There's a Rare Chemical Compound Lurking in The Icy Depths of Neptune

It's in the slushy 'ice mantle'.

18 AUG 2017

Scientists have used complex computer simulations to figure out what lies beneath Neptune's icy surface, finding a rare compound that gives us a greater understanding of how gas giants are evolving.

The simulations suggest a little-known compound called ammonia hemihydrate is lurking inside Neptune, a compound created when frozen water and ammonia mix together. The discovery could be invaluable in our future study of the planet and others like it.


With this knowledge the researchers from the University of Edinburgh in the UK suggest we can get a better idea of how ice giants like Neptune and Uranus are evolving and also help us to spot other similar planets outside our Solar System.

"This study helps us better predict what is inside icy planets like Neptune," says one of the team, Andreas Hermann. "Our findings suggest that ammonia hemihydrate could be an important component of the mantle in ice giants, and will help improve our understanding of these frozen worlds."

Getting a probe out to the surface of Neptune isn't feasible for the time being, and replicating the very low temperatures and high pressures of the planet in a lab here on Earth isn't much easier.

Neptune has a rather chilly average temperature of -214 degrees Celsius (-353 degrees Fahrenheit), mainly because it gets very little sunlight. It also has some of the most spectacular weather in the Solar System, with winds travelling as fast as 2,200 kilometres-per-hour (or 1,370 miles-per-hour).

In the absence of probes or a mini-Neptune-in-a-lab, scientists can turn to computer simulations, algorithms based on experiments measured here on Earth.

Using what we already know about Neptune's mantle as a starting point – that it has large amounts of water, methane, and ammonia – the researchers ran their sums to determine that ammonia hemihydrate is being formed.


That's based on the chemical reactions taking place and the high pressures they would be under.

Neptune doesn't actually have a "surface" as we would understand it: instead, there's the mantle, a slushy layer of water and ice. Astronomers set the "surface" level at the point where the pressure is the same as it is at sea level on Earth, around 1 bar.

If you stood on this so-called "surface", you'd sink down into its gaseous depths, and it's this mantle area that the researchers behind this study have been looking at.

As you dropped, temperatures and pressures would start to rise very fast.

Last year scientists used computer algorithms to look deeper into the cores of planets like Neptune, finding evidence for carbonic acid and an extremely rare compound of molecular hydrogen and methane deep below the atmosphere.

Because conditions on Neptune are so different to Earth, chemicals and compounds that are rare here could well be commonplace on the icy giant.


Now we can add ammonia hemihydrate to the mix of materials we'll probably find on Neptune, if we ever pay a visit – perhaps one day the computer simulations will get so good and so accurate that we won't need to.

"Computer models are a great tool to study these extreme places, and we are now building on this study to get an even more complete picture of what goes on there," says Hermann.

The research has been published in the Proceedings of the National Academy of Sciences.


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