Radioactivity powers volcanoes of salty ice on Ceres

Ceres is the largest object in the asteroid belt, the wide expanse between Mars and Jupiter littered with debris left over from the solar system’s planet-making process. Ceres used to be called an asteroid, but is so large — over 900 kilometers in diameter — that planetary scientists now call it a protoplanet, meaning it was on its way to becoming an actual planet until the raw materials it was growing from ran out.