Scientists explore Neptune’s interior without actually going there

Neptune, the outermost planet since the demotion of Pluto, has only ever been explored by one spacecraft. That was Voyager 2, which got a brief look as it flew past in August 1989 on its journey out of the Solar System. And of course it was using technology developed well before the probe was launched in 1977.

Voyager 2’s image of planet Neptune, showing a dark spot and a few other features. Image credit: NASA/JPL

That means we know a lot less about Neptune, and its inner neighbour Uranus, another ice giant, than we do about all the other planets, which have had probes orbiting them for years.

However, a team of scientists have helped discover what lies beneath Neptune’s cloudy, blue surface by mounting a virtual mission, using powerful computers. These simulated conditions in Neptune’s interior, within its mantle between the surface and the planet’s core.

The scientists, from the University of Edinburgh, working with colleagues at Jilin University, China, already knew that this thick mantle was made up of frozen mixtures of water, ammonia and methane, but not the way in which these chemicals were stored.

Related: More about Neptune

Related: Uranus and Neptune – ice twins that fell out

However, by testing how such chemicals react with each other at extreme conditions of high pressures and low temperatures, using large-scale computer simulations, the scientists were able to suggest which compounds form within the mantle.

The team found that frozen mixtures of water and ammonia inside Neptune are likely to form a compound called ammonia hemihydrate, which has been little-studied. Similar processes probably occur within fellow ice giant Uranus, they say.

The researchers say that, as well as influencing the future study of these ice giants, the findings could also help astronomers classify newly discovered exoplanets orbiting other stars. The study is published in the journal Proceedings of the National Academy of Sciences.

An artist’s impression of a veteran Voyager spacecraft out in the depths of space. Image credit: NASA

Dr Andreas Hermann, of the University of Edinburgh’s Centre for Science at Extreme Conditions, said in a statement: “This study helps us better predict what is inside icy planets like Neptune. 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.

“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.”

Neptune’s visible surface does not show as much detail as giant planet Jupiter, with its many belts and bands. When Voyager 2 flew past, it recorded deep blue bands and a giant dark spot that marked a temporary raging storm, plus bright clouds racing around the planet at 1,100 km per hour. 

Since then, some features have been spotted by ground-based professional and amateur observers. For example, astronomer Mike Brown has taken dramatic infrared images. And earlier this year, astronomers recorded a new bright spot erupting in the cloud tops.

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