Chemistry and structural properties of C-H-O mixtures at conditions comparable to the interiors of ice giants

Abstract:

The interiors of the ice giants Uranus and Neptune are dominated by a warm dense mixture of light elements, which presumably results in complex chemistry and phase transitions. New experimental capabilities, such as the combination of high-energy lasers and X-ray free electron lasers, allow for investigating such states of matter with unprecedented detail in the laboratory [1-6]. Here we show results from C-H-O samples dynamically compressed to pressures around 200 GPa and heated to temperatures of few thousand kelvins. Structural changes recorded by both in situ X-ray diffraction and small angle X-ray scattering show phase separation of carbon, which is evidently not retarded or prevented by the presence of oxygen. In contrast, we find that carbon is transferred to diamond more effectively in C-H-O in comparison to C-H samples brought to similar conditions. These findings are in line with quantum simulations predicting that the presence of oxygen more supports than prevents the phase separation of carbon and hydrogen. Besides carbon precipitation, this process may also trigger the creation of isolated metallic hydrogen or superionic ices and therefore can have significant consequences for the internal structure of icy giant planets.

[1] D. Kraus et al., Nature Astronomy 1, 606-611 (2017)
[2] D. Kraus et al., Physics of Plasmas 25, 056313 (2018)
[3] N. J. Hartley et al., Physical Review Letters 121, 245501 (2018)
[4] N. J. Hartley et al., Scientific Reports 9, 4196 (2029)
[5] A. K. Schuster et al., Physical Review B 101, 054301 (2020)
[6] S. Frydrych et al., Nature Communications 11, 2620 (2020)