On the secrets of Saturn’s strangest moon and the story of the evolution of minerals on earth

The preamble to the latest This weeks finds in Mathematical Physics (before the post goes on to talk about Pontryagin duality and such stuff) has a discussion on some fascinating aspects of celestial mechanics and mineralogy:

Last week we visited the geysers of Saturn’s moon Enceladus. Afterwards, George Musser pointed me to an article on this subject by Carolyn Porco, leader of the imaging team for the Cassini-Huygens mission – the team that’s been taking the photos I showed you. It’s a great article, leading up to some intriguing theories about what powers these geysers:

1) Carolyn Porco, Enceladus: secrets of Saturn’s strangest moon, Scientific American, November 2008, available at http://www.sciam.com/article.cfm?id=enceladus-secrets

And it’s free online! – at least for now. I’ve criticized the Scientific American before here, but if they keep coming out with articles like this, I’ll change my tune. For one thing, it’s well-written:

There is obviously a tale writ on the countenance of this little moon that tells of dramatic events in its past, but its present, we were about to find out, is more stunning by far. In its excursion over the outskirts of the south polar terrain, Cassini’s dust analyzer picked up tiny particles, apparently coming from the region of the tiger stripes. Two other instruments detected water vapor, and one of them delivered the signature of carbon dioxide, nitrogen and methane. Cas­si­ni had passed through a tenuous cloud.What is more, the thermal infrared imager sensed elevated temperatures along the fractures – possibly as high as 180 kelvins, well above the 70 kelvins that would be expected from simple heating by sunlight. These locales pump out an extraordinary 60 watts per square meter, many times more than the 2.5 watts per square meter of heat arising from Yellowstone’s geothermal area. And smaller patches of surface, beyond the resolving power of the infrared instrument, could be even hotter.

For another, it tackles a fascinating mystery. Where does all this power come from? The geysers near the south pole of Enceladus emit about 6 gigawatts of heat. Enceladus is too small to have that much radioactive heating at its core – only about 0.3 gigawatts, probably. The rest must come from tidal heating. This happens when stuff sloshes back and forth in a changing gravitional field: friction converts this motion to heat.

So, what causes tides on Enceladus? It may be important that Enceladus has a 2:1 resonance with Dione: it orbits Saturn twice for each orbit of that larger moon. This sort of resonance is known to cause tidal heating. For example, in “week269“, I showed you how Jupiter’s moon Io is locked in resonances with Europa and Ganymede. The resulting tidal heat powers its mighty volcanos.

Unfortunately, the resonance with Dione doesn’t seem powerful enough to produce the heat we see on Enceladus. Unless something funny is going on, there should only be 0.1 gigawatts of tidal heating – not nearly enough! At least that’s what Porco estimated in 2006:

But now I’d like to zoom in closer to home and quickly tell the history of the Earth, focusing on an aspect you may never have thought about. You see, Kevin Kelly recently pointed me to this fascinating paper on “mineral evolution”:

5) Robert M. Hazen, Dominic Papineau, Wouter Bleeker, Robert T. Downs, John M. Ferry, Timothy J. McCoy, Dmitri A. Sverjensky and Henxiong Yang, Mineral evolution, American Mineralogist 91 (2008), 1693-1720.

Ever since it was formed, the number of different minerals on Earth has kept going up — and ever since life ran wild, it’s soared! Some examples are obvious: seashells become limestone, which gets squashed into marble. Some are less so: for example, there wasn’t much clay before the advent of life.

Must-read of the day (probably this week!)

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