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Discovered by Galileo Galilei
Discovered on January 7, 1610
Orbital characteristics
Mean radius 421,700 km (0.002819 AU)
Eccentricity 0.0041
Periapsis 420,000 km (0.002807 AU)
Apoapsis 423,400 km (0.002830 AU)
Revolution period 1.769137786 d (152,853.5047 s)
Orbital circumference 2,649,600 km (0.018 AU)
Orbital velocity max: 17.406 km/s
mean: 17.334 km/s
min: 17.263 km/s
Inclination 2.21° (to the ecliptic)
0.05° (to Jupiter's equator)
Is a satellite of Jupiter
Physical characteristics
Mean diameter 3642.6 km (0.286 Earths)
(3660.0×3637.4×3630.6 km)
Surface area 41,910,000 km2 (0.082 Earths)
Volume 2.53×1010 km3 (0.023 Earths)
Mass 8.9319×1022 kg (0.015 Earths)
Mean density 3.528 g/cm3
Surface gravity 1.796 m/s2 (0.183 g)
Escape velocity 2.558 km/s
Rotation period synchronous
rotation velocity
271 km/h
Axial tilt zero
Albedo 0.63
Apparent Magnitude 5.0
Surface temp.
min mean max
90 K 130 K 2000 K
Atmospheric characteristics
Atmospheric pressure trace
Sulfur dioxide 90%

Io (eye'-oe, ˈaɪoʊ, Greek Ῑώ) is the innermost of the four Galilean moons of Jupiter. Io shines at magnitude 5.0 in the night sky. It is named after the Greek mythological figure Io, one of the many lovers of Zeus (who is also known as Jupiter in the Roman mythology).

Although the name "Io" was suggested by Simon Marius soon after its discovery in 1610, this name and the names of the other Galilean satellites fell into disfavor for a considerable time, and were not revived in common use until the mid-20th century. In much of the earlier astronomical literature, Io is simply referred to by its Roman numeral designation as "Jupiter I", or simply as "the first satellite of Jupiter".

History of observation and exploration

The moon Io is believed to have been discovered on 7 January, 1610 by Galileo. In his Mundus Jovialis, published in 1614, Simon Marius claimed to have discovered Io and the other moons of Jupiter in 1609, one week before Galileo's discovery. Galileo doubted this claim and catalogued the work of Marius as plagiarism.

In the middle of the 20th Century observations were made suggesting that the polar regions of Io were red. With the passage of the space probe, Pioneer in the 1970s, little was discovered about Io. Pioneer 10 was not able to obtain any clues regarding the radiation of Io. But, Pioneer 11 was able to verify that the polar region had an orange color, contrasting with the whitish equator. At this altitude, it was already known that Io had an atmosphere.

When the space probe, Voyager 1 sent its first images near Io in 1979, the scientists hoped to find numerous craters. Contrary to what they had expected, Io had almost no craters. They found a relatively young surface caused by intense volcanic activity that covered any signs of craters. Voyager 1 was able to observe nine active volcanoes on the surface, and later Voyager 2 observed eight of the nine active volcanoes.

The Galileo spacecraft arrived at Jupiter in 1995, and flew by Io at the end of 1999. Galileo approached Io closer than any other probe, took many photographs, observed volcanoes erupting, and discovered that Io has a large iron core, just like the rocky planets of the inner solar system.[1]



Io is most noteworthy for its volcanic nature; due to tidal heating it is the most volcanically active body in the Solar System. In February 2001, the largest recorded volcanic eruptions in the solar system occurred on Io.[2] Like volcanoes on Earth, Ionian volcanoes emit sulfur and sulfur dioxide. Originally it was thought that many lava flows consisted of sulfurous substances. However, it is now believed that many of them are molten silicate rock as on the Earth.

The energy for this activity probably derives from tidal interactions among Io, Jupiter, and two other moons of Jupiter, Europa and Ganymede. The three moons are locked into Laplace-resonant orbits such that Io orbits twice for each orbit of Europa, which in turn orbits twice for each orbit of Ganymede; furthermore, Io always keeps the same face towards Jupiter. The gravitational interaction of Europa, Ganymede and Jupiter cause Io to "stretch" and "bend" by as much as 100 meters, a process which generates heat through internal friction.

Some of Io's volcanic plumes have been measured rising over 300 km above the surface before falling back, with material ejected from the surface at approximately one kilometre per second. The volcanic eruptions change rapidly; in just four months between the arrivals of Voyager 1 and Voyager 2 some eruptions stopped and others began. The deposits surrounding the vents also changed visibly during this time.

