Neptune’s Icy Insides Contain A Mysterious Chemical Compound

Neptune’s frozen guts are probably made of a compound mixture of ammonia and water, researchers say, and this finding could help scientists better understand exoplanets as they discover them.

In the mantle of Neptune, between its core and its atmosphere, is water, ammonia and methane, like other icy planets. But it is unclear what forms these chemicals take when exposed to the frigid temperature and high pressure present on these planets known as ice giants.

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“To understand [the planets’] interior structure, it is crucial to study these ices at the extreme pressure conditions they likely experience,” a study in the journal Proceedings of the National Academy of Sciences says. “Although it is known that their mantles comprise large amounts of water, ammonia, and methane ices, it is unclear how these organize themselves within the planets — as homogeneous mixtures, with continuous concentration gradients, or as well-separated layers of specific composition.”

The researchers focused on how the icy compounds of water and ammonia would work together within Neptune — the farthest planet in our solar system, at 2.8 billion miles away from the sun — rather than how they work on an individual basis. Their computer simulations, which subjected the virtual ingredients to the extreme conditions of Neptune’s mantle, show the water and ammonia come together in an unexpected way: The chemicals create a mixture called ammonia hemihydrate, which is a rare and little understood compound.

“This study helps us better predict what is inside icy planets like Neptune,” researcher Andreas Hermann said in a statement from the University of Edinburgh. “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.”

Although this study focused on the composition of Neptune, that planet’s inward neighbor, Uranus, is also an ice giant.

Scientists have additionally found planets orbiting other stars that are likely ice giants, so these findings could give them some clues about what those faraway exoplanets are like, as they are difficult to observe.

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The solar system is full of analogs like this. There are exoplanets, for example, that have similar compositions to Jupiter but orbit much closer to their stars, earning them the nickname “hot Jupiters.” Scientists studying the real Jupiter don’t just learn more about the storms that choke its atmosphere and other conditions on the gas giant, but they also gain information that could help them predict what hot Jupiters are like.

For the team investigating ice giants, the work is not over.

“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,” Hermann said.