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.

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. 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.

The 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

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). 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, was artificially created in a
campus laboratory by a team led by the late Cornell astronomer Carl

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

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.

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.

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.