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Radar shows mirror-smooth sea on Saturn moon

March 20, 2014
Courtesy of Stanford University
and World Science staff

Mea­sure­ments by NASA’s Cas­si­ni space­craft show a mir­ror-smooth sur­face on a large sea of Ti­tan, a moon of Sat­urn, sci­en­tists say.

The ra­dar read­ings, tak­en last year, show the sur­face of Li­ge­ia Ma­re, Ti­tan’s sec­ond larg­est sea.

An ar­ti­fi­cial­ly col­ored im­age of the sur­face of Ti­tan made us­ing ra­dar mea­sure­ments made by NA­SA's Cas­si­ni space­craft, show­ing Li­ge­ia Ma­re, Ti­tan's sec­ond larg­est sea. (Cred­it: How­ard Ze­bker)


“If you could look out on this sea, it would be really still. It would just be a to­tally glassy sur­face,” said How­ard Zeb­ker, a geo­phys­i­cist at Stan­ford Uni­vers­ity in Cal­i­for­nia who is the lead au­thor of a new study de­tail­ing the re­search.

That could be due to lack of winds at the time of the mea­sure­ments, though wind is cer­tainly pos­si­ble on Ti­tan since the moon has a thick at­mos­phere, he added. The find­ings, re­cently pub­lished on­line in Geo­phys­i­cal Re­search Let­ters, al­so sug­gest the sol­id ter­rain sur­round­ing the sea is made of sol­id or­gan­ic ma­te­ri­als.

Sat­urn’s sec­ond larg­est moon, Ti­tan has a thick, planet-like at­mos­phere and large seas made of meth­ane and eth­ane. Meas­ur­ing roughly 260 miles (420 km) by 217 miles (350 km), Li­ge­ia Ma­re is larg­er than Lake Su­pe­ri­or 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. Its thick cloud cov­er makes it hard to get clear sur­face im­ages, so sci­en­tists use ra­dar, which can see through the clouds.

This in­volved bounc­ing ra­di­o waves off the sea’s sur­face and then an­a­lyz­ing the ech­o. The strength of the re­flected sig­nal in­di­cat­ed how much wave ac­tion was hap­pen­ing. To un­der­stand why, Zeb­ker said, im­ag­ine sun­light re­flect­ing off of a lake. “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.”

He added that “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.”

Anoth­er pos­si­ble ex­plana­t­ion for the sea’s calm­ness, he said, is that a thin lay­er of some ma­te­ri­al is sup­press­ing wave ac­tion. “For ex­am­ple, on Earth, if you put oil on top of a sea, you sup­press a lot of small waves.”

Ti­tan’s si­m­i­lar­i­ties to Earth make it a good mod­el for our own plan­et’s early ev­o­lu­tion, Zeb­ker said. “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.”


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Measurements by NASA’s Cassini spacecraft show a mirror-smooth surface on a large sea of Titan, a moon of Saturn, scientists say. The radar readings, taken last year, show the surface of Ligeia Mare, Titan’s second largest sea. “If you could look out on this sea, it would be really still. It would just be a totally glassy surface,” said Howard Zebker, a geophysicist at Stanford University in California who is the lead author of a new study detailing the research. That could be due to lack of winds at the time of the measurements, though wind is certainly possible on Titan since the moon has a thick atmosphere, he added. The findings, recently published online in Geophysical Research Letters, also suggest the solid terrain surrounding the sea is made of solid organic materials. Saturn’s second largest moon, Titan has a thick, planet-like atmosphere and large seas made of methane and ethane. Measuring roughly 260 miles (420 km) by 217 miles (350 km), Ligeia Mare is larger than Lake Superior 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. Its thick cloud cover makes it hard to get clear surface images, so scientists use radar, which can see through the clouds. This involved bouncing radio waves off the sea’s surface and then analyzing the echo. The strength of the reflected signal indicated how much wave action was happening. To understand why, Zebker said, imagine sunlight reflecting off of a lake. “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.” He added that “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.” Another possible explanation for the sea’s calmness, he said, is that a thin layer of some material is suppressing wave action. “For example, on Earth, if you put oil on top of a sea, you suppress a lot of small waves.” Titan’s similarities to Earth make it a good model for our own planet’s early evolution, Zebker said. “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.”