[M-IS11] Aqua planetology

[MIS11-P14] Why does Titan have an atmosphere, and why not Ganymede?—The origin of Titan-Ganymede dichotomy

*Yasuhito Sekine¹, Yuhiko Aoyama², Masahiro Ikoma², Takayuki Tanigawa³ (1.Earth-Life Science Insitute, Tokyo Institute of Technology, 2.Dept. of Earth and Planetary Sci., University of Tokyo, 3.Section of Liberal Arts and Sciences, National Inst. of Tech., Ichinoseki College)

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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Lunine and Stevenson (1982) Icarus 52, 14
Niemann et al. (2005) Nature 438, 779
Oka et al. (2011) Astrophys. J. 738, 141
Sekine et al. (2011) Nature Geosci. 4, 359