When they look at Io, one of Jupiter's moons, researchers think they could be seeing that the Earth might have looked like in its early history, before the development of plate tectonics - 200 to 500 million years after the planet formed. Volcanically active Io is being constantly pulled by Jupiter and another of its moons, Europa, so plate tectonics have never taken hold. On Earth, volcanic hotspots form at the edges of plates, but on Io they are circular.