March 20, 2014
Cassini Radar Data Shows Calm Seas On Saturnian Moon Titan
Lawrence LeBlond for redOrbit.com - Your Universe Online
Several presentations this past week at the Lunar and Planetary Science Conference (LPSC) talked about the possibility of waves rippling across the seas of Saturn's largest moon Titan. Data taken from NASA/ESA/ASI’s Cassini mission revealed some pixilation in the sea of Punga Mare that led researchers to assume rippling was occurring on the liquid hydrocarbon lake.Some new research, published online in Geophysical Research Letters, offers up some different opinions about one of the moon’s other bodies of water, Ligeia Mare, the second largest sea on Saturn’s neighboring satellite.
New radar measurements of Ligeia Mare offers planetary scientists insights into the weather patterns and landscape composition of Titan. The measurements were made in 2013 by the Cassini orbiter and reveal that the sea possesses a “mirror-like smoothness, possibly due to lack of winds.”
"If you could look out on this sea, it would be really still. It would just be a totally glassy surface," said study lead author Howard Zebker, professor of geophysics and of electrical engineering at Stanford University.
His team’s findings also indicate that the solid terrain surrounding the sea is made of solid organic material and not frozen water, as suggested by some researchers.
Despite being a moon, Titan’s atmosphere is very planet-like and has large seas made of methane and ethane. Ligeia Mare, the moon’s second-largest lake, is about 56,400 square miles in area – about 20 percent larger than Lake Superior here on Earth.
"Titan is the best analog that we have in the solar system to a body like the Earth because it is the only other body that we know of that has a complex cycle of solid, liquid, and gas constituents," Zebker said in a statement.
The thick cloud cover of Titan makes it difficult to obtain clear optical images of the surface, so scientists rely on radar, which is able to peer through the clouds and image the ground.
To obtain a radar picture of Ligeia Mare, the Cassini spacecraft bounced radio waves off the sea’s surface and then analyzed the returning echo. The strength of the echo determined how much wave action was occurring on the surface of the sea.
Zebker used sunlight’s reflection on an earthen lake as an example to understand what was happening on Ligeia Mare.
"If the lake were really flat, it would act as a perfect mirror and you would have an extremely bright image of the sun," he said. "But if you ruffle up the surface of the sea, the light gets scattered in a lot of directions, and the reflection would be much dimmer. We did the same thing with radar on Titan."
The radar data picked up from Cassini indicates that Ligeia Mare has an eerily still surface, at least at the time of the measurements it did.
"Cassini's radar sensitivity in this experiment is one millimeter, so that means if there are waves on Ligeia Mare, they're smaller than one millimeter. That's really, really smooth," Zebker said.
Zebker said the calmness observed on the sea could be explained by a lack of winds blowing across that particular region of the moon when Cassini flew by and made its measurements. Another possibility is that a thin layer of some material could be suppressing wave action.
"For example, on Earth, if you put oil on top of a sea, you suppress a lot of small waves," Zebker said.
Cassini also measured microwave radiation emitted by the materials that make up Titan’s surface, said the team. By analyzing these measurements and accounting for factors such as temperature and pressure, Zebker’s team confirmed that the terrain around Ligeia Mare is composed of solid organic material, likely the same methane and ethane that makes up the sea.
"Like water on Earth, methane on Titan can exists as a solid, a liquid, and a gas all at once," Zebker said, noting how Titan’s similarities to our own planet make it a good model for learning about Earth’s early evolution.
"Titan is different in the details from Earth, but because there is global circulation happening, the big picture is the same," he added. "Seeing something in two very different environments could help reveal the overall guiding principles for the evolution of planetary bodies, and help explain why Earth developed life and Titan didn't."