Seeing Deep Inside Icy Giant Planets

Water is one of the most abundant molecules in “icy” planets and moons in our solar system and probably in extrasolar planets, such as icy “super Earths” with masses from ∼ 2 to 10 times the mass of Earth, and “hot Neptunes” with masses comparable to or smaller that than the mass of Neptune, which is 15 times the mass of Earth. Planetary “ice” is a mixture of H2O, CH4, and NH3, whether frozen solid on the surface or as a fluid in the hot interior of an “icy” planet or moon. Ice in Neptune and Uranus, for example, exists only as a fluid. Ice is mostly H2O, and generally treated as such, which is why the equation of state (EOS) of water is so important for modeling icy planets [1]. Because of their large sizes and low thermal conductivities, planetary interior pressures range up to several 100 gigapascals (GPa) and temperatures of several 1000 K [2]. As reported in Physical Review Letters, Marcus Knudson at Sandia National Laboratories, New Mexico, and colleagues have now made substantial advances in probing the properties of water under such conditions [3].