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New Models Suggest Titan Lakes Are Explosion Craters Using radar
data from NASA's Cassini spacecraft, recently published research presents a new
scenario to explain why some methane-filled lakes on Saturn's moon Titan are
surrounded by steep rims that reach hundreds of feet high. The models suggests that
explosions of warming nitrogen created basins in the moon's crust.
Titan is
the only planetary body in our solar system other than Earth known to have
stable liquid on its surface. But instead of water raining down from clouds and
filling lakes and seas as on Earth, on Titan it's methane and ethane - hydrocarbons
that we think of as gases but that behave as liquids in Titan's frigid climate.
Most
existing models that lay out the origin of Titan's
lakes show liquid methane dissolving the moon's bedrock of ice
and solid organic compounds, carving reservoirs that fill with the liquid. This
may be the origin of a type of lake on Titan that has sharp boundaries. On
Earth, bodies of water that formed similarly, by dissolving surrounding limestone,
are known as karstic lakes.
The new,
alternative models for some of the smaller lakes (tens of miles across) turns
that theory upside down: It proposes pockets of liquid nitrogen in Titan's
crust warmed, turning into explosive gas that blew out craters, which then filled
with liquid methane. The new theory explains why some of the smaller lakes near
Titan's north pole, like Winnipeg Lacus, appear in radar imaging to have very steep
rims that tower above sea level - rims difficult to explain with the karstic
model.
The
radar data were gathered by the Cassini Saturn Orbiter - a mission managed by
NASA's Jet Propulsion Laboratory in Pasadena, California - during its last close
flyby of Titan, as the spacecraft prepared for its final plunge into Saturn's
atmosphere two years ago. An international team of scientists led by Giuseppe
Mitri of Italy's G. d'Annunzio University became convinced that the karstic
model didn't jibe with what they saw in these new images.
"The
rim goes up, and the karst process works in the opposite way," Mitri said.
"We were not finding any explanation that fit with a karstic lake basin.
In reality, the morphology was more consistent with an explosion crater, where
the rim is formed by the ejected material from the crater interior. It's
totally a different process."
The
work, published Sept. 9 in Nature Geosciences, meshes with other Titan climate
models showing the moon may be warm compared to how it was in earlier Titan
"ice ages."
Over
the last half-billion or billion years on Titan, methane in its atmosphere has
acted as a greenhouse gas, keeping the moon relatively warm - although still
cold by Earth standards. Scientists have long believed that the moon has gone
through epochs of cooling and warming, as methane is depleted by solar-driven
chemistry and then resupplied.
In the
colder periods, nitrogen dominated the atmosphere, raining down and cycling
through the icy crust to collect in pools just below the surface, said Cassini
scientist and study co-author Jonathan Lunine of Cornell University in Ithaca,
New York.
"These
lakes with steep edges, ramparts and raised rims would be a signpost of periods
in Titan's history when there was liquid nitrogen on the surface and in the
crust," he noted. Even localized warming would have been enough to turn
the liquid nitrogen into vapor, cause it to expand quickly and blow out a
crater.
"This
is a completely different explanation for the steep rims around those small
lakes, which has been a tremendous puzzle," said Cassini Project Scientist
Linda Spilker of JPL. "As scientists continue to mine the treasure trove
of Cassini data, we'll keep putting more and more pieces of the puzzle together.
Over the next decades, we will come to understand the Saturn system better and
better."
The Cassini-Huygens mission is a cooperative project of NASA,
ESA (the European Space Agency) and the Italian Space Agency. JPL, a division
of Caltech in Pasadena, manages the mission for NASA's Science Mission
Directorate in Washington. JPL designed, developed and assembled the Cassini
orbiter. The radar instrument was built by JPL and the Italian Space Agency,
working with team members from the U.S. and several European countries.
More information about Cassini can be found here:
https://solarsystem.nasa.gov/cassini
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