Are We Just ‘Lucky’ to See Activity on Enceladus?

by Nancy Atkinson on January 18, 2010

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Caption: Geysers on Enceladus. Credit: NASA, JPL, Space Science Institute

One of the most exciting but unexpected discoveries of the Cassini mission is seeing the activity taking place on Saturn’s small moon Enceladus. Between the active geysers, the unusual “tiger stripes” and the surprisingly young surface near the moon’s south pole, Enceladus has surprised scientists with almost all the images and data the gathered by the spacecraft. But is the moon always active, or are we just in the right place at the right time, lucky to be catching it during an active phase? A recent paper outlines a model in which the kind of geologic eruptions now visible on Enceladus only occur every billion years or so.

“Cassini appears to have caught Enceladus in the middle of a burp,” said Francis Nimmo, a planetary scientist at the University of California Santa Cruz. “These tumultuous periods are rare and Cassini happens to have been watching the moon during one of these special epochs.”

Nimmo and co-author Craig O’Neill of Macquarie University in Sydney, Australia propose that blobs of warm ice that periodically rise to the surface and churn the icy crust on Saturn’s moon Enceladus explain the quirky heat behavior and intriguing surface of the moon’s south polar region.

The most interesting features by far in the south polar region of Enceladus are the fissures known as “tiger stripes” that spray water vapor and other particles out from the moon. While Nimmo and O’Neill’s model doesn’t link the churning and resurfacing directly to the formation of fissures and jets, it does fill in some of the blanks in the region’s history.

Enceladus. Credit: NASA/JPL/Space Science Institute

“This episodic model helps to solve one of the most perplexing mysteries of Enceladus,” said Bob Pappalardo, Cassini project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif., of the research done by his colleagues. “Why is the south polar surface so young? How could this amount of heat be pumped out at the moon’s south pole? This idea assembles the pieces of the puzzle.”

But not everyone is convinced this model answers all the questions about Enceladus. Carolyn Porco, who leads the imaging team for Cassini said via Twitter regarding this paper, “Beware! Several different models out there say different things.”

About four years ago, Cassini’s composite infrared spectrometer instrument detected a heat flow in the south polar region of at least 6 gigawatts, the equivalent of at least a dozen electric power plants. This is at least three times as much heat as an average region of Earth of similar area would produce, despite Enceladus’ small size. The region was also later found by Cassini’s ion and neutral mass spectrometer instrument to be swiftly expelling argon, which comes from rocks decaying radioactively and has a well-known rate of decay.

Calculations told scientists it would be impossible for Enceladus to have continually produced heat and gas at this rate. Tidal movement – the pull and push from Saturn as Enceladus moves around the planet – cannot explain the release of so much energy.

The surface ages of different regions of Enceladus also show great diversity. Heavily cratered plains in the northern part of the moon appear to be as old as 4.2 billion years, while a region near the equator known as Sarandib Planitia is between 170 million and 3.7 billion years old. The south polar area, however, appears to be less than 100 million years old, possibly as young as 500,000 years.

O’Neill had originally developed the model for the convection of Earth’s crust. For the model of Enceladus, which has a surface completely covered in cold ice that is fractured by the tug of Saturn’s gravitational pull, the scientists stiffened up the crust. They picked a strength somewhere between that of the malleable tectonic plates on Earth and the rigid plates of Venus, which are so strong, it appears they never get sucked down into the interior.

These drawings depict explanations for the source of intense heat that has been measured coming from Enceladus' south polar region. Credit: NASA/JPL


Their model showed that heat building up from the interior of Enceladus could be released in episodic bubbles of warm, light ice rising to the surface, akin to the rising blobs of heated wax in a lava lamp. The rise of the warm bubbles would send cold, heavier ice down into the interior. (Warm is, of course, relative. Nimmo said the bubbles are probably just below freezing, which is 273 degrees Kelvin or 32 degrees Fahrenheit, whereas the surface is a frigid 80 degrees Kelvin or -316 degrees Fahrenheit.)

The model fits the activity on Enceladus when the churning and resurfacing periods are assumed to last about 10 million years, and the quiet periods, when the surface ice is undisturbed, last about 100 million to two billion years. Their model suggests the active periods have occurred only 1 to 10 percent of the time that Enceladus has existed and have recycled 10 to 40 percent of the surface. The active area around Enceladus’s south pole is about 10 percent of its surface.

Source: JPL

About 

Nancy Atkinson is Universe Today's Senior Editor. She also is the host of the NASA Lunar Science Institute podcast and works with Astronomy Cast. Nancy is also a NASA/JPL Solar System Ambassador.

Jon Hanford January 18, 2010 at 7:03 PM

This ‘we got lucky’ hypothesis seems a little premature at this stage. Did we also get lucky at Triton and Io, both known to exhibit active cryovolcanism? I realize the individual cases differ in many respects, but this is too good to be true….times three!

I think I’ll echo Carolyn Porco’s sentiment and wait and see.

Astrofiend January 18, 2010 at 9:31 PM

“Beware! Several different models out there say different things.”

