Ab initiosimulations of hot dense methane during shock experiments

Abstract

Using density functional theory molecular dynamics simulations, we predict shock Hugoniot curves of precompressed methane up to 75$\phantom{\rule{0.16em}{0ex}}$000 K for initial densities ranging from 0.35 to 0.70 g$\phantom{\rule{0.16em}{0ex}}$cm${}^{\ensuremath{-}3}$. At 4000 K, we observe the transformation into a metallic, polymeric state consisting of long hydrocarbon chains. These chains persist when the sample is quenched to 300$\phantom{\rule{0.16em}{0ex}}$K, leading to an increase in shock compression. At 6000$\phantom{\rule{0.16em}{0ex}}$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.