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A modern dynamo at Triton as a consequence of its capture by Neptune

Nature Astronomy (2026) Cite this article

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Abstract

Triton, Neptune’s largest moon, is presumed to be a trans-Neptunian object that became gravitationally bound to the giant planet and is one of the highest-priority candidate ocean worlds for exploration. Its capture and orbital circularization may have caused intense heating that led to internal differentiation and the formation of a metallic core. In this work, we argue that this ‘hot start’ may lead to Triton hosting an active dynamo today, driven by ongoing convection within its metallic core. Using thermal evolution models, we explore three potential regimes of core crystallization to assess their impact on the likelihood of dynamo action. Our nominal model, with solid iron sulfide crystallizing from the top of the core down, implies that Triton, like Ganymede, may possess an ongoing dynamo that generates a magnetic field with a strength of the order of ~1 μT at the surface. If confirmed, the existence of a dynamo would provide a unique constraint on Triton’s internal structure, composition, capture history and evolution. Strong dynamo fields would complicate efforts to characterize the subsurface ocean via magnetometry with a single flyby. A dedicated mission to the Neptune system would allow for more detailed measurements of Triton’s complex magnetic environment and putative ocean.

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Fig. 1: Cartoon of Triton’s formation and internal structure.
The alternative text for this image may have been generated using AI.
Fig. 2: Cartoons of three regimes of crystallization in the metallic core.
The alternative text for this image may have been generated using AI.
Fig. 3: Output of our nominal model with FeS crystallization.
The alternative text for this image may have been generated using AI.
Fig. 4: Sensitivity test of how key model parameters affect predicted dynamo lifetime.
The alternative text for this image may have been generated using AI.

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Data availability

The thermal and magnetic evolution datasets generated by the models in this study are available on Zenodo (https://zenodo.org/records/18610655)64.

Code availability

All code used to generate models and figures are available on GitHub via Zenodo (https://zenodo.org/records/18610655)64.

References

  1. Stone, E. C. & Miner, E. D. The Voyager 2 encounter with the Neptunian system. Science 246, 1417–1421 (1989).

    Article  ADS  Google Scholar 

  2. Shock, E. L. & McKinnon, W. B. Hydrothermal processing of cometary volatiles—applications to Triton. Icarus 106, 464–477 (1993).

    Article  ADS  Google Scholar 

  3. Mousis, O., Anderson, S. E., Luspay-Kuti, A., Mandt, K. E. & Vernazza, P. in Triton and Pluto: the Long Lost Twins of Active Worlds (eds Luspay-Kuti, A. & Mandt, K. E.) Ch. 1 (IOP Publishing, 2025).

  4. Nimmo, F., Bierson, C. & McKinnon, W. B. in Triton and Pluto: the Long Lost Twins of Active Worlds (eds Luspay-Kuti, A. & Mandt, K. E.) Ch. 2 (IOP Publishing, 2025).

  5. Moran, S. E. et al. Triton haze analogs: the role of carbon monoxide in haze formation. J. Geophys. Res. Planets 127, e2021JE006984 (2022).

    Article  ADS  Google Scholar 

  6. Grundy, W. M. et al. Near-infrared spectral monitoring of Triton with IRTF/SpeX II: spatial distribution and evolution of ices. Icarus 205, 594–604 (2010).

    Article  ADS  Google Scholar 

  7. Quirico, E. et al. Composition, physical state, and distribution of ices at the surface of Triton. Icarus 139, 159–178 (1999).

    Article  ADS  Google Scholar 

  8. Cruikshank, D. P. et al. Water ice on Triton. Icarus 147, 309–316 (2000).

    Article  ADS  Google Scholar 

  9. Smith, B. A. et al. Voyager 2 in the Uranian system: imaging science results. Science 233, 43–64 (1986).

    Article  ADS  Google Scholar 

  10. Croft, S. K. et al. in Neptune and Triton (ed. Cruikshank, D. P.) 879–947 (Univ. Arizona Press, 1995).

  11. Hansen, C. J. et al. Triton: fascinating moon, likely ocean world, compelling destination!. Planet. Sci. J. 2, 137 (2021).

    Article  Google Scholar 

  12. Soderblom, L. A. et al. Triton’s geyser-like plumes: discovery and basic characterization. Science 250, 410–415 (1990).

    Article  ADS  Google Scholar 

  13. Hansen, C. J., McEwen, A. S., Ingersoll, A. P. & Terrile, R. J. Surface and airborne evidence for plumes and winds on Triton. Science 250, 421–424 (1990).

    Article  ADS  Google Scholar 

  14. Schenk, P. M. et al. Triton: topography and geology of a probable ocean world with comparison to Pluto and Charon. Remote Sens. 13, 3476 (2021).

