Deep within Neptune and Uranus, it rains diamonds—or so astronomers and physicists have suspected for nearly 40 years. The outer planets of our Solar System are hard to study, however. Only a single space mission, Voyager 2, has flown by to reveal some of their secrets, so diamond rain has remained only a hypothesis.
Because the molecules that make up Neptune and Uranus' outer two layers are composed of hydrogen and helium, they are referred to as the "ice giants" of our solar system. The planets are known to be "icy" due to the presence of methane (CH4), ammonia (NH3), and water (H2O) in astronomical slang, which defines "ice" as any combination of light elements and hydrogen. Methane traces in their atmospheres give both planets their beautiful bluish hue. But the "ice" in the deep central layers is what makes them unique. For example, an ice layer 17,500 kilometers deep on Neptune sits beneath a 3,000-kilometer-thick hydrogen-helium atmosphere. According to experiments, the "ices" in this middle layer are compressed by gravity to high densities, and the heat from within causes the temperature inside to rise to several thousand kelvins. The so-called ices are compressed into a hot, dense fluid by pressures more than a million times higher than Earth's atmospheric pressure, despite the immense temperature.
Marvin Ross of Lawrence Livermore National Laboratory introduced the diamond-rain idea in a 1981 article in Nature titled, “The Ice Layer of Uranus and Neptune—Diamonds in the Sky?” He suggested that the carbon and hydrogen atoms of hydrocarbons such as methane separate at the high pressures and high temperatures inside the ice giant planets. Clusters of isolated carbon atoms would then be squeezed into a diamond structure, which is the most stable form of carbon under such conditions.
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