on a different note:
New Evidence of Liquid Methane on Saturn's Moon
By JOHN NOBLE WILFORD
Published: January 3, 2007
As scientists predicted but have had a hard time proving, the
surface of Titan, Saturn's largest moon, appears to be dotted with
an abundance of lakes of liquid methane. The lakes are more
intriguing evidence of the active phenomena at play on the only moon
in the solar system that has a dense atmosphere.
The Lakes of Titan (Nature)
The discovery, reported today by an international team of
researchers, was made by a radar survey of Titan's high northern
latitudes by the Cassini spacecraft, which has been orbiting Saturn
and its retinue of satellites since July 2004. One of the mission's
major objectives is the investigation of Titan's environment,
thought to be similar to conditions on the primordial Earth.
In a detailed description of the find scheduled for publication
Thursday in the journal Nature, the scientists said the radar
imaging system detected more than 75 dark patches in the landscape
toward Titan's northern polar region. The patches, they said,
indicated smooth surfaces in an otherwise rugged topography,
suggesting lake beds either partly dry or filled with liquid.
These smooth surfaces, more or less circular and ranging in diameter
from 2 to 40 miles, are associated with channels that appear to have
been formed by flowing liquids, presumably tributaries to the lakes.
Methane exists in Titan's atmosphere and, in the extreme cold of
high latitudes, is expected to rain on the surface and be present as
liquids in subsurface reservoirs.
The discovery team concluded that the radar images, made on a close
pass of the moon in July, "provide definitive evidence for the
presence of lakes on the surface of Titan."
When the spacecraft conducted its first radar search above 70
degrees north latitude, Ellen R. Stofan, leader of the team, said in
an interview, "We saw a huge swath of the surface just covered with
lakes, like Minnesota."
Dr. Stofan is a planetary scientist at Proxemy Research, a
contractor in Virginia to the National Aeronautics and Space
Administration, and at University College London. Other team members
include researchers at the Jet Propulsion Laboratory in Pasadena,
Calif., which directs the Cassini mission; the University of
Arizona, and several European institutions.
In an accompanying article in the journal, Christophe Sotin, a
planetary scientist at the University of Nantes in France who was
not involved in the research, wrote that the lakes and other mission
findings were strong evidence that "methane on Titan plays the role
of water on Earth." It is the second most abundant component of
Titan's atmosphere, after nitrogen.
The discovery, Dr. Sotin concluded, adds to the "weight of evidence"
that processes on and beneath Titan's surface must be similar to
those dominating the early evolution of any Earth-like planet.
"As far as we know, there is only one planetary body that displays
more dynamism than Titan," he wrote. "Its name is Earth."
Finding large bodies of liquid methane and probably ethane on Titan,
in lakes or perhaps vast seas, had long been hypothesized, based on
telescope observations of the moon's smoggy methane-rich atmosphere
and by the two Voyager spacecraft that passed close some 30 years
ago. But the Cassini remote-sensing instruments had failed to detect
an ocean, though they and the European Space Agency's Huygens lander
did pick up traces of the channels where liquids had apparently
flowed across the surface.
On Cassini's previous encounters with Titan, the smog-penetrating
radar found no smaller liquid bodies, until it observed the northern
latitudes from 70 to 83 degrees. At present, it is winter in Titan's
north. Methane is one of the few molecules to exist as a liquid in
such cold conditions, scientists said, and is the most plausible
explanation for the smooth, dark surfaces in the radar images.
Dr. Stofan said that the lake depressions could be volcanic craters
or sinkholes; very few craters gouged out by meteorite impacts have
been detected on Titan. Their roughly circular shapes and steep-
sided rims are typical of volcanic depressions. Surface collapses,
as in sinkholes, could fill the lakes with liquids from interior
chambers, like aquifers on Earth.
In both cases, scientists said, methane vapor condensing and
precipitating out of the cold Titan sky was probably a significant
source of the liquid in the lakes.
Jonathan I. Lunine of the University of Arizona, another member of
the discovery team, described in an interview what the lakes
probably look like. The methane liquid would be transparent, enough
to see the dark hydrocarbon sediments on the floor of shallow lakes.
The liquid would be less viscous than water, perhaps like gasoline.
Overhead, aerosols in the upper atmosphere presumably cast a dim
orange light on the lake. In the dark of winter, one would need a
flashlight to walk the shore.
The Cassini spacecraft has completed half of its planned mission at
Saturn. Twenty-two more fly-bys of Titan are scheduled, a few of
which may afford opportunities to find more lakes and observe
changes in their liquid levels over time.
Officials of the project are currently considering an extension of
the mission, if the spacecraft remains healthy. Then it should be
possible to broaden the radar survey, perhaps looking for similar
lakes in the high latitudes of the southern hemisphere.
http://www.nytimes.com/2007/01/03/science/03cnd-titan.html?
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