A Summary of Facts About Io

Io Quick-Look Statistics

 
Discovery:                             Jan 7, 1610 by Galileo Galilei
Diameter (km):                              3,630
Mass (kg):                                  8.94x10?22 
Mass (Earth = 1)                            0.014960 
Surface Gravity (Earth = 1):                0.183 
Mean Distance from Jupiter (km):            421,600 
Mean Distance From Jupiter (Rj):            5.905 
Mean Distance from Sun (AU):                5.20 
Orbital period (days):                      1.769138 
Rotational period  (days):                  1.769138 
Density (gm/cm?3)                           3.57 
Orbit Eccentricity:                         0.0041 
Orbit Inclination (degrees):                0.040 
Orbit Speed (km/sec):                       17.34 
Escape velocity (km/sec):                   2.56 
Visual Albedo:                              0.61 
Typical Subsolar Temperature (K)            ~135 
Typical Hotspot Temperature (K)             ~300
Surface Composition:                        Sulphur

The Galilean Moons
Ganymede, Callisto, Io and Europa

Io is the innermost of the Galilean satillites. Io's size and density is very similar to our own Moon, and it the most dense of the Galilean satellites.

Model of Io's Interior

When the Galileo spacecraft flew by Io in December 1995, it discovered that Io has an iron inner core. A high-altitude ionosphere was also revealed by the Galileo flyby.

Voyager 1 Image of an Eurupting Volcano (March 1979)

In 1979, the Voyager spacecraft flew through the Jovian system, and one of the most exciting discoveries made by the Voyager spacecraft was the presence of active erupting volcanoes on Io. It was discovered that Io was the most volcanically active body in the solar system, even more active than the Earth. The volcanism on Io is due to the internal heat generated by the tidal tug-of-war between Jupiter, Europa and Ganymede.

Voyager (1979) & Galileo (1996) Images of Pele

The largest volcano on Io is named Pele. Pele was the first volcano discovered on Io and it was actively erupting with a plume an astonishing 300 km high at the time of its discovery. The Voyager spacecraft observed eleven active volcanoes during their flyby. Hundreds of volcanic calderas have also been observed.

Galileo Images of Io (June 1996)

There are no impact craters on Io. The surface of Io must be younger than a millions years old, and is continually being resurfaced by volcanic activity. Also, the surface is very colorful, mottled with red, yellow, white and orange black markings. The surface composition on Io consists largely of sulfur with deposits of frozen sulfur dioxide. The surface on Io is mostly flat plains rising no more than 1km. Moutain ranges up to 9 km high have also been observed.

A torus of sodium gas along with sulfur ions is spread out over Io's orbit. This torus is so large that it has been observed from Earth.

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Last updated: 3/22/99 by C.Lawshe