2024-01-11 00:17:32
Planetary gemstone rain: Diamonds are raining inside the ice planets Uranus and Neptune, as an experiment has now confirmed. Accordingly, the pressure and temperature in the two ice giants are sufficient to break down hydrocarbons and compress the carbon into diamond crystals. The crystals sink toward the planet’s core and might explain why Uranus and Neptune are warmer than they should be. Their unusual, multipolar magnetic field might also be due to the diamond rain, as the team reports in “Nature Astronomy”.
Uranus and Neptune are icy exotics: They are the only planets in the solar system to have a four-pole magnetic field and are neither rocky planets nor gas giants. Instead, beneath their dense gas shell lies a mantle of water ice, ammonia, rock fragments and various hydrocarbons – including exotic compounds that do not exist on Earth. They arise because there is more heat and pressure in the interior of the two planets than in the depths of our much smaller home world.
Schematic structure of the ice giants Uranus and Neptune. © NASA
From methane and co to diamonds
Now it turns out that Uranus and Neptune even have a real treasure inside them: it’s probably raining diamonds inside them, as an experiment has now confirmed. A team led by Mungo Frost from the SLAC National Accelerator Laboratory in California conducted it to study the conditions under which hydrocarbons decay in the interior of the two ice planets. Previous studies had already suggested that diamonds might then be formed from the released carbon.
But how much pressure and heat is necessary? Are the conditions inside Uranus and Neptune sufficient? “So far there has been a lot of disagreement on this,” explain the researchers. Experiments with laser-driven shock compression of methane showed that diamonds only form at more than 140 gigapascals of pressure and a good 3,700 degrees of heat. Other experiments, however, came to much lower values, which some researchers believe might be due to contamination by metallic laser absorbers.
High-pressure experiment as an ice giant analogue
To clarify the discrepancy, Frost and his team conducted new experiments at the European XFEL X-ray laser. To do this, they exposed the hydrocarbon polystyrene, which is considered a suitable model – a common basis for foam and Styrofoam – to increasing pressures in a diamond stamp cell. At the same time, they heated the sample using short X-ray laser pulses and monitored the structure of the material using X-ray scattering. To avoid metal contamination, they used the non-reactive gold as a laser absorber.
The result: The hydrocarbon converted into diamond at around 2,300 degrees and pressures of 19 to 27 gigapascals. After around 30 microseconds, a very fine diamond powder is created. “After around 40 microseconds, larger diamond crystals begin to form, as can be seen from clearly visible spots in the scattering pattern,” report Frost and his team. Overall, these diamonds are created at lower pressure and less heat than previously assumed.
The diamonds formed from decaying hydrocarbons sink from the upper mantle of the ice planets into the depths. © European XFEL / Tobias Wüstefeld
Diamond rain affects heat and magnetic field
For Uranus and Neptune, this means: “Conditions already exist in the upper layers of their mantle in which we observed diamond formation,” report Frost and his colleagues. Inside these two ice giants there are diamonds that sink down into the deep interior – it rains diamond crystals.
This diamond rain has an impact on the heat balance and possibly also the magnetic field of the two ice giants. The reason for the former: When hydrocarbons are broken down and converted into diamond, this releases heat. “The shallow formation depth of the diamonds also causes them to sink and generates even more potential heat,” explain the researchers. This might explain why the planets are relatively warm despite their distance from the sun.
A second effect: The diamond rain also drags gas and ice into the depths and can thus cause convection currents in the mantle of the two ice planets. Because conductive molecules also start moving, they might contribute to the unusual magnetic fields of Neptune and Uranus. “Convection and the resulting dynamo activity are considered to be the origin of the complex, multipolar magnetic fields of the ice giants,” write Frost and his team.
Diamonds also in extrasolar mini-Neptunes
The new findings also have significance for exoplanets and other celestial bodies: If hydrocarbons turn into diamond at the lower pressures and temperatures now determined, then this might also happen on smaller celestial bodies than previously thought. “Although there are ten gigapascals more than the ice moons of the solar system, many exoplanets have already been discovered whose density matches an ice-rich composition and whose sizes lie between Earth and Neptune,” explains the team.
This means that, for example, there might also be a diamond rain inside the relatively common sub-Neptunes. “These mini-Neptunes are of particular interest because of their abundance and water abundance,” write Frost and his colleagues. “If diamonds are also formed in them, this might influence their geodynamics, atmospheric composition and planetary evolution. (Nature Astronomy, 2024; doi: 10.1038/s41550-023-02147-x)
Quelle: Nature, European XFEL
January 11, 2024 – Nadja Podbregar
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