Condensed matter physics of planets: Puzzles, progress and predictions

Despite recent advances in observations, experiment and theory, there are many major unresolved issues concerning planetary interiors. This paper is not a comprehensive review but seeks to highlight these issues. Emphasis is on the cosmically most abundant materials, the dominant constituents of the giant planets. The important issues include: (1) What are the atomic and electronic degrees of freedom in hydrogen at high pressure and temperature and how do they behave? (2) To what extent does helium dissolve in hydrogen? (3) What is the behavior of water at megabar pressures and what is the H_2-H_2O phase diagram? (4) How does carbon behave at high pressures, in the presence of oxygen and hydrogen? (5) What happens to clathrates (e.g. CH_4·5-3/4H_2O) at high pressure? (6) How does the volatile ice assemblage expected in Titan H_2O- NH_3-CH_4-N_2-CO?) behave at P ~ 20-40 kbar? (7) What is the nature of the core alloy in the Earth and the core-mantle phase boundary? (8) What are the electrical conductivities of all of the above