Chinese scientists have discovered graphene – a form of pure carbon – in lunar soil samples collected four years ago by the Chang’e-5 mission, a finding that could challenge a prevalent theory of the moon’s origin.
According to the Jilin University-led researchers, the presence of carbon challenges an assumption behind the commonly held view that the moon was formed in a collision between the Earth and another small planet.
“The prevalent giant impact theory has been strongly supported by the notion of [a] carbon-depleted moon derived from the early analysis of Apollo samples,” they said, in an unedited manuscript published online by the National Science Review.
The researchers said that a recent study from Japan had also challenged the giant impact theory by showing the moon had emission fluxes of carbon ions all over it, “suggesting the presence of indigenous carbon”.
To understand the origins of this carbon, the Chinese research team said a study of young lunar samples – just 2 billion years old – could help them to “unravel the crystalline structure of the indigenous carbon” present on the moon.
After analysing the graphene found in the sample, the researchers concluded that the moon may actually have a carbon capturing process on its surface that could explain its presence.
The team, which included scientists from the Shenyang National Laboratory for Materials Science and China Deep Space Exploration Lab, said the findings “may reinvent the understanding of chemical components … and the history of the moon”.
Graphene is a carbon allotrope that consists of a single layer of atoms arranged on a hexagonal lattice nanostructure to form the thinnest and strongest material in the world.
In 2010, Nasa researchers found graphite – a mineral made up of stacked layers of graphene – in lunar samples collected nearly 40 years earlier by the Apollo 17 mission. After ruling out the effects of solar winds, they attributed the finding to the impact of meteor strikes on the moon.
The Chinese scientists acknowledged that the impact of meteorites could also lead to the formation of graphitic carbon, as proposed by the Nasa researchers.
“Further in-depth property investigation of natural graphene would provide more information on the geologic evolution of the moon,” the Chinese team wrote.
According to the Chinese scientists, their study – which utilised a variety of characterisation techniques – is the “first … to verify the presence of natural graphene in lunar soil samples by examining its microstructure and composition”.
Graphene “plays an increasingly important role in extensive areas including planetary and space science,” they wrote.
And because graphene can be artificially prepared with different techniques that yield distinct properties, studying the structure of natural graphene can provide insight into the process that led to its formation, the researchers said.
The team used Raman spectroscopy – a chemical analysis technique that can determine structure and crystallinity of samples – along with other microscopy techniques to study a lunar soil sample measuring around 2.9 x 1.6mm (0.1 x 0.06 ins).
The sample, retrieved in 2020 by the Chang’e-5 mission, is a relatively young 2 billion years old and comes from a volcanic region on the moon’s near side that “has not been heavily affected by human interference”, the paper said.
According to the researchers, graphene was found in the form of individual flakes, as well as part of a “carbon shell” enclosing elements like sodium, magnesium, aluminium, silicon, calcium, tin and iron.
“Further examination under high magnification confirms the graphite carbon detected by Raman spectra to be few-layer graphene”, made up of two to seven layers of graphene, they said.
Iron found in the sample was only present in regions that also contained carbon, suggesting it played a role in the graphitisation of carbon as a catalyst for carbon-rich precursor materials, they added.
The structure of the graphene suggests it was formed as a result of “high-temperature processes resulting from volcanic eruptions” which may have allowed the iron-bearing lunar soil to interact with carbon-containing gas molecules in the solar winds, leading to mineral catalysis.
“Importantly, this mechanism suggests the presence of a carbon-capture process on the moon, which might lead to the gradual accumulation of indigenous carbon,” the team said.
According to the researchers, the findings could also shed light on how to develop low-cost and scalable synthesis techniques for creating high quality graphene.