Liquid Lake Confirmed on Titan

July 31, 2008 / Written by: NASA Press Release

NASA scientists have concluded that at least one
of the large lakes observed on Saturn’s moon Titan contains liquid
hydrocarbons, and have positively identified the presence of ethane.
This makes Titan the only body in our solar system beyond Earth known
to have liquid on its surface.

Scientists made the discovery using data from an instrument aboard the
Cassini spacecraft. The instrument identified chemically different
materials based on the way they absorb and reflect infrared light.
Before Cassini, scientists thought Titan would have global oceans of
methane, ethane and other light hydrocarbons. More than 40 close
flybys of Titan by Cassini show no such global oceans exist, but
hundreds of dark lake-like features are present. Until now, it was
not known whether these features were liquid or simply dark, solid
 material.

“This is the first observation that really pins down that Titan has a
surface lake filled with liquid,” said Bob Brown of the University of
Arizona, Tucson. Brown is the team leader of Cassini’s visual and
mapping instrument. The results will be published in the July 31
issue of the journal Nature.

Ethane and several other simple hydrocarbons have been identified in
Titan’s atmosphere, which consists of 95 percent nitrogen, with
methane making up the other 5 percent. Ethane and other hydrocarbons
are products from atmospheric chemistry caused by the breakdown of
methane by sunlight.

Some of the hydrocarbons react further and form fine aerosol
particles. All of these things in Titan’s atmosphere make detecting
and identifying materials on the surface difficult, because these
particles form a ubiquitous hydrocarbon haze that hinders the view.
Liquid ethane was identified using a technique that removed the
interference from the atmospheric hydrocarbons.

The visual and mapping instrument observed a lake, Ontario Lacus, in
Titan’s south polar region during a close Cassini flyby in December
2007. The lake is roughly 7,800 square miles in area, slightly larger
than North America’s Lake Ontario.

“Detection of liquid ethane confirms a long-held idea that lakes and
seas filled with methane and ethane exist on Titan,” said Larry
Soderblom, a Cassini interdisciplinary scientist with the U.S.
Geological Survey in Flagstaff, Ariz. “The fact we could detect the
ethane spectral signatures of the lake even when it was so dimly
illuminated, and at a slanted viewing path through Titan’s
atmosphere, raises expectations for exciting future lake discoveries
by our instrument.”

The ethane is in a liquid solution with methane, other hydrocarbons
and nitrogen. At Titan’s surface temperatures, approximately 300
degrees Fahrenheit below zero, these substances can exist as both
liquid and gas. Titan shows overwhelming evidence of evaporation,
rain, and fluid-carved channels draining into what, in this case, is
a liquid hydrocarbon lake.

Earth has a hydrological cycle based on water and Titan has a cycle
based on methane. Scientists ruled out the presence of water ice,
ammonia, ammonia hydrate and carbon dioxide in Ontario Lacus. The
observations also suggest the lake is evaporating. It is ringed by a
dark beach, where the black lake merges with the bright shoreline.
Cassini also observed a shelf and beach being exposed as the lake
 evaporates.

“During the next few years, the vast array of lakes and seas on
Titan’s north pole mapped with Cassini’s radar instrument will emerge
from polar darkness into sunlight, giving the infrared instrument
rich opportunities to watch for seasonal changes of Titan’s lakes,”
Soderblom said.

Launched in Oct. 1997, Cassini’s 12 instruments have returned a daily
stream of data from Saturn’s system. The mission is a cooperative
project of NASA, the European Space Agency and the Italian Space
 Agency.

Source: [NASA Press Release]