo radar observations of Titan were Gregory Black, the University of Virginia; Lynn Carter, Cornell graduate student; and Steven Ostro, Jet Propulsion Laboratory. The Arecibo Observatory part of NAIC which is operated by Cornell University under a cooperative agreement with the NSF. NASA provides partial support for Arecibo's planetary radar program. The Robert C. Byrd Green Bank Telescope is part of the National Radio Astronomy Observatory, an NSF supported institution operated under cooperative agreement by Associated Universities Inc.
Related Web Pages
When Eden Comes to Titan
Chronology of a Scientific Safari
Long, Strange Trips
Titan's Icy Bedrock
Saturn-- JPL Cassini Main Page
Alien Landers: Extreme Explorers Hall of Fame
Titan: Biological Birthplace?
Solar System Bodies: Titan (NASA JPL)
The Probe Mission (NASA JPL)
Why Titan? (ESA)