[P-PS01_30PM1] Toward JUICE and future explorations of outer solar system

[PPS01-04] Longevity of an internal ocean in Ganymede

*木村 淳¹、Vance Steven²、Hussmann Hauke³、栗田 敬⁴ (1.東京工業大学地球生命研究所、2.ジェット推進研究所、3.ドイツ航空宇宙研究所、4.東京大学地震研究所)

キーワード：satellite, evolution, ocean, habitability, ice, Ganymede

The outer solar system may provide a potential habitat of extra-terrestrial life. Most moons orbiting planets in the outer Solar System, at orbits beyond the snow line, such as Jupiter or Saturn, are covered with water ice and are referred to as "icy moons". Galileo's detection of induced magnetic fields combined with imaged surface characteristics and thermal equilibrium modeling of the moons, support that the Jovian icy moons Europa and Ganymede, and possibly Callisto, may harbor liquid water oceans underneath the icy crusts. The presence of internal oceans in the icy moons means that a deep habitat different from Earth's biosphere may exist, located beyond the "habitable zone" of the Sun. Evidence for oceans is not definitive, however, and awaits confirmation measurements. Also, the depth and composition of the oceans remain unclear, as do their variability through time.Here we focus on Ganymede, the largest moon in the Solar System and the primary target of a new mission to the Jupiter system, the Jupiter Icy Moons Explorer (JUICE), which is planned by the European Space Agency (ESA). The bulk density of Ganymede, 1.936 g/cc, indicates a composition of approximately equal amounts of rocky material and water. Previous measurements of Ganymede's gravitational field and intrinsic magnetic field by the Galileo orbiter suggest that its interior is completely differentiated into three layers, a convecting metallic core at the center, a rocky mantle surrounding the core, and an outermost water-ice shell. The water-ice layer in total has a thickness of 800-1000 km. A layer of melted, salty water that lies beneath the icy crust would be the best way to explain the signal of magnetic induction.To investigate the lifetime of an ocean (thickness change through time) assumed to be initially in an entirely liquid state, we performed numerical simulations for the internal thermal evolution using a spherically symmetric model for the convective and conductive heat transfer with radial dependence of viscosity, heat source distribution, and other material properties. Here we take into account the energy due to decay of long-lived radioactive elements and also evaluate the effect of tidal heating. If the ocean were composed of pure water, a primordial ocean would have disappeared (completely solidified) within 1 Gyr even if tidal heating for the current orbital state were included. Consistent with previous predictions , this result indicates that significant tidal heating in the past, or strong antifreeze components (e.g., salts or ammonia), are needed if the presence of the internal ocean in Ganymede would be confirmed from future JUICE observations. We numerically investigate their effect on the lifetime of the ocean.