Earth's early warmth may be explained by methane-making reaction

A recently-discovered chemical process could have led to widespread formation of the greenhouse gas methane when Earth was young. The reaction doesn’t involve living organisms, so could have occurred early in our planet’s history.

“We identify a potential new source of methane prior to the origin of life,” says Johannes Rebelein at the Max Planck Institute for Terrestrial Microbiology in Marburg, Germany.

If enough methane was formed, it could help explain why Earth stayed warm at a time when the sun was dimmer than today.

The discovery could also further complicate the search for life on other planets. Methane in the air of a planet is thought to be a signature of life, but astronomers will need to rule out the new process as an explanation for any detection of this gas.

Methane is a common chemical compound: each molecule consists of a single carbon atom surrounded by four hydrogen atoms. It is a greenhouse gas that traps the sun’s heat, warming the planet.

Today, most methane is made by living organisms, which use complex molecules called enzymes to do so. However, in 2022, Rebelein and his colleagues identified a process by which methane can form in living organisms without enzymes. Chemicals containing carbon, sulphur and nitrogen were transformed into methane, driven by highly reactive substances, including electrically-charged iron and reactive oxygen species.

This prompted the team to wonder whether the same reaction could occur outside living cells. “There’s iron occurring in nature; there are reactive oxygen species in water,” says Rebelein.

The team set up small vials with a few millilitres of water containing starter chemicals at temperatures ranging from 37°C to 97°C (99°F to 207°F). “We added the iron to it, and then we just incubated them under either increased heat or under light,” says Rebelein.

Methane formed consistently, with rates increasing at higher temperatures or when the samples were illuminated – the light split some water molecules, forming reactive oxygen species.

In the dim past

Rebelein says the results could help explain a mystery called the faint young sun paradox. When the sun was newly formed, it was significantly dimmer than it is today – yet geological evidence suggests Earth was warm then and didn’t freeze over.

Researchers have long suspected that high levels of methane helped achieve this. “We might be able to close that gap a little bit by showing a new mechanism which could deliver methane,” says Rebelein. However, he says it isn’t clear how much methane the new process could actually make.

It was already known that methane can form in the absence of life, albeit by a different process. When water flows through rocks rich in iron and magnesium, the rocks are transformed into a green mineral called serpentinite, and methane is released as a by-product.

This process, called serpentinisation, is the dominant non-biological source of methane on Earth, and probably was for early Earth too, says Giada Arney at the NASA Goddard Space Flight Center in Maryland, who wasn’t involved in the study.

Rebelein says he is convinced that serpentinisation also played a big role in keeping the young Earth warm.

“The faint young sun paradox is much less of a paradox these days,” says Arney. Instead, it is “a question to which there are many plausible answers”, but which we may never be able to answer definitively.

Life on other worlds

The new find will further complicate the search for life beyond Earth. Because methane is made by living organisms, it has been proposed as a “biosignature”: if a planet has methane in its air, the argument goes, that is a sign of life. Methane has been repeatedly detected on Mars, and this has been interpreted as evidence of microbial life.

“Any planet with water on it should potentially also show this mechanism which we describe here,” says Rebelein. “Methane is not an ideal biosignature any more.”

The fact methane can be produced by serpentinisation already complicated its use as a biosignature, says Arney, and the new finding adds to that. “It’s adding to the pile of things that we’re going to have to think about.”

For Arney, the solution is to search for multiple lines of evidence that paint a coherent picture. Earth’s atmosphere contains oxygen, so methane molecules get destroyed in about 10 years. “So [methane] needs to be produced extremely rapidly [to remain in the atmosphere],” she says. “That rapid production rate is far higher than any abiotic processes we know of.” As a result, the mix of methane and oxygen is a better biosignature than methane alone, she says.