
Underneath a low-density crust of 30-50 km thickness exists a global magma layer (asthenosphere) with a thickness exceeding 50 km and a rock melt fraction of a few tens of percent.
Io's center is in constant flux: Jupiter's enormous gravity pulls on the moon's "slushie-like interior," UCLA geophysicist Krishan Khurana says, and all that friction produces an enormous amount of thermal energy, through what's called tidal heating. The heat melts rock to create magma, which spews out of the 400 or so volcanoes on Io's crust. Previously, however, scientists weren't sure exactly how that magma was distributed. Some thought it might have come from isolated underground pockets or wellsprings, as is the case on Earth, rather than a global layer of magma.











