The interiors of the two ice giants of the outer solar system — Uranus, at about 2.9 billion kilometres from the Sun, and Neptune, at about 4.5 billion — are difficult to study by any direct means. The only spacecraft ever to visit either planet was Voyager 2, which flew past Uranus in January 1986 and Neptune in August 1989, observed both worlds for a few hours each, and continued on into interstellar space. No probe has entered either planet’s atmosphere, and none is planned within this decade. What scientists know about the interiors of Neptune and Uranus comes almost entirely from theoretical models, indirect observations of magnetic fields and gravitational moments, and laboratory experiments designed to recreate the extreme pressures and temperatures believed to exist at depth.
One of the more striking predictions of these models is that, somewhere between 5,000 and 10,000 kilometres below the planets’ visible cloud tops, the atmospheric pressure becomes so extreme that the methane molecules abundant in the atmosphere are torn apart. The methane (CH₄) decomposes into its constituent carbon and hydrogen atoms; the carbon atoms then arrange themselves into the diamond crystal lattice and crystallise out as solid diamond particles. The particles, being denser than the surrounding fluid, sink toward the planetary core under the pull of gravity. According to the 2017 SLAC National Accelerator Laboratory announcement of the first direct laboratory observation of this process, the phenomenon has been theorised for nearly four decades but had never been directly observed in any experimental setup until that year.
How you make diamond rain in a laboratory
The technical challenge of recreating Neptune’s deep interior is substantial. The pressures involved are on the order of 1.5 to 3 million times Earth’s atmospheric pressure. The temperatures are several thousand kelvin. Sustaining these conditions in any kind of bulk sample is impossible with current technology — no container could survive the pressures, and no heating system could maintain the temperatures without destroying everything around it. The trick the SLAC team developed is to recreate the conditions for only an extremely brief instant, just long enough for a single chemical reaction to occur and be observed.
The technique works as follows. According to a Lawrence Livermore National Laboratory technical summary of the experiment, a thin sample of polystyrene plastic — a substance whose chemical composition (carbon and hydrogen in long molecular chains) approximates that of the hydrocarbon compounds in Neptune’s atmosphere — is placed in a target chamber at SLAC’s Matter in Extreme Conditions instrument. The sample is then struck simultaneously by two pulses of an extremely high-powered optical laser, which together create two compression shock waves that travel through the plastic. The shock waves briefly compress the sample to roughly Neptune-deep-interior conditions of 1.5 to 2 million atmospheres of pressure and 5,000 to 6,000 K of temperature, sustaining these conditions for a few quadrillionths of a second.
During this femtosecond-long window, the SLAC team uses X-ray diffraction or X-ray scattering, fired through the same sample by the laboratory’s Linac Coherent Light Source X-ray free-electron laser, to monitor in real time what is happening to the carbon and hydrogen atoms inside the plastic. The diffraction patterns produced by the X-rays reveal the atomic arrangement of any crystalline material present. When the SLAC team performed the experiment in 2017, the X-ray diffraction patterns showed unambiguous evidence of diamond — the carbon atoms had separated from the hydrogen, organised themselves into the characteristic cubic crystal structure of diamond, and condensed into nanometre-sized crystalline grains, all in the brief moment of peak shock pressure.
What the experiments revealed
The 2017 results have been refined in subsequent experiments. According to a 2017 Eos article published by the American Geophysical Union, the simulated conditions corresponded to a depth of approximately 10,000 kilometres below Neptune’s surface — and produced nanodiamonds of unambiguous crystalline structure within the femtosecond observation window. Subsequent experiments at SLAC in 2020 and 2022 used variations of the technique, including replacing the polystyrene with plastics containing oxygen (more closely approximating the actual chemical mixture inside Neptune, which includes water and ammonia along with methane), and confirmed that the diamond formation persisted under these more realistic conditions.
The most recent refinement, published in Nature Astronomy in January 2024, contained a notable surprise. According to a SLAC announcement accompanying the 2024 paper, the diamond rain forms at substantially lower pressures and temperatures than the earlier experiments had suggested. The implication is that diamond formation occurs over a much larger region of Neptune’s and Uranus’s interiors than had been previously assumed, extending from much shallower depths than the originally calculated ~10,000 kilometres. The 2024 paper also linked the diamond rain to the unusual magnetic fields of Neptune and Uranus, which differ from those of Earth and Jupiter in being substantially offset from the planets’ rotational axes — possibly because the diamond rain disrupts the convective flows in the planets’ fluid interiors in ways that distort the magnetic field geometry.
The scale of the rain
The diamonds produced in the SLAC experiments are nanoscale — a few nanometres across — because the experimental shock lasts only femtoseconds. On Neptune itself, the same chemical process would operate over millions of years and produce diamonds of much larger size. Theoretical models suggest that individual diamond crystals could grow to weights of millions of carats before sinking toward the planet’s core. The total mass of diamond produced over the planet’s history could amount to a substantial fraction of the total mantle material, with some estimates suggesting that Neptune may have a layer of solid diamond hundreds of kilometres thick surrounding its rocky core.
The mass of diamond inside Neptune and Uranus, if these estimates are even approximately correct, would dwarf the total terrestrial diamond resource by many orders of magnitude. Diamond on Earth occurs in trace quantities in rocks called kimberlites and lamproites, with the global annual production of mined diamond on the order of 100 million carats per year, or about 20 tonnes. Neptune’s hypothesised diamond layer, if it exists at the scale current models suggest, would contain hundreds of millions of years of Earth’s diamond production in any given cubic kilometre. The diamonds would be inaccessible — Neptune is gaseous in its outer layers and the diamond rain region sits beneath thousands of kilometres of hydrogen and helium — but the resource itself, if any future technology could ever reach it, would be approximately the most abundant supply of pure crystalline carbon anywhere in the solar system.
What this tells us about other planets
The diamond-rain experiments have implications beyond Neptune and Uranus. Hydrocarbon-rich atmospheres are common in the universe. Many of the exoplanets discovered by the Kepler and TESS missions in the past 15 years are believed to be Neptune-like ice giants or larger “mini-Neptunes,” with atmospheres dominated by hydrogen, helium, and varying mixtures of methane, water, and ammonia. The same physics that produces diamond rain in Neptune should, in principle, operate in many of these exoplanetary atmospheres. If even a small fraction of the known ice giants in the galaxy have functioning diamond-rain processes in their interiors, the total mass of crystalline carbon distributed across exoplanetary mantles in the Milky Way may be enormous.
The next step in the experimental programme, according to the SLAC team, is to recreate the conditions found in still-deeper layers of these planets — pressures of 5 to 10 million atmospheres, temperatures approaching 10,000 K — and observe what other exotic chemistry occurs. The team has speculated that, at sufficient pressure, additional unusual phenomena may include the formation of metallic hydrogen, exotic compounds of helium and other noble gases, and possibly forms of carbon chemistry that have no Earth analogues at all. The diamond rain, in this view, is just the most accessible of a range of strange chemical processes that operate inside planets very different from Earth, and the SLAC experiments are the first direct experimental window into a kind of geology that exists nowhere on the visible surface of our own planet but is plausibly the dominant kind in much of the rest of the universe.
