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

General/Overview

• Possible Internal Structures of the Galilean Satellites (November 18, 1997)
• Galileo Finds Iron Core In Io (May 3, 1996)

Volcanic Changes

• Galileo Spacecraft Sees Volcanic Fireworks On Jupiter's Moon Io (July 2, 1998)
• Galileo Finds Arizona-Sized Volcanic Deposit on Io (November 5, 1997)
• Galileo Finds High-Altitude Ionosphere at Io (October 23, 1996)
• Galileo Finds Big Changes in Io (July 18, 1996)

Atmosphere

• New Jupiter Findings: Oxygen at Callisto's Surface, Sulfur Dioxide Sources At Io (December 10, 1997).
• Galileo Finds High-Altitude Ionosphere at Io (October 23, 1996)

More about Io

Diameter (km):
3,630

Mass (kg):
8.94x10²²

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/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:
0.61

Typical Subsolar
  Temperature (K):
~135

Typical Hotspot
  Temperature (K):
~300

Surface Composition:
Sulphur