Accretion of Phobos and Deimos in an extended debris disc stirred by transient moons

Abstract

Phobos and Deimos, the two small satellites of Mars, are thought either to be asteroids captured by the planet or to have formed in a disc of debris surrounding Mars following a giant impact1,2,3,4. Both scenarios, however, have been unable to account for the current Mars system1,2,3,5,6,7. Here we use numerical simulations to suggest that Phobos and Deimos accreted from the outer portion of a debris disc formed after a giant impact on Mars. In our simulations, larger moons form from material in the denser inner disc and migrate outwards due to gravitational interactions with the disc. The resulting orbital resonances spread outwards and gather dispersed outer disc debris, facilitating accretion into two satellites of sizes similar to Phobos and Deimos. The larger inner moons fall back to Mars after about 5 million years due to the tidal pull of the planet, after which the two outer satellites evolve into Phobos- and Deimos-like orbits. The proposed scenario can explain why Mars has two small satellites instead of one large moon. Our model predicts that Phobos and Deimos are composed of a mixture of material from Mars and the impactor.

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Figure 1: Mass distribution in the disc formed after the giant impact as obtained in our reproduction of SPH simulations5.
Figure 2: Formation of moons from the inner disc below the Roche limit.
Figure 3: Typical evolution of the outer disc since the inner moon’s formation, resulting in satellites similar to Phobos and Deimos.

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Acknowledgements

P.R. is financially supported by the Belgian PRODEX programme managed by the European Space Agency in collaboration with the Belgian Federal Science Policy Office. A.T. has been supported by the EC’s 7th Framework Programme (FP7/2008-2017) under grant agreement #263466. Calculations were performed on the clusters at the Institute of Physics of Rennes (IPR), at the Royal Observatory of Belgium, and at the S-CAPAD computational centre of IPGP (France). R.H. acknowledges the financial support by JSPS Grants-in-Aid for JSPS fellows (15J02110). S.C. thanks the Institut Universitaire de France (IUF) for financial support. We also acknowledge the financial support of the UnivEarthS Labex programme at Sorbonne Paris Cité (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02). We acknowledge E. Asphaug for his very helpful review. We would like to dedicate this paper to the memory of André Brahic.

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P.R. and S.C. developed the proposed scenario for the formation of Phobos and Deimos. S.C. also ran the one-dimensional model of massive moon formation at the Roche limit. K.M.D. and M.T.-D. built and ran the N-body code for accretion of small debris in the outer part of the disc. A.T. computed the mass repartition of debris and produced the animations in the Supplementary Information. A.T. and S.T. built and ran models of tidal evolution of the orbit of the two satellites after their formation. R.H. ran the SPH code of post-impact accretion-disc formation provided by H.G.

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Correspondence to Pascal Rosenblatt.

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Rosenblatt, P., Charnoz, S., Dunseath, K. et al. Accretion of Phobos and Deimos in an extended debris disc stirred by transient moons. Nature Geosci 9, 581–583 (2016). https://doi.org/10.1038/ngeo2742

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