Solution structure of Titan-relevant aqueous ammonia by neutron diffraction

In 2034, NASA Dragonfly will arrive at Titan’s Selk crater to study an environment where molten ice has potentially interacted with organics. Some models suggest that Titan has a sub-surface ocean enriched in ammonia, a molecule that forms a deep eutectic with water, implying that it strongly perturbs water’s intermolecular structure. In anticipation of the Dragonfly mission, and to understand the effects of the addition of ammonia to liquid water, we used neutrons to probe the structure of a 20.5 wt.% ammonia-water solution at 273 K and 298 K at 1 bar. We observed the formation of ice-like motifs in ammonia’s hydration shell, a result reminiscent of the ‘microscopic icebergs’ predicted to form around methane and non-polar solutes that were a feature of the original hypothesis for the hydrophobic effect. This result may have implications for the aqueous chemistry of Titan and ammonia-rich ocean worlds.