The interior of Io

Abstract

Io is the Galilean satellite nearest to Jupiter and it is therefore subject to the most intense tidal forces. These forces deform Io in a way that is determined by the properties of Io's interior, thus, we can use measurements of the deformation of Io to obtain information on Io's internal structure. Io's rotational and tidal deforma-tion was measured by imaging (Thomas et al., 1998) and by Earth-based Doppler tracking of the Galileo spacecraft during several ¯y-bys of the satellite between 1999 and 2002 and during insertion of the spacecraft into Jupiter orbit in 1995 (Anderson et al., 1996, 2001; Schubert et al., 2004). Even prior to the Galileo spacecraft's observa-tions of Io's tidal deformation, the moon's mass and radius and therefore density were known from Pioneer and Voyager spacecraft observations. The Galileo spacecraft data improved the accuracy of measurements of Io's mass and radius and determined Io's quadrupole gravitational coecients. Table 5.1 summarizes Io's basic physical properties. Io is about the size of the Earth's Moon, but it is considerably more dense (the lunar density is 3,341 kg m À3), indicating that there is more iron in Io than in the Moon. In fact, on the basis of density alone, it can be inferred that Io has an iron core of considerable size. Moreover, if the density is supplemented by shape information (the 3 ellipsoidal radii of the tidally and rotationally distorted Io) available from Voyager limb meas-urements, and it is assumed that Io is in hydrostatic equilibrium, the size of Io's iron core can be well constrained (Segatz et al., 1988). So, even before the Galileo mission, we had a good idea about the basic structure of Io's interior. The Galileo measure-ments of the tidally and rotationally distorted Ionian gravitational ®eld have veri®ed the equilibrium shape of Io and have provided better constraints on interior models of the satellite. Below, we will show that Io is a two-layer body consisting of a metallic core and a silicate mantle. Io's intensive volcanic activity and crater-free surface also suggests it likely that the satellite has di€erentiated a global crustal layer below which lies a partially molten asthenosphere. 5.1 TIDAL AND ROTATIONAL DEFORMATION Io is in synchronous rotation, meaning that its orbital period and rotational period are equal so that Io keeps one face to Jupiter at all times. Since Io is rotating, it experiences a centrifugal force that acts to ¯atten its shape. By keeping the same face to Jupiter, Io also experiences a steady tidal force that acts to elongate it along the line from Io to Jupiter. These forces are both nearly constant, since Io's rotation rate changes very slowly and its orientation toward Jupiter remains nearly ®xed. Io has therefore had time to relax into an equilibrium shape which, to ®rst order, is a triaxial ellipsoid, with the long axis pointing toward Jupiter and the short axis aligned with the rotation pole. In this section we show how the response of Io to tidal and rotational forces may be used to understand Io's interior. The steady forcing potentials due to rotation and tides È 2r and È 2t are given by (Kaula, 1964): È 2r ˆ ! 2 r