[P-PS01_30PM2] Toward JUICE and future explorations of outer solar system

[PPS01-07] 周惑星ガス円盤中で形成する巨大氷衛星の原始大気

*三上 峻¹、倉本 圭¹ (1.北海道大学理学院宇宙理学専攻)

キーワード：巨大氷衛星, 大気, 周惑星円盤

In spite of the great similarity in size and mean density, the giant icy satellites Ganymede, Callisto, and Titan have very different surface environments. In particular, only Titan holds a thick atmosphere dominated by N₂. Recent data of the Cassini spacecraft indicated that atmospheric N₂ is probably originated from other nitrogen-bearing species like NH₃. However, it still remains an open question when and how N₂ was generated. This is partly because the physical states of giant icy satellites have been poorly understood.According to a widely-accepted theory of regular satellites formation, the giant icy satellites were formed in subnebulae with low temperature and low pressure taking a long accretion time. Some models assert that their surfaces were kept too cold to induce significant differentiation during accretion. However, these satellites may capture a significant amount of subnebula gas, which possibly forms proto-atmospheres along with gases volatilized from icy components. Such a hybrid-type proto-atmosphere may have significant blanketing effect. Here, we numerically analyze the structure and effect of a hybrid-type proto-atmosphere. Our model atmosphere is hydrostatically connected with subnebula at the satellite Hill radius. It contains H₂ and He as nebula gas components, H₂O and NH3 as volatilized ice components. The radiative-convective equilibrium structure is solved as a function of surface temperature. The subnebula conditions are given by Canup and Ward (2002), the temperatures are 150 K at Ganymede, 120 K at Callisto, and 50 K at Titan, and the corresponding subnebula pressures are varied over 0.1-10 Pa. For all the boundary conditions, the proto-atmosphere is opaque due to water vapor, so that the outgoing thermal radiation (OTR) flux at top of the atmosphere is smaller than that of black body radiation without atmosphere when the surface temperature is higher than 273 K. When the surface temperature is lower, the OTR fluxes from the proto-atmospheres of Ganymede and Callisto are close to black-body radiation because these atmospheres have low surface pressure and are optically thin due to large scale height under high background temperature. On the other hand, the proto-atmosphere of Titan has another type of solution with the OTR fluxes significant lower than blackbody radiation under low surface temperature. This is due to the formation of optically thick atmosphere tightly bounded by gravity because of low background temperature. These results imply that a warm proto-atmosphere near 200 K could be kept on Titan for a long time after the end of accretion. Our stability analysis suggests that the proto-atmospheres of Ganymede and Callisto were lost associated with the dissipation of the Jovian subnebula, but that of Titan survived after the dissipation of the Saturnian subnebula. In the case, NH₃ vapor pressure would be kept high under the irradiation of the solar UV for a long time. The present atmospheric N₂ of Titan may be generated by photochemical reaction of NH₃ vapor in such a warm proto-atmosphere.