Evolution of Saturn’s mid-sized moons

Abstract

The orbits of Saturn’s inner mid-sized moons (Mimas, Enceladus, Tethys, Dione and Rhea) have been notably difficult to reconcile with their geology. Here we present numerical simulations coupling thermal, geophysical and simplified orbital evolution for 4.5 billion years that reproduce the observed characteristics of their orbits and interiors, provided that the outer four moons are old. Tidal dissipation within Saturn expands the moons’ orbits over time. Dissipation within the moons decreases their eccentricities, which are episodically increased by moon−moon interactions, causing past or present oceans to exist in the interiors of Enceladus, Dione and Tethys. In contrast, Mimas’s proximity to Saturn’s rings generates interactions that cause such rapid orbital expansion that Mimas must have formed only 0.1−1 billion years ago if it postdates the rings. The resulting lack of radionuclides keeps it geologically inactive. These simulations explain the Mimas−Enceladus dichotomy, reconcile the moons’ orbital properties and geological diversity, and self-consistently produce a recent ocean on Enceladus.

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Fig. 1: Evolution of the moons’ semi-major axes over time due to tidal dissipation inside Saturn.
Fig. 2: The simulated orbital, structural and thermal evolution of Saturn’s mid-sized moons for an initial Saturn Q of 80,000 decreasing linearly to today’s value of about 2,450 provides a close match to observational constraints.
Fig. 3: Orbital, structural, and thermal evolution of Saturn’s mid-sized moons for an initial Saturn Q of 2,452.8.

Data availability

All data generated or analysed during this study are included in this published article and Supplementary Information. The code used to generate those data is freely available at https://github.com/MarcNeveu/IcyDwarf.

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Acknowledgements

This research was funded by A.R.R.’s startup funds at Arizona State University and NASA’s Cassini Data Analysis Program award NNX16AI42G. We thank S. Desch for providing access to the computers on which the model was developed and simulations were run.

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M.N. developed the models and ran the simulations. M.N. and A.R.R. designed the research and interpreted the results.

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Correspondence to Marc Neveu.

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Neveu, M., Rhoden, A.R. Evolution of Saturn’s mid-sized moons. Nat Astron 3, 543–552 (2019). https://doi.org/10.1038/s41550-019-0726-y

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