Pluto’s ocean is capped and insulated by gas hydrates

Abstract

Many icy Solar System bodies possess subsurface oceans. On Pluto, Sputnik Planitia’s location near the equator suggests the presence of a subsurface ocean and a locally thinned ice shell. To maintain an ocean, Pluto needs to retain heat inside. On the other hand, to maintain large variations in its thickness, Pluto’s ice shell needs to be cold. Here we show, by thermal evolution and viscous relaxation calculations, that the presence of a thin layer of clathrate hydrates (gas hydrates) at the base of the ice shell can explain both the long-term survival of the ocean and the maintenance of shell thickness contrasts. Clathrate hydrates act as a thermal insulator, preventing the ocean from completely freezing while keeping the ice shell cold and immobile. The most likely clathrate guest gas is methane, derived from precursor bodies and/or cracking of organic materials in the hot rocky core. Nitrogen molecules initially contained and/or produced later in the core would probably not be trapped as clathrate hydrates, instead supplying the nitrogen-rich surface and atmosphere. The formation of a thin clathrate hydrate layer cap to a subsurface ocean may be an important generic mechanism to maintain long-lived subsurface oceans in relatively large but minimally heated icy satellites and Kuiper belt objects.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

Fig. 1: A schematic diagram of the interior structure of Pluto.
Fig. 2: Time evolution of the interior thermal profile above the rocky core.
Fig. 3: Timescale of viscous relaxation of the ice shell.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author on reasonable request.

Code availability

Codes for the thermal evolution and viscous relaxation calculations are available upon reasonable request from S.K.

References

  1. 1.

    Nimmo, F. & Pappalardo, R. T. Ocean worlds in the outer solar system. J. Geophys. Res. 121, 1378–1399 (2016).

    Article  Google Scholar 

  2. 2.

    Nimmo, F. et al. Reorientation of Sputnik Planitia implies a subsurface ocean on Pluto. Nature 540, 94–96 (2016).

    Article  Google Scholar 

  3. 3.

    Hussmann, H., Sotin, C. & Lunine, J. I. in Treatise in Geophysics 2nd edn, Vol. 10 (ed. Schubert, G.) 605–635 (Elsevier, 2015).

  4. 4.

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

    Article  Google Scholar 

  5. 5.

    Hammond, N. P., Barr, A. C. & Parmentier, E. M. Recent tectonic activity on Pluto driven by phase changes in the ice shell. Geophys. Res. Lett. 43, 6775–6782 (2016).

    Article  Google Scholar 

  6. 6.

    Durham, W. B., Prieto-Ballesteros, O., Goldsby, D. L. & Kargel, J. S. Rheological and thermal properties of icy materials. Space Sci. Rev. 153, 273–298 (2010).

    Article  Google Scholar 

  7. 7.

    Kamata, S. & Nimmo, F. Interior thermal state of Enceladus inferred from the viscoelastic state of the ice shell. Icarus 284, 387–393 (2017).

    Article  Google Scholar 

  8. 8.

    McKinnon, W. B., Simonelli, D. P. & Schubert, G. in Pluto and Charon (eds. Stern, S. A. & Tholen, D. J.) 295–346 (Univ. Arizona Press, 1997).

  9. 9.

    Le Roy, L. et al. Inventory of the volatiles on comet 67P/Churyumov–Gerasimenko from Rosetta/ROSINA. Astron. Astrophys. 583, A1 (2015).

    Article  Google Scholar 

  10. 10.

    Croft, S. K., Lunine, J. I. & Kargel, J. Equation of state of ammonia–water liquid: derivation and planetological applications. Icarus 73, 279–293 (1988).

    Article  Google Scholar 

  11. 11.

    Johnson, B. C., Bowling, T. J., Trowbridge, A. J. & Freed, A. M. Formation of the Sputnik Planum basin and the thickness of Pluto’s subsurface ocean. Geophys. Res. Lett. 43, 10068–10077 (2016).

    Article  Google Scholar 

  12. 12.

