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Carbon and hydrogen are among the most abundant elements in the universe, and are a major constituent of icy giant planets such as Uranus and Neptune. In the outer atmosphere, these atoms are found in the form of methane gas, but deeper inside the high pressure can lead to more complex hydrocarbon structures. Predicting the phases and structures that material takes at these conditions is one of the big questions of planetary research.
In order to better understand the structure of the ice giants, an international team led by the two HZDR researchers Dr. Nicholas Hartley and Dr. Dominik Kraus investigated two types of plastic in a laboratory experiment: polystyrene and polyethylene. These materials are similar in chemistry to the hydrocarbon inside the planets. At the SLAC National Accelerator Laboratory in the US, the scientists exposed the samples to conditions predicted to be present around ten thousand kilometers below the surface of Neptune and Uranus. At this depth, the pressure is almost as high as in the core of the earth and two million times higher than the atmospheric pressure on the earth's surface.
Pressures - Temperatures - Structure - Researchers - Diamond
At such high pressures and temperatures, the only possible structure that the researchers expected was diamond, or that the samples would be melted. Instead, they observed stable hydrocarbon structures up to the highest pressures reached, but only for the polyethylene samples. "We were very surprised by this result," says Hartley. "We did not expect the different initial state to make such a big difference at such extreme conditions. It's only recently, with the development of brighter X-ray sources, that we're able to study these materials. We were the first to think that it might be possible -- and it was."
Since the extreme conditions inside the ice giants on Earth can only be reached for a brief moment, the...
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