Short answers, pulled from the story.
Io is the most volcanically active world in the entire Solar System, possessing over 400 active volcanoes that constantly reshape its surface. This volcanic activity prevents the formation of impact craters, making Io's landscape geologically young and constantly renewing itself. The source of this energy is tidal friction generated by a gravitational tug-of-war with Jupiter and its neighboring moons Europa and Ganymede.
Galileo Galilei first observed Io as a distinct point of light on the 8th of January 1610. Although his early telescope could not separate it from Europa, the moon remained a fifth-magnitude star for the next two and a half centuries. The name Io was proposed by Simon Marius in 1614, naming the moon after the Greek priestess who was a lover of Zeus.
The Laplace resonance forces Io to complete two orbits around Jupiter for every one orbit of Europa and four orbits of Ganymede. This orbital choreography maintains a slight eccentricity that causes tidal bulges to rise and fall by as much as 100 meters. The constant flexing generates friction that produces heat up to 200 times greater than radioactive decay alone, melting the moon's mantle to create a subsurface magma ocean.
There are between 100 and 150 mountains on Io, with the tallest reaching 17.5 kilometers, which is more than twice the height of Mount Everest. These peaks form by the compression of the moon's crust as volcanic materials bury the surface, forcing the crust to buckle and fold. They are primarily composed of silicate rock and appear as flat-topped plateaus or tilted crustal blocks with steep scarps.
Io is the driest known body in the Solar System because the heat of the early Solar System drove off volatile materials like water in the vicinity of Jupiter. The moon is primarily made of silicate rock with a thin coating of sulfur and sulfur dioxide rather than water ice. This lack of water distinguishes it from other moons in the outer Solar System that are composed largely of water ice.
Io's atmosphere is primarily composed of sulfur dioxide and collapses when the moon passes into Jupiter's shadow. During this time, pressure drops by up to 80 percent as the sulfur dioxide sublimates from the frost and the atmosphere becomes less dense. The atmosphere is constantly replenished by volcanic plumes and sublimation of surface frost, creating a dynamic system tied directly to surface temperature.