Yes, 'diamond rain' on Uranus and Neptune

Despite their size, Uranus and Neptune are seldom mentioned; all the media attention goes to their larger siblings, Jupiter and Saturn. 

In the beginning, Uranus and Neptune appear to be bland, boring balls of indifferent molecules. There may be something spectacular lying beneath the outer layers of these worlds: an endless rain of diamonds.

In fact, astronomers refer to Uranus and Neptune as ice giants, rather than Tolkien-like creatures.

In spite of this, the name refers to nothing like ice in the sense that you would usually think of it - as in ice cubes in drinks. The distinction comes from what these planets are made of. Mercury and Venus are small, rocky planets. Jupiter and Saturn are giant gas giants. This vast planet was able to swell to its present size due to the rapid accretion of those elements.

Neptune, on the other hand, is mostly made up of methane, ammonia, and water. These molecules are commonly referred to as "ice" by astronomers, but there's no real reason for it, except that when the planets first formed, these elements were probably solids. 

Water, ammonia, and methane can be found deep within Uranus and Neptune's cloud tops. However, these ice giants probably contain rocky cores surrounded by exotic quantum states, resulting in compressed elements. Quantum weirdness eventually turns into a super-pressured "soup" that thins out as one approaches the surface.

When Voyager 2 whizzed past two of these worlds three decades ago, we got some very close-up views of them.

As a result, we have been able to observe Jupiter and Saturn through several orbiting probes, but we can only observe Uranus and Neptune through telescopes.

For astronomers and planetary scientists to try to figure out what's inside those planets, they need to combine meager data with laboratory experiments that simulate the conditions in those planets' interiors. Plus, they use some good old-fashioned math — a lot of it. With limited data, mathematical modeling allows astronomers to understand what's happening in a particular environment.

We realized Uranus and Neptune may have diamond rain after using both mathematical modeling and laboratory experiments.

Before the Voyager 2 mission launched in 1977, diamond rain was a concept that was first broached. Uranus and Neptune are both made of the same stuff, and the deeper you go into a planet, the hotter and denser the material becomes. In addition to providing details on the innermost areas of the mantles of these planets, the mathematical modeling indicates that they are likely to have temperatures somewhere around 7,000 kelvins (12,140 degrees Fahrenheit, or 6,727 degrees Celsius) and pressures six million times Earth's.

In those same models, we find that the outermost mantle layers are somewhat cooler than their innermost layers, which are 3,140 F and 1,727 C less intensely pressurized (200,000 times Earth's atmospheric pressure). As a result, it's natural to ask: What happens to water, ammonia, and methane when exposed to such intense temperatures and pressures?

In particular, methane molecules can be broken apart under intense pressure, releasing carbon. The carbon then forms long chains with its brethren. Diamonds are formed when these long chains squeeze together to form crystalline patterns.

The dense diamond formations then sink from the mantle into the bottom until the temperatures become too high, at which point they vaporize and float back up, thus the name "diamond rain."