U.S. scientists using first-principle molecular dynamics simulations have determined the interiors of Neptune, Uranus and Earth might contain some solid ice.

Lawrence Livermore National Laboratory scientists, together with University of California-Davis collaborators, used a two-phase approach to determine the melting temperature of ice VII -- a high-pressure phase of ice -- in pressures ranging from 100,000 to 500,000 atmospheres.

For pressures between 100,000 and 400,000 atmospheres, the team led by Eric Schwegler found ice melts as a molecular solid, similar to how it melts in a cold drink. But in pressures above 450,000 atmospheres, there is a sharp increase in the slope of the melting curve due to molecular disassociation and proton diffusion in the solid, prior to melting, which is typically referred to as a superionic solid phase.

"The sharp increase in the melting curves slope opens up the possibility that water exists as a solid in the deep interior of planets such as Neptune, Uranus and Earth," Schwegler said.

The research that included Francois Gygi, Giulia Galli and Manu Sharmae appears in early online edition of the Proceedings of the National Academy of Science.