Io, one of the big moons of Jupiter, might be the toughest solid body to map in the entire solar system. That’s because its surface is constantly changing – the result of major volcanic activity.

Io is about the same size as our own moon. And it’s the “rope” in a tug-of-war between Jupiter and two of its other big moons. They pull at Io, squeezing and heating its interior. That’s created about 400 major volcanic features – mountains, lava flows, and lakes of molten rock. Some of the volcanoes produce plumes that are up to 300 miles high, while the lakes are up to 300 miles long.

Scientists have wondered whether the volcanoes are fed by a global “ocean” of magma below the surface, or by individual magma chambers. Recent observations by the Juno spacecraft suggest the “individual” model.

Scientists tracked Juno’s radio waves as the craft flew less than a thousand miles from Io. The wavelength changed slightly as a result of Io’s gravity. The way it changed revealed details about Io’s interior. And the change didn’t match what scientists expected to see if there was a global ocean. Instead, Io’s volcanoes may be fed by hundreds of underground hotspots – chambers that are making it hard to keep the maps of Io up to date.

Jupiter is close to the Moon tonight. It looks like a brilliant star to the lower left of the Moon. Through binoculars, Jupiter’s four big moons look like tiny stars near the giant planet.

Script by Damond Benningfield