lunar mystery

A new study contradicts the Moon's familiar origin story

We've still got a lot to learn about our orbiting rock.

S. Yokota

The Moon is perhaps the most familiar celestial body in our skies. It's inspired countless generations and sparked a Space Race that still captures the imagination. And yet, there is still so much we don't know about Earth's lone natural satellite, such as how it came about in the first place.

A study, published Wednesday in the journal Science, contradicts a well-established theory about the composition of the Moon and dismisses a long-standing hypothesis about how the Moon formed in the Solar System.

The team of scientists behind the new study found traces of carbon ions across the lunar surface, despite a previously held theory that the Moon is completely void of volatile, readily vaporizing elements such as carbon.

The findings suggest that the volatile carbon has been there since the very formation of the Moon, around 4.5 billion years ago, or was somehow acquired shortly after the Moon formed. Therefore, these new results could have a major impact on scientists' current hypothesis of how the Moon came to be.

We've stared at the Moon for decades, but still can't figure out exactly how it formed.NASA

For years, scientists have believed that the Moon formed around 4.5 billion years ago, in the midst of the early chaos of a young solar system.

The widely-held giant-impact hypothesis suggests that the Moon formed from the ejected material of a major collision between a Mars-sized planetary body and a young Earth, also known as the proto-Earth, right after our planet formed its initial crust. The debris leftover from this impact would have then collected in an orbit around the Earth.

The giant-impact hypothesis was further cemented following the Apollo missions. After landing on the Moon, Apollo astronauts brought back over 22 kilograms of rock and dust collected from the lunar surface.

The Apollo samples revealed some striking similarities between the Earth and the Moon, suggesting that they have an almost identical chemical and isotopic composition, which showed that they indeed may have a shared history, or rather came from the same parent body.

The analysis of the Apollo samples also suggested that volatile elements, such as carbon, no longer existed on the Moon, and that the Moon was ultimately dry. However, recent studies have contradicted this idea of a 'dry' Moon.

In order to settle this debate, the team of researchers behind the new study used observations collected by the KAGUYA lunar orbiter.

The spacecraft was launched in September, 2007, as the second Japanese mission to explore the Moon from orbit. KAGUYA, or Moon Princess, spent almost two years in lunar orbit, collecting data on the origins and geological evolution of the Moon.

Using observation data taken by KAGUYA, the researchers were able to create a map of lunar carbon ion emissions from the Moon. The estimated emissions were far greater in quantity than other ongoing supplies of carbon, meaning that they likely did not come from other sources of carbon on the Moon, such as solar wind, or the flow of charged particles from the Sun, or collisions with volatile-rich micrometeoroids. Instead, the carbon ions were distributed across almost the entire lunar surface.

Additionally, the study notes regional differences in the carbon ion emissions. The Moon’s large, basaltic plains were emitting a lot more carbon than the highlands, which is likely due to indigenous storage of carbon that existed on the Moon for billions of years rather than carbon that came from outside sources.

With the latest study possibly challenging findings from the Apollo samples, it creates room for other competing hypotheses of how the Moon formed. The capture hypothesis suggests that the Moon was a wandering rock in space that was captured by Earth's gravity, while the accretion theory suggests that the Moon and Earth were both created at the same time, from the same disk of material.

Abstract: Carbon is a volatile element that has a considerable influence on the formation and evolution of planetary bodies, although it was previously believed to be depleted in the Moon. We present observations by the lunar orbiter KAGUYA of carbon ions emitted from the Moon. These emissions were distributed over almost the total lunar surface, but amounts were differed with respect to lunar geographical areas. The estimated emission fluxes to space were ~5.0 × 104 per square centimeter per second, which is greater than possible ongoing supplies from the solar wind and micrometeoroids. Our estimates demonstrate that indigenous carbon exists over the en- tire Moon, supporting the hypothesis of a carbon-containing Moon, where the carbon was embedded at its forma- tion and/or was transported billions of years ago.
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