Io

prop. n. [ Gr. 'Iw`. ] 1. In Greek mythology, the beautiful daughter of Inachus, king of Argos, Greece, who was changed by Hera (Juno), in a fit of jealousy, into a white heifer, and placed under the watch of Argus of the hundred eyes.
When Argus was killed by Hermes at the command of Zeus, the heifer was maddened by a terrible gadfly sent by Hera, and wandered about until she arrived in Egypt. There she recovered her original shape, and bore Epaphus to Zeus. Epaphus became the ancestor of Ægyptus, Damaus, Cepheus, and Phineus. She was identified by the Egyptians with Isis. According to another legend, Io was carried off by Phoenician traders who landed in Argos. The myth is generally explained to be Aah or the moon wandering in the starry skies, symbolized by the hundred-eyed Argus; her transformation into a horned heifer representing the crescent moon. [ Century Dict. 1906 ]

Greek mythology, too, knew her [ Astarte ] as Iô and Europa, and she was fitly symbolised by the cow whose horns resemble the supine lunar crescent as seen in the south. Seyce (Anc. Empires, p. 195). [ Century Dict. 1906 ]

2. One of the large moons of the planet Jupiter, remarkable for its intense volcanic activity, as observed in fly-bys of space probes. It was named after the mythological Io. The remarkable images of Io and Jupiter obtained by the Galileo 2 space probe to Jupiter and Hubble photographs are available at the web site of NASA. Reproduced here are:
Surface of Io.
Full disk of Io passing, over the clouds of Jupiter.
Jupiter with Io in transit.
More images are available at the NASA site. [ PJC ]