With a diameter of over 4,800 km (2,985 miles), Callisto is the third largest satellite in the solar system and is almost the size of Mercury. Callisto is the outermost of the Galilean satellites, and orbits beyonds Jupiter's main radiation belts. It has the lowest density of the Galilean satellites (1.86 grams/cubic centimeter). Its interior is probably similar to Ganymede except the inner rocky core is smaller, and this core is surrounded by a large icy mantle. Callisto's surface is the darkest of the Galileans, but it is twice as bright as our own Moon.

Landscape
Callisto is the most heavily cratered object in the solar system. It is thought to be a long dead world, with a nearly complete absence of any geologic activity on its surface. In fact, Callisto is the only body greater than 1000 km in diameter in the solar system that has shown no signs of undergoing any extensive resurfacing since impacts have molded its surface. With a surface age of about 4 billion years, Callisto has the oldest landscape in the solar system.

Science Findings

• Jupiter's Moon Callisto May Hide Salty Ocean (October 21, 1998)
• Galileo Mission Finds Strange Interior of Callisto (June 4, 1998)
• Galileo Returns New Insights into Callisto and Europa (May 23, 1997)
• New Jupiter Findings: Oxygen at Callisto's Surface, Sulfur Dioxide Sources at Io (December 10, 1997)

More about Callisto

Diameter (km):
4,806

Mass (kg):
1.077e23

Mass (Earth = 1)
1.807e-02

Surface Gravity
  (Earth = 1):
0.127

Mean Distance from
  Jupiter (km):
1,883,000

Mean Distance From
  Jupiter (Rj):
26.6

Mean Distance from
  Sun (AU):
5.203

Orbital period (days):
16.68902

Rotational period (days):
16.68902

Density (gm/cm³)
1.86

Orbit Eccentricity:
0.007

Orbit Inclination
  (degrees):
0.281

Orbit Speed (km/sec):
8.21