Abstract
Uranus and Neptune are generally assumed to have helium only in their gaseous atmospheres. Here, we report the possibility of helium being fixed in the upper mantles of these planets in the form of NH3–He compounds. Structure predictions reveal two energetically stable NH3–He compounds with stoichiometries (NH3)2He and NH3He at high pressures. At low temperatures, (NH3)2He is ionic with NH3 molecules partially dissociating into (NH2)− and (NH4)+ ions. Simulations show that (NH3)2He transforms into intermediate phase at 100 GPa and 1000 K with H atoms slightly vibrate around N atoms, and then to a superionic phase at ~2000 K with H and He exhibiting liquid behavior within the fixed N sublattice. Finally, (NH3)2He becomes a fluid phase at temperatures of 3000 K. The stability of (NH3)2He at high pressure and temperature could contribute to update models of the interiors of Uranus and Neptune.
Highlights
Uranus and Neptune are generally assumed to have helium only in their gaseous atmospheres
A widely accepted model for each of these planets is that the upper mantle comprises a mixture of ionized H2O, NH3–He compounds with stoichiometries (NH3), and CH45,8,11, whereas the lower mantle consists of metallic H2O, NH35,6 Much effort has been devoted to determine the ratio of the components in the interior of the ice planets[8,10,11], no consensus were reached
Our results show that He can react with NH3 to form (NH3)2He under extreme conditions, to a certain extent corresponding to the upper mantles of Uranus and Neptune, thereby providing information essential to the understanding of the interior models of these planets
Summary
Uranus and Neptune are generally assumed to have helium only in their gaseous atmospheres. An unusual layered ionic phase of NH3(H2O)[2] was predicted for a 1:2 mixture of NH3 and H2O; it was modeled to transform into a superionic phase at high pressure and high temperature (41 GPa and 600 K)[33]. Our results show that He can react with NH3 to form (NH3)2He under extreme conditions, to a certain extent corresponding to the upper mantles of Uranus and Neptune, thereby providing information essential to the understanding of the interior models of these planets.
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