Methane Reactivity in Deep Earth, Neptune, and Uranus

Work published by DCO scientists in Nature Communications sheds new light on the reactivity of methane, and addresses several ambiguities in the literature.

Under the high temperature and pressure conditions of Earth’s mantle, molecules don’t behave as they would above ground. Methane is no exception to this rule, and as one of the most abundant compounds in the universe (along with water and ammonia) scientific investigations into its chemical idiosyncrasies could unlock ubiquitously valuable information. Work published by DCO scientists in Nature Communications sheds new light on the reactivity of methane, and addresses several ambiguities in the literature [1].

“Our knowledge of physics and chemistry of volatiles inside planets is based mainly on observations of the fluxes at their surfaces. High-pressure, high-temperature experiments, which simulate conditions deep inside planets and provide detailed information about the physical state, chemical reactivity, and properties of the planetary materials, remain a big challenge for us,” said lead author Sergey Lobanov.

In order to address the question of how methane reacts to high pressures and temperatures, Lobanov and his colleagues, from institutions in Russia, USA, and China, monitored the activity of methane in a laser-heated diamond anvil cell. Previous theoretical modeling suggested that at pressures over 300 GPa and temperatures above 4,000K, methane would dissociate into diamond and molecular hydrogen [2]. However, experimental work lacked consensus agreement with this model, with some groups showing methane dissociation and hydrocarbon formation and others implying an absence of such reactivity [3,4,5].

The current study shows that under various deep plantary conditions methane does indeed react, and as pressure and temperature increase the chemical signature of the hydrocarbon fluid changes. The authors observed production of heavy hydrocarbons under deep mantle conditions, and also detected elemental carbon precipitation.

Such a nuanced appreciation for the complexity of hydrocarbon chemistry at high temperature and pressure impacts how we think about the reduced parts of Earth, as well as the icy, methane-rich planets, Neptune and Uranus. In particular, these data may have important implications for research into the formation of diamonds deep in the mantle, as it provides a new pathway for the origin of such important carbon reservoirs through chemical evolution of hydrocarbons under planetary conditions.

 

Image credit: Sergey Lobanov

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