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Capture of an ancient Charon around Pluto

Nature Geoscience volume 18, pages 37–43 (2025)Cite this article

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Abstract

Pluto and Charon are the largest binary system in the known population of trans-Neptunian objects in the outer Solar System. Their shared external orbital axis suggests a linked evolutionary history and collisional origin. Their radii, ~1,200 km and ~600 km, respectively, and Charon’s wide circular orbit of about 16 Pluto radii require a formation mechanism that places a large mass fraction into orbit, with sufficient angular momentum to drive tidal orbital expansion. Here we numerically model the collisional capture of Charon by Pluto using simulations that include material strength. In our simulations, friction distributes impact momentum, leading Charon and Pluto to become temporarily connected, instead of merging, for impacts aligned with the target’s rotation. In this ‘kiss-and-capture’ regime, coalescence of the bodies is prevented by strength. For a prograde target rotation consistent with the system angular momentum, Charon is then tidally decoupled and raised into a near-circular orbit from which it migrates outwards to distances consistent with its present orbit. Charon is captured relatively intact in this scenario, retaining its core and most of its mantle, which implies that Charon could be as ancient as Pluto.

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Fig. 1: Contrast in outcomes for a potential Charon-capturing collision when implementing strength.
Fig. 2: Time series of a potential Charon-capturing collision at 45°.
Fig. 3: Orbital and thermal evolution of Pluto and Charon during and after kiss-and-capture.
Fig. 4: Time series and orbital evolution of a potential Charon-capturing collision with more Pluto- and Charon-like initial bodies.

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Data availability

All model parameters used to run our simulations are available in the main text and Supplementary Information. The full video of the simulation shown in Figs. 1b and 2 is included as Supplementary Video 1, with the input file and first and last dumps uploaded to the Harvard Dataverse (https://doi.org/10.7910/DVN/ARMAJ1). A lower-resolution video of the simulation shown in Fig. 4 is included as Supplementary Video 2, with the input file and first and last dumps found in the above repository; the repository also hosts csv files containing the processed orbital and momentum data shown in Figs. 3 and 4. Additional data, such as additional intermediate dump files from simulations, are available from the corresponding author on reasonable request.

Code availability

The sphlatch SPH code is available via GitHub at https://github.com/andreasreufer/sphlatch. The algorithm for implementing strength in sphlatch is completely described in section 2 of ref. 12.

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Acknowledgements

We acknowledge S. Cambioni and N. Baijal for their helpful suggestions in an early version of the paper, and J. T. Keane for the original name ‘kiss-and-run’ for the hypothesis. We acknowledge the University of Arizona High Performance Computing (HPC) for a generous resource allocation where all simulations were performed.

Author information

Authors and Affiliations

  1. Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA

    C. Adeene Denton, Erik Asphaug & Robert Melikyan

  2. Southwest Research Institute, Boulder, CO, USA

    C. Adeene Denton

  3. Space Research and Planetary Science, University of Bern, Bern, Switzerland

    Alexandre Emsenhuber

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Contributions

C.A.D. led the project, ran the impact simulations, performed initial analysis and wrote the draft paper. E.A. assisted in the conceptualization of the initial idea and contributed to the paper. R.M. performed the dynamical analysis. A.E. assisted in the conceptualization of the initial idea and provided the collisional modelling framework. All authors contributed to the improvement of the analysis and the paper.

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Correspondence to C. Adeene Denton.

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Nature Geoscience thanks Philip Carter, Sébastien Charnoz and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: Tamara Goldin, in collaboration with the Nature Geoscience team.

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Extended data

Extended Data Fig. 1 Geometry of the collision.

Colors as in Figs. 1, 2, and 4.

Extended Data Fig. 2 Comparison of pre-impact tidal deformation.

Comparison of the tidal deformation that occurs prior to impact for bodies without strength (top), and with strength (bottom), in which the deformation is greatly reduced. Time of impact is at 0 h.

Extended Data Fig. 3 Role of pre-impact rotation.

Final states of collisions with strength, with otherwise identical impact parameters (θcoll = 45∘, vcoll/vesc ~ 1.1), as a function of target rotation: no spin (left), 6 h prograde period (middle), and 3 h prograde period (right, the same simulation as Fig. 1a and 2).

Extended Data Fig. 4 Role of pre-impact thermal structure.

Contrast between the final states of collisions with identical impact parameters (θcoll = 45∘, vcoll/vesc ~ 1.1) with varying internal thermal structures. Right shows the nominal solid target of Figs. 1a and 2). Left shows result for a hot target (majority of interior > 300 K, mostly liquid, left). Center is for a warm target (majority of interior > 230 K, center, more readily deformable ice shell).

Extended Data Fig. 5 Contrast between the final states of the Pluto-Charon-like collision shown in Figs. 1 and 2 (θcoll = 45∘, vcoll/vesc ~ 1.1), left, and the same collision parameters for an Orcus-Vanth-like system, right.

For our Orcus and Vanth simulation, both bodies are scaled down to 40% diameters of the Pluto-Charon case, with the same rock/ice mass distribution.

Extended Data Table 1 Parameters used for material strength of ice and rock
Full size table
Extended Data Table 2 Simulation parameters and outcomes for rocky interior structures
Full size table
Extended Data Table 3 Simulation parameters and outcomes for initial exploration of Charon-mass impactors
Full size table

Supplementary information

Supplementary Information

Supplementary Figs. 1 and 2 and Table 1.

Supplementary Video 1

Video of the potential Charon-capturing collision shown in Fig. 2 (θcoll = 45°, vcoll/vesc ≈ 1.1, 3 h prograde rotation). Colours as in Figs. 1 and 2.

Supplementary Video 2

Video of the potential Charon-capturing collision shown in Fig. 4, using an impactor that is 55% rock by mass (θcoll = 45°, vcoll/vesc ≈ 1.2, 3 h prograde rotation). Colours as in Figs. 1 and 2.

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Denton, C.A., Asphaug, E., Emsenhuber, A. et al. Capture of an ancient Charon around Pluto. Nat. Geosci. 18, 37–43 (2025). https://doi.org/10.1038/s41561-024-01612-0

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