Diamond and other forms of elemental carbon in Saturn’s deep atmosphere

The energetic lightning storms in the Saturn atmosphere will dissociate molecules into atoms, ions and plasma. Specifically, methane will be dissociated into elemental carbon, most probably in an amorphous form, such as fluffy turbostratic carbon or irregular soot particles. Once formed, this non-crystalline carbon sinks down through the atmosphere reaching an altitude of similar density. Amorphous carbon is converted to graphite under pressure. Graphite has a density of ~2.2 g/cc at room temperature. The density of diamond is ~3.3 g/cc at STP. However, at much higher pressures, the density of diamond increases dramatically, up to 9 grams/cm3 at P=1500 GPa (15 Mbar). As carbon descends through the atmosphere, amorphous carbon becomes graphite which then is converted into diamond, creating various strata of carbon allotropes according to their densities. Densities of the planets increase with depth. Eventually, at great depths, diamond will melt, forming liquid diamond. The melting point of diamond varies with pressure, reaching a high of ~ 8000 K at 500 GPa (5 Mbar). Using updated adiabats and equation-of-state data from Nettelmann et al. (2011), we determined the altitude at which diamond reaches its melting point on each planet. Combining these adiabats with new data for the carbon phase diagram from high-pressure shockwave experiments indicates that diamond may be a stable layer in the atmospheres of Jupiter and Saturn. Previously, only Uranus and Neptune were thought to have conditions in their interiors that would allow the formation of diamond at their cores. It appears that the interior of Jupiter gets hot enough to reach the liquid diamond region of the carbon phase diagram, whereas the interior of Saturn includes regions of temperature and pressure where carbon could exist as solid diamond. At the boundaries (locations of sharp increases in density) on Jupiter and Saturn, there may be diamond rain or diamond oceans sitting as a layer. However, in Uranus and Neptune, the temperatures never reach as high as 8000 K. The cores are ~5000K, too cold for diamond to melt on these planets. Therefore, it appears that diamonds are forever on Uranus and Neptune but not on Jupiter and Saturn.