Lava Lake Loki on Jupiter's moon Io, up close
Io, the innermost of the four moons of Jupiter discovered by Galileo Galilei in 1610 and only slightly bigger than our own moon, is the most geologically active body in our solar system. Hundreds of volcanic areas dot its surface, which is mostly covered with sulfur and sulfur dioxide. The largest of these volcanic features, named Loki after the Norse god often associated with fire and chaos, is a volcanic depression called patera in which the denser lava crust solidifying on top of a lava lake episodically sinks in the lake, yielding a raise in the thermal emission that has been regularly observed from Earth. Loki, only 124 miles in diameter and at least 373 million miles from Earth, was, up until recently, too small to be looked at in detail from any ground-based optical/infrared telescope.