Constraints on the presence of volatiles in Ganymede and Callisto from an evolutionary turbulent model of the Jovian subnebula

We describe an evolutionary turbulent one-dimensional model of the Jovian subnebula, based on the previous models of the solar nebula of Dubrulle (Icarus 106 (1993) 59), and of Drouart et al. (Icarus 140 (1999) 129), as well as on the evolutionary turbulent model of the subnebula of Saturn of Mousis et al. (Icarus 156 (2002a) 162). We show that the conversion of N2 to NH3 and that of CO to CH4 were inhibited in the Jovian subnebula, in conflict with the conclusions of Prinn and Fegley (Astrophys. J. 249 (1981) 308). We argue that grains from which ultimately formed Galilean satellites were initially produced in the cooling feeding zone of Jupiter prior to the formation of the subdisk surrounding the giant planet. It is assumed that hydrates of NH3 and clathrate hydrates of CO, CH4, and N2 formed in the feeding zone (Astrophys. J. Lett. 550 (2001a) L227; Astrophys. J. Lett. 559 (2001b) L183) were incorporated in planetesimals embedded in the cold outer part of the Jovian subnebula. Under the assumption that planetesimals which formed Ganymede and Callisto migrated from the outer region and did not outgas during this migration, the per mass abundances of NH3, N2, CO, and CH4 with respect to H2O in the interiors of these satellites are estimated. Calculated values depend upon the poorly known relative abundances of these species in the solar nebula. However, they provide an interpretation of the presence of NH3 suspected in subsurface oceans of Ganymede and Callisto, and which is consistent with the measurement of the internal magnetic field of these satellites measured by the Galileo mission (Geophys. Res. Lett. 24 (1997) 2155; J. Geophys. Res. 104 (1999) 4609).