Jupiter’s moons Io, Europa, and Ganymede have an orbital resonance ratio of 1:2:4. For every one orbit Ganymede completes around Jupiter, Europa completes two and Io completes four.
This pattern is called the Laplace resonance. The moons’ orbital periods are approximately 1.77 days for Io, 3.55 days for Europa, and 7.15 days for Ganymede. Their periods therefore form a near-perfect sequence in which each outer moon takes about twice as long to orbit as the next moon inward.
The resonance is more than a numerical coincidence. The moons repeatedly line up in related positions, allowing their mutual gravity to exchange orbital energy and maintain small eccentricities rather than perfectly circular orbits. Io’s orbit is especially important because those repeated gravitational tugs help generate internal friction and intense tidal heating.
That heating powers Io’s spectacular volcanic activity and may also contribute to the subsurface ocean believed to exist beneath Europa’s icy crust. The ratio is sometimes written in reverse as 4:2:1 when the moons are listed from the inside out, but with Io, Europa, and Ganymede named in that order, 1:2:4 is the standard period ratio.