Evidence for ultra-water-rich ammonia hydrates stabilized in icy exoplanetary mantles

TL;DR

This study reveals the formation of ultra-water-rich ammonia hydrates in icy exoplanet mantles at high pressures and temperatures, suggesting they influence planetary structure and evolution.

Contribution

It provides experimental evidence for stable ultra-water-rich ammonia hydrates, including a novel NH3.6H2O phase, under conditions relevant to exoplanetary interiors.

Findings

Water-rich ammonia hydrates are stable up to 30 GPa and 1600 K.

A new hydrate phase NH3.6H2O was discovered.

Hydrates can be quenched to room temperature, indicating potential for planetary modeling.

Abstract

Understanding the behavior of the water-ammonia system at high pressure-high temperature conditions is important for modeling the internal dynamics of exoplanet icy mantles. Conventionally, mixtures of ammonia hemihydrate AHH (2:1 ammonia-water molar ratio) and H2O ice VII have been regarded as the ultimate solid phase assembly in the system. Here we report evidence for chemical reactions between AHH and ice VII above 750 K and 16 GPa that stabilize water-rich ammonia hydrates, including a novel ultra-water rich hydrate NH3.6H2O (1:6 ratio) coexisting with ammonia dihydrate ADH (1:2 ratio) and excess ice VII. This assembly is stable up to at least 30 GPa and 1600 K and can be quenched to room temperature. Our results demonstrate that water-rich ammonia hydrates are favored in the icy mantle of 1-2 MEarth exoplanets regardless of the ammonia content of the hydrate crystallized during accretion and/or evolution as long as excess H2O ice is available. The buoyancy contrast between water-rich hydrates and ice VII may lead to chemical stratification in exoplanet icy mantles, hence affecting their thermal evolution.