Io is the closest to Jupiter of the 4 Galilean Satellites, a little less than 6 Jupiter Radii out, and finishes a complete orbit in only 1.77 days. (Of course, in that time Jupiter rotates more than 4 times.) It's the most volcanic body in the solar system, with active volcanoes powered by Jupiter's tides. Io is by far the densest of the Galilean moons, with a density nearly twice that of Callisto. As a result, Io has a surface gravity larger than Ganymede's, even though Ganymede is the largest moon in the solar system.

Caption: The image is centered on the side of Io that always faces away from Jupiter. The color in the image is composed of data taken in the near-infrared, green and violet filters of Galileo's solid-state imaging camera, and has been enhanced to emphasize the extraordinary variations in color and brightness that characterize Io's volcano-pocked face. The black and bright red materials correspond to the most recent volcanic deposits, probably no more than a few years old. The near-infrared filter makes Jupiter's atmosphere look blue. The active volcano Prometheus is seen near the right-center of the disk.

More about Io can be found here.