Letter | Published:

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

Nature Geoscience volume 9, pages 581583 (2016) | Download Citation


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.

Access optionsAccess options

Rent or Buy article

Get time limited or full article access on ReadCube.

from $8.99

All prices are NET prices.


  1. 1.

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

  2. 2.

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

  3. 3.

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

  4. 4.

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

  5. 5.

    , & Formation of Phobos and Deimos via a giant impact. Icarus 252, 334–338 (2015).

  6. 6.

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

  7. 7.

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

  8. 8.

    , & The recent formation of Saturn’s moons from viscous spreading of the main rings. Nature 465, 752–754 (2010).

  9. 9.

    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).

  10. 10.

    , & Geophysical consequences of planetary-scale impacts into a Mars-like planet. Icarus 211, 960–985 (2011).

  11. 11.

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

  12. 12.

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

  13. 13.

    & Oligarchic growth of protoplanets. Icarus 131, 171–178 (1998).

  14. 14.

    , & Formation of multiple-satellite systems from low-mass circumplanetary particle discs. Astrophys. J. 799, 40 (2015).

  15. 15.

    Tidal rigidity of Phobos. Icarus 439, 327–346 (1982).

  16. 16.

    , , , & in Asteroids III (eds Bottke, B., Cellino, A., Paolocchi, P. & Binzel, R.) 501–515 (Univ. Press of Arizona, 2002).

  17. 17.

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

  18. 18.

    , , & in Asteroids IV (eds Michel, P., DeMeo, F. E. & Bottke, W. F.) 451–467 (Univ. Press of Arizona, 2015).

  19. 19.

    , , & Composition of surface materials on the moons of Mars. Planet. Space Sci. 102, 144–151 (2014).

  20. 20.

    , & Merging criteria for giant impacts of protoplanets. Astrophys. J. 744, 137–144 (2012).

  21. 21.

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

  22. 22.

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

  23. 23.

    & A method for the numerical calculations of hydrodynamics shocks. J. Appl. Phys. 21, 232–237 (1950).

  24. 24.

    , & Evolution of a circumterrestrial disc and formation of a single moon. Icarus 148, 419–436 (2000).

  25. 25.

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

  26. 26.

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

  27. 27.

    & The demise of Phobos and development of a Martian ring system. Nature Geosci. 8, 913–917 (2015).

Download references


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.

Author information

Author notes

    • Stéven Toupin

    Present address: Université Libre de Bruxelles, 1050 Brussels, Belgium.


  1. Royal Observatory of Belgium, 1180 Brussels, Belgium

    • Pascal Rosenblatt
    • , Antony Trinh
    •  & Stéven Toupin
  2. Institut de Physique du Globe/Université Paris Diderot/CEA/CNRS, 75005 Paris, France

    • Sébastien Charnoz
    •  & Ryuki Hyodo
  3. Institut de Physique de Rennes, Université de Rennes 1/CNRS, 35042 Rennes Cedex, France

    • Kevin M. Dunseath
    •  & Mariko Terao-Dunseath
  4. Kobe University, 567-8501 Kobe, Japan

    • Ryuki Hyodo
  5. Earth Life Science Institute/Tokyo Institute of Technology, 2-12-1 Tokyo, Japan

    • Hidenori Genda


  1. Search for Pascal Rosenblatt in:

  2. Search for Sébastien Charnoz in:

  3. Search for Kevin M. Dunseath in:

  4. Search for Mariko Terao-Dunseath in:

  5. Search for Antony Trinh in:

  6. Search for Ryuki Hyodo in:

  7. Search for Hidenori Genda in:

  8. Search for Stéven Toupin in:


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.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Pascal Rosenblatt.

Supplementary information

PDF files

  1. 1.

    Supplementary Information

    Supplementary Information

Image files

  1. 1.

    Supplementary Information

    Supplementary Information

  2. 2.

    Supplementary Information

    Supplementary Information

  3. 3.

    Supplementary Information

    Supplementary Information

About this article

Publication history