Frigid Future for Ocean in Saturn's Moon

The tide may be changing for the ocean suspected under theicy shell of Enceladus. Recent research has shown that this small moon ofSaturn does not produce enough heat in its present configuration to keep waterfrom freezing down to its core.

"There is no possible combination of parameters thatallow for a thermally stable ocean," said James Roberts of the University of California, Santa Cruz.

Roberts and his colleague Francis Nimmo, also from UCSC,calculated the tidal heating expected inside Enceladusfrom the uneven tugging of nearby Saturn.

In all the models studied, the moon could not sustain anocean for more than around 30 million years.

This could mean the ocean froze up long ago. Or perhaps morelikely, Enceladus generated a greater amount of heat at some point during thepast 30 million years by being in a more eccentric orbit than it is in now, theauthors claim in a recent article for the journal Icarus.

Thermal surprise

Scientists did not always think Enceladus had an ocean.

"Normally the smaller a planet or satellite is, thecolder it is," Roberts explained. "Since Enceladus is roughly thesize of Great Britain, everyone assumed it would be dull and boring."

But observations during several flybys of NASA's Cassinispacecraft in 2005 showed surprising thermal activity on the south pole.

Plumes of water vapor were detected eruptingout of the surface, and infrared measurements showed 5.8 gigawatts of heatemanating from several narrow ridges called "tigerstripes."

"It's a no-brainer that tidal heating is happening on Enceladus,"said William McKinnon of Washington University in Saint Louis. "I canconceive of no other explanation for the south polar thermal anomaly."

Tidal heating results from the elliptical orbit ofEnceladus. As the moon's distance to Saturn varies, the tidal pull from theplanet causes the moon to squeeze in and stretch out. The friction from thisstrain generates heat.

The phenomenon is well-known on the moons Io and Europa. Iois one of the most volcanically active bodies in the solar system thanks totidal heating from its orbit around Jupiter.

However, significant tidal heating on Enceladus is onlypossible if there is an ocean separating the icy shell from the solid siliconcore. Without that liquid lubricant, the moon would be too rigid to flex in andout.

And as a catch-22, the ocean will freeze without significanttidal heating. This is because the only other source of heat — radioactivedecay inside the core — is expected to be an insufficient 0.32 gigawatts.

Freeze down

Roberts and Nimmo were interested in probing how far downEnceladus' proposed ocean would likely be.

They tested various models, varying both the thickness ofthe icy shell and its deformation properties. They also looked at differentways that heat might flow through Enceladus.

Enceladus' tidalheating has been calculated before, but Roberts and Nimmo are the first toshow how the heating is distributed through the shell, with more at the polesthan the equator.

They were surprised to find that heat escapes Enceladusfaster than it is generated. Therefore, if Enceladus's orbital eccentricity hasalways been what it is today, the moon would have frozen completely solidbillions of years ago.

"And once it froze, tidal heating would have been shutoff forever," Roberts said.

Lunar anti-freeze

But in all likelihood, there is some kind of ocean onEnceladus, so something must have prevented Enceladus from fully freezing.

"There has to be an ocean to allow the flexing — that'sa well-founded conclusion," McKinnon said.

Although McKinnon thinks this new work is important, he doesnot think it covers all possible scenarios. For one, he believes the waterin the Enceladus ocean will be full of salts and maybe ammonia. Such amixture will freeze at a lower temperature than pure water.

McKinnon also said that the ocean itself will generate tidalheating by sloshing back and forth like the ocean tides do on our planet.

But even including these other elements, Enceladus may stillnot retain enough heat to allow for an ocean to exist today. This is whyRoberts and Nimmo consider the possibility that the moon has migrated from amore eccentric orbit in the past.

If Enceladus' orbit had been three times more eccentric thanit is now, the tidal heating would have been enough to keep an ocean. Assumingthis high eccentricity (high heating) phase ended within the last 30 millionyears, the moon would not yet have had enough time to freeze up. Theresearchers speculate that the moon's eccentricity and tidal heating may havefluctuated up and down many times.

The orbital history of Enceladus is not known, but migrationof satellites is not unheard of, McKinnon said.

Astrobiologists considering the suitability of Enceladus tolife may have to incorporate a dynamic ocean that has shrunk and expanded overmillion-year timescales in response to the moon's fluctuating eccentricity.

Scientists will not be able to confirm these predictionsanytime soon as a mission to penetrate Enceladus' icy depths would be extremelydifficult, Roberts said.

"The field is still going to be model-driven forawhile," he said.

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