1610

Io

Galileo Galilei (1564–1642), Simon Marius (1573–1624)

Galileo’s January 8, 1610, drawing of Jupiter’s four bright satellites, with Io’s position on that night highlighted by the red arrow.

When Galileo first trained his telescope on Jupiter on January 7, 1610, he noticed what he described as “three fixed stars, totally invisible by their smallness.” The stars were right next to Jupiter (two on one side and one on the other) and all in a straight line that passed through the middle of Jupiter’s disk. It is fun to imagine his amazement when he saw four tiny stars on the next night instead of three—again all along the same line—and as he continued to watch over subsequent weeks, the little stars moved relative to the planet. It didn’t take long for him to spot the pattern: they were orbiting Jupiter.

Galileo had discovered four new worlds, the first known moons of another planet besides our own. As their discoverer, he earned the right to name them, and in a politically astute move he decided to name them the Medician stars, after his patron and funding source Cosimo II de’ Medici. Other contemporary astronomers hated the idea. The German astronomer Simon Marius, who claimed to have discovered the moons before Galileo, proposed names from Greek mythology: Io (a nymph who was seduced by Zeus), Europa, Ganymede, and Callisto. Galileo hated other naming options and eventually started calling the moons Jupiter I through IV. Astronomers used those designations well into the twentieth century, until Marius’s more romantic naming scheme finally stuck. In honor of their discoverer, though, they are now collectively called the Galilean satellites.

Io (pronounced EYE-oh) is the closest in of the four large moons of Jupiter, with an orbital period of about 42 hours. Seven space missions (Pioneers 10 and 11, Voyagers 1 and 2, Galileo, Cassini, and New Horizons) have now studied Io up close, revealing it to have a diameter of 2,275 miles (3,660 kilometers)—slightly larger than Earth’s Moon—and a surprisingly “rocky” density of 3.5 grams per cubic centimeter. The biggest surprise, however, was Voyager 1’s discovery of active volcanoes on Io, responsible for the moon’s young surface being covered with red, orange, and black sulfur and silicate lava, and its thin atmosphere of sulfur dioxide. Strong tidal forces cause constant eruptions on Io, making it the most volcanically active world in the solar system.

Jupiter’s innermost moon, Io, photographed against the backdrop of Jupiter’s clouds in 1996 by the NASA Galileo Jupiter orbiter spacecraft.