Hydrogen-carbon doubly superionic conduits of carbonic acids in planetary ices
2026-05-12
Recent astronomical observations show that carbon dioxide (CO 2 ) is widespread on planetary bodies, often coexisting with water (H 2 O). Crystalline carbonic acid (H 2 CO 3 ), produced from CO 2 -H 2 O 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 CO 2 -H 2 O planetary ices across pressures from 0 to 500 gigapascals. Our simulations demonstrate that H 2 CO 3 and orthocarbonic acid (H 4 CO 4 ) 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 H 2 CO 3 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.