[meteorite-list] Saturn's Moon Titan Has A Liquid Surface Lake

[Sterling K. Webb]

First confirmed existence of a lake of any liquid on the
surface of any planet or body in the solar system -- the
envelope, please! -- goes to Titan. The lake is liquid 
ethane.

There are seas, lake, and rivers seen in the region of
the North Pole, and the prevailing suspicion (but no 
proof) has been that they are methane, but now ethane
may seem more likely.

This lake is located near the South Pole, is named 
Ontario Lactus, or Lake Ontario. It's about the same 
size as Earth's Lake Ontario; the existence of a Canada 
on its north shore has not been confirmed. 

No lakes or seas have been seen outside the polar regions.
Evidence suggests that "rain" falls only at the Poles, where
the lakes are. Oddly, cold as Titan is, it may be that it is
too "hot" for rain and bodies of water except at the Poles
and that the hazy atmosphere is, in effect, cold "steam."

Full text of the press release is below.

Sterling K. Webb
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http://www.sciencedaily.com/releases/2008/07/080730140726.htm

Scientists have confirmed that at least one body in our 
solar system, other than Earth, has a surface liquid lake. 
Using an instrument on NASA's Cassini orbiter, they 
discovered that a lake-like feature in the south polar 
region of Saturn's moon, Titan, is truly wet. The lake is 
about 235 kilometers, or 150 miles, long.

The visual and infrared mapping spectrometer, or VIMS, an 
instrument run from The University Arizona, identifies the 
chemical composition of objects by the way matter reflects 
light.

When VIMS observed the lake, named Ontario Lacus, it 
detected ethane, a simple hydrocarbon that Titan experts 
have long been searching for. The ethane is in liquid 
solution with methane, nitrogen and other low-molecular 
weight hydrocarbons.

"This is the first observation that really pins down that 
Titan has a surface lake filled with liquid," VIMS 
principal investigator and professor Robert H. Brown of 
UA's Lunar and Planetary Laboratory said. Brown and his 
team report their results in the July 31 issue of the 
journal Nature.

"Detection of liquid ethane in Ontario Lacus confirms a 
long-held idea that lakes and seas filled with methane and 
ethane exist on Titan," said Larry Soderblom of the U.S. 
Geological Survey, Flagstaff, Ariz.

The fact that the VIMS could detect the spectral 
signatures of ethane on the moon's dimly lit surface while 
viewing at a highly slanted angle through Titan's thick 
atmosphere "raises expectations for exciting future lake 
discoveries by the infrared spectrometer," Soderblom, an 
interdisciplinary Cassini scientist, said.

The ubiquitous hydrocarbon haze in Titan's atmosphere 
hinders the view to Titan's surface. But there are 
transparent atmospheric "windows" at certain infrared 
light wavelengths through which Cassini's VIMS can see to 
the ground. VIMS observed Ontario Lacus on Cassini's 38th 
close flyby of Titan in December 2007.

The lake is roughly 20,000 square kilometers, or 7,800 
square miles, just slightly larger than North America's 
Lake Ontario, Brown said. Infrared spectroscopy doesn't 
tell the researchers how deep the lake is, other than it 
must be at least a centimeter or two, or about 
three-quarters of an inch, deep.

"We know the lake is liquid because it reflects 
essentially no light at 5-micron wavelengths," Brown said. 
"It was hard for us to accept the fact that the feature 
was so black when we first saw it. More than 99.9 percent 
of the light that reaches the lake never gets out again. 
For it to be that dark, the surface has to be extremely 
quiescent, mirror smooth. No naturally produced solid 
could be that smooth."

VIMS observations at 2-micron wavelengths shows the lake 
holds ethane. The scientists saw the specific signature of 
ethane as a dip at the precise wavelength that ethane 
absorbs infrared light. Tiny ethane particles almost as 
fine as cigarette smoke are apparently filtering out of 
the atmosphere and into the lake, Brown said.

Ethane is a simple hydrocarbon produced when ultraviolet 
light from the sun breaks up its parent molecule, methane, 
in Titan's methane-rich, mostly nitrogen atmosphere.

Before the Cassini mission, several scientists thought 
that Titan would be awash in global oceans of ethane and 
other light hydrocarbons, the byproducts of photolysis, or 
the action of ultraviolet light on methane over 4.5 
billion years of solar system history. But 40 close flybys 
of Titan by the Cassini spacecraft show no such oceans 
exist.

The observations also suggest the lake is evaporating. The 
lake is ringed by a dark beach, where the black lake 
merges with the bright shoreline.

"We can see there's a shelf, a beach, that is being 
exposed as the lake evaporates," Brown said.

That the beach is darker than the shoreline could mean 
that the "sand" on the beach is wet with organics, or it 
could be covered with a thin layer of liquid organics, he 
said.

The VIMS measurements rule out the presence of water ice, 
ammonia, ammonia hydrate and carbon dioxide in Ontario 
Lacus.

The VIMS result gives researchers new insight on Titan's 
chemistry and weather dynamics.

Titan, which is one-and-a-half times the size of Earth's 
moon and bigger than either Mercury or Pluto, is one of 
the most fascinating bodies in the solar system when it 
comes to exploring environments that may give rise to 
life.

Cassini cameras and radar and the UA-built camera aboard 
the European Space Agency's Huygens probe that landed on 
Titan in January 2005 have shown that methane saturates 
and drains from Titan's atmosphere, creating river-like 
and lake-like features on the surface. Just as water 
cycles through the hydrologic regime on Earth, methane 
cycles through a methanological cycle on Titan.