Online Astronomy eText: Satellites (Moons)
The Satellites (Moons) of Jupiter: Io Link for sharing this page on Facebook
(also see The Galilean Satellites of Jupiter, Europa, Ganymede and Callisto)

Io casts its shadow on Jupiter.
(Cassini Imaging Team, Cassini Project, NASA, apod021207)

Another view of Io, passing in front of Jupiter, by the Cassini spacecraft on its way to Saturn.
(Cassini Imaging Team, Cassini Project, NASA, apod020706)

True color image of Io. The farside of the moon is on the left and the nearside is on the right.
(Galileo Project, JPL, NASA, apod010715)

Photomosaic of farside of Io considerably enhanced to emphasize variations in brightness and colors.
Red and black colored areas are lava flows, yellow areas are sulfur patches from explosive plumes.
Culann Patera (shown below) is below right center in this image.
(Galileo Project, JPL, NASA, apod021019)

Enlargement of area near Culann Patera, an active volcano on Io.
(Galileo Project, JPL, NASA, apod021218)

Eruptive plumes on Io (enlarged views, inset).
(Galileo Project, JPL, NASA, apod070211)

More eruptive plumes (on the left side of the picture) on Io.
(Voyager Project, JPL, NASA, apod960805)

Aurorae in the atmosphere of Io. Although Io has no atmosphere as such, gases lost by its volcanic activity are swept around its orbit by Jupiter's magnetic field and pile up near the moon as the ionized gases near Io slow down and concentrate the magnetic field lines. As a result of Jupiter's powerful magnetic field and intense trapped radiation field, the thin gases near the moon produce auroral displays. The display on the left was imaged by the Galileo spacecraft while Io was in Jupiter's shadow, and lit only by its aurorae (in red and green) and the blue glow of volcanic gases electrified by Jupiter's electromagnetic field. (Features on the portion of Io facing the spacecraft at the time are shown on the right.) (Galileo Project, University Of Arizona (PIRL), JPL, NASA, apod981016)

Closeup of the caldera of Tupan Patera. The caldera is approximately 50 miles across.
(Galileo Project, JPL, NASA, apod020327)

Closeup of another volcanic area
(Galileo Project, JPL, NASA, apod000606)

Mongibello Mons (on far left), a sharp ridge higher than most mountains on Earth, casts shadows at sunset on Io. (Galileo Project, JPL, NASA, apod030120)

Comparison of possible internal structures of the Galilean satellites of Jupiter
( Calvin Hamilton)

Relative size of various large moons and Pluto. (NASA, Wikipedia Commons)