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.