Moon of Jupiter, one of the Galeleian Moons. Named after - in Greek mythology - the daughter of king Inachos.

Being close to Jupiter and the large moons Europa and Ganymede, Io suffers from enourmous tidal waves, sometimes more than 100 meters (300+ feet). Beacause of this, the surface cracks open and Io is covered in lava, with about nine major active volcanos. Io is also highly charged, electrically, affecting the atmosphere of Jupiter.

As can be seen from the data below, it has an almost circular orbit, only slightly elliptical. Just like our moon, the period of rotation equals the period of revolution, meaning that it is always the same side facing Jupiter.

The moon has a heavy impact on the magnetosphere of Jupiter. A current of approximately 1 mega ampere flows between the ionosphere of Jupiter and Io. This supports the theory that Io - much like Earth - has a melted core of metal, and also explains the volcanic activity. It also explains the eruptions of longwave electromagnetic radiation from Jupiter that correlate with Io's orbit.

Discovery:                             Jan 7, 1610 by Galileo Galilei
Diameter (km):                         3,630                 
Mass (kg):                             8.94e22 kg 
Mass (Earth = 1)                       0.014960 
Surface Gravity (Earth = 1):           0.183
Mean Distance from Jupiter (km):       421,000
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/cm3)                       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:                         0.01
Subsolar Temperature (K):              ?
Equatorial Subsurface Temperature (K): ?
Surface Composition:                   Sulphur
Data from NASA

2005.5.17@4:55 jrn says re Io: The orbital resonance node has one reason for Io's volcanic activity.