The origin of Ganymede: implications for volatile content

We use an evolutionary turbulent model of Jupiter’s subnebula to constrain the composition of ices incorporated in its regular icy satellites [1]. We consider CO2, CH3OH, CO, CH4, N2, NH3, H2S, Ar, Kr and Xe as the major volatile species existing in the gasphase of the solar nebula. All these volatile species, except CO2 which crystallized as a pure condensate, are assumed to be trapped by H2O to form hydrates or clathrate hydrates in the solar nebula. Once condensed, these ices were incorporated into the growing planetesimals produced in the feeding zone of protoJupiter. Some of these solids then flowed from the solar nebula to the subnebula, and have been accreted by the forming Jovian regular satellites. We show that ices embedded in solids entering at early epochs into the Jovian subdisk were all vaporized. This leads us to consider two different scenarios of regular icy satellite formation in order to estimate the composition of the ices they contain. In the first scenario, icy satellites were accreted from planetesimals that have been produced in Jupiter’s feeding zone without further vaporization, whereas, in the second scenario, icy satellites were accreted from planetesimals produced in the Jovian subnebula. In this latter case, we study the evolution of carbon and nitrogen gas-phase chemistries in the Jovian subnebula and we show that the conversions of N2 to NH3, of CO to CO2, and of CO to CH4 were all inhibited in the major part of the subdisk. Finally, we assess the mass abundances of the major volatile species with respect to H2O in the interiors of the Jovian regular icy satellites, including Ganymede. Chemistry in the Jovian subnebula The net reactions relating CO, CH4, CO2, N2 and NH3 in a gas dominated by H2 are CO + H2O = CO2 + H2 (1) CO + 3H2 = CH4 + H2O (2) N2 + 3H2 = 2NH3 (3) which all proceed to the right with decreasing temperature at constant pressure. Reaction (1) has a rate coefficient which is very low, even at temperatures as high as 2000 K [1]. Such a high temperature range is only reached at distances quite close to Jupiter and at early epochs in the Jovian subnebula. As a result, the amount of carbon species produced through this reaction is insignificant during the whole lifetime of the subnebula. Reactions (2) and (3) are illustrated by Figs. 1 and 2. At equilibrium, CO:CH4 and N2:NH3 ratios depend only on local conditions of temperature and pressure. CO:CH4 and N2:NH3 ratios of 1000, 1 and 0.001 are plotted in Figs. 1 and 2, and are compared to our turbulent model at three different epochs (0 yr, 0.56 Myr and 0.6 Myr). These figures show that, when the kinetics of chemical reactions are not considered, CH4 and NH3 progressively dominate with time in the major part of our turbulent model of the Jovian subnebula rather than CO and N2. However, the actual CO:CH4 and N2:NH3 ratios depend on the chemical timescales, which characterize the rates of CO to CH4 and N2 to NH3 conversions in our model of the Jovian subnebula. Taking into account the kinetics of chemical reactions, we have inferred that the efficiency of the conversion is limited only to the inner part of the Jovian subnebula and at early times of its first phase. All these calculations imply that CO:CH4, CO2/CO and N2:NH3 gas phase molecular ratios remain almost constant during the whole lifetime of the Jovian subnebula. In these conditions, the composition of the ices produced within the Jovian subnebula is then close to that of the ices produced in the solar nebula [2] (see Table 1).