Interior Structure Models of the Galilean Satellites: Implications for Differences Between Their Chemical Compositions

Details Interior Structure Models of the Galilean Satellites: Implications for Differences Between Their Chemical Compositions Interior Structure Models of the Galilean Satellites: Implications for Differences Between Their Chemical Compositions

Dec 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999baas...31.1587s&link_type=abstract

American Astronomical Society, DPS Meeting #31, late abstracts, #59.20; Bulletin of the American Astronomical Society, Vol. 31,

Astronomy and Astrophysics

Astronomy

Scientific paper

The recent determinations of the moment-of-intertia factors of the Galilean Satellites of Jupiter allow new and improved models of the interior structure of these moons and estimates of their bulk chemical compositions. Two- and three-layer structure models are constructed and ice, rock, and metal concentrations are calculated. We take the rock to consist of an Fe-rich and an Mg-rich component which, for simplicity and lack of a detailed knowledge, are represented by the olivine (ol) end members fayalite (fa) and forsterite (fo), respectively. Self- compression and temperature of the interior are accounted for by using a Murnaghan equation of state. If it is assumed that Io's mantle is supersolidus, a mantle composition of approximately 80 wt.% fo and 20 wt.% fa is calculated, consistent with recent Galileo spectral imaging results. In general, the concentration of fa increases with increasing temperature. Europa's water (ice+liquid) shell must have a thickness of at least 120 km comprising about 10 wt.% of its mass to allow silicate mantle densities consistent with ol compositions. With increasing water shell thickness, the fa content increases at the expense of fo. Both moons have similar rock contents of about 80 to 90 wt.%. Io's core mass fraction amounts to 10 to 20 wt.%, while Europa's metallic core takes up to 15 wt.% of the total mass. Ganymede consists of an ice mass fraction of about 50 wt.% and has much less well constrained rock (20 to 50 wt.%) and metal (up to 35 wt.%) concentrations. The bulk composition of Callisto ranges roughly between 40 and 60 wt.% ice, 30 and 60 wt.% rock and up to 15 wt.% metal. The differences between the inner pair, Io and Europa, and the outer pair, Ganymede and Callisto, become most obvious if their bulk iron- to-silicon ratios Fe/Si are compared. For Io and Europa, Fe/Si ratios plot below the CI carbonaceous chondrite value of 1.7 +/- 0.1, whereas Ganymede and Callisto Fe/Si ratios cover a broad range above the chondritic value due to their wide variation of permissible rock and metal concentrations. This finding may indicate substantial differences in iron-silicate fractionation during the formation of both pairs of satellites in the protojovian nebula.

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