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.
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.
- Unmasking Titan: Volcano Spotted on Saturn's Smoggy Moon
- Titan's Methane Not Produced by Life, Scientists Say
- SPACE.com Special Report: Cassini-Huygens at Saturn and Titan