Diamond formation from hydrocarbon mixtures in planets

TL;DR

This study uses first-principles calculations to determine the conditions under which diamonds can form from hydrocarbon mixtures inside planets, revealing a depletion zone that influences planetary structure and evolution.

Contribution

It provides the first detailed phase boundary and nucleation rate estimates for diamond formation from hydrocarbons at planetary interior conditions.

Findings

Diamond formation is thermodynamically favorable in a specific depletion zone at high pressures and low temperatures.

Neptune's interior conditions favor diamond formation more than Uranus.

The results help explain the low luminosity of Uranus and improve understanding of planetary evolution.

Abstract

Hydrocarbon mixtures are extremely abundant in the Universe, and diamond formation from them can play a crucial role in shaping the interior structure and evolution of planets. With first-principles accuracy, we first estimate the diamond nucleation rate in pure liquid carbon, and then reveal the nature of chemical bonding in hydrocarbons at extreme conditions. We finally establish the pressure-temperature phase boundary where diamond can form from hydrocarbon mixtures with different atomic fractions of carbon. Notably, we find a depletion zone at pressures above 200~GPa and temperatures below 3000~K-3500~K where diamond formation is thermodynamically favorable regardless of the carbon atomic fraction, due to a phase separation mechanism. The cooler condition of the interior of Neptune compared to Uranus means that the former is much more likely to contain the depletion zone. Our findings can help explain the dichotomy of the two ice giants manifested by the low luminosity of Uranus, and lead to a better understanding of (exo-)planetary formation and evolution.