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Compelling evidence found that Saturn moon hides vast ocean, scientists say

June 22, 2011
Courtesy of the University of Colorado at Boulder
and World Science staff

Sam­ples of icy spray shoot­ing from Sat­urn’s moon En­cel­a­dus show the strongest ev­i­dence yet that its sur­face con­ceals a vast, salt­wa­ter ocean un­der­ground, ac­cord­ing to a stu­dy.

The find­ing was made dur­ing the Cas­sini-Huy­gens mis­sion to Sat­urn, a col­la­bora­t­ion of NASA, the Eu­ro­pe­an Space Agen­cy and the Ital­ian Space Agen­cy. Launched in 1997, the mis­sion’s space­craft ar­rived at the Sat­urn sys­tem in 2004 and has been tour­ing the gi­ant ringed plan­et and its vast moon sys­tem ev­er since.

Enceladus geysers. (Cre­dit: Cas­sini Im­ag­ing Team, SSI, JPL, ESA, NA­SA)
“En­cel­a­dus is a ti­ny, icy moon” in a re­mote re­gion “where no liq­uid wa­ter was ex­pected to ex­ist be­cause of its large dis­tance from the sun,” said Ni­co­las Al­to­belli, the Eu­ro­pe­an Space Agen­cy’s proj­ect sci­ent­ist for the Cas­sini-Huy­gens mis­sion. So the find­ing is “a cru­cial new piece of ev­i­dence show­ing that en­vi­ron­men­tal con­di­tions fa­vor­a­ble to the emer­gence of life may be sus­tain­a­ble on icy bod­ies or­bit­ing gas gi­ant plan­ets.”

The craft re­vealed foun­tains of wa­ter va­por and ti­ny ice grains in­to space em­a­nat­ing from En­cel­a­dus, one of Sat­urn’s 19 known moons, in 2005. The plumes came from so-called “ti­ger stripe” fea­tures at the moon’s south pole, be­lieved to be breaks in the sur­face, and ap­par­ently fur­nished the ma­te­ri­al for Sat­urn’s faint “E ring” which traces the moon’s or­bit. Three to four years af­ter the gey­sers’ dis­cov­ery, an on­board in­stru­ment called the Cos­mic Dust An­a­lys­er meas­ured the make­up of freshly ejected grains. The icy flecks hit the de­tec­tor at up to 11 miles per sec­ond, va­porizing them so their con­stituents could be sep­a­rat­ed and an­a­lyzed.

The study found the ice grains fur­ther out from En­cel­a­dus are small and mostly ice-poor, closely match­ing the com­po­si­tion of the “E Ring.” But nearer the moon, larg­er, salt-rich grains dom­i­nate.

“There cur­rently is no plau­si­ble way to pro­duce a steady out­flow of salt-rich grains from sol­id ice across all the ti­ger stripes oth­er than the salt wa­ter un­der En­cel­a­dus’ icy sur­face,” said Frank Post­berg of the Un­ivers­ity of Hei­del­berg in Ger­ma­ny, lead au­thor of the stu­dy, be­ing pub­lished in the jour­nal Na­ture on June 23.

“The study in­di­cates that ‘salt-poor’ par­t­i­cles are be­ing ejected from the un­der­ground ocean through cracks in the moon at a much high­er speed than the larg­er, salt-rich par­t­i­cles,” added co-au­thor Sascha Kempf of the Un­ivers­ity of Col­o­rad­o at Boul­der.

Ac­cord­ing to the re­search­ers, the salt-rich par­t­i­cles have an “o­cean-like” com­po­si­tion that in­di­cates most or all of the ex­pelled ice comes from the eva­pora­t­ion of liq­uid salt wa­ter rath­er than from the icy sur­face. When salt wa­ter freezes slowly the salt is “squeezed out,” leav­ing pure wa­ter ice be­hind. If the plumes were com­ing from the sur­face ice, there should be very lit­tle salt in them, but there’s a lot, ac­cord­ing to the re­search team.

