"Long before it's in the papers"
September 16, 2015


Ocean underlies whole surface of Saturn moon, study concludes

Sept. 16, 2015
Courtesy of NASA
and World Science staff

A glob­al ocean lies be­neath the icy crust of Sat­urn’s moon En­cel­a­dus, ac­cord­ing to new re­search us­ing da­ta from NASA’s Cas­si­ni space probe.

Re­search­ers con­clud­ed that the moon’s dis­tinc­tive, tiny wob­ble must be due to a body of liq­uid sit­ting be­neath the out­er ice shell, all the way around.

Diagram of layers believed to make up Saturn's moon Enceladus. Thickness of layers is not shown to scale. (Credits: NASA/JPL-Caltech)

This ocean lay­er pro­vides a squishi­ness that en­ables the moon’s icy crust to wob­ble some­what in­de­pend­ently of its hard co­re, they ar­gue; it al­so feeds a spray of va­por, icy par­t­i­cles and sim­ple or­gan­ic mo­le­cules that burst from cracks near the south pole.

The re­search is pre­sented in a pa­per pub­lished on­line this week in the jour­nal Ic­a­rus.

Pre­vi­ous anal­y­sis of Cas­si­ni da­ta sug­gested a lens-shaped body of wa­ter might un­der­lie the south po­lar re­gion. 

But gra­vity da­ta col­lect­ed dur­ing the space­craft’s sev­er­al close pas­ses over that ar­ea sug­gested the sea might be glob­al. The new re­sult­s—derived us­ing an in­de­pend­ent line of ev­i­dence based on Cas­si­ni’s im­ages—con­firm this, the in­ves­ti­ga­tors ar­gued.

“This was a hard prob­lem that re­quired years of ob­serva­t­ions, and cal­cula­t­ions in­volv­ing a di­verse col­lec­tion of dis­ci­plines, but we are con­fi­dent we fi­nally got it right,” said Pe­ter Thom­as, a Cas­si­ni im­ag­ing team mem­ber at Cor­nell Un­ivers­ity in Ith­a­ca, New York, and lead au­thor of the pa­per.

Cas­si­ni sci­en­tists an­a­lyzed more than sev­en years’ worth of im­ages of the ge­o­log­ic­ally ac­tive moon, snapped by the space­craft, which has been or­bit­ing Sat­urn since mid-2004. They mapped the fea­tures on En­cel­a­dus—mostly crater­s—across hun­dreds of im­ages, in or­der to meas­ure changes in the moon’s rota­t­ion very ex­act­ly.

The re­sult showed that En­cel­a­dus wob­bles very slightly as it or­bits Sat­urn. Be­cause the icy moon is­n’t per­fectly round—and be­cause it goes slightly faster and slower dur­ing dif­fer­ent parts of its or­bit around Sat­urn—the gi­ant ringed plan­et subtly rocks En­cel­a­dus back and forth as it spins.

The team plugged their meas­urement of the wob­ble, called a libra­t­ion, in­to dif­fer­ent mod­els for how En­cel­a­dus might be ar­ranged on the in­side, in­clud­ing ones in which the moon was fro­zen all the way to the co­re.

But if that were the case, “the co­re would pro­vide so much dead weight the wob­ble would be far smaller,” said Mat­thew Tis­careno, a Cas­si­ni par­ti­ci­pat­ing sci­ent­ist at the SETI In­sti­tute, Moun­tain View, Cal­i­for­nia, and a co-au­thor of the pa­per. “This proves that there must be a glob­al lay­er of liq­uid sep­a­rat­ing the sur­face from the co­re.”

The mys­tery is why the ocean is­n’t fro­zen, he added. Thom­as and his col­leagues sug­gest a few ideas, in­clud­ing the sur­pris­ing pos­si­bil­ity that tid­al forc­es due to Sat­urn’s gra­vity could be gen­er­at­ing much more heat with­in En­cel­a­dus than pre­vi­ously thought.

