Scientists have modeled the composition of Neptune’s ice

A mixture of water and ammonia ice in neptunian conditions should form ammonium hemihydrate, from which the solid part of the planet consists.

Extremely low temperatures and very high pressures on Neptune and similar planets seem to mean that an appreciable fraction of the matter lying below the visible atmosphere must be in a solid state. Thick frozen layer lies between the atmosphere of the planet and its core, the temperature of which is likely to be quite high, which again makes the picture difficult to predict.
Neptunian ice consists of a mixture of water, ammonia and methane. More precisely, it should be so, but modern science can not guarantee it. No one has yet seen this picture, and it is obviously impossible to recreate such conditions, primarily pressure, in the terrestrial laboratory with the current level of technology development.
A team of scientists from Edinburgh conducted a computer simulation of the conditions in which, most likely, the substance is located on Neptune, and the results of the substance’s stay in these conditions. According to scientists, the mathematical model is an excellent tool for knowing what can not be measured directly.
The main result of the simulation is the assumption that a frozen mixture of water and ammonia on Neptune and similar ice giants like Uranus probably forms ammonium hemihydrate – a compound that exists at low temperatures and high pressures, not found on Earth and poorly studied. At least in the latter it’s easy to see for yourself, trying to find on the Web a clear description of this hemihydrate in Russian or English.
The authors of the study believe that their results will be of significant importance for further study of the ice giants, including Neptune. Details of their work can be found in an article published in the PNAS.