Saturn's
smoggy moon Titan has hundreds of times more natural gas and other liquid
hydrocarbons than all the known oil and natural gas reserves on Earth,
scientists said today.
The
hydrocarbons rain
from the sky on the miserable moon, collecting in vast deposits that form
lakes and dunes. This much was known. But now the stuff has been quantified
using observations from NASA's Cassini spacecraft.
"Titan
is just covered in carbon-bearing material — it's a giant factory of organic
chemicals," said Ralph Lorenz, a Cassini radar team member from the Johns
Hopkins University Applied Physics Laboratory. "This vast carbon inventory
is an important window into the geology and climate history of Titan."
At minus
179 degrees Celsius (minus 290 degrees Fahrenheit), Titan would be an awful
place to live. Instead of water, liquid hydrocarbons in the form of methane and
ethane are present on the moon's surface, and tholins probably make up its
dunes. The term "tholins" was coined by Carl Sagan in 1979 to
describe the complex organic molecules at the heart of prebiotic chemistry.
Titan has
long been viewed as a place that might be somewhat like Earth just before
biology got going.
Cassini has
mapped about 20 percent of Titan's surface with radar. Several hundred lakes and
seas have been observed, with each of several dozen estimated to contain
more hydrocarbon liquid than Earth's oil and gas reserves, according to a NASA
statement. The dark dunes that run along the equator contain a volume of
organics several hundred times larger than Earth's coal reserves.
Proven
reserves of natural gas on Earth total 130 billion tons, enough to provide 300
times the amount of energy the entire United States uses annually for residential heating, cooling and
lighting, according to the release. Dozens of Titan's lakes individually have
the equivalent of at least this much energy in the form of methane
and ethane.
"This
global estimate is based mostly on views of the lakes in the northern polar
regions," Lorenz said. "We have assumed the south might be similar,
but we really don't yet know how much liquid is there."
Cassini's
radar has observed the south polar region only once, and only two small lakes
were visible.
The
findings are detailed in the Jan. 29 issue of the journal Geophysical Research
Letters.
Scientists
estimated Titan's lake depth by making some general assumptions based on lakes
on Earth. They took the average area and depth of lakes on Earth, taking into
account the nearby surroundings, like mountains. On Earth, the lake depth is
often 10 times less than the height of nearby terrain.
"We
also know that some lakes are more than 10 meters or so deep because they
appear literally pitch-black to the radar. If they were shallow we'd see the
bottom, and we don't," Lorenz said.
The
question of how much liquid is on the surface is an important one because
methane is a strong greenhouse gas on Titan as well as on Earth, but there is
much more of it on Titan. If all the observed liquid on Titan is methane, it
would only last a few million years, because as methane escapes into Titan's
atmosphere, it breaks down and escapes into space.
If the methane
were to run out, Titan could become much colder. Scientists believe that
methane might be supplied to the atmosphere by venting from the interior in cryovolcanic
eruptions. If so, the amount of methane, and the temperature on Titan, may have
fluctuated dramatically in Titan's past.
"We
are carbon-based life, and understanding how far along the chain of complexity
towards life that chemistry can go in an environment like Titan will be
important in understanding the origins of life throughout the universe,"
Lorenz said.
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