Sci­en­tists first de­tected signs of the moon’s icy plume in early 2005, and fol­lowed up with a se­ries of disco­veries about the ma­te­ri­al gush­ing from warm frac­tures near its south pole. They an­nounced strong ev­i­dence for a re­gional sea in 2014, and this year, they shared re­sults that sug­gest hy­dro­ther­mal, or heating, ac­ti­vity is tak­ing place on the ocean floor.

Cas­si­ni is sched­uled to make a close fly­by of En­cel­a­dus on Oct. 28, in the mis­sion’s deepest-ever dive through the moon’s ac­tive plume of icy ma­te­ri­al. The space­craft will pass just 30 miles (49 kilo­me­ters) above the moon.

The Cas­si­ni-Huygens mis­sion is a proj­ect of NASA, the Eu­ro­pe­an Space Agen­cy and the Ital­ian Space Agen­cy. NASA’s Je­t Pro­pul­sion Lab­o­r­a­to­ry in Pas­a­de­na, Cal­if., man­ages the mis­sion for the agen­cy’s Sci­ence Mis­sion Di­rec­to­rate in Wash­ing­ton.

* * *

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A global ocean lies beneath the icy crust of Saturn’s moon Enceladus, according to new research using data from NASA’s Cassini space probe. Researchers concluded that the moon’s distinctive, very slight wobble must be due to a body of liquid sitting beneath its outer ice shell, all the way around. This ocean layer provides a squishiness that enables the moon’s icy crust to wobble somewhat independently of its hard core, they argue; it also feeds a spray of vapor, icy particles and simple organic molecules that burst from cracks near the moon’s south pole. The research is presented in a paper published online this week in the journal Icarus. Previous analysis of Cassini data suggested the presence of a lens-shaped body of water, or sea, underlying the moon’s south polar region. But gravity data collected during the spacecraft’s several close passes over that area suggested the sea might be global. The new results—derived using an independent line of evidence based on Cassini’s images—confirm this, the investigators argued. “This was a hard problem that required years of observations, and calculations involving a diverse collection of disciplines, but we are confident we finally got it right,” said Peter Thomas, a Cassini imaging team member at Cornell University in Ithaca, New York, and lead author of the paper. Cassini scientists analyzed more than seven years’ worth of images of the geologically active moon, snapped by the spacecraft, which has been orbiting Saturn since mid-2004. They mapped the features on Enceladus—mostly craters—across hundreds of images, in order to measure changes in the moon’s rotation very exactly. The result showed that Enceladus wobbles very slightly as it orbits Saturn. Because the icy moon isn’t perfectly round—and because it goes slightly faster and slower during different parts of its orbit around Saturn—the giant ringed planet subtly rocks Enceladus back and forth as it spins. The team plugged their measurement of the wobble, called a libration, into different models for how Enceladus might be arranged on the inside, including ones in which the moon was frozen from surface to core. “If the surface and core were rigidly connected, the core would provide so much dead weight the wobble would be far smaller than we observe it to be,” said Matthew Tiscareno, a Cassini participating scientist at the SETI Institute, Mountain View, California, and a co-author of the paper. “This proves that there must be a global layer of liquid separating the surface from the core.” The mystery is why the ocean isn’t frozen, he added. Thomas and his colleagues suggest a few ideas, including the surprising possibility that tidal forces due to Saturn’s gravity could be generating much more heat within Enceladus than previously thought. Scientists first detected signs of the moon’s icy plume in early 2005, and followed up with a series of discoveries about the material gushing from warm fractures near its south pole. They announced strong evidence for a regional sea in 2014, and more recently, in 2015, they shared results that suggest hydrothermal activity is taking place on the ocean floor. Cassini is scheduled to make a close flyby of Enceladus on Oct. 28, in the mission’s deepest-ever dive through the moon’s active plume of icy material. The spacecraft will pass just 30 miles (49 kilometers) above the moon. The Cassini-Huygens mission is a project of NASA, the European Space Agency and the Italian Space Agency. NASA’s Jet Propulsion Laboratory in Pasadena, California, manages the mission for the agency’s Science Mission Directorate in Washington.