    Article  ADS  Google Scholar 

  15. Schenk, P. M. & Zahnle, K. On the negligible surface age of Triton. Icarus 192, 135–149 (2007).

    Article  ADS  Google Scholar 

  16. Stern, S. A. & McKinnon, W. B. Triton’s surface age and impactor population revisited in light of Kuiper belt fluxes: evidence for small Kuiper belt objects and recent geological activity. Astron. J. 119, 945–952 (2000).

    Article  ADS  Google Scholar 

  17. Tyler, G. L. et al. Voyager radio science observations of Neptune and Triton. Science 246, 1466–1473 (1989).

    Article  ADS  Google Scholar 

  18. McKinnon, W. B. On the origin of Triton and Pluto. Nature 311, 355–358 (1984).

    Article  ADS  Google Scholar 

  19. Goldreich, P., Murray, N., Longaretti, P. Y. & Banfield, D. Neptune’s story. Science 245, 500–504 (1989).

    Article  ADS  Google Scholar 

  20. Agnor, C. B. & Hamilton, D. P. Neptune’s capture of its moon Triton in a binary–planet gravitational encounter. Nature 441, 192–194 (2006).

    Article  ADS  Google Scholar 

  21. Nogueira, E., Brasser, R. & Gomes, R. Reassessing the origin of Triton. Icarus 214, 113–130 (2011).

    Article  ADS  Google Scholar 

  22. Canup, R. M. & Ward, W. R. A common mass scaling for satellite systems of gaseous planets. Nature 441, 834–839 (2006).

    Article  ADS  Google Scholar 

  23. Ćuk, M. & Gladman, B. J. Constraints on the orbital evolution of Triton. Astrophys. J. 626, L113–L116 (2005).

    Article  ADS  Google Scholar 

  24. Rufu, R. & Canup, R. M. Triton’s evolution with a primordial Neptunian satellite system. Astron. J. 154, 208 (2017).

    Article  ADS  Google Scholar 

  25. Ross, M. N. & Schubert, G. The coupled orbital and thermal evolution of Triton. Geophys. Res. Lett. 17, 1749–1752 (1990).

    Article  ADS  Google Scholar 

  26. Trinh, K. T., Bierson, C. J. & O’Rourke, J. G. Slow evolution of Europa’s interior: metamorphic ocean origin, delayed metallic core formation, and limited seafloor volcanism. Sci. Adv. 9, eadf3955 (2023).

    Article  Google Scholar 

  27. Trinh, K. T., Bierson, C. J. & O’Rourke, J. G. A critical core size for dynamo action at the Galilean satellites. Geophys. Res. Lett. 51, e2024GL110680 (2024).

    Article  ADS  Google Scholar 

  28. Frazier, W. et al. Trident: the path to Triton on a Discovery budget. In 2020 IEEE Aerospace Conference 1–12 (IEEE, 2020).

  29. Hendrix, A. R. et al. The NASA Roadmap to Ocean Worlds. Astrobiology 19, 1–27 (2019).

    Article  ADS  Google Scholar 

  30. Triton Ocean World Surveyor: a Neptune Orbiter and Triton Explorer https://tinyurl.com/2p88fx4f (NASA, 2022).

  31. Cochrane, C. J. et al. Single- and multi-pass magnetometric subsurface ocean detection and characterization in icy worlds using principal component analysis (PCA): application to Triton. Earth Space Sci. 9, e2021EA002034 (2022).

    Article  ADS  Google Scholar 

  32. Khurana, K. K. et al. Dual-frequency electromagnetic sounding of a Triton ocean from a single flyby. Philos. Trans. R. Soc. A 382, 20240087 (2024).

    Article  ADS  Google Scholar 

  33. Kivelson, M. G., Khurana, K. K. & Volwerk, M. The permanent and inductive magnetic moments of Ganymede. Icarus 157, 507–522 (2002).

    Article  ADS  Google Scholar 

  34. Jia, X., Kivelson, M. G., Khurana, K. K. & Walker, R. J. Improved models of Ganymede’s permanent and induced magnetic fields based on Galileo and Juno data. J. Geophys. Res. Planets 130, e2024JE008309 (2025).

    Article  ADS  Google Scholar 

  35. Stevenson, D. J. Planetary magnetic fields. Earth Planet. Sci. Lett. 208, 1–11 (2003).

    Article  ADS  Google Scholar 

  36. Christensen, U. R. Dynamo scaling laws and applications to the planets. Space Sci. Rev. 152, 565–590 (2010).

    Article  ADS  Google Scholar 

  37. Desch, S. J., Kalyaan, A. & Alexander, C. M. O. The effect of Jupiter’s formation on the distribution of refractory elements and inclusions in meteorites. Astrophys. J. Suppl. Ser. 238, 11 (2018).

