This was suspected by astronomers and physicists for almost 40 years, but only now it was possible to confirm the theory experimentally. The outer planets of our solar system are difficult to study. Only one space mission, Voyager 2, flew past to uncover some of their secrets, so the diamond rain remained just a hypothesis.
Apart from the lingering mystery of the diamond rain, we cannot study Uranus and Neptune inside and out. This limits our understanding of the solar system and the galaxy because planets of this size have proven to be extremely common in the Milky Way.
The number of planets similar in size to Uranus and Neptune is about nine times greater than the number of much larger planets similar in size to Jupiter and Saturn. The outer planets also bear scars that can reveal much about the formation of our own solar system. Not surprisingly, there is a growing need to study Neptune and Uranus. This will help to better understand where and how planetary systems form, as well as answer the question of where to look for planets capable of supporting life.
Although we have been limited by the capabilities of spacecraft and ground-based telescopes, advances in lab modeling offer a startlingly new understanding of what is going on in understanding what causes diamond rain. Discoveries like these show the complexity of the chemical processes involved in the evolution of these planets. Our simulations define the intrinsic nature of worlds far beyond the solar system that we will never see directly.
Neptune and Uranus are called the «ice giants» of our solar system because their outer two layers are made up of compounds of hydrogen and helium. In astronomical slang, ice refers to all compounds of the light elements that contain hydrogen, which is why the water (H2O), ammonia (NH3) and methane (CH4) of planets make them «icy». The beautiful bluish hue of both planets is the result of traces of methane in their atmospheres.
It is the «ice» in the deep middle layers that really shapes their properties. For example, on Neptune, under a hydrogen-helium atmosphere that is 3,000 kilometers thick, there is an ice layer 17,500 kilometers thick. Modeling suggests that gravity compresses the «ices» in this middle layer to high densities, and internal heat raises the internal temperature to several thousand Kelvin. Despite the heat, pressure more than a million times atmospheric pressure on Earth compresses the so-called ice into a hot, dense liquid.
At this heat and pressure, ammonia and methane are reactive. The scientists modeled exotic processes, including the formation of diamonds, that occur between joints deep in layers of ice. Marvin Ross of the Livermore National Laboratory. Lawrence first introduced the idea of diamond rain in a 1981 paper titled «The Ice Layer of Uranus and Neptune — Diamonds in the Sky?». He proposed that carbon and hydrogen atoms separate at high pressures and high temperatures inside the ice giants. Clusters of isolated carbon atoms will then be compressed into a diamond structure, which is the most stable form of carbon under such conditions.
Diamond is denser than the methane, ammonia, and water left in the ice layer, so the carbon crystal will begin to sink towards the planet’s core. It will build up new layers as it falls, touching other isolated carbon atoms or diamonds, allowing individual blocks of diamonds to be several meters in diameter. As a result, a thick layer of carbon surrounds the rocky cores of Uranus and Neptune. This carbon layer may consist of blocks of solid diamond or, if the temperature is extremely high (as suggested by some planetary models), it may turn into liquid carbon or a mixture of solid carbon and liquid carbon.
If the layer is a mixture of solid and liquid carbon, the solid carbon will have a lower density than the liquid carbon, causing large «diamond icebergs» to float on the surface of the liquid carbon ocean. Each possible composition of the carbon layer — solid, liquid or mixed — affects the core of the planet in a different way. Hard diamond, for example, is electrically insulating and has a rigid crystal lattice, while liquid carbon is a metallic conductor. Determining the properties of the carbon layer could reveal whether Neptune and Uranus formed from a protoplanet’s rocky core billions of years ago.
Although Ross’s idea was exciting, at the time it was mostly hypothetical and needed to be verified by observation. It is not possible with any conceivable technology to design and build a probe that can go deep into Neptune or Uranus and directly observe the formation of diamonds.
Scientists instead tried to recreate the extreme conditions of planetary interiors in their laboratories. Even this more limited goal is extremely challenging, as we need to reliably generate and measure pressures of several million atmospheres and temperatures of several thousand kelvins to simulate their effects on the elements found inside ice giants. In fact, we need to build a piece of the planet in the laboratory.
Institutions around the world solve this problem by compressing a sample material, such as methane, between two diamond anvils with very small tips that press against the sample. The same pressure build-up effect can be seen on a different scale by putting something under the heel of a high heeled shoe. Although diamond anvils can generate several megabars of pressure (comparable to the pressure that would be generated by placing several thousand African elephants on top of these high-heeled shoes), the sample must also be heated by electric currents or lasers to simulate hot planetary interiors.
Using such a setup, some experiments actually formed a diamond. However, in these facilities, materials representing layers of planetary ice — methane, ammonia or water — begin to react with diamond anvils and gaskets. These reactions can greatly alter and contaminate the results.
That changed in 2009 with the creation of the world’s first X-ray free electron laser: the Linac coherent light source at Stanford University. Combining this machine with a powerful pulsed laser system allows us to study chemical reactions under conditions comparable to those found in the deep interiors of giant planets in real time. Plastics, which are mostly made of carbon and hydrogen, are useful substances for simulating the mixture of materials in the ice layers of Neptune and Uranus. You can also read about whether life exists on Jupiter’s moon Europa.
In such experiments, a high-energy pulsed laser is focused on a spot 200 micrometers in diameter, heating a thin surface layer of an 80 micrometer thick plastic sample. Its surface instantly turns into an extremely hot plasma with a temperature of several million kelvins. This plasma vapor expands rapidly. The resulting force of extreme pressure presses on the remaining plastic material and sends strong compression waves into the sample. With the right settings, the experiment can accurately mimic the pressure and temperature conditions predicted inside the ice giant.
Understanding the internal processes of ice giants gives insight into the features of these planets. For example, the precipitation of a diamond releases gravitational energy, which is converted into heat by friction between the diamonds and the surrounding material. This effect may explain why Neptune radiates more energy than it receives from the Sun. Such an internal energy source could help explain the origin of unexpectedly violent storms that are observed on the surface of the planet.
The diamond formation could also explain why the ice giant’s magnetic fields are so exotic. Unlike the Earth’s magnetic field, the fields around Uranus and Neptune are not symmetrical and do not extend from each pole. These properties suggest that the fields of ice giants do not originate in the core, but in a thin, rather variable layer of conductive material, such as metallic hydrogen, formed as a by-product of diamond production.
Scientists will continue to study these phenomena in the lab, but a new space exploration mission to Neptune or Uranus could add a wealth of information about planetary processes and how such planets formed in our solar system. NASA is currently considering such a mission. In 2030, the planets in our solar system will be aligned favorably for the launch of a spacecraft capable of reaching Neptune in 2040. The next random alignment of the planets won’t come for another two generations, so now is the time to start thinking about exploring the ice giants and exploring the solar system’s intriguing diamond worlds.