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Emily LakdawallaMarch 17, 2006

LPSC: Wednesday afternoon: Cassini at Enceladus

So after those two rover talks I skipped over to the other large room to listen to what the Cassini science teams had to say about Enceladus. I had missed the first two talks, which covered the discovery of the atmosphere by the magnetometer team, and the fracture/tectonics patterns surrounding the south pole, which I had already read about in the Science paper. But I came in to check out John Spencer's talk on CIRS (here's the abstract). I'd talked to John when that Enceladus news broke last week, and he primarily covered the same material. There were a few interesting additional facts though.

While CIRS does see a noticeable correlation between their hottest temperature detections and the locations of tiger stripe cracks, they also have footprints that don't cross tiger stripes but still have significantly higher temperatures than the Enceladus background. In other words, not all the heat is coming from the cracks. John did a calculation to figure out exactly how much power is being radiated from the south pole, and found it to be a little more than 3 Gigawatts, or just a tiny fraction (6 · 10-5) of Io's. This amount of power "is plausible for tidal heating in the current orbit if the Q is very low." "Q" is the tidal dissipation, essentially a measure of how squishy the interior is, how efficiently it turns tidal energy into heat. Very stiff bodies have higher Q. But, he continued, "the puzzle has always been that Mimas, because it has a much more eccentric orbit, should have a tidal heating rate ten times Enceladus. So Enceladus is in a self-maintaining warm state; the only trick is to kick-start Enceladus into a warm state."

There were several comments on John's talk. Carolyn Porco stood up and said that there must be temperatures higher than he is detecting in order to support liquid water reservoirs close to the surface for geysers. Jeff Kargel stood up and pointed out that folded mountains on Earth require rheologically layered materials -- that is, stacked layers of materials that have different mechanical properties (some more stiff, some more prone to bending or faulting). He said "that kind of structure could make tidal heating much more effective." Bill McKinnon stood up and said "There is all too much hand-wringing about Mimas; the answer for Enceladus is tidal heating, period."

Next, Dennis Matson stood up and talked about the hydrothermal geochemistry of the geysers (here's the abstract). First he showed some models where he tried to figure out what initial conditions would be necessary to keep Enceladus hot down to the present time, and he concluded that Enceladus needed to form early, as early as Iapetus, in order to retain enough short-lived radioactive isotopes in its interior to get it up to an initially hot state. This model gave a compositionally stratified Enceladus with a molten interior. One interesting element of his models is that a body as small as Enceladus could have been porous initially, but heating it up can make that pore space collapse, shrinking the whole body.

If you're not sure what I mean by "models," building a geophysical model of a planet basically involves writing down a few differential equations that relate the bulk density, pressure, temperature, and viscosity of a body's materials and that can incorporate heat flow by either conduction or convection. These kinds of models can have multiple layers. They are usually defined on paper but then plugged into a computer that runs the equations forward in time to predict how all of those physical properties will evolve. I did a little of this kind of work in graduate school; it's actually surprisingly powerful at predicting the global-scale behavior of planet-sized bodies, if you choose your parameters right.

I should also add in here a conversation I had with Dennis at the poster session on Tuesday. I had told him I was looking forward to the Enceladus talks, and he told me, "Tell us what acetylene means on Enceladus and you win a prize." Apparently one of the instruments (I would guess INMS, I have to finish reading the Science papers to find out for sure) has detected acetylene, among other interesting things, in Enceladus' plume. Acetylene is C2H2, and those two carbons are joined by a triple bond. "Forming acetylene requires the breakdown of long-chain hydrocarbons or temperatures of 1,770 Kelvin, which you don't have," Dennis said. "So we think we must have catalytic chemistry down there, which could mean there's all kinds of interesting things we're not detecting" so far in the chemistry of Enceladus' geysers. Cool.

Next, there was a talk by Bob Pappalardo called "Diapir-induced reorientation of Enceladus." (here's the abstract.) Basically, Bob's thesis is that if you make a plume, or diapir, of hot material inside Enceladus through internal heating and convection, the density contrast between that plume and the rest of the moon could have caused the whole thing to reorient, putting the plume at the south pole. (These kinds of hot plumes are theorized to exist on Earth beneath Hawaii, Yellowstone, Iceland, etc.) This idea sounds a little crazy but it's pretty much accepted that the gigantic volcanic deposits of the Tharsis rise on Mars have caused that planet to reorient to put Tharsis at the equator; this is called "true polar wander." Tharsis is at the equator on Mars because there is extra mass there, and Mars' rotation is most stable with that mass anomaly located at its equator. By contrast, Bob outlined, a plume on Enceladus could produce a negative mass anomaly, which would be most rotationally stable sitting at one of the two poles. Bob considered both the possibilities of a silicate (that is, rock) diapir within Enceladus' core and an ice diapir within Enceladus' mantle. In his models he found that a silicate diapir was more efficient than an ice diapir in reorienting Enceladus, and that it was most efficient if a silicate diapir was coupled to an ice diapir. However, he pointed out an interesting problem: in order for a silicate diapir to reorient the whole of Enceladus, then there must be no global ocean, because if there was a global ocean then the solid icy crust of Enceladus would be totally mechanically decoupled from the rocky core, and the core could reorient completely independently of the crust. That's a pretty important prediction for the history of Enceladus. Bob pointed out some tests that could be done for reorientation. One is that the crust should show more craters on the leading side than the trailing side of the moon; if the crater patterns don't match that, then there's a good case for reorientation. Also, he said, there should be a large gravity anomaly at the pole, so "we urge the Cassini project to consider gravity-only tracking paths across the south pole" in the extended mission.

After that I took a break for a little while, then I moved on to Deep Impact. But right at this moment, I want to run back to check out some more of this morning's Hayabusa talks -- and then I'll have to run off to the airport. I'll probably write up some more stuff on the plane and post it tonight or tomorrow.

Read more: Enceladus, Cassini, Saturn's moons, conference report

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Emily Lakdawalla

Senior Editor and Planetary Evangelist for The Planetary Society
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