The Moon is a “dead” world. It trembles with a few small moonquakes, and there may be occasional “burps” of gas. But for the most part, not much happens inside it.

That’s definitely not the case for one of the moons of the giant planet Jupiter. Io is the most volcanically active world in the solar system. It’s covered by hundreds of volcanoes and pools of hot lava. Some of the volcanoes are larger than anything on Earth, and the lava is much hotter. The volcanoes can send gas and ash hundreds of miles high. Some of this material escapes Io completely – about one ton every second. It forms a wide “doughnut” around Jupiter.

The activity is powered by a gravitational tug-of-war between Jupiter and some of its other big moons. They pull on Io in different directions. That heats Io’s interior, melting some of its rocks.

A couple of recent studies found that Io has been at least this active since it was born. That suggests that Io and the other big moons have been locked into their current configuration since shortly after the birth of Jupiter itself. If that’s the case, then Io has been caught in a terrific tug-of-war for four and a half billion years.

Jupiter rises above our moon this evening. The planet looks like a brilliant star – only the Moon and Venus outshine it. But you need binoculars to pick out Io and the planet’s other big moons.

Tomorrow: gravitational “rings” around a galaxy.

Script by Damond Benningfield