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Scientists have discovered naturally formed graphene flakes in lunar soil samples, which may shed light on the mystery of the moon’s formation.
The samples were collected from the moon in 2020 by Chang’e-5 (CE-5), the fifth lunar exploration mission of China’s Lunar Exploration Program and China’s first mission to return samples to Earth.
Graphene is a form of carbon, or “allotrope,” that consists of a single layer of atoms arranged in a honeycomb structure. It was first discovered by Andre Geim and Konstantin Novoselov in 2004. In 2010, the duo received the Nobel Prize in Physics for the discovery due to the remarkable physical properties of graphene and its potential applications in electronics, energy storage, sensing and biomedicine, among many others.
“Graphene has researched condensed matter physics and materials science with its novel physical phenomena and unusual properties,” the lunar sample team wrote in a paper recently published in the journal National Science Review.
But why were scientists looking for graphene on the moon in the first place?
Could graphene show how the moon was formed?
The origin of the moon is still debated, although many theories have been advanced over the years. One theory in particular, the mass effect hypothesis, has gained popularity.
This hypothesis suggests that Earth collided with a Mars-sized planet about 4.5 billion years ago, producing debris in Earth’s orbit that eventually formed the moon. Massive impacts like this were common in the inner solar system when the Earth was coming together in the early turbulent times of our planetary system.
Related: The moon’s thin atmosphere is made by constant bombardment of meteorites
Although the composition of the Moon is similar to that of Earth, samples brought back from NASA’s Apollo missions have shown that our planet’s lunar companion is relatively depleted in volatile elements (elements that evaporate easily), including carbon, in compared to our planet.
The scientists said the intense heat generated by such a massive collision would cause volatile elements to vaporize and escape, leaving behind a carbon-depleted body. The Apollo samples also showed a similarity in isotope composition between Earth and the moon. Atoms of the same element and different numbers of neutrons in their nuclei are atoms of the same element, so these results support the idea that the moon was made of material from Earth after a major impact.
These new data from CE-5, along with recent observations that have detected fluxes of carbon ions coming from the moon, indicate that native or indigenous carbon may be present there. This challenges the existing consensus regarding this theory.
“The crystalline structure of the is highly desirable [moon’s] native carbon,” wrote the team. This is because carbon is a fundamental element in understanding the formation and evolution of planetary bodies, and its shape and structure are determined by the formation process.
“Since graphene is regularly prepared using artificial techniques with specific morphologies and properties as determined by the specific formation process, the composition and characterization of the structure of natural graphene would provide rich information on the geological evolution of the parent bodies,” said the team.
The team behind the discovery of graphene in lunar samples analyzed an “olive-shaped” lunar soil sample 2.9 millimeters long and 1.6 mm wide.
To search for graphene carbon within this sample, the researchers used various characterization techniques often used by chemists, including scanning electron microscopy (SEM) and Raman spectroscopy.
Raman spectroscopy is a light scattering technique that allows scientists to study vibrational, rotational and low-frequency modes of bonds within molecules. Therefore, this technique provides insights into the structure and composition of these molecules. SEM, on the other hand, creates high-resolution images of a material’s surface using a focused electron beam. SEM can be used to determine elemental composition.
With these tools, the scientists identified embedded flakes of graphene in carbon-rich areas of the sample, between two and seven layers thick. The team also noted that the layered graphene forms a shell structure that includes a core of complex compounds.
This suggests a bottom-up synthesis process rather than creation by exfoliation, the separation of the layers by breaking the bonds between them, which usually involves a high-temperature reaction.
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In addition to finding graphene in the returned CE-5 lunar soil samples, the scientists only found a compound containing iron present in areas of the sample where carbon was found.
This is interesting because iron-containing minerals on the moon, such as olivine and pyroxene, may have played a role in catalyzing the transformation of carbon to graphene.
Volcanic activity, solar winds, or meteorite impacts could provide the necessary energy for this transformation, which could be created by the high-pressure, high-temperature environments needed to convert carbon into graphene.
“This result could reconstruct the understanding of chemical components, geological episodes and history of the moon,” said the team behind this discovery.