    Durham, W. B., Stern, L. A., Kubo, T. & Kirby, S. H. Flow strength of highly hydrated Mg- and Na-sulfate hydrate salts, pure and in mixtures with water ice, with application to Europa. J. Geophys. Res. 110, E12010 (2005).

    Article  Google Scholar 

  13. 13.

    Durham, W. B., Kirby, S. H. & Stern, L. A. Effects of dispersed particulates on the rheology of water ice at planetary conditions. J. Geophys. Res. 97, 20883–20897 (1992).

    Article  Google Scholar 

  14. 14.

    Zolotov, M. Y. Aqueous fluid composition in CI chondritic materials: chemical equilibrium assessments in closed systems. Icarus 220, 713–729 (2012).

    Article  Google Scholar 

  15. 15.

    Sloan, E. D. & Koh, C. A. Clathrate Hydrates of Natural Gases 3rd edn (CRC, 2007).

  16. 16.

    Choukroun, M., Kieffer, S. W., Lu, X. & Tobie, G. in The Science of Solar System Ices (eds Gudipati, M. S. & Castillo-Rogez, J.) 409–454 (Springer, 2013).

  17. 17.

    Waite, W. F., Stern, L. A., Kirby, S. H., Winters, W. J. & Mason, D. H. Simultaneous determination of thermal conductivity, thermal diffusivity and specific heat in sI methane hydrate. Geophys. J. Int 169, 767–774 (2007).

    Article  Google Scholar 

  18. 18.

    Durham, W. B., Kirby, S. H., Stern, L. A. & Zhang, W. The strength and rheology of methane clathrate hydrate. J. Geophys. Res. 108, 2182 (2003).

    Google Scholar 

  19. 19.

    Tobie, G., Choblet, G. & Sotin, C. Tidally heated convection: constraints on Europa’s ice shell thickness. J. Geophys. Res. 108, 5124 (2003).

    Article  Google Scholar 

  20. 20.

    Barr, A. C. & McKinnon, W. B. Convection in ice I shells and mantles with self-consistent grain size. J. Geophys. Res. 112, E02012 (2007).

    Article  Google Scholar 

  21. 21.

    Moore, J. M. et al. The geology of Pluto and Charon through the eyes of New Horizons. Science 351, 1284–1293 (2016).

    Article  Google Scholar 

  22. 22.

    Keane, J. T., Matsuyama, I., Kamata, S. & Steckloff, J. K. Reorientation and faulting of Pluto due to volatile loading within Sputnik Planitia. Nature 540, 90–93 (2016).

    Article  Google Scholar 

  23. 23.

    White, O. L. et al. Geological mapping of Sputnik Planitia on Pluto. Icarus 287, 261–286 (2017).

    Article  Google Scholar 

  24. 24.

    Mousis, O. et al. Methane clathrates in the Solar System. Astrobiology 15, 308–326 (2015).

    Article  Google Scholar 

  25. 25.

    Lunine, J. I. & Stevenson, D. J. Thermodynamics of clathrate hydrate at low and high pressures with application to the outer Solar System. Astrophys. J. Suppl. 58, 493–531 (1985).

    Article  Google Scholar 

  26. 26.

    McCollom, T. M. & Seewald, J. S. Experimental constraints on the hydrothermal activity of organic acids and acid anions: I. Formic acid and formate. Geochim. Cosmochim. Acta 67, 3625–3644 (2003).

    Article  Google Scholar 

  27. 27.

    Herri, J.-M. et al. Gas hydrate equilibria for CO2–N2 and CO2–CH4 gas mixtures—Experimental studies and thermodynamic modelling. Fluid Phase Equilib. 301, 171–190 (2011).

    Article  Google Scholar 

  28. 28.

    Petuya, C. & Desmedt, A. Revealing CO-preferential encapsulation in the mixed CO–N2 clathrate hydrate. J. Phys. Chem. C 123, 4871–4878 (2019).

    Article  Google Scholar 

  29. 29.

