[M-IS11] 水惑星学

[MIS11-P14] なぜタイタンには大気があり、ガニメデにはないのか？―タイタンとガニメデの二分性の起源

*関根 康人¹、青山 雄彦²、生駒 大洋²、谷川 享行³ (1.東京工業大学地球生命研究所、2.東京大学大学院理学系研究科、3.一関工業高等専門学校総合科学)

キーワード：氷衛星、惑星大気、惑星形成

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.

Dodson-Robinson et al. (2009) Icarus 200, 672
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