…it’s the hottest moon in the solar system. Why’s that?
I’ll have to ask my brother to make sure (he did his doctoral dissertation on atmospheric modeling of Io) but I pretty sure it has to do with the active volcanos which keep the atmosphere liquid. I’ll fire off an email to get the real scoop… (unless some more astronomy minded straight doper beats me to the punch)
As I understand it, Io owes its vulcanism to the extreme tidal forces imparted on that moon by the strong gravity of Jupiter. Io gets squeezed one way, then another as it rotates and revolves, and this squeezing heats the moon’s interior.
It’s caused by the interplay of gravity around Jupiter. In addition to the gravitational pull of Jupiter itself, the moons influence each other to varying degrees, dragging each other up and down in their orbits. Thanks to the influence of some of the other moons, Io sort of oscillates in its orbit, and the shifting tidal forces produce a lot of internal heat, keeping the core molten. That’s why Io has such a high level of volcanic activity.
Several of Jupiter’s other moons have also been shown to be warmer than one might expect. Europa, Ganymede, and Callisto are believed to have subsurface layers of liquid (salty) water.
tremorviolet, thanks. I’d like to hear what your brother has to say.
Redsland: If the tidal forces can affect Io in that way, how come this doesn’t happen with, say, Europa? Or is Europa out of range?
Thanks, Balance. That makes sense.
It does happen with Europa. That is why it may have liquid oceans underneath its crust of ice, as Balance said.
From this Nasa page:
Io is the innermost of Jupiter’s four main moons. Since gravity’s influence falls off so rapidly with distance, I would surmise that Io feels stronger tidal forces simply because it is closer to the planet.
Balance and Redsland: By Jove, I think you’re right!
Gravity falls off with the square of the distance. Tides fall of with cube.
Correct. Which means that distance is even more important for tidal effects than it is for straight gravitational effects.
Europa is the second-closest of the four major moons, so it feels the second-strongest tidal effects. And we’resure that Europa has a liquid ocean under its crust. The others are farther away and cooler, and we’re less certain about liquid water in them.
We’re sure? Not to contradict you, but I’m just curious. I haven’t been keeping up on it, but the reports I remember just had the experts saying that Europa “possibly” or “probably” or “likely” has liquid water. Did Galileo (or some other source) provide definitive confirmation?
If you’re really interested, I recommend you read John S. Lewis’s Worlds Without End for a planetary scientist’s concise and well-written summary of our current understanding: physical materials, orbital and gravitational effects, and so on.
Don’t forget that there are four satellites inside the orbit of Io. Three are new discoveries by Voyager I and II. The fourth is the respectably sized Almathea (J5). It also gives off more heat than it receives from the sun.
Cervaise, I checked out Worlds Without End on amazon and it looks interesting. I’ll certainly be on the lookout for it 
Thanks everyone.
What exactly is the technical term for the tidal effect that causes the planet to heat up? I know it’s “Tidal _________” something or other. Anyone?
The technical term for the tidal effect that causes Io to heat up is “Tidal heating”. Not very glamorous, is it? But technical terms are designed to be easier to understand, not harder. Closely related is “tidal stressing”, which refers to the forces (strictly, forces per area) inside the moon caused by tides, and “tidal strain”, which is how the moon changes shape, twists, or otherwise moves around due to the tidal stresses.
And bryanmcc, no scientist will ever say he’s completely sure of anything. But there are a great many observations which are all consistent with the hypothesis that Europa has a liquid ocean under the ice, and are not consistent with any other reasonably simple hypothesis. Moreover, the liquid Europan ice is a very simple hypothesis, and theoretically predicted by our models of tidal heating. Put it all together, and we’re pretty darn sure that there’s liquid water there.
Thanks Chronos. I understand about the hedging of scientific bets, but I was just wondering if there had been more recent findings that I had not heard of. Some direct observational evidence of the presence of liquid water, rather than just theoretical. Although I’m prefectly willing to accept that the presence of water is likely on purely theoretical grounds, it’ll be cool when someone first “sees” water there.