Research On High-Pressure Physics Of H-O Compounds Inside Ice Giant Planets

In 1986 and 1989,NASA's Voyager 2 probe have measured the magnetic fields of Uranus and Neptune and surprisingly find that,these magnetic fields are distinctly different from other planets in the Solar System.The geomagnetic field is predominantly dipolar,just like a bar magnet.And the north-south axis of the geomagnetic field aligns nearly along the rotation axis.However,the magnetic fields of Uranus and Neptune are not predominantly dipolar,but also possess quadrupole component.And the north-south axis is tripped over from the rotation axis by about 59°and 47°for Uranus and Neptune,respectively.The magnetic fields of planets are proposed to be generated by dynamo.The dipolar magnetic fields are generated in thick shell of conducting fluid.The origin of the abnormal magnetic fields of Uranus and Neptune have long been a puzzle.Large progress has been made recently.The abnormal magnetic fields of Uranus and Neptune are recently proposed to be generated in thin shell of conducting fluid.But the key materials for the dynamo in Uranus and Neptune,i.e.the conducting fluid materials,have remain controversial.It is necessary to figure out the states of compounds inside Uranus and Neptune.Due to the extreme high pressure inside Uranus and Neptune??800 GPa near the core?,theoretical calculations are more suited than experimental methods for investigating their interiors.Therefore,we have performed crystal structure predictions combined with ab initio total energy calculations for H-O system,as well as ab initio molecular dynamics.The details are listed below:1.The interiors of Uranus and Neptune are under high temperature and high pressure conditions.The compositions of the interiors are thought to be H₂O?56%?,CH₄?36%?and NH₃?8%?in molar fraction.Previous studies have reported that CH₄would decompose into diamond and hydrogen under the interior conditions of giant planets.We thus conclude that H₂O may react with hydrogen and form new kinds of compounds in a hydrogen-rich environment.We have used our in-house developed CALYPSO method to explore H-O compounds under high pressure and have found unusual stoichiometric H₃O which is stable from 450 to 600 GPa.Further phonon related calculations indicate that H₃O is also dynamically stable.These results expand the class of compounds inside Uranus and Neptune.2.Ab initio molecular dynamics simulations and electronic structure calculations have been used to construct the P-T phase diagram of H₃O.Further analyses indicate that the isentropes of Uranus and Neptune pass through the metallic fluid region in the phase diagram of H₃O.Therefore,H₃O may exist in the interiors of Uranus and Neptune as metallic fluid.These results are important for explaining the unusual magnetic fields of Uranus and Neptune as well as the structure of other ice giants in exoplanets.3.H₃O exist in the inner region of“ice layer”inside Uranus and Neptune.And CH₄ decomposes into diamond and hydrogen inside“ice layer”.Due to the larger density of diamond,it will sink into the region of fluid H₃O.In this work,we investigate the mixture of H₃O and diamond via ab inito molecular dynamics simulations.The molecular dynamics simulations indicate that H₃O would react with diamond under high pressure and temperature.C element has improved the electronic density of states near the fermi level significantly and will enhance the conductivity.What's more,the increase of the concentration of C will improve the electronic density of states.These results may have great impact on the abnormal magnetic fields of Uranus and Neptune.