Ice giants Uranus and Neptune rarely make headlines. In general, all attention is focused on the other two huge balls of gas in our system, namely Jupiter and Saturn. However, these two worlds have a lot to offer us, like real showers of diamonds.
The last (and only) time we got close-up data from Uranus and Neptune was three decades ago, when Voyager 2 went into route to the outer edges of the Solar System. Since then, we have had to make do with a few telescope observations, while other planets, moons and asteroids are the subject of real exploration missions. These two seemingly bland and boring worlds hide real treasures.
Uranus and Neptune are "ice giants". However, this confusing name has nothing to do with ice cream in the proper sense. The distinction comes from the composition of these planets. Jupiter and Saturn, the other two gas giants in the Solar System, are almost entirely composed of gas (hydrogen and helium), while Uranus and Neptune are mainly made up of water, ammonia and methane . Astronomers commonly refer to these molecules as "ice", but there's not really a good reason for that, other than that these elements may have still been in solid form when these two planets formed.
Uranus and Neptune therefore offer a lot of water, ammonia and methane. For the rest, we don't know much. To try to understand it, astronomers and planetary scientists must therefore rely on the meager data collected by Voyager 2, then combine them with mathematical models to try to reproduce the conditions inside these planets in the laboratory, which takes us to these famous "treasures". It is indeed thanks to these laboratory works that we realized that Uranus and Neptune could wipe diamond rains .
According to these models, the outermost layers of the mantle of these two planets offer temperatures around 1,700°C and a pressure about 200,000 times more crushing than that of the Earth at atmospheric level . Under these conditions, the methane molecules break apart, releasing their carbon. These carbon atoms then come together to form long chains which in turn tighten to form crystal patterns like diamonds.
These very dense diamond structures then fall through the mantle layers of Uranus and Neptune until it gets too hot (the innermost layers have temperatures above 6000°C). They then vaporize and rise. The cycle then repeats itself, hence the term "diamond rain".
We have also been able to observe this phenomenon on Earth, in the laboratory. A few years ago, researchers fired powerful lasers at targets to very briefly reproduce the temperatures and pressures found inside these two planets. An experiment with polystyrene had then made it possible to manufacture nanometric-sized diamonds .
Admittedly, Uranus and Neptune do not contain large amounts of polystyrene, but this material was much easier to handle than methane and, according to the researchers, it behaves very similarly. Furthermore, we know that Uranus and Neptune can maintain these pressures much longer as astrophysicist Paul M. Sutter points out on Space.com, so diamonds could theoretically get much bigger.
The best way to find out for sure would be to send a ship directly to the scene. Unfortunately, that won't be anytime soon.
