Jupiter and Its Moons Photograph courtesy NASA This family portrait, a composite of the Jovian system, includes the edge of Jupiter (with the Great Red Spot visible) and Jupiter's four largest moons, known as the Galilean satellites. From top to bottom are Io, Europa, Ganymede, and Callisto. The smallest of these four moons, Europa is about the size of Earth's moon.
Jupiter’s moon Io and its tiny shadow sweep across the giant planet’s face back in 1999, as snapped by the Hubble Space Telescope. [Credit: John Spencer (Lowell Observatory) and NASA] Mona Evans, "Jupiter's Galilean Moons" http://www.bellaonline.com/articles/art42279.asp
Io is the first Galilean moon of Jupiter, it is slightly larger than Earth’s moon. Io experiences intense tidal heating due to its elliptical orbit and orbital resonance with Europa and Ganymede. This makes Io the most geologically active moon in our solar system.
Hubble Space Telescope observations show water vapor on Jupiter's moon Europa. If this is coming through surface cracks from Europa's subsurface ocean, it could have a lot of information on what's there. This is an artist's impression showing Europa with Jupiter in the background. (Image credit: NASA, ESA, and M. Kornmesser) Mona Evans, "Jupiter's Galilean Moons" http://www.bellaonline.com/articles/art42279.asp