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Io (moon)

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True-color image taken by the Galileo probe
Galileo spacecraft true-color image of Io. The dark spot just left of the center is the erupting volcano Prometheus. The whitish plains on either side of it are coated with volcanically deposited sulfur dioxide frost. The yellower regions contain a higher proportion of sulfur
Discovered by Galileo Galilei
Discovery time January 7, 1610
Other names Jupiter I
Adjective Ionian
Shortest distance from what it orbits around 420,000 km (0.002 807 AU)
Longest distance from what it orbits around 423,400 km (0.002 830 AU)
Avgdistance from the center of its orbital path 421,700 km (0.002 819 AU)
How egg-shaped its orbit is
How long it takes to complete an orbit 1.769 137 786 d (152 853.504 7 s, 42 h)
Average speed 17.334 km/s
Angle above the reference plane
2.21° (to the ecliptic)
0.05° (to Jupiter's equator)
What it orbits Jupiter
Size and Other Qualities
Measures 3,660.0 × 3,637.4 × 3,630.6 km[1]
Average distance from its center to its surface 1,821.3 km (0.286 Earths)[1]
Area of its surface 41,910,000 km2 (0.082 Earths)
Volume inside it 2.53×1010 km3 (0.023 Earths)
Mass 8.9319×1022 kg (0.015 Earths)
Average density 3.528 g/cm3
Gravity at its surface 1.796 m/s2 (0.183 g)
Slowest speed able to escape into space
("escape velocity")
2.558 km/s
How long it takes to turn around one time synchronous
Turning speed 271 km/h
How much light it reflects 0.63 ± 0.02[2]
Surface temp. Min. Avg. Max.
Surface 130 K 200 K
Seeming brightness
("apparent magnitude")
5.02 (opposition)[3]
Pressure trace
Make up 90% sulfur dioxide

Io is a moon of the planet Jupiter. It is Jupiter's third biggest moon with a diameter of 3642 km, being slightly bigger than Earth's moon. Io has about 400 active volcanos.

Io is the most volcanically active body in the Solar System. Volcanoes erupt massive volumes of silicate lava, sulphur and sulphur dioxide, constantly changing Io's appearance. This new basemap of Jupiter's moon Io was produced by combining the best images from both the Voyager 1 and Galileo Missions. Although the subjovian hemisphere of Io was poorly seen by Galileo, superbly detailed Voyager 1 images cover longitudes from 240 W to 40 W and the nearby southern latitudes.

In the same way that the Moon always has the same side facing Earth, Io always has the same side facing Jupiter. The movie shows two speeded-up rotations of Io (a single rotation really takes 1.77 days), and begins with a view of the Jupiter-facing hemisphere. With rotation in an easterly direction, after two seconds the volcano Prometheus (on the equator) comes into view. The massive red deposit around Pele (seconds 5-10) is the most distinctive expression of volcanic activity on Io, and just to the north-west is the horse shoe-shaped Loki Patera, the most powerful volcano on Io.


  1. 1.0 1.1 Thomas, P. C.; et al. (1998). "The Shape of Io from Galileo Limb Measurements". Icarus 135 (1): 175–180. doi:10.1006/icar.1998.5987 .
  2. Yeomans, Donald K. (July 13, 2006). "Planetary Satellite Physical Parameters". JPL Solar System Dynamics. Retrieved 2007-11-05.
  3. "Classic Satellites of the Solar System". Observatorio ARVAL. Retrieved 2007-09-28.