Ab Initio Simulations of Hot, Dense Methane During Shock Experiments

TL;DR

This study uses ab initio molecular dynamics to simulate shock compression of methane, revealing phase transitions to metallic, polymeric, and plasma states with implications for planetary interiors and high-pressure experiments.

Contribution

First ab initio simulations of shock-compressed methane across a wide temperature and density range, identifying phase transitions and potential superionic states.

Findings

Methane transforms into a metallic, polymeric state at 4000 K.

At 6000 K, methane becomes a plasma with short-lived species.

Simulations provide insights into planetary interior conditions.

Abstract

Using density functional theory molecular dynamics simulations, we predict shock Hugoniot curves of precompressed methane up to 75000 K for initial densities ranging from 0.35 to 0.70 g/cc. At 4000 K, we observe the transformation into a metallic, polymeric state consisting of long hydrocarbon chains. These chains persist when the sample is quenched to 300 K, leading to an increase in shock compression. At 6000 K, the sample transforms into a plasma composed of many, short-lived chemical species. We conclude by discussing implications for the interiors of Uranus and Neptune and analyzing the possibility of creating a superionic state of methane in high pressure experiments.