The scientists be­lieve that pe­rhaps 50 miles be­neath the sur­face crust is a lay­er of wa­ter kept liq­uid by gravita­t­ionally driv­en tid­al forc­es cre­at­ed by Sat­urn and sev­eral neigh­bor­ing moons, and by heat from the de­cay of ra­di­o­ac­t­ive el­e­ments. Ac­cord­ing to the sci­ent­ists, roughly 440 pounds of wa­ter va­por is lost ev­ery sec­ond from the plumes. Cal­cula­t­ions in­di­cate the liq­uid ocean must have a siz­a­ble eva­porating sur­face or it would easily freeze over, halt­ing the spray. “This study im­plies that nearly all of the mat­ter in the En­cel­a­dus plumes orig­i­nates from a salt­wa­ter ocean that has a very large eva­porating sur­face,” said Kempf.

Salt in the rock dis­solves in­to the wa­ter, which ac­cu­mu­lates in a liq­uid ocean be­neath the icy crust, ac­cord­ing to the re­search­ers. When the crust cracks open, the res­er­voir is ex­posed to space. The pres­sure drop causes the liq­uid to eva­porate, with some of it “flash-freezing” in­to salty ice grains, which sub­se­quently cre­ates the plumes, the sci­ence team be­lieves.

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Samples of icy spray shooting from Saturn’s moon Enceladus show the strongest evidence yet that its surface conceals a vast, saltwater ocean underground, according to a study. The finding was made during the Cassini-Huygens mission to Saturn, a collaboration of NASA, the European Space Agency and the Italian Space Agency. Launched in 1997, the mission’s spacecraft arrived at the Saturn system in 2004 and has been touring the giant ringed planet and its vast moon system ever since. “Enceladus is a tiny, icy moon” in a remote region “where no liquid water was expected to exist because of its large distance from the sun,” said Nicolas Altobelli, the European Space Agency’s project scientist for the Cassini-Huygens mission. So the finding is “a crucial new piece of evidence showing that environmental conditions favorable to the emergence of life may be sustainable on icy bodies orbiting gas giant planets.” The craft revealed fountains of water vapor and tiny ice grains into space emanating from Enceladus, one of Saturn’s 19 known moons, in 2005. The plumes came from so-called “tiger stripe” features at the moon’s south pole, believed to be breaks in the surface, and apparently furnished the material for Saturn’s faint “E ring” which traces the moon’s orbit. Three to four years after the plumes’ discovery, an onboard instrument called the Cosmic Dust Analyser measured the makeup of freshly ejected grains. The icy particles hit the detector at up to 11 miles per second, vaporizing them so their constituents could be separated and analyzed. The study found the ice grains further out from Enceladus are small and mostly ice-poor, closely matching the composition of the “E Ring.” But nearer the moon, larger, salt-rich grains dominate. “There currently is no plausible way to produce a steady outflow of salt-rich grains from solid ice across all the tiger stripes other than the salt water under Enceladus’ icy surface,” said Frank Postberg of the University of Heidelberg in Germany, lead author of the study, being published in the journal Nature on June 23. “The study indicates that ‘salt-poor’ particles are being ejected from the underground ocean through cracks in the moon at a much higher speed than the larger, salt-rich particles,” added co-author Sascha Kempf of the University of Colorado at Boulder. According to the researchers, the salt-rich particles have an “ocean-like” composition that indicates most, if not all, of the expelled ice comes from the evaporation of liquid salt water rather than from the icy surface. When salt water freezes slowly the salt is “squeezed out,” leaving pure water ice behind. If the plumes were coming from the surface ice, there should be very little salt in them, which was not the case, according to the research team. The researchers believe that perhaps 50 miles beneath the surface crust is a layer of water kept liquid by gravitationally driven tidal forces created by Saturn and several neighboring moons, and by heat from the decay of radioactive elements. According to the scientists, roughly 440 pounds of water vapor is lost every second from the plumes. Calculations indicate the liquid ocean must have a sizable evaporating surface or it would easily freeze over, halting the formation of the plumes. “This study implies that nearly all of the matter in the Enceladus plumes originates from a saltwater ocean that has a very large evaporating surface,” said Kempf. Salt in the rock dissolves into the water, which accumulates in a liquid ocean beneath the icy crust, according to the researchers. When the outermost layer of the Enceladus crust cracks open, the reservoir is exposed to space. The drop in pressure causes the liquid to evaporate into a vapor, with some of it “flash-freezing” into salty ice grains, which subsequently creates the plumes, the science team believes.