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Enceladus: Heat Output a Surprise

What do we make of the ‘tiger stripes’? The intriguing terrain in the south polar region of Saturn’s moon Enceladus is geologically active and one of the most fascinating finds of the Cassini mission. The ‘stripes’ are actually four trenches, more or less in parallel, that stretch 130 kilometers, each about 2 kilometers wide. What Cassini showed us was that geysers of ice particles and water vapor are being ejected into space from the interior of the moon, setting off astrobiological speculations that elevated Enceladus to a new and deeply intriguing status.

Is there liquid water under the surface? You could make that case based on recent work showing that some ice particles ejected from the moon are rich in salt, a sign that they may be frozen droplets from a saltwater ocean in contact with Enceladus’ mineral-rich core. Interesting place indeed — we’ve got a serious possibility of liquid water and an energy source in the form of tidal effects from Enceladus’ orbit as it changes in relation to Dione and Saturn itself. We’ve also got observations of carbon-rich chemicals in the plumes streaming from the moon.

Given all that, new observations of the heat output from the south polar region of Enceladus are provocative. An analysis recently published in the Journal of Geophysical Research draws on data from Cassini’s composite infrared spectrometer to show that heat output from the ‘tiger stripes’ region is much greater than previously thought. In fact, the internal heat-generated power is thought to be about 15.8 gigawatts, more than an order of magnitude higher than predicted. Enceladus’ south polar region is giving us 2.6 times the power output of all the hot springs in the Yellowstone region, using mechanisms that are not yet understood.

Image: This graphic, using data from NASA’s Cassini spacecraft, shows how the south polar terrain of Saturn’s moon Enceladus emits much more power than scientists had originally predicted. Data from Cassini’s composite infrared spectrometer indicate that the south polar terrain of Enceladus has an internal heat-generated power of about 15.8 gigawatts. Credit: NASA/JPL/SWRI/SSI.

A 2007 study of the internal heat of Enceladus, which was based largely on tidal interactions between the moon and Dione, had found no more than 1.1 gigawatts averaged over the long term, with heating from natural radioactivity within Enceladus adding another 0.3 gigawatts. The new work covers the entire south polar region, raising speculation that the unusually high output is due to periodic surges in the tidal heating as resonance effects change over time. But the paper acknowledges “The mechanism capable of producing such a high endogenic power remains a mystery and challenges the current models of proposed heat production.” Whatever the case, the higher heat flow finding makes the presence of liquid water that much more likely.

The paper is Howett et al., “High heat flow from Enceladus’ south polar region measured using 10–600 cm−1 Cassini/CIRS data,” Journal of Geophysical Research 116 , E03003 (2011). Abstract available. This JPL news feature summarizes the paper’s findings.

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Comments on this entry are closed.

  • connie March 9, 2011, 16:00

    Paul-
    I know this is probably very elemental to many of your readers, but I know zip about how tidal forces work except that the ocean level rises and lowers. Please excuse my ignoranace, but I have some buring questions: What happens inside a planet or moon that would cause heating from tidal forces? There must be a lot of friction going on, but friction against what kind of molecules/atoms? Do the tidal forces we have on earth also cause heat?

    thanks

  • Adam March 9, 2011, 16:24

    Got to wonder if the heat source isn’t a downed alien spacecraft’s antimatter reactor slowly losing confinement and trickling out some propellant. Or one of Louise Riofrio’s primordial black holes spitting out Hawking radiation as it decays. A big ingot of enriched uranium.

  • Paul Gilster March 9, 2011, 21:18

    Adam: Well, it’s a good SF scenario anyway, particular the second of the two.

    Connie: I’ll let some of our resident astronomers pass along specifics, but in the case of a world like Enceladus, gravitational stresses caused by the moon’s interactions with Dione and Saturn itself should cause tidal distortions of the outer shell. An ocean under the surface would allow even greater distortions and thus increase the efficiency of the heating effect. Here’s the basic model as explained in an article in Astrobiology Magazine:

    This led scientists to consider the role of tidal heating as a way to keep Enceladus warm enough for liquid water to remain under its surface. Enceladus’ orbit around Saturn is slightly oval-shaped. As it travels around Saturn, Enceladus moves closer in and then farther away. When Enceladus is closer to Saturn, it feels a stronger gravitational pull from the planet than when it is farther away. Like gently squeezing a rubber ball slightly deforms its shape, the fluctuating gravitational tug on Enceladus causes it to flex slightly. The flexing, called gravitational tidal forcing, generates heat from friction deep within Enceladus.

