TY - JOUR
T1 - Stability of hydrides in sub-Neptune exoplanets with thick hydrogen-rich atmospheres
AU - Kim, Taehyun
AU - Wei, Xuehui
AU - Chariton, Stella
AU - Prakapenka, Vitali B.
AU - Ryu, Young Jay
AU - Yang, Shize
AU - Shim, Sang Heon
N1 - Publisher Copyright:
© 2023 the Author(s).
PY - 2023
Y1 - 2023
N2 - Many sub-Neptune exoplanets have been believed to be composed of a thick hydrogendominated 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 Fe2+ in (Mg,Fe)O to Fe0 metal at the pressure-temperature conditions relevant to the atmosphere-interior boundary. However, it is unclear whether 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 3,500 to 4,900 K (close to or above their melting temperatures) in an H medium leads to the formation of Mg2FeH6 and H2O at 8 to 13 GPa. At 26 to 29 GPa, the behavior of the system changes, and Mg-H in an H fluid and H2O were detected with separate FeHx . 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.
AB - Many sub-Neptune exoplanets have been believed to be composed of a thick hydrogendominated 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 Fe2+ in (Mg,Fe)O to Fe0 metal at the pressure-temperature conditions relevant to the atmosphere-interior boundary. However, it is unclear whether 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 3,500 to 4,900 K (close to or above their melting temperatures) in an H medium leads to the formation of Mg2FeH6 and H2O at 8 to 13 GPa. At 26 to 29 GPa, the behavior of the system changes, and Mg-H in an H fluid and H2O were detected with separate FeHx . 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.
KW - exoplanets
KW - hydride
KW - hydrogen
KW - magma
KW - sub-Neptunes
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U2 - 10.1073/pnas.2309786120
DO - 10.1073/pnas.2309786120
M3 - Article
C2 - 38109550
AN - SCOPUS:85180619506
SN - 0027-8424
VL - 120
JO - Proceedings of the National Academy of Sciences of the United States of America
JF - Proceedings of the National Academy of Sciences of the United States of America
IS - 52
ER -