For two days, the entire scientific world has had its eyes fixed on Chang’e 6, the Chinese probe that has just returned to Earth after a historic journey to the far side of the Moon. But while waiting for the results of the first analyses of its samples, other researchers continue to study those that were brought back by its big sister Chang’e 5 four years ago. And this work continues to produce fascinating results.
In a study spotted by the South China Morning Post (SCMP), researchers revealed that the material collected by Chang’e 5 contains graphenea material made of a single layer of pure carbon atoms. A conclusion that could call into question the leading theory about the formation of the Moon.
Carbon to explain the origin of the Moon
Although there are several possible scenarios, it is now commonly accepted that the Moon was born approximately 4.5 billion years ago during a cataclysmic collision between Earth and Theia, a protoplanet of a size comparable to that of Mars. The impact would have torn off a huge quantity of material which would have spread out into a vast orbital disk before condensing into a new celestial body.
If this so-called giant impact hypothesis has been so prevalent since the end of the 20th century, it is because it is particularly consistent with studies conducted over the past few decades. For example, we know from the samples brought back by the Apollo missions that our planet and its neighbor are surprisingly close geochemicallyThe isotopes that the Moon is composed of are very similar to those found in the Earth’s mantle, pointing to a common origin.
Another important factor that largely contributed to this theory is that a very important element is missing on the Moon: the coals. This element is extremely abundant on Earth, and is even a fundamental component of life as we know it. On the other hand, NASA found that it was much less abundant in the Apollo samples, and the giant impact theory could explain this low concentration.
Indeed, models of the event have shown that such an impact would have instantly vaporized an immense quantity of material to form a synestia – a huge “donut” of rapidly rotating pulverized material. In gaseous form, all the carbon contained in the debris would have completely escaped into space before the synestia agglomerated to form the Moon. To explain the fact that NASA samples still contain traces of it, the researchers concluded that it probably came from sources called “ exogenous “, like meteorites.
Of ” carbone native » and capture mechanisms
But new evidence is now forcing researchers to reconsider this theory. In 2020, a team of Japanese researchers showed that there were small emissions of carbon ions all over the Moon (see this research paper).
According to the authors, this suggests that there are so-called carbon sources native ». In this context, the term refers to carbon that originated directly on the Moon, as opposed to the exogenous carbon mentioned above. This may seem trivial, but the presence of indigenous carbon has quite profound implications; it means that researchers may need to refine the currently generally accepted model of Moon formation, or even reconsider it entirely.
This is why the discovery of graphene in the Chang’e 5 samples is so exciting: this material is an excellent attack surface to study these different hypotheses. The Chinese researchers therefore focused on the structure of the samples on a small scale. Using several microscopy and spectrometry techniques, they were able to determine that this graphene was “only” two billion years old, which is much younger than the Moon. And above all, they found that the structure of graphene is consistent with “ high temperature processes resulting from volcanic eruptions ».
According to the SCMP, this could have allowed the iron contained in these relatively carbon-rich areas to interact with carbon molecules. More specifically, this would have given rise to a “ mineral catalysis ” likely to form graphene. What is important is that this suggests the existence of a carbon capture mechanism on the Moon.
The latter could explain the accumulation of indigenous carbon in soils. Further studies will be needed to better understand all the nuances of these phenomena as well as their implications. But this is already a very interesting starting point. According to the researchers, these findings could revolutionize our understanding of lunar chemistry and, by extension, its history.
The hidden face of the Moon, the missing piece of the puzzle?
And the most exciting thing is that new evidence could very soon allow researchers to make great progress on these questions. Indeed, the latest Chinese probe, Chang’e 6, has just returned home with fresh samples.
The difference is that this material comes from the far side of the Moon – a region that is both radically different and very poorly understood at the moment.
We can therefore expect fascinating new results to emerge very quickly, with potentially profound implications for the future of planetary science.
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