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Io - The Volcanic Moon


Onward to Io! That's the rallying cry now that NASA's Galileo spacecraft has successfully completed its fourth and final flyby of Jupiter's pockmarked moon, Callisto. The Callisto encounters were designed to lower Galileo's orbit to bring it closer to the fiery moon Io, the most volcanic body in the solar system. Galileo will fly by Io in October and then again in November. More.

Io's Surface
There are no impact craters on Io. The surface of Io must be younger than a Million 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 1 km. Moutain ranges up to 9 km high have also been observed.

The Io Plasma Torus
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. As Jupiter rotates, it takes its magnetic field around with it, sweeping past Io and stripping off about 1,000 kilograms (1 ton) of Io's material every second! This material becomes ionized in the magnetic field and forms a doughnut-shaped cloud of intense radiation referred to as a plasma torus. Some of the ions are pulled into Jupiter's atmosphere along the magnetic lines of force and create auroras in the planet's upper atmosphere. It is the ions escaping from this torus that inflate Jupiter's magnetosphere to over twice the size we would expect.

Science Findings


Volcanic Changes


More about Io

Quick Facts about Io

Jan 7, 1610 by Galileo Galilei

Diameter (km):

Mass (kg):

Mass (Earth = 1)

Surface Gravity
  (Earth = 1):


Mean Distance from
  Jupiter (km):


Mean Distance From
  Jupiter (Rj):


Mean Distance from
  Sun (AU):


Orbital period (days):

Rotational period (days):

Density (gm/cm3)

Orbit Eccentricity:

Orbit Inclination


Orbit Speed (km/sec):

Escape Velocity (km/sec):

Visual Albedo:

Typical Subsolar
  Temperature (K):


Typical Hotspot
  Temperature (K):


Surface Composition:

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Last updated 10/01/01.

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