Jupiter’s moon Io stands alone among the solar system’s moons. It has more than 400 active volcanoes, and its surface is home to more than 100 massive volcanic mountains, some of which are taller than Mt. Everest. Its surface is painted and repainted with the sulfur-rich fallout from these volcanoes, coloring the moon in red, yellow, and orange hues. Vast lakes of lava—the largest of which is 200 km across—also dot its surface. All this on a world that’s only about one quarter as large as Earth.