    Glein, C. R. & Waite, J. H. Primordial N2 provides a cosmochemical explanation for the existence of Sputnik Planitia, Pluto. Icarus 313, 79–92 (2018).

    Article  Google Scholar 

  30. 30.

    Claypool, G. E. & Kvenvolden, K. A. Methane and other hydrocarbon gases in marine sediment. Annu. Rev. Earth Planet. Sci. 11, 299–327 (1983).

    Article  Google Scholar 

  31. 31.

    McKinnon, W. B. & Mueller, S. Pluto’s structure and composition suggest origin in the solar, not a planetary, nebula. Nature 335, 240–243 (1988).

    Article  Google Scholar 

  32. 32.

    Bardyn, A. et al. Carbon-rich dust in comet 67P/Churyumov–Gerasimenko measured by COSIMA/Rosetta. Mon. Not. R. Astron. Soc. 469, S712–S722 (2017).

    Article  Google Scholar 

  33. 33.

    Seewald, J. S. Organic–inorganic interactions in petroleum-producing sedimentary basins. Nature 426, 327–333 (2003).

    Article  Google Scholar 

  34. 34.

    Stopler, D. A. et al. Formation temperatures of thermogenic and biogenic methane. Science 344, 1500–1503 (2014).

    Article  Google Scholar 

  35. 35.

    Behar, F., Gillaizeau, B., Derenne, S. & Largeau, C. Nitrogen distribution in the pyrolysis products of a type II kerogen (Cenomanian, Italy). Timing of molecular nitrogen production versus other gases. Energy Fuels 14, 431–440 (2000).

    Article  Google Scholar 

  36. 36.

    Sekine, Y. et al. High-temperature water–rock interactions and hydrothermal environments in the chondrite-like core of Enceladus. Nat. Commun. 6, 8604 (2015).

    Article  Google Scholar 

  37. 37.

    Skiba, S. S., Larionov, E. G., Manakov, A. Y., Kolesov, B. A. & Kosyakov, V. I. Investigation of hydrate formation in the system H2−CH4−H2O at a pressure up to 250 MPa. J. Phys. Chem. B 111, 11214–11220 (2007).

    Article  Google Scholar 

  38. 38.

    Grundy, W. M. et al. Surface compositions across Pluto and Charon. Science 351, aad9189 (2016).

    Article  Google Scholar 

  39. 39.

    Gladstone, G. R. et al. The atmosphere of Pluto as observed by New Horizons. Science 351, aad8866 (2016).

    Article  Google Scholar 

  40. 40.

    Barucci, M. A., Brown, M. E., Emery, J. P. & Merlin, F. in The Solar System Beyond Neptune (eds. Barucci, M. A. et al.) 143–160 (Univ. Arizona Press, 2008).

  41. 41.

    Hussmann, H., Sohl, F. & Spohn, T. Subsurface oceans and deep interiors of medium-sized outer planet satellites and large trans-Neptunian objects. Icarus 185, 258–273 (2006).

    Article  Google Scholar 

  42. 42.

    Waite, J. H. et al. Liquid water on Enceladus from observations of ammonia and 40Ar in the plume. Nature 460, 487–490 (2009).

    Article  Google Scholar 

  43. 43.

    Kamata, S. One-dimensional convective thermal evolution calculation using a modified mixing length theory: Application to Saturnian icy satellites. J. Geophys. Res. 123, 93–112 (2018).

    Article  Google Scholar 

  44. 44.

    Nimmo, F. et al. Mean radius and shape of Pluto and Charon from New Horizons images. Icarus 287, 12–29 (2017).

    Article  Google Scholar 

  45. 45.

    Leliwa-Kopystyński, J., Maruyama, M. & Nakajima, T. The water–ammonia phase diagram up to 300 MPa: Application to icy satellites. Icarus 159, 518–528 (2002).

    Article  Google Scholar 

  46. 46.

    Hobbs, P. V. Ice Physics (Oxford Univ. Press, 1974).

  47. 47.

