Researchers from the SLAC National Accelerator Laboratory have demonstrated that diamonds can form through hydrocarbon demixing at pressures ranging from 20 to 80 GPa, simulating conditions 6,000 kilometers beneath the surfaces of Uranus and Neptune. The study, published in Physical Review B, suggests these diamonds would sink toward the planetary cores, creating “diamond rain” that generates internal heat and alters magnetic fields.
Diamond formation at 80 GPa exceeds Earth mantle pressures
Diamonds on Earth typically form in the upper mantle between 150 and 250 kilometers below the surface. In contrast, the environments within ice giants like Uranus and Neptune involve pressures far exceeding those found in Earth’s interior.
To test these conditions, SLAC researchers used a diamond anvil cell to reach pressures of 20-80 GPa. For comparison, standard lab-grown diamonds are often created at 5-6 GPa. The pressure used in the ice-giant simulation is roughly 10 times higher than the pressure required for diamond formation on Earth.
Did you know? 80 GPa is equivalent to the pressure experienced at the bottom of the ocean if 500 additional oceans were stacked on top of it.
Hydrocarbon demixing creates nanometer diamonds
The mechanism for diamond creation in ice giants differs from the process of crushing carbon-rich materials. The SLAC study focused on “hydrocarbon demixing,” where molecules composed of hydrogen and carbon split apart under extreme pressure, leaving carbon to crystallize into diamonds.
The experimental process involved placing a micrometer-sized sample of hydrocarbon material mixed with gold powder between two diamond anvils, each 200 µm wide. Researchers used X-rays to heat and melt the gold, which in turn heated the hydrocarbons. This process successfully produced nanometer-sized diamonds, which were detected using X-ray analysis.
Sinking diamonds alter planetary magnetic fields
Because diamonds are heavier than the surrounding materials in the interiors of Uranus and Neptune, they are expected to sink toward the center of the planets. This movement converts gravitational energy into heat.
This internal heating affects the molten metals that generate planetary magnetic fields. Uranus and Neptune already possess unconventional magnetic fields due to electrically conducting liquids in their interiors; the addition of heat from sinking diamonds would further complicate these magnetic structures.
| Location | Formation Pressure | Primary Mechanism |
|---|---|---|
| Earth Mantle | 5-6 GPa | Crushing carbon materials |
| Ice Giants (Simulated) | 20-80 GPa | Hydrocarbon demixing |
Ice giant interiors remain largely unobserved
Direct observation of these processes is currently impossible. Uranus and Neptune have each been visited once, and current technology cannot peer into the deep layers 6,000 kilometers beneath their surfaces to verify the existence of diamond rain.
Common questions about ice giant diamonds
Why do diamonds rain on Neptune and Uranus but not Earth?
Earth’s internal pressure is insufficient to trigger the hydrocarbon demixing process seen in ice giants. On Earth, diamonds form in the mantle and stay there unless volcanic activity moves them toward the surface.
How do scientists prove diamonds exist in these planets?
Since direct observation is impossible, researchers use diamond anvil cells to recreate pressures of 20-80 GPa and X-ray detection to confirm that hydrocarbons split into diamonds under those specific conditions.
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