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.
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The data that support the plots within this paper and other findings of this study are available from the corresponding author on reasonable request.
Codes for the thermal evolution and viscous relaxation calculations are available upon reasonable request from S.K.
Nimmo, F. & Pappalardo, R. T. Ocean worlds in the outer solar system. J. Geophys. Res. 121, 1378–1399 (2016).
Nimmo, F. et al. Reorientation of Sputnik Planitia implies a subsurface ocean on Pluto. Nature 540, 94–96 (2016).
Hussmann, H., Sotin, C. & Lunine, J. I. in Treatise in Geophysics 2nd edn, Vol. 10 (ed. Schubert, G.) 605–635 (Elsevier, 2015).
Robuchon, G. & Nimmo, F. Thermal evolution of Pluto and implications for surface tectonics and a subsurface ocean. Icarus 216, 426–439 (2011).
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).
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).
Kamata, S. & Nimmo, F. Interior thermal state of Enceladus inferred from the viscoelastic state of the ice shell. Icarus 284, 387–393 (2017).
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).
Le Roy, L. et al. Inventory of the volatiles on comet 67P/Churyumov–Gerasimenko from Rosetta/ROSINA. Astron. Astrophys. 583, A1 (2015).
Croft, S. K., Lunine, J. I. & Kargel, J. Equation of state of ammonia–water liquid: derivation and planetological applications. Icarus 73, 279–293 (1988).
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).
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).
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).
Zolotov, M. Y. Aqueous fluid composition in CI chondritic materials: chemical equilibrium assessments in closed systems. Icarus 220, 713–729 (2012).
Sloan, E. D. & Koh, C. A. Clathrate Hydrates of Natural Gases 3rd edn (CRC, 2007).
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).
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).
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).
Tobie, G., Choblet, G. & Sotin, C. Tidally heated convection: constraints on Europa’s ice shell thickness. J. Geophys. Res. 108, 5124 (2003).
Barr, A. C. & McKinnon, W. B. Convection in ice I shells and mantles with self-consistent grain size. J. Geophys. Res. 112, E02012 (2007).
Moore, J. M. et al. The geology of Pluto and Charon through the eyes of New Horizons. Science 351, 1284–1293 (2016).
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).
White, O. L. et al. Geological mapping of Sputnik Planitia on Pluto. Icarus 287, 261–286 (2017).
Mousis, O. et al. Methane clathrates in the Solar System. Astrobiology 15, 308–326 (2015).
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).
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).
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).
Petuya, C. & Desmedt, A. Revealing CO-preferential encapsulation in the mixed CO–N2 clathrate hydrate. J. Phys. Chem. C 123, 4871–4878 (2019).
Glein, C. R. & Waite, J. H. Primordial N2 provides a cosmochemical explanation for the existence of Sputnik Planitia, Pluto. Icarus 313, 79–92 (2018).
Claypool, G. E. & Kvenvolden, K. A. Methane and other hydrocarbon gases in marine sediment. Annu. Rev. Earth Planet. Sci. 11, 299–327 (1983).
McKinnon, W. B. & Mueller, S. Pluto’s structure and composition suggest origin in the solar, not a planetary, nebula. Nature 335, 240–243 (1988).
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).
Seewald, J. S. Organic–inorganic interactions in petroleum-producing sedimentary basins. Nature 426, 327–333 (2003).
Stopler, D. A. et al. Formation temperatures of thermogenic and biogenic methane. Science 344, 1500–1503 (2014).
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).
Sekine, Y. et al. High-temperature water–rock interactions and hydrothermal environments in the chondrite-like core of Enceladus. Nat. Commun. 6, 8604 (2015).
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).
Grundy, W. M. et al. Surface compositions across Pluto and Charon. Science 351, aad9189 (2016).
Gladstone, G. R. et al. The atmosphere of Pluto as observed by New Horizons. Science 351, aad8866 (2016).
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).
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).
Waite, J. H. et al. Liquid water on Enceladus from observations of ammonia and 40Ar in the plume. Nature 460, 487–490 (2009).
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).
Nimmo, F. et al. Mean radius and shape of Pluto and Charon from New Horizons images. Icarus 287, 12–29 (2017).
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).
Hobbs, P. V. Ice Physics (Oxford Univ. Press, 1974).
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).
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).
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).
Goldsby, D. L. & Kohlstedt, D. L. Superplastic deformation of ice: experimental observations. J. Geophys. Res. 106, 11017–11030 (2001).
Barr, A. C. & Pappalardo, R. T. Onset of convection in the icy Galilean satellites: influence of rheology. J. Geophys. Res. 110, E12005 (2005).
Christensen, U. R. Convection in a variable-viscosity fluid: Newtonian versus power-law rheology. Earth Planet. Sci. Lett. 64, 153–162 (1983).
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).
Anderson, G. K. Enthalpy of dissociation and hydration number of methane hydrate from the Clapeyron equation. J. Chem. Thermodyn. 36, 1119–1127 (2004).
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).
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).
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).
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.
The authors declare no competing interests.
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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
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