    Article  ADS  Google Scholar 

  38. Bermingham, K. R., Füri, E., Lodders, K. & Marty, B. The NC–CC isotope dichotomy: implications for the chemical and isotopic evolution of the early Solar System. Space Sci. Rev. 216, 133 (2020).

    Article  ADS  Google Scholar 

  39. Kruijer, T. S., Kleine, T. & Borg, L. E. The great isotopic dichotomy of the early Solar System. Nat. Astron. 4, 32–40 (2020).

    Article  ADS  Google Scholar 

  40. Breuer, D., Rueckriemen, T. & Spohn, T. Iron snow, crystal floats, and inner-core growth: modes of core solidification and implications for dynamos in terrestrial planets and moons. Prog. Earth Planet. Sci. 2, 39 (2015).

    Article  ADS  Google Scholar 

  41. Rückriemen, T., Breuer, D. & Spohn, T. The Fe snow regime in Ganymede’s core: a deep-seated dynamo below a stable snow zone. J. Geophys. Res. Planets 120, 1095–1118 (2015).

    Article  ADS  Google Scholar 

  42. Rückriemen, T., Breuer, D. & Spohn, T. Top-down freezing in a Fe–FeS core and Ganymede’s present-day magnetic field. Icarus 307, 172–196 (2018).

    Article  ADS  Google Scholar 

  43. Bland, M. T., Showman, A. P. & Tobie, G. The production of Ganymede’s magnetic field. Icarus 198, 384–399 (2008).

    Article  ADS  Google Scholar 

  44. Hauck, S. A. II, Aurnou, J. M. & Dombard, A. J. Sulfur’s impact on core evolution and magnetic field generation on Ganymede. J. Geophys. Res. Planets 111, (2006).

  45. Williams, Q. A note on the direction of core solidification in asteroids, the iron melting curve, and phase equilibria parameterizations. Icarus 427, 116381 (2025).

    Article  Google Scholar 

  46. Morard, G. et al. Liquid properties in the Fe–FeS system under moderate pressure: tool box to model small planetary cores. Am. Mineral. 103, 1770–1779 (2018).

    ADS  Google Scholar 

  47. Buono, A. S. & Walker, D. The Fe-rich liquidus in the Fe–FeS system from 1bar to 10GPa. Geochim. Cosmochim. Acta 75, 2072–2087 (2011).

    Article  ADS  Google Scholar 

  48. Van Hoolst, T. et al. Geophysical characterization of the interiors of Ganymede, Callisto and Europa by ESA’s JUpiter ICy moons Explorer. Space Sci. Rev. 220, 54 (2024).

    Article  ADS  Google Scholar 

  49. Cascioli, G., Mazarico, E., Dombard, A. J. & Nimmo, F. Leveraging the gravity field spectrum for icy satellite interior structure determination: the case of Europa with the Europa Clipper mission. Planet. Sci. J. 5, 45 (2024).

    Article  Google Scholar 

  50. Vance, S. D. et al. Geophysical investigations of habitability in ice-covered ocean worlds. J. Geophys. Res. Planets 123, 180–205 (2018).

    Article  ADS  Google Scholar 

  51. Christensen, U. R. A deep dynamo generating Mercury’s magnetic field. Nature 444, 1056–1058 (2006).

    Article  ADS  Google Scholar 

  52. Trumbo, S. K. et al. Hydrogen peroxide at the poles of Ganymede. Sci. Adv. 9, eadg3724 (2023).

    Article  Google Scholar 

  53. Lodders, K. Solar System abundances and condensation temperatures of the elements. Astrophys. J. 591, 1220–1247 (2003).

    Article  ADS  Google Scholar 

  54. Robuchon, G. et al. Coupling of thermal evolution and despinning of early Iapetus. Icarus 207, 959–971 (2010).

    Article  ADS  Google Scholar 

  55. Robuchon, G. & Nimmo, F. Thermal evolution of Pluto and implications for surface tectonics and a subsurface ocean. Icarus 216, 426–439 (2011).

    Article  ADS  Google Scholar 

  56. Olson, P. & Christensen, U. R. Dipole moment scaling for convection-driven planetary dynamos. Earth Planet. Sci. Lett. 250, 561–571 (2006).

    Article  ADS  Google Scholar 

  57. Littleton, J. A. H., Secco, R. A. & Yong, W. Electrical resistivity of FeS at high pressures and temperatures: implications of thermal transport in the core of Ganymede. J. Geophys. Res. Planets 126, e2020JE006793 (2021).