    The gravitational tides also produce stress that cracks the surface ice in certain regions, like the south pole, and may be reworking those cracks daily. Tidal stress can pull these cracks open and closed while shearing them back and forth. As they open and close, the sides of the south polar cracks move as much as a few feet, and they slide against each other by up to a few feet as well. This movement also generates friction, which (like vigorously rubbing your hands together) releases extra heat at the surface at locations that should be predictable with our understanding of tidal stress.

    Hope this helps. It’s a fascinating topic.

  • Rob Henry March 10, 2011, 17:53

    Excuse my ignorance, but exactly how significant is it that one moderately sized moon of Saturn is releasing heat 10X quicker than its long term average. I know that in the very different case of the large Laplace resonance moons of Jupiter, significant cyclical variations in tidal heating output are expected in some models. I also know that, according to Showman and Malhotra, when Ganymede first joined the resonance its tidal heating could have been several hundred times higher than this average. Are the two cases so dissimilar that I am raising a red herring?

    Excuse my further ignorance, but I thought that the actual heat output of a resonant satellite also depended on how chewy or crunchy its core was. Surely, in the shifting, and much more complex resonances of Saturn’s moons, this is even more of an issue that it is on Jupiter. Perhaps the bit I am missing is that it is know to be virtually impossible for a world below a certain density or size to have a chewy core.

  • ljk March 10, 2011, 18:05

    Maybe it is an engine and it is starting up.

  • Tarmen March 12, 2011, 19:46

    There is too much heat there. I suspect that a large comet or a large asteroid has hit it fairly recently, 1000 or 10 000 yrs. It somehow ‘ignited’ a mildly radioactive core.
    Either that or the world was once warm enough for very stubborn heat-emitting life to establish itself. Or “Maybe it is an engine and it is starting up.”

  • ericSECT March 15, 2011, 5:09

    Connie;
    Enceladus is in a slightly oval orbit, deformed from a perfect circle and perturbed by the gravity of other nearby moons. She is tidally locked to Saturn and keeps the same face pointing at Saturn… which would work if the orbit was a perfect circle. But because it is an oval, the entire moon stresses as it rocks back and forth through the orbit, this stress induces heat.

    But why does it all seem to come out at the south pole tiger stripes?

    Is it possible that there is some electrical inductance heating going on? As Enceladus cuts across Saturn’s magnetic field, perhaps some kind of conductor (salt water?) has a current induced, with resistance to current flow generating heat? If so could be a heat source for many icy moons.

  • Connie March 15, 2011, 17:33

    Paul and ericSECT –

    Thanks to both of you for your answers to my questions! This really is a facsinating subject. Being a lover of sci-fi, I was amused and intrigued by Adam’s comments about an alien space ship. I honestly believe anything can be possible – be it something we consider plausible because we have experiencess and observation that form the foundations for our assumptions (such as an eccentric orbit with strong tidal forces) or something we consider fiction because it is not within our set of observations/experiences (such as the core of a wrecked alien star ship melting down). If we can think of it, then it probably will happen some day, some way. The possibilities are endless, hence the intrigue and lust for space travel. I digress. I loved your responses and now I’m off to do more reading!

  • Louise March 15, 2011, 18:27

    Thanks Adam! A tiny Black Hole in Enceladus’ core would make the moon’s centre a maelstrom of charged particles. Like Earth and most other solar system objects, the core would rotate anti clockwise as seen from the North. This would create a dipolar magnetic field with the “positive” pole in the South, as within Earth.

    Charged particles would spiral toward the poles like the twin jets of a Black Hole. Negatively charged electrons would spiral Northward and be absorbed by Enceladus’ interior. Heavier, positively charged protons would penetrate these layers to warm the South Pole. A little Black Hole would easily account for all Enceladus’ interior heat.