Keywords:icy satellite, planetary atmosphere, planetary formation
One of the long-standing issues in planetary science is why Saturn’s moon Titan possesses a thick atmosphere and why Jupiter’s moon Ganymede not. These satellites are similar in size and mass; however, only Titan has active surface environments, characterized as a thick N2 atmosphere and liquid CH4 cycles. Since Titan’s atmospheric N2 originates from NH3 (Niemann et al., 2005; Sekine et al., 2011), the building materials of Titan may have contained both NH3 and CH4 ices in addition to H2O ice. Previous studies hypothesized that no condensation of NH3 and CH4 would have occurred in the circum-Jovian disk since proto-Jupiter might have formed in a relatively warm region of the protoplanetary disk (e.g., disk temperature > 100 K) (Lunine and Stevenson, 1982). However, recent disk models show that even at a few au, the disk temperature would have become sufficiently low (i.e., < 100 K) to form NH3 and CH4 ices in the disk (e.g., Dodson-Robinson et al., 2009; Oka et al., 2011), thereby calling for a new explanation. Here we show that gas and solids infalling onto massive proto-Jupiter would have experienced extensive shock heating (~104 K) upon accretion. The shock heating is sufficient to dissociate primordial NH3 and CH4 to thermochemically stable N2 and CO, which cannot condensate in the circum-Jovian disk due to their low condensation temperatures. On the other hand, dissociation of NH3 and CH4 proceeds only incompletely upon accretion onto less massive proto-Saturn. Accordingly, the building materials of Saturn’s icy moons contain abundance of survived NH3 and CH4 as ices. We suggest that giant planet’s mass is a critical factor to determine the chemical compositions, surface environments, and potential habitability of the icy moons.
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Sekine et al. (2011) Nature Geosci. 4, 359