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Scientists have discovered naturally formed graphene flakes in lunar soil samples, potentially shedding light on the mystery surrounding 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 the Chinese Lunar Exploration Program and the first Chinese 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 in 2004 by Andre Geim and Konstantin Novoselov. In 2010, the duo received the Nobel Prize in Physics for the discovery due to graphene’s remarkable physical properties and its potential applications in electronics, energy storage, sensing, and biomedical engineering, among many others.
“Graphene has revolutionized research in condensed matter physics and materials science with its novel physical phenomena and extraordinary properties,” the lunar sample team wrote in a recent paper published in the journal National Science Review.
But why were scientists looking for graphene on the moon in the first place?
Can graphene reveal how the moon formed?
The origin of the moon is still a matter of debate, although many theories have been proposed over the years. One theory in particular, the giant impact hypothesis, has gained popularity.
This hypothesis states that Earth collided with a Mars-sized planet about 4.5 billion years ago, creating debris in Earth’s orbit that eventually formed the moon. Giant impacts like this were common in the inner solar system when Earth came together in the turbulent early era of our planetary system.
Related: The Moon’s thin atmosphere is formed by a constant bombardment of meteorites
Although the Moon’s composition is similar to that of Earth, samples brought back by NASA’s Apollo missions show that the Moon, which has been around our planet for a long time, contains relatively few volatile elements (elements that evaporate easily), including carbon, compared to our planet.
Scientists hypothesized that the intense heat generated by such a massive collision would have caused volatile elements to vaporize and escape, leaving behind a body low in carbon. The Apollo samples also showed a similarity in isotopic composition between the Earth and the Moon. Isotopes are atoms of the same element with different numbers of neutrons in their nuclei, so these results further support the idea that the Moon formed from material flung away from Earth after a giant impact.
These new data from CE-5, along with recent observations that have detected carbon ion fluxes emanating from the moon, indicate that native carbon may be present, challenging the existing consensus around this theory.
“It is highly desirable to determine the crystal structure of the [moon’s] native carbon,” the team wrote. This is because carbon is a fundamental element for understanding the formation and evolution of planetary bodies, with shape and structure determined by the process of formation.
“Since graphene is routinely prepared using artificial techniques with specific morphologies and properties as determined by the specific formation process, characterization of the composition and structure of natural graphene could provide much information about the geological evolution of parent bodies,” the team said.
The team behind the discovery of graphene in lunar samples analyzed an “olive-shaped” lunar soil sample measuring 2.9 millimeters long and 1.6 millimeters wide.
To search for graphene carbon in this sample, the researchers used several characterization techniques commonly used by chemists, including scanning electron microscopy (SEM) and Raman spectroscopy.
Raman spectroscopy is a light scattering technique that allows scientists to study the vibrational, rotational, and low-frequency modes of bonds within molecules. This technique thus provides insight into the structure and composition of these molecules. On the other hand, SEM creates high-resolution images of the surface of a material using a focused electron beam. SEM can be used to determine the elemental composition.
Using these tools, the scientists identified embedded flakes of graphene in carbon-rich regions of the sample, ranging from two to seven layers thick. The team also observed that the layered graphene forms a shell structure that encloses a core of complex compounds.
This suggests a bottom-up synthesis process rather than creation by exfoliation, the separation of layers by breaking the bonds between them, which normally involves a high-temperature reaction.
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In addition to finding graphene in the lunar soil samples returned from CE-5, the scientists found an iron-bearing compound that was present only in the parts of the sample where carbon was found.
This is interesting because iron-bearing minerals on the Moon, such as olivine and pyroxene, may have played a role in the transformation of carbon into graphene.
Volcanic activity, solar winds, or meteorite impacts could have provided the energy needed for this transformation, potentially creating the high-pressure, high-temperature environments needed to convert carbon into graphene.
“This discovery could lead to new insights into the Moon’s chemical composition, geographic events and history,” said the team behind the discovery.