UC Berkeley

The interiors of Uranus, Neptune, and ice giant–like exoplanets remain poorly constrained, particularly regarding the high-pressure chemical processes that govern their structure, evolution, and energy balance. To address this gap, we conducted high-pressure experiments on a range “Synthetic Uranus” (water, ammonium hydroxide, and isopropanol) compositions formulated to approximate cosmic carbon, hydrogen, oxygen, and nitrogen ratios relevant to ice-giant mantles. In particular, we studied carbon-poor compositions with <10% carbon and correspondingly larger abundances of oxygen or nitrogen.Using laser-heated diamond anvil cells with in situ X-ray diffraction and Raman spectroscopy, we observed diamond precipitation between 14 and 55 GPa at temperatures as low as 1,500 K, and for compositions with as little as ~2% carbon. These pressures, temperatures, and carbon abundances are significantly lower than reported for simpler hydrocarbon mixtures. Our results indicate that oxygen- and nitrogen-bearing species can reduce the barrier for carbon dissociation, enabling diamond formation at comparatively moderate conditions. The presence of these volatiles therefore suggests that “diamond hail” may occur at shallower depths and lower C concentrations than previously estimated, leading to consequences for internal heat transport, stratification, convective stability, and magnetic field generation. More broadly, these findings provide new constraints on the chemical pathways expected in water–ammonia–methanol–rich planetary interiors and contribute to ongoing efforts to refine evolution models for Uranus, Neptune, and analogous exoplanets.As a secondary study, we investigated cyclohexane under dynamic compression to examine how molecular structure and hydrogen content influence hydrocarbon dissociation, carbon clustering, and metallization pathways. Cyclohexane, while not a direct compositional analog to Synthetic Uranus, serves as a clean model for isolating fundamental high-pressure reaction mechanisms that are otherwise obscured in multicomponent mixtures. We see evidence of changes in molecular structure and possible dissociation as expressed in the measured equation of state. Together, the static and dynamic results provide a more comprehensive picture of hydrocarbon and mixed-volatile chemistry at planetary interior pressures and temperatures, informing models of material behavior across a diverse range of icy and carbon-rich worlds.