    Choukroun, M. & Grasset, O. Thermodynamic data and modeling of the water and ammonia-water phase diagrams up to 2.2 GPa for planetary geophysics. J. Chem. Phys. 133, 144502 (2010).

    Article  Google Scholar 

  48. 48.

    Circone, S., Kirby, S. H. & Stern, L. A. Thermodynamic calculations in the system CH4−H2O and methane hydrate phase equilibria. J. Phys. Chem. B 110, 8232–8239 (2006).

    Article  Google Scholar 

  49. 49.

    Yao, C., Deschamps, F., Lowman, J. P., Sanchez-Valle, C. & Tackley, P. J. Stagnant lid convection in bottom-heated thin 3-D spherical shells: influence of curvature and implications for dwarf planets and icy moons. J. Geophys. Res. 119, 1895–1913 (2014).

    Article  Google Scholar 

  50. 50.

    Goldsby, D. L. & Kohlstedt, D. L. Superplastic deformation of ice: experimental observations. J. Geophys. Res. 106, 11017–11030 (2001).

    Article  Google Scholar 

  51. 51.

    Barr, A. C. & Pappalardo, R. T. Onset of convection in the icy Galilean satellites: influence of rheology. J. Geophys. Res. 110, E12005 (2005).

    Article  Google Scholar 

  52. 52.

    Christensen, U. R. Convection in a variable-viscosity fluid: Newtonian versus power-law rheology. Earth Planet. Sci. Lett. 64, 153–162 (1983).

    Article  Google Scholar 

  53. 53.

    Dumoulin, C., Doin, M.-P. & Fleitout, L. Heat transport in stagnant lid convection with temperature- and pressure-dependent Newtonian or non-Newtonian rheology. J. Geophys. Res. 104, 12759–12777 (1999).

    Article  Google Scholar 

  54. 54.

    Anderson, G. K. Enthalpy of dissociation and hydration number of methane hydrate from the Clapeyron equation. J. Chem. Thermodyn. 36, 1119–1127 (2004).

    Article  Google Scholar 

  55. 55.

    Kamata, S., Sugita, S. & Abe, Y. A new spectral calculation scheme for long-term deformation of Maxwellian planetary bodies. J. Geophys. Res. 117, E02004 (2012).

    Article  Google Scholar 

  56. 56.

    Choukroun, M. & Grasset, O. Thermodynamic model for water and high-pressure ices up to 2.2 GPa and down to the metastable domain. J. Chem. Phys. 127, 124506 (2007).

    Article  Google Scholar 

  57. 57.

    Duan, Z. & Mao, S. A thermodynamic model for calculating methane solubility, density and gas phase composition of methane-bearing aqueous fluids from 273 to 523 K and from 1 to 2000 bar. Geochim. Cosmochim. Acta 70, 3369–3386 (2006).

    Article  Google Scholar 

Download references

Acknowledgements

This study was supported by KAKENHI from the Japan Society for Promotion of Science (grant nos. JP16K17787, JP17H06456 and JP17H06457) and by the Astrobiology Center Program of the National Institutes of Natural Sciences.

Author information

Affiliations

Authors

Contributions

S.K. developed the idea of this study, conducted thermal evolution and viscous relaxation calculations, created all figures and was the primary author of the manuscript. F.N. participated in numerous discussions and co-wrote the manuscript. Y.S. and K.K. provided information on gas production mechanisms and likely guest gas species of clathrate hydrates. N.N. provided detailed information on clathrate hydrates and calculated their densities. J.K. participated in numerous discussions on thermal evolution models. A.T. provided detailed information on clathrate hydrates formation. All of the authors participated in interpretation of the results.

Corresponding author

Correspondence to Shunichi Kamata.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

Supplementary Figs. 1–9, Table 1 and discussion.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Kamata, S., Nimmo, F., Sekine, Y. et al. Pluto’s ocean is capped and insulated by gas hydrates. Nat. Geosci. 12, 407–410 (2019). https://doi.org/10.1038/s41561-019-0369-8

Download citation

Further reading

Search

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing