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March 24, 2008

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Distant moon may have hidden ocean, scientists say

March 20, 2008
Courtesy NASA
and World Science staff

NASA’s Cas­si­ni space­craft has found ev­i­dence of an un­der­ground ocean of wa­ter and am­mo­nia on Sat­urn’s moon Ti­tan, sci­en­tists say.

The find­ings, made us­ing ra­dar mea­sure­ments of Ti­tan’s rota­t­ion, will ap­pear in the March 21 is­sue of the re­search jour­nal Sci­ence. 

The  moon Titan floating nearby Saturn and its rings. (Courtesy NASA Cassini imaging Team) 
“With its or­gan­ic dunes, lakes, chan­nels and moun­tains, Ti­tan has one of the most var­ied, ac­tive and Earth-like sur­faces in the so­lar sys­tem,” said Ralph Lo­renz, lead au­thor of the pa­per and Cas­si­ni ra­dar sci­ent­ist at the Johns Hop­kins Ap­plied Phys­ics Lab­o­r­a­to­ry in Lau­rel, Md. 

“Now we see changes in the way Ti­tan ro­tates, giv­ing us a win­dow in­to Ti­tan’s in­te­ri­or.”

Mem­bers of the mis­sion’s sci­ence team col­lect­ed ra­dar da­ta from 19 passes over Ti­tan be­tween Oc­to­ber 2005 and May 2007. The ra­dar can de­tect the sur­face un­der Ti­tan’s ha­zy, methane-rich at­mos­phere. 

Re­search­ers mapped 50 rec­og­niz­a­ble land­marks based on early fly­bys. They then searched for these same lakes, canyons and moun­tains in the reams of da­ta from lat­er fly­bys. They found prom­i­nent fea­tures had shifted from their ex­pected po­si­tions by up to 19 miles (31 km). This sys­tem­at­ic dis­place­ment would be hard to ex­plain, re­search­ers ar­gued, un­less an in­ter­nal ocean sep­a­rat­ed the moon’s icy crust from its co­re, let­ting the crust move eas­i­ly. 

“We be­lieve that about 62 miles [100 km] be­neath the ice and or­gan­ic-rich sur­face is an in­ter­nal ocean of liq­uid wa­ter mixed with am­mo­nia,” said Bry­an Stiles of NASA’s Jet Pro­pul­sion Lab­o­r­a­to­ry in Pas­a­de­na, Ca­lif., a co-au­thor of the pa­per.

The study of Ti­tan is a ma­jor goal of the space agen­cy’s Cas­si­ni-Huy­gens mis­sion be­cause the moon may pre­serve, in deep-freeze, many of the chem­i­cal in­gre­di­ents of life on Earth. Ti­tan is the only moon in the so­lar sys­tem with a dense at­mos­phere—this is 1.5 times thicker than Earth’s. Ti­tan is the larg­est of Sat­urn’s moons, big­ger than the plan­et Mer­cu­ry.

A moon of Jupi­ter, Eu­ro­pa, is also thought to pos­sib­ly have a sub­surface ocean. 

On Titan, “the com­bina­t­ion of an or­gan­ic-rich en­vi­ron­ment and liq­uid wa­ter is very ap­peal­ing to as­tro­bi­ol­o­gists,” who study the pos­si­bil­ity of life in space, Lo­renz said. “Fur­ther study of Ti­tan’s rota­t­ion will let us un­der­stand the wa­tery in­te­ri­or bet­ter, and be­cause the spin of the crust and the winds in the at­mos­phere are linked, we might see sea­son­al varia­t­ion in the spin in the next few years.”

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NASA’s Cassini spacecraft has found evidence of an underground ocean of water and ammonia on Saturn’s moon Titan, scientists say. The findings, made using radar measurements of Titan’s rotation, will appear in the March 21 issue of the journal Science. “With its organic dunes, lakes, channels and mountains, Titan has one of the most varied, active and Earth-like surfaces in the solar system,” said Ralph Lorenz, lead author of the paper and Cassini radar scientist at the Johns Hopkins Applied Physics Laboratory in Laurel, Md. “Now we see changes in the way Titan rotates, giving us a window into Titan’s interior.” Members of the mission’s science team collected radar data from 19 passes over Titan between October 2005 and May 2007. The radar can detect the surface under Titan’s hazy, methane-rich atmosphere. Researchers mapped 50 recognizable landmarks based on early flybys. They then searched for these same lakes, canyons and mountains in the reams of data from later flybys. They found prominent features had shifted from their expected positions by up to 19 miles (31 km). This systematic displacement would be hard to explain, researchers argued, unless an internal ocean separated the moon’s icy crust from its core, letting the crust move easily. “We believe that about 62 miles (100 km) beneath the ice and organic-rich surface is an internal ocean of liquid water mixed with ammonia,” said Bryan Stiles of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. Stiles also is a contributing author to the paper. The study of Titan is a major goal of the space agency’s Cassini-Huygens mission because the moon may preserve, in deep-freeze, many of the chemical ingredients of life on Earth. Titan is the only moon in the solar system with a dense atmosphere—it’s 1.5 times thicker than Earth’s. Titan is the largest of Saturn’s moons, bigger than the planet Mercury. “The combination of an organic-rich environment and liquid water is very appealing to astrobiologists,” those who research the possibility of life on other worlds, Lorenz said. “Further study of Titan’s rotation will let us understand the watery interior better, and because the spin of the crust and the winds in the atmosphere are linked, we might see seasonal variation in the spin in the next few years.”