Thursday, May 21, 2015

Subsurface Ocean of Liquid Water on Callisto

Figure 1: Callisto (bottom left), Jupiter (top right) and Europa (below and left of Jupiter’s Great Red Spot) as viewed in 2000 by NASA’s Cassini spacecraft on its way to Saturn. Credit: NASA/JPL/University of Arizona.

Callisto was discovered by Galileo Galilei, an Italian astronomer, in January 1610 along with three other large moons around Jupiter - Io, Europa and Ganymede. These four large moons are known as the Galilean satellites of Jupiter and Callisto is the outermost member. With a diameter of 4,821 km, Callisto is the third largest moon in the Solar System and is nearly as large as the planet Mercury. Callisto orbits far enough from Jupiter that it does not participate in the orbital resonance that the three inner Galilean satellites are in. As a result, Callisto has never experienced any appreciable tidal heating.

With a mean density of 1.83 g/cm³, Callisto’s bulk composition is roughly half rocky material and half water-ice. Unlike the three other Galilean satellites, Callisto never got warm enough to fully differentiate into a rocky core and an icy mantle. Instead, Callisto is only partially differentiated, with the proportion of rocky material increasing with depth. Nevertheless, interior models of Callisto based on measurements of its density and moment of inertia do not rule out the existence of a tiny rocky core in the center. However, the size of such a rocky core cannot exceed a diameter of about 1,200 km.

Figure 2: A model of the internal structure of Callisto with an internal ocean. The icy crust is 135 to 150 km thick; the thickness of the water layer is ~120 to 180 km, and the total thickness of the water-ice shell is ~270 to 315 km. The solid curve and the dashed one are for densities of the rock-iron component equal to 3.62 and 3.15 g/cm³, respectively. Kuskov & Kronrod (2005).

When NASA’s Galileo spacecraft was in orbit around Jupiter, it detected the presence of an induced magnetic field around Callisto. This is evidence for the existence of a conducting layer just beneath the surface of Callisto. Such a conducting layer may be best explained by the presence of a salty subsurface ocean of liquid water. However, theoretical models have difficulties showing how such an ocean could have remained liquid till now. This is because Callisto does not experience any significant tidal heating. Furthermore, solid-state convection in the ice layer overlying the subsurface ocean would drive convective heat loss, leading to complete freezing of the ocean in ~100 million years.

One way such an ocean can be kept from freezing completely is the presence of substances that lower the melting temperature of water-ice. For example, the presence of ammonia can lower the melting temperature of water-ice to ~176 K and the presence of chloride salts or sulphuric acid can lower the melting temperature to ~210 K. A study by Javier Ruiz (2001) suggests that even without the presence of substances to depress the melting temperature of water-ice, the subsurface ocean on Callisto can be kept from freezing completely if the overlying ice layer is more rigid than commonly assumed, thereby greatly reducing the rate of heat loss through convection.

A rigid, non-convecting ice shell overlying an internal ocean of liquid water appears to be consistent with the ancient surface geology of Callisto. The surface of Callisto is one of the most heavily cratered in the Solar System and shows a lack of endogenic geological processes. A stable and non-convecting ice shell can explain Callisto’s geologically inactive surface. Callisto’s salty subsurface ocean of liquid water lies beneath an icy shell that is estimated to be between 100 to 200 kilometres in thickness. If icy shells are more rigid and more difficult to undergo convection than commonly assumed, then the existence of subsurface oceans of liquid water on the numerous icy worlds in the Solar System could be more common than thought.

- Kuskov & Kronrod, “Models of the Internal Structure of Callisto”, Solar System Research, Vol. 39, No. 4, 2005, pp. 283-301
- K. Khurana et al., “Induced magnetic fields as evidence for subsurface oceans in Europa and Callisto”, Nature 395, 777-780 (22 October 1998)
- Javier Ruiz, “The stability against freezing of an internal liquid-water ocean in Callisto”, Nature 412, 409-411 (26 July 2001)