How do scientists know Io is the most volcanically active world in the Solar System?

Io sits deep in Jupiter’s gravity field, and its orbit is constantly stretched and squeezed by Jupiter and the moons Europa and Ganymede. That tidal flexing generates intense internal friction, which melts rock and fuels volcanism. Voyager, Galileo, and later observatories observed many active eruptions, lava flows, and giant plumes on the moon’s surface. Io is now recognized as the most volcanically active body in the Solar System, with resurfacing so rapid that impact craters are relatively rare.