High pressures on Saturn could be squeezing soot into diamonds, which then falls as rain.
NASA/JPL/SPACE SCIENCE INSTITUTE

Researchers have long wondered whether the high pressures inside the giant planets could turn carbon into diamond2, and even though some researchers dispute their claim, Mona Delitsky of California Specialty Engineering in Flintridge, and Kevin Baines of the University of Wisconsin-Madison, now say it is possible. Forget diamonds in the sky — it may actually be raining diamonds on Saturn and Jupiter, according to two planetary scientists1.
In their scenario, lightning zaps molecules of methane in the upper atmospheres of Saturn and Jupiter, liberating carbon atoms. These atoms then stick onto each other, forming larger particles of carbon soot, which the Cassini spacecraft may have spotted in dark storm clouds on Saturn3. As the soot particles slowly float down through ever-denser layers of gaseous and liquid hydrogen towards the planets' rocky cores, they experience ever greater pressures and temperatures. The soot is compressed into graphite, and then into solid diamonds before reaching a temperature of about 8,000 °C, when the diamond melts, forming liquid diamond raindrops, they say.

Precious rain

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IMAGE COURTESY OF MICHAEL CARROLL ALIEN SEAS: OCEANS IN SPACE (SPRINGER 2013 )
Inside Saturn, the conditions are right for diamond 'hail' to form, beginning at a depth of about 6,000 kilometres into the atmosphere and extending for another 30,000 km below that, says Baines. He estimates that Saturn may harbour about 10 million tonnes of diamond produced this way, with most of it made up of rocks no bigger than a millimetre and perhaps some chunks spanning 10 centimetres.