Stabilization of ammonia-rich hydrate inside icy planets

The interior structure of the giant ice planets Uranus and Neptune, but also of newly discovered exoplanets, is loosely constrained, because limited observational data can be satisfied with various interior models. Although it is known that their mantles comprise large amounts of water, ammonia, and...

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Veröffentlicht in:Proceedings of the National Academy of Sciences - PNAS 2017-08, Vol.114 (34), p.9003-9008
Hauptverfasser: Robinson, Victor Naden, Wang, Yanchao, Ma, Yanming, Hermann, Andreas
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container_issue 34
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container_title Proceedings of the National Academy of Sciences - PNAS
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creator Robinson, Victor Naden
Wang, Yanchao
Ma, Yanming
Hermann, Andreas
description The interior structure of the giant ice planets Uranus and Neptune, but also of newly discovered exoplanets, is loosely constrained, because limited observational data can be satisfied with various interior models. Although it is known that their mantles comprise large amounts of water, ammonia, and methane ices, it is unclear how these organize themselves within the planets—as homogeneous mixtures, with continuous concentration gradients, or as well-separated layers of specific composition. While individual ices have been studied in great detail under pressure, the properties of their mixtures are much less explored. We show here, using first-principles calculations, that the 2:1 ammonia hydrate, (H₂O)(NH₃)₂, is stabilized at icy planet mantle conditions due to a remarkable structural evolution. Above 65 GPa, we predict it will transform from a hydrogen-bonded molecular solid into a fully ionic phase O 2 − ( NH 4 + ) 2 , where all water molecules are completely deprotonated, an unexpected bonding phenomenon not seen before. Ammonia hemihydrate is stable in a sequence of ionic phases up to 500 GPa, pressures found deep within Neptune-like planets, and thus at higher pressures than any other ammonia–water mixture. This suggests it precipitates out of any ammonia–water mixture at sufficiently high pressures and thus forms an important component of icy planets.
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subjects Ammonia
Chemical bonds
Concentration gradient
Density
Extrasolar planets
Homogeneous mixtures
Hydrogen bonding
Hydrogen bonds
Neptune
Phase transitions
Physical Sciences
Planet detection
Planetary interiors
Planetary mantles
Planets
Precipitates
Studies
Uranus
Water chemistry
title Stabilization of ammonia-rich hydrate inside icy planets
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