Uranus and Neptune, the icy giant planets of the Solar System, have an atmosphere rich in carbon molecules like methane. The extreme pressure the atmospheres experience suggests that diamonds could naturally form from the hydrogen and carbon squeezed together and rain down on the planets. Now scientists have been able to get some experimental evidence.
An international team of researchers has used the SLAC National Accelerator Laboratory to simulate the severe conditions of an icy giant planet. Methane (which is carbon and hydrogen) on such a planet would first form polymers (what plastic is made of) and then it will eventually form a diamond, freeing the hydrogen from the carbon. As reported in Nature Astronomy, the researchers used an intense optical laser to create shockwaves inside plastics and were able to form diamonds. Diamond rain could totally be happening inside icy giant planets.
"Understanding planetary interiors is crucial to understand how solar systems, planets and, with that also conditions favoring life, can form out of stardust. That is true for our solar system, but also for all the other planetary systems that are now found at other stars," lead author Dr. Domink Kraus, from the Helmholtz-Zentrum Dresden-Rossendorf, told IFLScience.
The researchers created a pair of shockwaves in polystyrene. The first wave was smaller and slower than the second one. So when the second one caught up it would interfere in such a way that the pressure would spike and the diamond would form. The researchers used the powerful Linac Coherent Light Source (LCLS) to take snapshots (one every half a billionth of a second) of the diamond forming. Since the shockwaves' interaction is so brief, the diamonds only survive for a short time.
"The key result of this paper is that we produce diamonds out of plastic," co-author Professor Siegfried Glenzer told IFLScience. "Previously what people have done is to use graphite or pure carbon. Here instead we have formed them from two species, carbon and hydrogen."
The diamonds formed at a pressure of about a million atmospheres and at temperatures of several thousands of degrees. This pressure was at least 10 times higher than what previous attempts at creating them had envisioned. The conditions are similar to what you'd be expected to find 10,000 kilometers (over 6,000 miles) deep inside the icy giant planets.
"Pressure is the main driving force and our experiments really show that we need pretty high pressures," Dr Kraus explained. "In principle, the hydrocarbon separation can happen at nearly absolute zero when you have enough pressure (3 million times atmospheric pressure), according to theoretical calculations. When the temperature is higher, this pressure threshold is reduced."
The diamonds obtained in the experiment are a few nanometers in diameter. On Uranus and Neptune, diamonds wouldn’t stay that small, though. Researchers expect them to grow to hundreds of kilograms in size. The interior environment of these planets is a solid core surrounded by a dense slush of different ices. Researchers have long suspected macrodiamonds would form and then, possibly, over thousands of years sink through the ice and accumulate around the core. Neptune might actually have a diamond encrusted core.
There are still many things we need to understand about the processes that go on inside these giant planets. Researchers plan to use the same method to find out what else is going on the atmospheres of these worlds. They are already conducting experiments shocking material made of carbon, hydrogen, and oxygen.
"Those icy giant planets have many constituents. We have just completed an experiment that used a different type of plastic that contained oxygen and we investigated if diamonds form as well in those conditions." Professor Glenzer added.
The research not only has consequences in planetary science but also on many earthly fields. Medicine and engineering use nanodiamonds in several applications and shocked plastic is one of the approaches used to ignite hydrogen in certain fusion reactors. A better understanding of the icy giants might have many interesting repercussions on Earth's technology and energy production.