Another source of energy is Jupiter's magnetic field lines, which Io crosses, generating an electric current. Though not a large source of energy compared to the tidal heating, this current may carry more than 1,000 gigawatts with a potential of 400 kilovolts. It also strips ionized atoms from Io at the rate of a thousand kilograms per second. Due to the rapid rotation of Jupiter's magnetic field, these particles are swept along the orbit in front of Io where they form a torus of intense radiation around Jupiter that emits bright ultraviolet light. Particles escaping from this torus are partially responsible for Jupiter's unusually large magnetosphere, their outward pressure inflating it from within. Recent data from the Galileo orbiter indicate that Io might have its own magnetic field.[3][4][5]

The location of Io with respect to the Earth and Jupiter has a strong influence on the Jovian radio emissions as seen from the earth: When Io is visible, radio signals from Jupiter increase considerably.

Physical characteristics

File:Iosurface gal.jpg

Unlike most moons in the outer solar system, Io may be somewhat similar in bulk composition to the terrestrial planets, primarily composed of molten silicate rock. Recent data from the Galileo orbiter indicates that Io has a core of iron (perhaps mixed with iron sulfide), the core's radius being at least 900 km.[4]

When Voyager 1 first returned images of Io in 1979, scientists expected to see numerous craters, the density of which across Io's surface would give clues to the moon's age. However, they were surprised to discover that Io's surface is almost completely lacking in craters, due to the tremendous amount of volcanic activity constantly reshaping the landscape. Since the surface features visible today were formed relatively recently, the Ionian surface is described as "young", as is the terrestrial surface. In contrast, celestial bodies with heavily cratered features, such as Earth's Moon, are considered to have "old" surfaces, since they have remained in their current state for billions of years.

File:Tvashtar Catena.jpg
File:PIA01129 Interior of Io.jpg

In addition to volcanoes, Io's surface includes nonvolcanic mountains, numerous lakes of molten sulfur, calderas up to several kilometres deep, and extensive flows hundreds of kilometres long of low-viscosity fluid (possibly some form of molten sulfur or silicate). Sulfur and its compounds take on a wide range of colors and are responsible for Io's variegated appearance.

Analysis of the Voyager images led scientists to believe that the lava flows on Io's surface were composed mostly of various compounds of molten sulfur. However, subsequent ground-based infrared studies indicate that they are too hot for liquid sulfur; some of the hottest spots on Io may reach temperatures as high as 2000 K, 1300 K higher than the boiling point of sulfur, though the average is much lower, at around 130 K. One current theory is that Io's lavas are molten silicate rock. Recent Hubble Space Telescope observations indicate that the material may be rich in sodium. There may be a variety of different materials in different locations.[6][7]

Io has a thin atmosphere composed of sulfur dioxide and perhaps other gases. The pressure near equatorial latitude is 0.2 to 0.35 nbar of sulfur dioxide.[8]

Unlike the other Galilean satellites, Io has little or no water. This is probably because Jupiter was hot enough early in the evolution of the solar system to drive off the volatile elements in the vicinity of Io, but not hot enough to do so farther out.

See also


  1. R. M. C. Lopes, D. A. Williams (2005). "Io after Galileo". Rep. Prog. Phys. 68: 303–340. DOI:10.1088/0034-4885/68/2/R02.
  2. EXCEPTIONALLY BRIGHT ERUPTION ON IO RIVALS LARGEST IN SOLAR SYSTEM. Keck Observatory News. Archived from the original on 2005-04-12. Retrieved on 2006-03-04.
  3. Sarson, G. R. et al, "Magnetoconvection Dynamos and the Magnetic Fields of Io and Ganymede" - May 16, 1997, Science Vol. 276. no. 5315, pp. 1106 - 1108. URL accessed April 15, 2006.
  4. 4.0 4.1 "NASA'S GALILEO FINDS GIANT IRON CORE IN JUPITER'S MOON IO" - May 3, 1996 NASA Press release. URL accessed Arpil 15, 2006
  5. Krimigis, S. M. et al, "A nebula of gases from Io surrounding Jupiter" - 28 February, 2002 Nature 415, 994-996. URL accessed 15 April, 2006.
  6. Roesler, F. L. et al, "Far-Ultraviolet Imaging Spectroscopy of Io's Atmosphere with HST/STIS" - January 15, 1999 Science Vol. 283. no. 5400, pp. 353 - 357. URL accessed Arpil 15, 2006.
  7. Geissler, P. E. et al, "Galileo Imaging of Atmospheric Emissions from Io" - August 6, 1999 Science Vol. 285. no. 5429, pp. 870 - 874. URL accessed April 15, 2006.
  8. D. F. Strobel B. C. Wolven (2001). "The Atmosphere of Io: Abundances and Sources of Sulfur Dioxide and Atomic Hydrogen". Astrophysics and Space Science 277: 271 - 287. DOI:10.1023/A:1012261209678.

External links

(Jupiter's natural satellite navigator) Thebe | Io | Europa | ...

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