    Article  ADS  Google Scholar 

  58. Schekochihin et al. Critical magnetic Prandtl number for small-scale dynamo. Phys. Rev. Lett. 92, 054502 (2004).

    Article  ADS  Google Scholar 

  59. Sanderson, H. R., Bryson, J. F. J., Nichols, C. I. O. & Davies, C. J. Unlocking planetesimal magnetic field histories: a refined, versatile model for thermal evolution and dynamo generation. Icarus 425, 116323 (2025).

    Article  Google Scholar 

  60. Turner, J. S. Buoyancy Effects in Fluids (Cambridge Univ. Press, 1973).

  61. Pommier, A., Davies, C. J. & Zhang, R. A joint experimental-modeling investigation of the effect of light elements on dynamos in small planets and moons. J. Geophys. Res. Planets 125, e2020JE006492 (2020).

    Article  ADS  Google Scholar 

  62. Bagheri, A. et al. Post-capture thermal and orbital evolution of Triton and implications for present-day ocean properties. In 56th Lunar and Planetary Science Conference 2524 (2025).

  63. McComas, D. J. et al. Pluto’s interaction with the solar wind. J. Geophys. Res. Space Phys. 121, 4232–4246 (2016).

    Article  ADS  Google Scholar 

  64. Tilke, L. J. A modern dynamo at Triton as a consequence of its capture by Neptune. Zenodo https://zenodo.org/records/18610655 (2026).

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Acknowledgement

J.G.O. thanks the NSF (Astronomy and Astrophysics grant 2308186) for support.

Author information

Authors and Affiliations

  1. School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA

    Lana J. Tilke, Kevin T. Trinh & Joseph G. O’Rourke

  2. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA

    Kevin T. Trinh

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  1. Lana J. Tilke
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Contributions

L.J.T. and J.G.O. designed the study and wrote the manuscript. L.J.T. built the models and created the figures. K.T.T. provided the methodology for the thermal evolution of the mantle and core. All authors made edits during the revision process.

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Correspondence to Lana J. Tilke.

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The authors declare no competing interests.

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Nature Astronomy thanks Sarah Anderson and Christophe Gissinger for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data

Extended Data Fig. 1 Output of our nominal model with iron snow crystallization.

a, Thermal evolution of the mantle and core, with illustrative ice and ocean layers drawn for scale but not modelled in detail. Temperature contours are plotted in the mantle, whereas shading is used in the core to denote when the core is liquid but stagnant (that is, no convection) and when the iron snow activates. b, Heat flow across the core–mantle boundary (red) compared to the adiabatic heat flow in the core (black). c, Temperature at the core–mantle boundary (red) compared to that at the mid-mantle depth (orange). d, Magnetic Reynolds number in the core. e, Surface strength of the dipolar component of the magnetic field on the equator.

Extended Data Fig. 2 Output of our nominal model with inner core crystallization.

a, Thermal evolution of the mantle and core, with illustrative ice and ocean layers drawn for scale but not modelled in detail. Temperature contours are plotted in the mantle, whereas shading is used in the core to denote when the core is liquid but stagnant (that is, no convection) and when the Earth-like inner core forms. b, Heat flow across the core–mantle boundary (red) compared to the adiabatic heat flow in the core (black). c, Temperature at the core–mantle boundary (red) compared to that at the mid-mantle depth (orange). d, Magnetic Reynolds number in the core. e, Surface strength of the dipolar component of the magnetic field on the equator.

Extended Data Fig. 3 Sensitivity test of how key model parameters affect predicted dynamo lifetime at higher critical Rem.

Points show the maximum lifetime of Triton’s dynamo after the start of each model for a critical Rem a, 100; b, 150; and c, 200. This sensitivity test also shows the sensitivity of our results to the assumed magnetic diffusivity. For example, doubling the magnetic diffusivity is equivalent to doubling the critical Rem, in terms of the predicted dynamo lifetime.

Extended Data Fig. 4 Flyby field strength.

a, A simplified trajectory of a hypothetical flyby of Triton. b, The predicted strength of a dynamo field during a notional flyby with a closest approach of 400 km to Triton’s surface, based on the nominal FeS layer model at 4.5 Gyr, compared to the simulated strength of an ocean-induced magnetic field. c, A more detailed view of the simulated ocean-induced magnetic field strength. Panel c adapted from ref. 31 under a Creative Commons license CC BY-NC-ND 4.0. Credit: a, Voyager icon, NASA/JPL/SPL; image of Triton, NASA/JPL—NOIRLab.

Extended Data Table 1 Dimensionless numbers
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Tilke, L.J., Trinh, K.T. & O’Rourke, J.G. A modern dynamo at Triton as a consequence of its capture by Neptune. Nat Astron (2026). https://doi.org/10.1038/s41550-026-02868-9

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