Conditions like those inside Neptune cause diamond formation

Enlarge / That lovely blue exterior could be hiding a heart of diamond.

chemistry starts to get a bit weird. Do these chemicals survive the crushing interiors of these planets?

One intriguing idea is that methane doesn’t survive. As pressure and temperature increase, methane should start condensing into more complex hydrocarbons. Then, as pressures increase further, calculations indicate the hydrogen and carbon should separate out, leaving pure carbon to sink to the depths of these planets. As a result, it’s been hypothesized that, close to their core, planets like Neptune and Uranus have a layer of pure diamond.

While some evidence supporting this theory has surfaced over the years, it’s been hard to precisely replicate the temperatures and pressures found inside the planets. Now, new work done at the SLAC X-ray laser facility supports the idea that these planets are full of diamonds. But the work indicates the diamonds only form at greater depths than we’d previously thought.

The conditions inside places like Neptune have been mimicked in a number of ways. Compressing and heating methane is easy when you use lasers or accelerate samples of the gas into collisions. But mimicry seldom got the conditions just right; when pressures were high enough, the temperatures were often unrealistically high. And the relevant conditions only persist for a fraction of a second, making it hard to tell what chemicals are being formed.

An alternative is diamond anvil cells, which crush small samples between two diamonds. Here, it’s possible to control the temperatures and pressures well. But one of the components of methane—the hydrogen—can diffuse through diamond at the relevant pressures. Removing the hydrogen may unnaturally increase the probability that diamonds can form.

A large international research team figured out how to solve all of these issues. They used lasers to heat and compress a sample of hydrocarbon (polystyrene) using a two-stage process. The first blast of laser would heat and compress the sample, while a second would greatly accelerate the compression without raising the temperature as much as earlier attempts. This led to conditions that are similar to what you’d expect to see 10,000km beneath the surface of Neptune, or about 40 percent of the way down to its core. (That means pressures of about 150 Gigapascal and temperatures below 6,000 Kelvin.)

One of the experiment rooms that receives X-ray pulses from the SLAC free electron laser.
Enlarge / One of the experiment rooms that receives X-ray pulses from the SLAC free electron laser.
John Timmer

Monitoring what happens under these conditions is why the experiment was performed at the SLAC facility. SLAC uses an electron accelerator to generate multiple short, intense pulses of X-rays in quick succession. That allows the molecular details of a sample to be probed every few nanoseconds using X-ray diffraction. Essentially, you can make a molecular movie of a tiny sample, with frames separated by a fraction of a nanosecond.

In these experiments, the initial laser pulse vaporized the aluminum frame holding the polystyrene, but some signals from the aluminum persisted out to about 7.5 nanoseconds. The polystyrene was transformed into a compressed hydrocarbon liquid. As the second laser pulse arrived (six nanoseconds after the first), a new signal started to appear: compressed diamond. This stuck around until about 10 nanoseconds, after which it started to be replaced by uncompressed diamond, indicating that the compressed sample had rebounded outward.

While it didn’t have time to happen in this sample, the formation of diamond would liberate hydrogen, which would be able to escape upwards to areas of lower pressure. The diamond, over time, would sink downward, essentially precipitating out of the hydrocarbon soup.

This has some major implications for planetary interiors. To begin with, layers of diamond wouldn’t engage in the sort of convection that carries heat around through the liquid and gas layers of these planets. This should mean that more of the initial heat of gravitational collapse is trapped in the core of the planet. It also means that planets rich in hydrocarbons will experience a sudden change once they become large enough for diamond formation in the interior, which will alter how the planet’s radius grows with its mass.

In addition to all that, the authors note that, should you be in need of a source of nanometer-sized diamonds, they know just how to make them.

Nature Astronomy, 2017. DOI: 10.1038/s41550-017-0219-9  (About DOIs).

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