IoIo was a priestess of the Greek goddess Hera. Hera was the jealous wife of Zeus, the king of the gods. Zeus was indeed very unfaithful. When Zeus fell in love with Io, he transformed himself into the shape of a dark cloud to hide himself from his jealous wife.
However, noticed the small cloud and suspected that the cloud was one of Zeus's tricks. Thus, she approached to check the true nature of the cloud. As soon as Hera arrived, Zeus immediately transformed Io into a white cow to avoid his wife's wrath. But Hera guessed the intrigue and asked if Zeus wanted the cow as a gift. Zeus could not refuse such a little gift without giving himself away.
Thus, Hera tied the poor cow and sent her faithful servant Argus to watch over Io. Argus had a hundred eyes and only a few were ever closed at any time. To free Io, Zeus sent his son Mercury to sing and tell boring stories to make Argus sleep with all his eyes. Mercury told so many stories that finally Argus close all his hundred eyes. Only then did Mercury kill Argus and untie Io, who ran home free.
Yet when Hera discovered what had occurred, she was so furious that she sent a vicious gadfly to sting the cow forever. Moreover, to honor the memory of her faithful servant, Hera put the hundred eyes of Argus on the tail of her favorite bird, the peacock. The hundred eyes could not see any more but beautifully decorate the tail of the peacock.
Meanwhile, Io, who was still prisoner into the shape of a cow, could not get rid of the malicious gadfly. Finally, after Zeus vowed to no longer pursue his beloved Io, Hera released Io from her inhuman prison, and Io settled in Egypt, becoming the first queen of Egypt.
The Jovian moon Io was named for the mythological character Io by Johannes Kepler, and Simon Marius. And finally, when Voyager 1 passed Io in March 1979 and imaged the surface, the image clearly showed the hoof print of a heifer!