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'Diamond rain' on Uranus and Neptune appears to be a possibility.

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  • Tip Bones

Uranus and Neptune, the icy giants, don't get nearly enough attention; all of the focus is on their larger siblings, strong Jupiter and gorgeous Saturn.

Uranus and Neptune appear to be drab, monotonous spheres of uninteresting molecules at first view. However, something extraordinary may be hidden beneath the outer layers of such worlds: a continual downpour of diamonds.

"Ice giants" may conjure up images of Tolkien-esque creatures, but it's the phrase scientists use to classify the solar system's farthest planets, Uranus and Neptune.

However, the name is perplexing because it has nothing to do with ice in the traditional sense — as in, for instance, ice crystals in your drink. The differentiation stems from the composition of these planets. Jupiter and Saturn, the system's gas giants, are almost entirely comprised of gas: hydrogen and helium. These massive planets grew to their current size due to the rapid accumulation of certain components.

Uranus and Neptune, on the other hand, are largely comprised of water, ammonia, and methane. These molecules are usually referred to as "ices" by astronomers, but there is no convincing rationale for this other than that when the planets originally formed, those elements were most likely solid.

Into the (not-so-frozen) depths

There's a lot of water, ammonia, and methane deep beneath the green or blue cloud tops of Uranus and Neptune. These ice giants, however, are expected to have rocky cores surrounded by elements that are squeezed into strange quantum states. At some point, the quantum weirdness transforms into a super-pressurized "soup" that thins down as you got closer to the surface.

To be honest, we don't know much about the innards of the ice giants. The last time we saw those two worlds up close was three decades ago when Voyager 2 flew by on its historic journey.

Since then, other orbiting missions have visited Jupiter and Saturn, but our knowledge of Uranus and Neptune has been limited to telescope studies.

To try to grasp what's inside those planets, astronomers and planetary scientists must combine scant data with laboratory studies that attempt to duplicate the circumstances of those planets' innards. They also use a lot of good old-fashioned math. Based on limited data, mathematical modelling assists astronomers in understanding what is happening in a specific situation.

It's pouring diamonds.

The concept of diamond rain was first proposed before the launch of the Voyager 2 mission in 1977. The argument was straightforward: we know what Uranus and Neptune are comprised of, and we know that stuff grows hotter and denser the deeper you go into a planet. The mathematical modelling helps fill in the specifics, such as the fact that the innermost portions of these planets' mantles are anticipated to be roughly 7,000 kelvins (12,140 degrees Fahrenheit, or 6,727 degrees Celsius) and have pressured 6 million times that of Earth's atmosphere.

According to the same simulations, the outermost layers of the mantles are slightly colder — 2,000 K (3,140 F or 1,727 C) — and less highly pressured (200,000 times Earth's atmospheric pressure). As a result, it's natural to wonder what happens to water, ammonia, and methane at such high temperatures and pressures.

The extreme pressures, in particular with methane, can tear the molecule apart, releasing the carbon. The carbon then finds its brothers and sisters, producing lengthy chains. The lengthy chains then bind together to form crystalline patterns like diamonds.

The dense diamond formations then fall down the mantle layers until they get too hot, at which point they evaporate and float back up, repeating the cycle — hence the term "diamond rain."

Diamonds created in laboratories

Sending a spacecraft to Uranus or Neptune would be the best method to validate this theory. That isn't going to happen anytime soon, so we'll have to settle for the second-best option: laboratory experiments.

On Earth, we can fire strong lasers at targets to simulate the temperatures and pressures found inside the ice giants for a fraction of a second. One experiment with polystyrene (also known as Styrofoam) produced nano-sized diamonds. No, Uranus and Neptune do not contain large amounts of polystyrene, but the material proved much easier to handle in the laboratory than methane and, presumably, acts similarly.

Furthermore, Uranus and Neptune can sustain those pressures for much longer than a laboratory laser, implying that the diamonds might grow to be much larger than nano-sized.

What was the result? Diamond rain is a very real thing, based on all we know about the composition of the ice giants, their internal architecture, laboratory studies, and our mathematical modelling.