Short answers, pulled from the story.
Io is the innermost and second-smallest of Jupiter's four Galilean moons, slightly larger than Earth's Moon. It is the most geologically active object in the Solar System, with over 400 active volcanoes driven by tidal heating from friction generated as Io is pulled between Jupiter, Europa, and Ganymede.
Galileo Galilei discovered Io along with the other Galilean moons in 1610, first recording it on the 7th of January 1610 through a 20-power refracting telescope at the University of Padua. The IAU uses the 8th of January 1610 as the discovery date, when Io and Europa were first seen as separate bodies.
Io has the least amount of water of any known body in the Solar System, with little to no water indicated by its high density and compositional mapping. This dryness likely traces to the early Solar System, when Jupiter ran hot enough to drive volatiles like water out of Io's vicinity but not hot enough to do so farther out.
Several volcanoes on Io produce plumes of sulfur and sulfur dioxide as high as 500 km above the surface. Io also has 100 to 150 mountains averaging 6 km in height, with the tallest, South Boösaule Montes, reaching 17.5 km, higher than Mount Everest.
The Io plasma torus is a doughnut-shaped ring of ionized sulfur, oxygen, sodium, and chlorine that orbits with Io but co-rotates with Jupiter's magnetosphere at 74 km per second. It forms when neutral atoms from the cloud surrounding Io are ionized, and it helps inflate Jupiter's unusually large magnetosphere from within.
Loki Patera is the largest volcanic depression on Io at 202 km across and consistently its strongest volcano, contributing on average 25 percent of Io's global heat output. On the 27th of December 2024, Juno's JIRAM instrument observed an even more energetic outburst in the southern hemisphere producing 140 to 260 terawatts.