Callisto

Callisto is the second-largest moon of Jupiter and the outermost of the four Galilean satellites. This icy moon was discovered on 7^th January, 1610, by Galileo Galilei. Callisto has a mean diameter of 4,818 km, a mass of 1.077 × 10²³ kg, and an estimated density of 1.85 g/cm3 (1,850 kg/m3) - the lowest density of the four Galilean satellites. The acceleration of gravity on the surface of Callisto is 1.24 m/s² (Earth has a value of 9.8 m/s² at sea level), and the escape velocity is 2.4 km/s. Callisto orbits Jupiter at a distance of 1.883 × 10⁶ km with a period of 16.689 days. The moon’s orbit is further characterized by an eccentricity of 0.007 and an inclination of 0.51°. Since Callisto has a rotation period of 16.689 days, it is in synchronous rotation and therefore keeps the same hemisphere facing Jupiter.

Planetary scientists believe that Callisto, sometimes called Jiv, consists of equal portions of rock and water-ice. Callisto lacks mountains and is the most heavily cratered moon in the solar system. The equatorial subsurface temperature is 168 K and the surface composition is dirty ice. Callisto has a visual albedo of 0.19. The moon’s crust appears very ancient, probably dating back about 4 billion years to just after the formation of the solar system.

Magnetometer data collected in the late 1990s by NASA’s Galileo spacecraft suggest to scientists that something interesting is hidden below Callisto’s icy, ancient surface - quite possibly a salty ocean. Specifically, these Galileo spacecraft data indicate that Callisto’s magnetic field, like Europa’s, is variable, which can be explained by the presence of varying electrical currents associated with Jupiter that flow near Callisto’s surface. Because Callisto’s atmosphere is extremely tenuous and lacks charged particles, scientists examining these magnetic field data believe this wisp of an atmosphere (primarily carbon dioxide) is not responsible for variations of the moon’s magnetic field. The likely candidate currently is the possible presence of a layer of salty, melted ice beneath the surface.

Questions to Ponder

• What is the nature of Callisto’s Atmosphere?
• What makes Callisto different from Jupiter’s other moons?