The Jovian moons of Europa and Io are a true song of ice and fire. The former is a frigid world concealing a massive ocean that potentially contains life, or at least the precursors to it; the latter is the most volcanic object known to science, one whose atmosphere collapses and re-inflates every time it moves in and out of Jupiter’s shadow.
They are, inarguably, two of the most fascinating and important members of our Solar System, so you can’t blame NASA’s Juno spacecraft for hankering after capturing the two as they engaged in their celestial dance. Taken during the satellite’s eighth flyby of the gas giant on September 1, the red-yellow Io can be seen closer to the colossus, with Europa orbiting a little further out.
Apart from being a lovely photograph, it’s a reminder that their close orbits are actually vital to each other’s geological evolution.
Thanks to Jupiter’s powerful gravitational well, but partly thanks to the close and chaotic orbits of the inner moons themselves, Europa and Io have internal heat sources driven by this gravitational tug-of-war. Without this method of heat source generation, Io wouldn’t be a volcanic monster and Europa probably wouldn’t have an ocean hiding beneath its criss-crossed icy shell.
