Hydrogen-carbon doubly superionic conduits of carbonic acids in planetary ices

Recent astronomical observations show that carbon dioxide (CO₂) is widespread on planetary bodies, often coexisting with water (H₂O). Crystalline carbonic acid (H₂CO₃), produced from CO₂-H₂O interactions, has been predicted and synthesized under high pressure, yet their dynamic behaviors under planetary interior conditions remain poorly understood. Here, we investigate the stability of CO₂-H₂O planetary ices across pressures from 0 to 500 gigapascals. Our simulations demonstrate that H₂CO₃ and orthocarbonic acid (H₄CO₄) become dominant C─O─H compounds and evolve from molecular crystals into three-dimensional solids with increasing pressures. Particularly, both carbonic acids transform into a H-diffusive superionic phase and subsequently a C─H doubly superionic conduit enabled by interconnected oxygen polyhedral voids, where correlated motions between ions enhance the ionic conductivities. Moreover, the hydrogen in H₂CO₃ exhibits strong anisotropic behaviors that may contribute to the nonaxisymmetric magnetic fields of ice giants. These findings further suggest that carbonic acid ices could sustain hydrogen-carbon transport, potentially enhancing convective volatile cycling in giant planet interiors.