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


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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  1. Burns, J. in Mars (eds Kieffer, H. H., Jakosky, B. M., Snyder, C. W. & Matthews, M. S.) 1283–1301 (Univ. Press of Arizona, 1992).

    Google Scholar 

  2. Peale, S. J. in Treatise on Geophysics Vol. 10 (eds Schubert, G. & Spohn, T.) 465–508 (Elsevier B. V., 2007).

    Book  Google Scholar 

  3. Rosenblatt, P. The origin of the Martian moons revisited. Astron. Astrophys. Rev. 19, 44 (2011).

    Article  Google Scholar 

  4. Safranov, V. S. et al. in Satellites (eds Burns, J. A. & Matthews, M. S.) 89–116 (Univ. Press of Arizona, 1986).

    Google Scholar 

  5. Citron, R. I., Genda, H. & Ida, S. Formation of Phobos and Deimos via a giant impact. Icarus 252, 334–338 (2015).

    Article  Google Scholar 

  6. Craddock, R. A. Are Phobos and Deimos the result of a giant impact? Icarus 211, 1150–1161 (2011).

    Article  Google Scholar 

  7. Rosenblatt, P. & Charnoz, S. On the formation of the Martian moons from a circum-martian accretion disc. Icarus 221, 806–815 (2012).

    Article  Google Scholar 

  8. Charnoz, S., Salmon, J. & Crida, A. The recent formation of Saturn’s moons from viscous spreading of the main rings. Nature 465, 752–754 (2010).

    Article  Google Scholar 

  9. Charnoz, S. et al. Accretion of Saturn’s mid-sized moons during the viscous spreading of young massive rings: solving the paradox of silicate-poor rings vs silicate-rich moons. Icarus 216, 535–550 (2011).

    Article  Google Scholar 

  10. Marinova, M. M., Aharonson, O. & Asphaug, E. Geophysical consequences of planetary-scale impacts into a Mars-like planet. Icarus 211, 960–985 (2011).

    Article  Google Scholar 

  11. Wetherill, G. W. & Stewart, G. R. Formation of planetary embryos—Effects of fragmentation low relative velocity and independent variation of eccentricity and inclination. Icarus 106, 190–209 (1993).

    Article  Google Scholar 

  12. Kokubo, E. & Ida, S. Formation of protoplanets from planetesimals in the Solar nebula. Icarus 143, 15–27 (2000).

    Article  Google Scholar 

  13. Kokubo, E. & Ida, S. Oligarchic growth of protoplanets. Icarus 131, 171–178 (1998).

    Article  Google Scholar 

  14. Hyodo, R., Ohtsuki, K. & Takeda, T. Formation of multiple-satellite systems from low-mass circumplanetary particle discs. Astrophys. J. 799, 40 (2015).

    Article  Google Scholar 

  15. Yoder, C. F. Tidal rigidity of Phobos. Icarus 439, 327–346 (1982).

    Article  Google Scholar 

  16. Richardson, D. C., Leinhardt, Z. M., Melosh, H. J., Bootke, W. F. & Asphaug, E. in Asteroids III (eds Bottke, B., Cellino, A., Paolocchi, P. & Binzel, R.) 501–515 (Univ. Press of Arizona, 2002).

    Google Scholar 

  17. Andert, T. P. et al. Precise mass determination and the nature of Phobos. Geophys. Res. Lett. 37, L09202 (2010).

    Article  Google Scholar 

  18. Murchie, S., Thomas, P., Rivkin, A. & Chabot, N. in Asteroids IV (eds Michel, P., DeMeo, F. E. & Bottke, W. F.) 451–467 (Univ. Press of Arizona, 2015).

    Google Scholar 

  19. Pieters, C. M., Murchie, S., Thomas, N. & Britt, D. Composition of surface materials on the moons of Mars. Planet. Space Sci. 102, 144–151 (2014).

    Article  Google Scholar 

  20. Genda, H., Kokubo, E. & Ida, S. Merging criteria for giant impacts of protoplanets. Astrophys. J. 744, 137–144 (2012).

    Article  Google Scholar 

  21. Monaghan, J. J. Smoothed particle hydrodynamics. Annu. Rev. Astron. Astrophys. 30, 543–574 (1991).

    Article  Google Scholar 

  22. Tillotson, J. H. Metallic Equations of State for Hypervelocity Impact Report No. GA-3216, July 18 (General Atomic, San Diego, California, 1962).

  23. Von Neumann, J. & Richtmyer, R. D. A method for the numerical calculations of hydrodynamics shocks. J. Appl. Phys. 21, 232–237 (1950).

    Article  Google Scholar 

  24. Kokubo, E., Ida, S. & Makino, J. Evolution of a circumterrestrial disc and formation of a single moon. Icarus 148, 419–436 (2000).

    Article  Google Scholar 

  25. Kinoshita, H., Yoshida, H. & Nakai, H. Symplectic integrators and their application to dynamical astronomy. Celest. Mech. Dyn. Astron. 50, 59–71 (1991).

    Article  Google Scholar 

  26. Everhart, E. in The Dynamics of Comets: Their Origin and Evolution (eds Carusi, A. & Valsecchi, G. B.) 185–202 (Reidel, 1985).

    Book  Google Scholar 

  27. Black, B. A. & Mittal, T. The demise of Phobos and development of a Martian ring system. Nature Geosci. 8, 913–917 (2015).

    Article  Google Scholar 

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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).

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