Redox-Controlled Carbon Speciation and Cycling in Planetary Interiors: From Ice Giants to Rocky Planets

Carbon and hydrogen speciation and cycling in planetary interiors fundamentally shapes planetary structure and evolution. However, the behaviors of C-H system under extreme pressure-temperature (P-T) conditions across diverse planetary redox conditions remains poorly constrained. Through high P-T experiments spanning ice giant to terrestrial planet conditions, we investigated the C-H-O speciation, transport mechanisms, and implications for volatile budgets in differentiated bodies throughout the solar system and beyond. Our laser-heated diamond anvil cell experiments combined with X-ray diffraction up to 108 GPa and 3,410 K revealed pronounced melting-point depression of diamond in the presence of H₂O, demonstrating that carbon speciation and phase stability in deep planetary interiors—from ice giants like Neptune and Uranus to carbon-rich exoplanets—are strongly influenced by volatile interactions. Multi-anvil press experiments on carbon-saturated Fe-Si alloys (4-27 wt.% Si) at 5-20 GPa constrained carbon solubility variations with silicon conent in the Fe-Si-C liquids. This relationship reveals how redox conditions controls carbon partitioning during core-mantle differentiation in reduced planetary environments like Mercury, potentially driving carbon exsolution and transport from metallic to silicate reservoirs to form distinct diamond layer on top of core-mantle boundary. For more oxidized and hydrated planetary mantle, such as the Earth’s mantle, our high P-T experiments examining the reaction of iron carbides (Fe₃C, Fe₇C₃) with hydrous minerals such as brucite produce elemental carbon, in form of diamond, and (Mg,Fe)O, demonstrating that redox reactions between reduced carbon-bearing phases and hydrous minerals can generate diamond and redistribute carbon during magma ocean crystallization and slab-mantle interactions. These findings illuminate the speciation and cycling of C-H under varying redox conditions controls volatile budgets and distribution across planetary bodies, influences the core-mantle-crust carbon cycling through diverse planetary processes in our solar system and exoplanetary systems.