Stability of Hydrides in Sub-Neptune Exoplanets with Thick Hydrogen-Rich Atmospheres (2401.02637v1)
Abstract: Many sub-Neptune exoplanets have been believed to be composed of a thick hydrogen-dominated atmosphere and a high-temperature heavier-element-dominant core. From an assumption that there is no chemical reaction between hydrogen and silicates/metals at the atmosphere-interior boundary, the cores of sub-Neptunes have been modeled with molten silicates and metals (magma) in previous studies. In large sub-Neptunes, pressure at the atmosphere-magma boundary can reach tens of gigapascals where hydrogen is a dense liquid. A recent experiment showed that hydrogen can induce the reduction of Fe${2+}$ in (Mg,Fe)O to Fe$0$ metal at the pressure-temperature conditions relevant to the atmosphere-interior boundary. However, it is unclear if Mg, one of the abundant heavy elements in the planetary interiors, remains oxidized or can be reduced by H. Our experiments in the laser-heated diamond-anvil cell found that heating of MgO + Fe to 3500-4900 K (close to or above their melting temperatures) in a H medium leads to the formation of Mg$_2$FeH$_6$ and H$_2$O at 8-13 GPa. At 26-29 GPa, the behavior of the system changes, and Mg-H in an H fluid and H$_2$O were detected with separate FeH$_x$. The observations indicate the dissociation of the Mg-O bond by H and subsequent production of hydride and water. Therefore, the atmosphere-magma interaction can lead to a fundamentally different mineralogy for sub-Neptune exoplanets compared with rocky planets. The change in the chemical reaction at the higher pressures can also affect the size demographics (i.e., "radius cliff") and the atmosphere chemistry of sub-Neptune exoplanets.
- Lambrechts M, Johansen A (2012) Rapid growth of gas-giant cores by pebble accretion. A&A 544:A32.
- Astron Astrophys Rev 26(1):2.
- Space Sci Rev 216(5):86.
- Space Sci Rev 216(8):129.
- Journal of Geophysical Research: Planets 126.
- ApJ 869(2):163.
- ApJ 891(2):111.
- Zeng L, et al. (2021) New Perspectives on the Exoplanet Radius Gap from a Mathematica Tool and Visualized Water Equation of State. ApJ 923(2):247.
- Olson P, Sharp ZD (2018) Hydrogen and helium ingassing during terrestrial planet accretion. Earth and Planetary Science Letters 498:418–426.
- Planet. Sci. J. 3(5):127.
- Planet. Sci. J. 4(2):30.
- Nature 616:306–311.
- Fulton BJ, Petigura EA (2018) The California- Kepler Survey. VII. Precise Planet Radii Leveraging Gaia DR2 Reveal the Stellar Mass Dependence of the Planet Radius Gap. AJ 156(6):264.
- ApJ 887(2):L33.
- Shinozaki A, et al. (2014) Formation of SiH4 and H2O by the dissolution of quartz in H2 fluid under high pressure and temperature. American Mineralogist 99(7):1265–1269.
- The Astrophysical Journal 825:86.
- Nat Commun 8(1):15735.
- Prakapenka VB, et al. (2008) Advanced flat top laser heating system for high pressure research at GSECARS: Application to the melting behavior of germanium. High Pressure Res. 28(3):225–235.
- Goncharov AF, et al. (2010) X-ray diffraction in the pulsed laser heated diamond anvil cell. Review of Scientific Instruments 81(11):113902.
- Nature 615(7953):646–651.
- Piet H, et al. (2023) Superstoichiometric Alloying of H and Close-Packed Fe-Ni Metal Under High Pressures: Implications for Hydrogen Storage in Planetary Core. Geophysical Research Letters 50(5):e2022GL101155.
- Physical Review B 105:104111.
- High Pressure Research 27(4):431–446.
- High Pressure Research 39(3):457–470.
- Okuchi T (1997) Hydrogen Partitioning into Molten Iron at High Pressure: Implications for Earth’s Core. Science 278(5344):1781–1784.
- Sakamaki K, et al. (2009) Melting phase relation of FeH up to 20 GPa: Implication for the temperature of the Earth’s core. Physics of the Earth and Planetary Interiors 174(1-4):192–201.
- Shinozaki A, et al. (2013) Influence of H22{}_{2}start_FLOATSUBSCRIPT 2 end_FLOATSUBSCRIPT fluid on the stability and dissolution of Mg22{}_{2}start_FLOATSUBSCRIPT 2 end_FLOATSUBSCRIPTSiO44{}_{4}start_FLOATSUBSCRIPT 4 end_FLOATSUBSCRIPT forsterite under high pressure and high temperature. American Mineralogist 98(8-9):1604–1609.
- Efimchenko VS, et al. (2019) Destruction of fayalite and formation of iron and iron hydride at high hydrogen pressures. Phys Chem Minerals 46(8):743–749.
- Kim T, et al. (2021) Atomic-scale mixing between MgO and H2O in the deep interiors of water-rich planets. Nat Astron 5(8):815–821.
- (AIP, Pasadena, California (USA)), Vol. 341, pp. 211–220.
- J. Chem. Phys. 145(12):124315.
- Phys. Chem. Chem. Phys. 19(9):6677–6687.
- Kuzovnikov M, et al. (2013) Raman scattering study of α𝛼\alphaitalic_α-MgH2 and γ𝛾\gammaitalic_γ-MgH2. Solid State Communications 154:77–80.
- Science 253(5018):421–424.
- Yang SZ, et al. (2019) Direct cation exchange in monolayer MoS22{}_{2}start_FLOATSUBSCRIPT 2 end_FLOATSUBSCRIPT via recombination-enhanced migration. Physical review letters 122(10):106101.
- Contributions to Mineralogy and Petrology 112:490–500.
- ApJ 733(1):2.
- International Journal of Hydrogen Energy 34(8):3410–3416.
- Inorg. Chem. 23(13):1953–1957.
- International Journal of Hydrogen Energy 40(30):9306–9313.
- Phys. Scr. 86(1):015701.
- American Mineralogist 84(4):677–680.
- Rahim W, et al. (2020) Geometric Analysis and Formability of the Cubic A2BX6 Vacancy-Ordered Double Perovskite Structure. Chem. Mater. 32(22):9573–9583.
- AJ 158(3):109.
- Astronomy & Astrophysics 634:A43.
- Earth and Planetary Science Letters 345–348:38–48.
- (National Academies Press, Washington, D.C.), p. 26141.
- Astronomy & Astrophysics 646:A17.
- International Journal of Hydrogen Energy 32(9):1121–1140.
- Duan D, et al. (2017) Structure and superconductivity of hydrides at high pressures. National Science Review 4(1):121–135.
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