FORMATION OF A HOT PROTO-ATMOSPHERE ON THE ACCRETING GIANT ICY SATELLITE - IMPLICATIONS FOR THE ORIGIN AND EVOLUTION OF TITAN, GANYMEDE, AND CALLISTO

Judging from accretion energy and accretion time for the giant icy satellites it is suggested that a proto-atmosphere is formed by the evaporation of icy materials during accretion of these bodies around the proto-gaseous giant planets. We study the blanketing effect of proto-atmosphere during accretion of these satellites in the gas-free environment. We use a gray atmosphere model in which the condensation of H2O in a convective atmosphere is taken into account. The numerical results strongly suggest that the accretion energy flux is large enough to increase the surface temperature higher than approximately 500 K during accretion due to the blanketing effect of proto-atmosphere as long as the accretion time is shorter than 10(5) years. Such a high surface temperature causes the formation of a deep water-rich ocean due to the melting of icy materials. Also a rocky core should be eventually formed by sinking of rocky materials through the water-rich ocean during accretion. Therefore, the apparent difference in the surface geologic features between Ganymede and Callisto can hardly be explained by whether or not these bodies have experienced the formation of rocky core. Stability of hydrostatic structure of the proto-atmosphere is also studied. Vigorous escape of the proto-atmosphere is likely to occur under high surface temperature. A large portion of accretional energy is possibly consumed by the vigorous escape during accertion. Thus, the giant icy satellites may lose a significant amount of icy materials during their accretions. This can explain the ice-depleted composition of Titan inferred from the observed mean densities of Saturnian satellites, if the accretion occurs within 10(4) - 10(5) years. Such a significant loss of icy materials is also expected for Ganymede and Callisto. The escape and catalytic reaction in the hot proto-atmosphere may play an important role in formation of the present N2-abundant and CO-depleted atmosphere of Titan.