Europas Ocean

With a diameter of 3120 km, Europa is one of the four largest satellites of Jupiter, the so called "Galilean" satellites which were discovered by Galileo Galilei in 1610. With the "grand tour" of the Voyager mission in the outer solar system, a tremendous amount of new information has been obtained on the giant planets and their

*François Raulin—LISA, Universités Paris 12 et Paris 7, CNRS, 61 Av. du Général de Gaulle, 94010, Créteil, France. Email: [email protected]

Europa

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Figure 3.1. Possible internal structure of the four Galilean satellites (adapted from NASA/JPL).

Ganymede

Figure 3.1. Possible internal structure of the four Galilean satellites (adapted from NASA/JPL).

Figure 3.2. Image of Europa taken during Voyager 2 closest approach, in July 1979. The surface is young and shows complex array of streaks indicating that the crust has been fractured and filled by materials released from the interior. Credit: NASA/JPL.

satellites (Fig. 3.2). In particular, from the data collected during the flyby ofJupiter by the two Voyager spacecrafts in 1979, models of the internal structure of the Galilean satellites started to be developed, some suggesting the possible presence of an internal ocean in several of them, particularly in Europa (Fig. 3.1). However, most of our current knowledge on this satellite comes from the Galileo mission which explored the Jupiter system from 1995 to 2003. The high resolution images of Europa's surface showed chaotic terrains, poorly craterized, suggesting resurfacing processes and with many deep fractures indicative of a relatively fluid sub-surface (Fig. 3.3). These observations re-enforced the assumption of the presence of an internal ocean and suggested that it may be not very far from the surface. The mean density (compared to water) of Europa, is about 3, suggesting that it is made of silicates (density of 3-3.5) and water ice (density of ~0.9). The Galileo mission detected a strong induc-

Surface Europa
Figure 3.3. High resolution image of Europa's surface (area of about 34 x 42 km) taken by the Galileo mission in 1997, from a distance of 53490 km. It shows crustal plates broken apart and shifted, resembling features seen on polar ice seas on Earth. Credit: NASA/JPL.
Figure 3.4. Artist view of a cryobot exploring Europa's ocean. Credit: NASA/JPL.

tive magnetic field in Europa. It can be explained by the presence either of an iron core, or of a water ocean more than 10 km thick and with a conductivity similar to that of the terrestrial oceans.7 So far the available data do not allow discriminating between the two possibilities, but the presence of an internal water ocean that may be located (relatively) close to the surface crust of water ice seems likely.

Over the past thirty years, many papers have been published on this subject (see for instance refs. 7, 8 for a review). The subsurface ocean could be about 100 km thick, covered by an ice shell ofabout 15 km and with a water temperature of 4°C.9 More recently Hand and Chyba10 have re-examined the possible properties of Europa's ocean on the basis ofthe Galileo data. Some ofthe main conclusions of their work indicate the possibility that the European ocean is a near-saturation magnesium sulphate aqueous solution, with an ice shell thickness of 4 km or less.

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