The Cassini proble, which has been orbiting Saturn for just over four years now, is scheduled to pass over Enceladus (en-sell'-a-dus) in 16 1/2 from the time of this writing. Not that the precise timing makes a whole lot of difference.
Enceladus presents an interesting problem. One of the bands in Saturn's rings appeared to be associated with this moon, but it wasn't clear how. In an earlier pass, Cassini managed to image jets of water ice spraying from some structures (referred to as tiger stripes) near the moon's south polar regions. This phenomenon can be thought of as water volcanism- here on earth, we tend to think in terms of silicate volcanism- most lavas are silicate-based, and cool to form silicate minerals. In rare cases- for exampe in the rift valley of eastern Africa- there are carbonate based lavas. But water volcanism is not what we generally think of when we imagine a volcano (though I think that technically a geyser could be considered a water volcano).
At any rate, this upward spraying "snow" apparently accumulates to form the aforementioned ring. Now we know of volcanism elsewhere in the solar system. Io, a moon of Jupiter, is the most volcanically active body known. Since Io's orbit is not perfectly round, there is a point in its orbit where it's coser to Jupiter and an opposite point when it's farther away. "Tidal forces" is simply a way of saying that when an object is closer to another massive body, there is a difference in pull between the closer face and the farther face- the first object is stretched out. When the first object is farther away in its orbit, it relaxes a little. In the case of Io, this stretching and relaxing (often referred to as gravitational kneading) creates enough internal friction to generate heat that in turn drives its intense volcanism.
In the case of Enceladus, it's far enough away from Saturn, and small enough, that such gravitational kneading shouldn't be able to generate enough heat to drive volcanism. So what's the heat source?
We don't know.
Damn, I love those words. Yay! A puzzle!
According to this article, the probe will pass only 50 km (30 mi) over Enceladus' south pole, and be able to imagery with resolutions with as small as 7 m (22 ft) per pixel. The previous pass in March was optimized to use the fields and particles instruments; this pass is optimized to capture imagery. That means we should get some really exciting pictures tomorow. I do recommend reading the above lined article if your interested in this sort of stuff; I wish more science journalism was of this quality- not too much jargon, but clearly the author assumes some background knowledge. She recognizes that 1) I'm not an idiot, and 2) it's important to know her stuff. Most science reporters, unfortunately, make the opposite assumptions on both points.
You can follow the action at the Cassini website, (The lead picture is from here) or check in at a later date to check out the cool pictures.
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