Resemblance and difference between the constitution of the Moon and Io

The Moon and Io are comparable in the mean density, the mass and moment of inertia in spite of the principle differences in bulk compositions and mass es of central planets. The Moon is the only extraterrestrial body for which we have information about the mantle seismic velocities as a function of d epth. Volcanic activity on the Moon was completed in the early stage of its evolution. Io is the most active of all known satellites and it is much ri cher in volatiles and iron than the Moon. In this work, internally consiste nt models of the constitution of the Moon and Io based on the geophysical ( the seismic velocities, the mass and moment of inertia) and geochemical (ch emical and phase composition) constraints are compared. Thermodynamic model ing of the phase relations and physical properties in the CaO-FeO-MgO-Al2O3 -SiO2-Fe-FeS system was used to estimate the bulk composition, the density distribution in the mantle and radius of a core from the geophysical data. The solution of the inverse problem is based on the minimization of the dev iations between the calculated and observed geophysical parameters. The sma llest and largest core radii are estimated to be: 500-655 km for an Fe core (6-10%) and 810-1080 km far an FeS core (12-28% of total mass) for Io; 290 -345 km for an Fe core (1.1-1.9%) and 450-530 km for an FeS core (2.4-4% of total mass) for the Moon. The atomic ratios of total iron to silicon for I o are estimated to be (Fe/Si)= 0.40-0.66 in comparison with 0.22-0.24 for t he Moon, 0.51-0.59 for the LL and L chondrites, and 0.74-0.8 for CV3 and CM 2 meteorites. The current results show that the proposed Io's compositions similar to that of carbonaceous chondrites do not satisfy to bulk Fe/Si rat ios inferred from the geophysical constraints. The bulk composition of Io i s estimated to be close to that of the L and LL chondrites having 8-13 wt.% of iron and iron sulfide; a change of the core composition from pure iron to pure iron sulfide leads to a change of the core radius from 600 to 625 k m (8-9%) to 930-965 km (18-20%); the radius of the eutectic Fe-FeS-core is estimated to be 840-870 km (14-16% of Io's mass; (Fe/Si)(at) = 0.55-0.57). If Io does have such a composition, then the core is probably Fe or Fe-rich , whereas a large FeS core is excluded by the composition of the L and LL c hondrites. A comparison of estimated bulk chemistry of Io with much more re liable composition of the Moon shows that Earth's and Jupiter's satellites have absolutely different bulk compositions. (C) 2000 Elsevier Science Ltd. All rights reserved.