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Dynamical evidence for Phobos and Deimos as remnants of a disrupted common progenitor

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The origin of the Martian moons, Phobos and Deimos, remains elusive. While the morphology and their cratered surfaces suggest an asteroidal origin1,2,3, capture has been questioned because of potential dynamical difficulties in achieving the current near-circular, near-equatorial orbits4,5. To circumvent this, in situ formation models have been proposed as alternatives6,7,8,9. Yet, explaining the present location of the moons on opposite sides of the synchronous radius, their small sizes and apparent compositional differences with Mars2 has proved challenging. Here, we combine geophysical and tidal-evolution modelling of a Mars–satellite system to propose that Phobos and Deimos originated from disintegration of a common progenitor that was possibly formed in situ. We show that tidal dissipation within a Mars–satellite system, enhanced by the physical libration of the satellite, circularizes the post-disrupted eccentric orbits in <2.7 Gyr and makes Phobos descend to its present orbit from its point of origin close to or above the synchronous orbit. Our estimate for Phobos’s maximal tidal lifetime is considerably less than the age of Mars, indicating that it is unlikely to have originated alongside Mars. Deimos initially moved inwards, but never transcended the co-rotation radius because of insufficient eccentricity and therefore insufficient tidal dissipation. Whereas Deimos is very slowly receding from Mars, Phobos will continue to spiral towards and either impact with Mars or become tidally disrupted on reaching the Roche limit in ≲39 Myr.

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Fig. 1: The orbital history of Phobos and Deimos.

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Data availability

The data that support the findings of this study are available from the corresponding author on request.

Code availability

The code for computing the orbital evolution is available on request from the corresponding author.


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We thank G. Kaplan for comments on an earlier version of this manuscript and M. Jutzi for informed discussion on the topic of impact disruption. This work was supported by a grant from the Swiss National Science Foundation (SNSF project 172508 ‘Mapping the internal structure of Mars’). This is InSight contribution number 96.

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Authors and Affiliations



A.B., A.K. and M.E. discussed the original idea; A.B. derived and implemented the orbital evolution model, with input from M.E. and A.K.; M.K. helped with implementation of the numerical time-stepping scheme in the orbital evolution model; A.B. performed the simulations and data analysis and produced the figures; the manuscript was written by A.K., A.B., M.E. and M.K., with input from D.G.

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Correspondence to Amirhossein Bagheri.

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The authors declare no competing interests.

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Peer review informationNature Astronomy thanks Gwenaël Boué, Hidenori Genda and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary Information

Supplementary Sections 1–13, Figs. 1–10 and Tables 1–8.

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Bagheri, A., Khan, A., Efroimsky, M. et al. Dynamical evidence for Phobos and Deimos as remnants of a disrupted common progenitor. Nat Astron 5, 539–543 (2021).

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