Molecular systems at high pressures and temperatures: Solar system astronomy in a physics laboratory

The molecular materials that make up most of Uranus and Neptune are profoundly different at the high temperatures and pressures beneath those planets' gas atmospheres than at standard temperature and pressure. H₂O and CH₄, which may account for ∼40% and ∼20% of the planetary mass respectively, both break down to produce pure carbon (diamond) or oxygen and hydrogen when compressed to pressures greater than several gigapascals (GPa) and then heated to temperatures greater than 2000 K. Heating-induced reactions were optically evident at every experimental pressure (between 6 and 43 GPa for H₂O and between 10 and 50 GPa for CH₄) and documented with x-ray diffraction and spectroscopy. In contrast, at similar conditions NH₃ (∼5% of Uranus or Neptune by mass) does not appear to undergo molecular transformation. A new phase of oxygen is produced in the H₂O dissociation reaction and in experiments in which pure oxygen was laser heated between 3 and 47 GPa. This phase is a structural distortion of the previously observed epsilon phase of oxygen, but 12–15% denser and less compressible (K_0T = 72 ± 2 GPa).; Both dissociation reactions help to relax timing constraints on planet formation by providing a mechanism to create a gas atmosphere after disk dispersal. In addition, gravitational settling of dense diamond to the core of Neptune or Uranus creates an energy reservoir of the same magnitude as Neptune's internal luminosity, while O-H instability forces a reinterpretation of the magnetic field source in Uranus and Neptune: perhaps convecting metallic oxygen produces the planets' dynamo.