Different models saying different things?! That’s absurd!

IVAN3MAN January 18, 2010 at 10:51 PM

At the penultimate paragraph:

(Warm is, of course, relative. Nimmo said the bubbles are probably just below freezing, which is 273 degrees Kelvin or 32 degrees Farenheit [sic], whereas the surface is a frigid 80 degrees Kelvin or -316 degrees Farenheit [sic].)

According to Wikipedia — Kelvin — the 13th General Conference on Weights and Measures (CGPM), in 1967–1968, changed the name of the Kelvin scale to simply “kelvin” (symbol K). The omission of “degree” indicates that it is not relative to an arbitrary reference point like the Celsius and Fahrenheit scales, but rather an absolute unit of measure which can be manipulated algebraically. Also, when reference is made to the unit kelvin (either a specific temperature or a temperature interval), kelvin is always spelled with a lowercase k unless it is the first word in a sentence.

Furthermore, regarding spelling, that should be Fahrenheit, not “Farenheit”. ;-)

Torbjorn Larsson OM January 18, 2010 at 11:32 PM

Funny though, I could swear I just read an article of Porco’s from 2008 where she lays out the same type of model. Seems she is on top of her world. So to speak.

The “coincidence card” is always a nerve-wrecker to play. What I really would like to know though is where that tantalizing layer of liquid water is a persistent feature of the model, or if it too goes away between periods. [AFAIU it is needed to arrive at the amount of heat generation observed, in much the same way that water on Earth helps tectonic processes in and out of cracks.]

IVAN3MAN, that is the good type of spell help. (If one can put such labels on it.)

It’s several kK on the Sun surface, not KK. I’ve seen the US spelling MG, is that mg or Mg? (Milligram or mega-gram? Or even “mega Gauss”?) The neat thing with SI is that it is context independent, so in science and technology more robust against error than haphazard spelling.

Ironically, it is the other way around in general language. It is rather robust against spelling errors, but not qualitative errors such as errors in units. (But we have had that discussion.)

Yesterday I had occasion to write to JPL for once, though. They, rather confusingly, in a posted news item referred to a mass spectrometer on the upcoming Curiosity mission as a “quadruple mass spectrometer”. Which at first glance looks really safe, backups on backups.

But even safer would be to send a “quadrupole mass spectrometer”, which works. Proof that spell checkers are still stupid, especially when you need them most! (No offense, I hope. :-o)

Paul Eaton-Jones January 19, 2010 at 12:43 AM

I’m always wary when someone says, ” aren’t we lucky to here at JUST the right time to see this/that event”. Something happens perhaps just once every billion years and we turn up at exactly right on time? Smacks to much of the Anthropic Principle to me.

ND January 19, 2010 at 7:32 AM

What about all those billion other 1/billionyears events we’ve missed? I think someone up there really doesn’t like us and is being passive aggressive.

gopher65 January 19, 2010 at 7:42 AM

Paul Eaton-Jones: It might happen once every 100 million to billion years, but it appears to last about 10 million. So the probability that we’d see this happening right now is between 1 and 10 percent.

That’s not bad, honestly. There are probably hundreds of rare events in the solar system that only occur 1% of the time. We’re bound to be experiencing a few of them at any given moment.

cipater January 19, 2010 at 1:18 PM

If we happened to exist and to have instruments observing Enceladus at just the right time, just imagine all the other wacky stuff that we’ve missed and will miss, and are missing even now!

solrey January 19, 2010 at 8:15 PM

Evidence For Enceladus Link To Saturn Ionosphere: Does The Plume Have An Auroral Footprint?

http://adsabs.harvard.edu/abs/2009DPS….41.6408R

It is increasingly well accepted that, despite its diminutive size, the tiny icy moon Enceladus is the dominant source of water group neutrals and charged particles throughout Saturn’s magnetosphere through the copious gas and dust emanations from its South pole. During two recent Cassini flybys the spacecraft plasma instruments were oriented such that they looked along a magnetic flux tube nominally connecting the Enceladus plume to Saturn’s ionosphere. Two of the remarkable discoveries from these observational campaigns were, 1) high energy (10s-100s of keV) field aligned ion beams propagating from Saturn toward the plume and 2) lower energy field aligned electron beams which were observed to ‘flicker’ in energy from 10s of eV to several 100 eV. Initial speculation was that this is evidence of an Alfven wing type interaction, such as exists at Io due to significant mass loading in the wake of the moon. It was subsequently realised that the magnetic field signature is not consistent with this simple picture, leading us to speculate that there exists a more filamentary Birkeland current system with the observed variability linked to the highly dynamic and variable nature of the Enceladus outgassing. Ions could be accelerated by wave activity or field-aligned potential drops just above the ionosphere, but we have yet to ascertain if either is sufficient to explain the observed very high energy ion beams. Additionally we will show that similar phenomena exist near the L-value of Enceladus, but away from the moon – implying the existence of a significant extended Enceladus plasma torus.

Ion beams, electron beams, two way particle flow, Birkeland currents and a plasma torus. Yep, no electrical activity here, move along folks. /sarc

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