Physics
Scientific paper
May 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agusm.p21a..01s&link_type=abstract
American Geophysical Union, Spring Meeting 2004, abstract #P21A-01
Physics
5724 Interiors (8147), 5734 Magnetic Fields And Magnetism, 6207 Comparative Planetology, 6218 Jovian Satellites
Scientific paper
The Galileo magnetic field observations at the Galilean Satellites present a number of open questions. Among those is the following: Why should Ganymede have a self-sustained magnetic field while both Io and Europa have not? For Io, the absence of a dynamo in the core may be explained by the enormous amount of tidal heating in the satellite's mantle that keeps the interior warm and the core from freezing. A dynamo driven by chemical convection upon inner core freeze out is then impossible. A dynamo driven by thermal convection is equally unlikely as Wienbruch and Spohn (1995) have already shown. For Europa and Ganymede, tidal heating, at present, is much less important or, respectively, of no importance at all. Because Europa is about 1000km smaller in radius than Ganymede, the former should be cooling faster than the latter and, if anything, Europa should grow an inner core and produce a magnetic field, but not Ganymede. We will argue that the explanation may lie with Ganymede's core having much less Sulphur than Europa's. Sulphur in the core acts to depress the melting point and may have frustrated inner core growth in Europa for the age of the Jovian system. We use the chemical model of Kuskov and Kronrod (2001) in which the three inner Galilean satellites are close to L and LL chondrites in composition and model the interior structure while satisfying the known masses and moment of inertia factors calculated from Galileo data. When stripped of their ice shells, Ganymede has a smaller moment of inertia factor and larger density than Europa. Thus with the same silicate mantle composition, Europa's core must be less dense than Ganymede's. We take the bulk concentration of Sulphur in the iron-silicate deep interiors of the satellites to be about 2 weight-% (Lodders and Fegley, 1998) and assume that the Sulphur is entirely in the core. We then find that Ganymede should have 15 to 20 weight-% Sulphur in its core while the Europan core typically has Sulphur concentrations of 20 weight-% or more. The latter concentration is approximately equal to the eutectic composition in the Fe-FeS system at Europa and Ganymede core pressures (Fei et al., 1997). For about 15 weight-% Sulphur, core freezing is feasible. At the eutectic composition, the melting temperature is about 1100 K (Fei et al. 1997) and core freezing is unlikely. For larger Sulphur concentrations the usual chemical dynamo will not work. The potential for dynamo action on the FeS rich part of the eutectic is largely unexplored, however. Fei et al. 1997, Science 275, 1621-1623; Lodders and Fegley 1998, The Planetary Scientists Companion, Oxford Univ. Press; Kuskov and Kronrod 2001, Icarus 151, 204-227. Wienbruch and Spohn 1995, Planet. Space Sci. 43, 1045-1057.
Breuer Doris
Schmidt Frederic
Spohn Tilman
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