The Cassini spacecraft studying Saturn and its satellites made the first of four planned Titan flybys Tuesday in a search for subsurface oceans.
The potential of a subsurface
ocean, possibly beneath a sheet of methane-rich ice, factors greatly for
astronomers hoping to pin down how Titan replenishes the methane in its
atmosphere.
"Methane is only around for a short
time geologically," said Gabriel Tobie, a researcher with the University of
Nantes in France. "The timescale is somewhere between 10 and 100 million
years."
In a study detailed in the March 2
issue of the journal Nature, Tobie and his colleagues found that Titan's
methane is likely locked in ice covering an ocean of water and ammonia, Tobie told
SPACE.com.
Methane makes up about two percent
of Titan's thick,
predominantly nitrogen-rich atmosphere, and is apparently replenished over
time through outgassing since it is destroyed by sunlight, he added.
Methane hunt
Before
Europe's Huygens probe landed
on Titan last year, astronomers believed the moon's methane resided in a liquid
hydrocarbon ocean. Huygens' images
of the surface found no signs of an ocean, but did suggest that liquid
methane once etched gullies across Titan.
"Finding the subsurface ocean will
not give us direct evidence of the outgassing of methane," Tobie said, adding
that a subsurface ocean covered in methane-rich ice is predicted by his study.
"But it will help us put constraints on our model."
Models developed by Tobie and his
colleagues suggest Titan's methane stemmed from three primary outgassing events
during the moon's 4.5-billion-year evolution.
The latest event occurred about 500
million years ago due convection within its ice crust, according to the study.
Convection in Titan's core also prompted a methane event about two billion
years ago, with the first outgassing occurring just after the moon formed, the
study suggests.
Methane locked within ice on Titan
could be freed by a cryovolcanic - or ice volcano - event, such as that
suggested in a 2005
study, Tobie said.
Data from the Cassini orbiter's future Titan flybys, redoubled modeling efforts, and comparison between the two
will be vital for astronomers studying the moon's thick atmosphere.
Shape shifting Titan
Cassini is using radio waves to make
detailed measurements of the moon's gravitational field, which astronomers hope
will help identify whether Titan sports an internal ocean covered in crust of
ice.
Arvydas Kliore,
team lead for Cassini's Radio Science Experiment,
told SPACE.com that scientists are
use slight differences in the probe's radio signals to Earth to make precise
measurements of Titan's shape and gravitational field. The measurements are
taken at different points in Titan's orbit around Saturn - from its most
distant to its closest - to see how much the moon's shape flexes over time, he
added.
"The shape would change more [with a
subsurface ocean] than if it's just a solid body," Kliore
said of Titan.
Cassini took its first two gravity
readings during the Titan flyby, the 11th of the mission, which ended on
Feb. 28. Additional measurements will be collected during Cassini's
22nd, 33rd and 38th Titan flybys - the last of
which is set for 2008 - to round out the experiment, Kliore
said.
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