Etymology
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Io

in Greek mythology, daughter of the river god Inachus, she was pursued by Zeus, who changed her to a heifer in a bid to escape the notice of Juno, but she was tormented by a gadfly sent by Juno.

The Jovian moon was discovered in 1610; the mythological names for all of them (objects of Jupiter's seductions in the myths) were proposed shortly thereafter but not widely used before mid-19c. (Compare Titan).

These bodies [the Jovian moons] have been called in the order of their distance from Jupiter, Hebe, Ganymede, Themis, and, Metis — these names are, however, little used at present, and they are distinguished by the order of their distance from Jupiter, the first being the nearest. [Dionysius Lardner and M. Argo, "Popular Lectures on Astronomy," New York, 1845]

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Definitions of Io

Io (n.)
(Greek mythology) a maiden seduced by Zeus; when Hera was about to discover them together Zeus turned her into a white heifer;
Io (n.)
the closest of Jupiter's moons; has active volcanoes;
From wordnet.princeton.edu