# Why does such a small moon like Io have current volcanic activity?

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m. charlee and Vincent G gave good answers, but I think it's interesting enough to look more closely at the reasons for Io's tidal heating. Io is in a forced orbital resonance with Ganymede and Europa, resulting in an orbital eccentricity of about 0.0041. Given Jupiter's massive gravitation and Io's proximity to it, that's sufficient to cause tidal heating in Io, which drives its volcanic activity. Hmm... interesting.

• Anonymous

Io is only slightly further away from Jupiter than our Moon is away from Earth. But since Jupiter has 317 times the mass of Earth, the gravitational pull the little moon Io experiences is roughly 250 times stronger than for our Moon. You can calculate this if you look at the force of gravity between two bodies:

F = G*m*M/r^2

Since the mass M is 317 times larger, and r_Io=421000km is only slightly larger than r_Moon=384000km, you get 317*(384000/421000)^2=263 times stronger gravitational forces on the same mass m.

Io has 1.5% of the mass of Earth, our moon is 1.23%... so Io and the Moon are very similar, especially since they are almost the same size and density, too.

As you can see, Earth's gravity was enough to deform the Moon so that its center of mass has moved towards Earth and it has been tidal locked. On Io, the same effect deposits so much energy that the interior stays molten. It is like as if Jupiter is kneading a giant loaf of lava.

The whole story is a bit more complex than what I told you because Io has an orbital resonance with Ganymede and Europa, which keeps the moon in a non-circular orbit at a constant distance. Without it, the tidal forces would slowly make the orbit circular, resulting in a much lower heat generation:

http://en.wikipedia.org/wiki/Io_%28moon%29

Io's orbit about Jupiter continually squeezes & contracts the little world. This generates heat - a lot of it. Much of Io's makeup is sulfer, which melts, then gets spewed as ejecta from it's surface volcanos.

If Io was twice as far from Jupiter as it is now, then it would still have some heat, but with only 1/4 the amount of gravitational effects acting on it, no volcanos would be erupting.

• 3 years ago

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Tides on Io raised by both Europa and massive Jupiter are so great that they continually heat and partially melt the interior of Io.

Because it is so close to Jupiter (and the orbit is not perfectly circular) it goes though various level of gravitational pull from the massive planet.

This is just like kneading dough, and this constant squishing and tidal heating creates heat that is the source of all this vocanic activity.