solar system


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.

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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!

Physical characteristics

Discovered by Galileo Galilei
Discovery date January 7, 1610
Alternate name Jupiter I
Adjective Ionian
Physical characteristics
Dimensions 3,660.0 W 3,637.4
W 3,630.6 km
Mean radius 1,821.3 km
(0.286 Earths)[1]
Surface area 41,910,000 km2
(0.082 Earths)
Volume 2.53 W 1010 km3
(0.023 Earths)
Mass 8.9319 W 1022 kg
(0.015 Earths)
Mean density 3.528 g/cm3
Equatorial surface gravity 1.796 m/s2 (0.183 g)
Escape velocity 2.558 km/s
Rotation period Synchronous
Equatorial rotation velocity 271 km/h
Albedo 0.63 1 0.02[2]
Surface temp. min mean max
Surface - 130 K 200 K
Apparent magnitude 5.02 (opposition)
Surface pressure trace
Composition 90% sulfur dioxide

Jupiter And Io

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 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.


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.

Orbital Characteristics - Io

Periapsis 420,000 km (0.002 807 AU)
Apoapsis 423,400 km (0.002 830 AU)
Mean orbit radius 421,700 km (0.002 819 AU)
Eccentricity 0.0041
Orbital period 1.769 137 786 d
(152 853.504 7 s, 42 h)
Average orbital speed 17.334 km/s
Inclination 2.21° (to the ecliptic)
0.05° (to Jupiter's equator)
Satellite of Jupiter

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