IV.

Discovery and Exploration

Io was discovered independently in 1610 by Italian astronomer Galileo and German astronomer Simon Marius. Io and the three other largest moons of Jupiter, all found in 1610, are called the Galilean moons. Marius named Io and the other Galilean moons for mythical lovers of the Greek god Zeus, whom the Romans renamed Jupiter. Zeus transformed Io into a cow to protect her from his jealous wife, Hera. Io’s volcanoes are named for gods of fire, the sun, and thunder from mythologies from all over the world.

The United States spacecraft Voyager 1 and 2, which passed Jupiter in 1979, and the Galileo spacecraft, which went into orbit around Jupiter in 1995, have provided much of the information astronomers know about Io. In 1999, for example, Galileo took detailed photographs of Io’s volcanic eruptions; scientists hope to use the observations to learn more about similar volcanic activity that occurred on Earth eons ago. The Hubble Space Telescope has also made important observations of Io’s volcanic activity.

The New Horizons spacecraft studied Io when the probe passed by Jupiter in 2007. The probe was able to observe Io’s atmosphere, auroras, and volcanoes, including an eruption by the volcano Tvashtar that sent a plume 330 km (200 mi) high. New Horizons used Jupiter’s gravitation to boost its velocity for a planned flyby of distant Pluto in 2015.