Abstract
Moons potentially harbouring a global ocean are tending to become relatively common objects in the Solar System1. The presence of these long-lived global oceans is generally betrayed by surface modification owing to internal dynamics2. Hence, Mimas would be the most unlikely place to look for the presence of a global ocean3. Here, from detailed analysis of Mimas’s orbital motion based on Cassini data, with a particular focus on Mimas’s periapsis drift, we show that its heavily cratered icy shell hides a global ocean, at a depth of 20–30 kilometres. Eccentricity damping implies that the ocean is likely to be less than 25 million years old and still evolving. Our simulations show that the ocean–ice interface reached a depth of less than 30 kilometres only recently (less than 2–3 million years ago), a time span too short for signs of activity at Mimas’s surface to have appeared.
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Data availability
Most astrometric data are already available from refs. 6,21 and references therein. The extra astrometric data of Mimas and Tethys that were obtained from three-dimensional complex-shape modelling are available on IMCCE FTP server at ftp://ftp.imcce.fr/pub/psf.
Code availability
All astrometric data derived from ISS images can be reproduced using our CAVIAR software available under Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International. The software is available at www.imcce.fr/recherche/equipes/pegase/caviar.
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Acknowledgements
V.L. and N.R. thank the FP7-ESPaCE European programme for funding under the agreement number 263466. G.T. acknowledges support from the ANR COLOSSe project. Q.Z. is supported by the Joint Research Fund in Astronomy (number U2031104) under cooperative agreement between the National Natural Science Foundation of China (NSFC) and Chinese Academy of Sciences (CAS).
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V.L. developed and fitted to the observations the full numerical model presented for the astrometric approach. N.R. developed the librational model and provided the solutions as function of interior structure. G.T. developed the thermo-orbital model of Mimas and performed the simulations of past evolution. N.C., V.L. and Q.Z. provided extra astrometric data. B.N. ran the N-body simulations involving a high eccentric Mimas. All authors contributed to the writing of the paper.
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Extended data figures and tables
Extended Data Fig. 1 Reprocessing of Mimas astrometry.
Here most of the satellite edge can be used for determining the center of figure of Mimas. The purple dots are the detected edge points on the image, while the turquoise ones represent the expected shape from spherical harmonics computation. The orange curve represents Mimas’s equator. Using 3-D complex shape modelling allows a more accurate center of figure to be obtained for Mimas.
Extended Data Fig. 2 Core radius for solid model.
Solutions for the 3-D geometric axes, polar radius (rp) and equatorial radius at longitude = pi/2 (re2) as function of the equatorial radius at longitude = 0 (re1). Each point represents an interior model where the core and mantle densities vary from [920–1100] kg m−3 and [1200–3600] kg m−3. In all cases, rp or re2 is negative. For this figure, Stoke’s coefficients are C20 = −0.101 and C22 = 0.0093.
Extended Data Fig. 3 Sensitivity of shape parameters.
This figure shows the Mimas’ libration and periapsis drift solutions for a range of equatorial and polar flattenings.
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Lainey, V., Rambaux, N., Tobie, G. et al. A recently formed ocean inside Saturn’s moon Mimas. Nature 626, 280–282 (2024). https://doi.org/10.1038/s41586-023-06975-9
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DOI: https://doi.org/10.1038/s41586-023-06975-9
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