Coevolution of moons and the spin axis of their host planet

Satellites alter the spin-axis precession rate of their host planet. The tidal migration of moons can therefore produce the crossing of resonances between the spin-axis and orbital precession modes of a planet. Once the planet is captured in resonance, the migration of its moons produces a large obliquity increase over billions of years (Saillenfest & Lari 2021).

This mechanism has direct applications in the Solar system. The current obliquity of Jupiter is small (3°), but Jupiter's spin axis will be gradually tilted in the future because of the migration of its moons (Saillenfest et al. 2020). Saturn's spin axis is currently inclined by 27° and this is due to the tidal migration of its most massive satellite, Titan (Saillenfest et al. 2021a, b). The extreme obliquity of Uranus (98°) could also be due to this phenomenon: an ancient massive moon could have converged to an unstable configuration, ultimately freezing Uranus's spin axis in its current orientation (Saillenfest et al. 2022).

References

• Saillenfest, Lari, Courtot, 2020, A&A 640, A11, The future large obliquity of Jupiter
• Saillenfest, Lari, Boué, 2021, Nature Astronomy 5, 345, The large obliquity of Saturn explained by the fast migration of Titan
• Saillenfest, Lari, Boué, Courtot, 2021, A&A 647, A92, The past and future obliquity of Saturn as Titan migrates
• Saillenfest, Lari, 2021, A&A 654, A83, Future destabilisation of Titan as a result of Saturn's tilting
• Saillenfest, Rogoszinski, Lari, Baillié, Boué, Crida, Lainey, 2022, A&A, Tilting Uranus via the migration of an ancient satellite

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