Io was first discovered by Galileo in 1610, making it one of the Galilean Satellites. Of the 17 moons it is the 5th closest to Jupiter, with a standoff distance of 421,600 km. It is the 4th largest moon of Jupiter, with a diameter about 2/3 the distance across the United States, of 3630 km (2256 miles).
Io is named after one of Jupiter's many lovers, from Roman mythology. It is the only moon known to have active volcanism, which is visible on the surface.
Io is mostly made of sulfur, iron, and rocky material, which means that Io is nothing at all like the other icy satellites of Jupiter, and has had a very different evolution.
The particles comprising Io's atmosphere readily find their way into the magnetosphere, and leave behind a donut-shaped cloud of dust. This cloud has a very important effect on Jupiter's magnetosphere.
Because of all the lava, the environment of Io is pretty unfriendly to life as we know it on earth.
Greek Mythology
Io was a priestess of the Greek goddess Hera. Hera was the jealous wife of Zeus, the king of the gods. Zeus was very unfaithful. When Zeus fell in love with Io, he changed himself into the shape of a dark cloud to hide himself from his jealous wife Hera.
However, Hera looked down on earth and noticed the small cloud. She knew it was her husband. As soon as Hera arrived, Zeus immediately transformed Io into a white cow to avoid his wife's wrath. But 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.
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!
Jupiter And Io
The Galilean satellite Io floats above the cloudtops of Jupiter in this image captured on the dawn of the new millennium, January 1, 2001 10:00 UTC (spacecraft time), two days after Cassini's closest approach. The image is deceiving: there are 350,000 kilometers -- roughly 2.5 Jupiters -- between Io and Jupiter's clouds. Io is the size of our Moon, and Jupiter is very big.
Jupiter And Io

This image taken by NASA's Cassini spacecraft on Dec. 1, 2000, shows details of Jupiter's Great Red Spot and other features that were not visible in images taken earlier, when Cassini was farther from Jupiter.
The picture is a color composite, with enhanced contrast, taken from a distance of 28.6 million kilometers (17.8 million miles). It has a resolution of 170 kilometers (106 miles) per pixel. Jupiter's closest large moon, Io, is visible at left.
The edges of the Red Spot are cloudier with ammonia haze than the spot's center is. The filamentary structure in the center appears to spiral outward toward the edge. NASA's Galileo spacecraft has previously observed the outer edges of the Red Spot to be rotating rapidly counterclockwise, while the inner portion was rotating weakly in the opposite direction. Whether the same is true now will be answered as Cassini gets closer to Jupiter and interior cloud features become sharper. Cassini will make its closest approach to Jupiter, at a distance of about 10 million kilometers (6 million miles), on Dec. 30, 2000.
The Red Spot region has changed in one notable way over the years: In images from NASA's Voyager and Galileo spacecraft, the area surrounding the Red Spot is dark, indicating relatively cloud-free conditions. Now, some bright white ammonia clouds have filled in the clearings. This appears to be part of a general brightening of Jupiter's cloud features during the past two decades.
Jupiter has four large moons and an array of tiny ones. In this picture, Io is visible. The white and reddish colors on Io's surface are due to the presence of different sulfurous materials while the black areas are due to silicate rocks. Like the other large moons, Io always keeps the same hemisphere facing Jupiter, called the sub-Jupiter hemisphere. The opposite side, much of which we see here, is the anti-Jupiter hemisphere. Io has more than 100 active volcanoes spewing very hot lava and giant plumes of gas and dust. Its biggest plume, Pele, is near the bottom left edge of Io's disk as seen here.
Prometheus
The volcano called Prometheus, found on Jupiter's moon Io, could be called the Old Faithful of the outer solar system, because its volcanic plume has been visible every time it has been observed since 1979. This particular image, one of the highest-resolution pictures ever taken of Io, was obtained by NASA's Galileo spacecraft as it approached Io on July 3, 1999.
The volcanic plume of Prometheus has been visible during observations by Galileo (1996-1999) and NASA's Voyager spacecraft (1979). No other volcano on Io has been so stable in its behavior. However, between the Voyager flybys and the time of Galileo's arrival at Jupiter, the source of the plume has shifted about 70 kilometers (44 miles) to the west.
This false color close-up was taken of Prometheus using the near-infrared, green and violet filters (slightly greater than the visible range) of the spacecraft's camera and processed to enhance subtle color variations.
The long-lived plume has produced a ring-like deposit of bright white and yellow material that is likely to be rich in sulfur dioxide frost. Also note the denser jets in the plume that point like spokes to its source. Galileo scientists do not yet know whether this long-lived plume is erupting from a vent at the west end of the lava flow, or if the plume is being produced by the advancing lava as it flows over ground rich in sulfur dioxide.