Scientists Explain Puzzling Lake Asymmetry on Titan


http://www.jpl.nasa.gov/news/news.cfm?release=2009-180

Scientists Explain Puzzling Lake Asymmetry on Titan
Jet Propulsion Laboratory
November 30, 2009

PASADENA, Calif. -- Researchers at the California Institute of
Technology, NASA's Jet Propulsion Laboratory, and other institutions
suggest that the eccentricity of Saturn's orbit around the sun may be
responsible for the unusually uneven distribution of lakes over the
northern and southern polar regions of the planet's largest moon, Titan.
A paper describing the theory appears in the Nov. 29 advance online
edition of Nature Geoscience.

Saturn's oblong orbit around the sun exposes different parts of Titan to
different amounts of sunlight, which affect cycles of precipitation and
evaporation in those areas. Similar variations in Earth's orbit also
drive long-term ice-age cycles on our planet.

As revealed by Synthetic Aperture Radar imaging data from NASA's Cassini
spacecraft, liquid methane and ethane lakes in Titan's northern high
latitudes cover 20 times more area than lakes in the southern high
latitudes. The Cassini data also show there are significantly more
partially filled and now-empty lakes in the north. (In the radar data,
smooth features -- like the surfaces of lakes -- appear as dark areas,
while rougher features -- such as the bottom of an empty lake-appear
bright.) The asymmetry is not likely to be a statistical fluke because
of the large amount of data collected by Cassini in its five years
surveying Saturn and its moons.

Scientists initially considered the idea that "there is something
inherently different about the northern polar region versus the south in
terms of topography, such that liquid rains, drains or infiltrates the
ground more in one hemisphere," said Oded Aharonson of Caltech, lead
author of the Nature Geoscience paper.

However, Aharonson notes that there are no substantial known differences
between the north and south regions to support this possibility.
Alternatively, the mechanism responsible for this regional dichotomy may
be seasonal. One year on Titan lasts 29.5 Earth years. Every 15 Earth
years, the seasons of Titan reverse, so that it becomes summer in one
hemisphere and winter in the other. According to this seasonal variation
hypothesis, methane rainfall and evaporation vary in different seasons
-- recently filling lakes in the north while drying lakes in the south.

The problem with this idea, Aharonson said, is that it accounts for
decreases of about one meter per year in the depths of lakes in the
summer hemisphere. But Titan's lakes are a few hundred meters deep on
average, and wouldn't drain (or fill) in just 15 years. In addition,
seasonal variation can't account for the disparity between the
hemispheres in the number of empty lakes. The north polar region has
roughly three times as many dried-up lake basins as the south and seven
times as many partially filled ones.

"How do you move the hole in the ground?" Aharonson asked. "The seasonal
mechanism may be responsible for part of the global transport of liquid
methane, but it's not the whole story." A more plausible explanation,
say Aharonson and his colleagues, is related to the eccentricity of the
orbit of Saturn -- and hence of Titan, its satellite -- around the sun.

Like Earth and other planets, Saturn's orbit is not perfectly circular,
but is instead somewhat elliptical and oblique. Because of this, during
its southern summer, Titan is about 12 percent closer to the sun than
during the northern summer. As a result, northern summers are long and
subdued; southern summers are short and intense.

"We propose that, in this orbital configuration, the difference between
evaporation and precipitation is not equal in opposite seasons, which
means there is a net transport of methane from south to north," said
Aharonson. This imbalance would lead to an accumulation of methane --
and hence the formation of many more lakes -- in the northern hemisphere.

This situation is only true right now, however. Over very long time
scales of tens of thousands of years, Saturn's orbital parameters vary,
at times causing Titan to be closer to the sun during its northern
summer and farther away in southern summers, and producing a reverse in
the net transport of methane. This should lead to a buildup of
hydrocarbon -- and an abundance of lakes -- in the southern hemisphere.

"Like Earth, Titan has tens-of-thousands-of-year variations in climate
driven by orbital motions," Aharonson said. On Earth, these variations,
known as Milankovitch cycles, are linked to changes in solar radiation,
which affect global redistribution of water in the form of glaciers, and
are believed to be responsible for ice-age cycles. "On Titan, there are
long-term climate cycles in the global movement of methane that make
lakes and carve lake basins. In both cases we find a record of the
process embedded in the geology," he added.

"We may have found an example of long-term climate change, analogous to
Milankovitch climate cycles on Earth, on another object in the solar
system," he said.

The paper's co-authors are Caltech graduate student Alexander G. Hayes;
Jonathan I. Lunine, Lunar and Planetary Laboratory, Tucson, Ariz.; Ralph
D. Lorenz, Applied Physics Laboratory at the Johns Hopkins University,
Laurel, Md.; Michael D. Allison, NASA Goddard Institute for Space
Studies, New York; and Charles Elachi, director of JPL. The work was
partially funded by the Cassini Project.

For more information about the Cassini-Huygens mission, visit:
http://www.nasa.gov/cassini or http://saturn.jpl.nasa.gov/index.cfm. The
Cassini-Huygens mission is a cooperative project of NASA, the European
Space Agency and the Italian Space Agency. JPL, a division of the
California Institute of Technology in Pasadena, manages the mission for
NASA's Science Mission Directorate in Washington, D.C.

Jia-Rui C. Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.
jia-rui.c.cook@xxxxxxxxxxxx

Stephen Cole 202-657-2194
Headquarters, Washington
stephen.e.cole@xxxxxxxx

2009-180

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