A probe sent to orbit the Moon in order to collect data about it registered something really weird. The natural satellite is discharging a constant stream of carbon ions from almost its overall surface, conflicting with the long-standing theory that the Moon has no carbon or other volatile elements.
As a matter of fact, it appears that carbon has been there since the Moon formed, or perhaps very shortly after – about 4.5 billion years ago. This means that the theory of the impact of the Moon’s formation, which mostly suggests a lack of volatiles, may need to be reviewed.
Unaccounted Carbon Emissions
The observations were noticed by the Japanese Aerospace Exploration Agency’s (JAXA) Selenological and Engineering Explorer spacecraft, also known as Kaguya. Ten years ago, it orbited the Moon for about a year and a half, collecting global-mapping data.
One of its tools was an ion mass spectrometer, which found and mapped lunar ions, including carbon. This information did not turn up in Apollo data, which made astronomers believe the Moon had very little amounts of them.
However, recent analyses have discovered signs of carbon and volatile water in volcanic lunar glasses, requiring changes to the impact formation model, so a team of scientists decided to reread the Kaguya data in order to find out where the carbon source came from.
“These emissions were distributed over almost the total lunar surface, but amounts were different with respect to lunar geographical areas,” the team wrote. “Our estimates demonstrate that indigenous carbon exists over the entire Moon, supporting the hypothesis of a carbon-containing Moon, where the carbon was embedded at its formation and/or was transported billions of years ago.”
The measurements discovered a concentration of carbon ions that could not be accounted for by the arrangement of carbon by the solar wind, not the distribution of carbon on micrometeorites. Both systems are known to supply the Moon with small quantities of carbon.
Moreover, the concentrations differed. The younger volcanic basalt plains on the surface close to the margins issued more carbon ions than the older highlands. This implies that the element is an integral part of the Moon.
Rethinking the Origins of the Moon
The reason this is an issue for the initial lunar impact formation model, which theorizes that a massive cosmic object known as Theia crashed with Earth in the first years of the Solar System, separating a piece and sending it into Earth orbit, is because volatiles have a low boiling point.
However, the Theia clash would have produced rather intense temperatures – 4,000 to 6,000 Kelvin – which should have partly evaporate the debris, and boiled the volatiles, creating what is known as a volatile-depleted ‘dry’ Moon.
The fact that the measurements instead depicted a volatile-rich ‘wet’ Moon implies that the temperatures produced by the impact could have been less powerful than researchers previously considered. Also, it could be that the impact model requires further revisions.
“It would be useful to further evaluate initial amounts of volatiles in the Moon, (for example, future isotope analyses of the C+ emissions from the lunar surface) to provide a quantitative estimation of the mass balance of indigenous carbon, the solar wind, and micrometeoroids,” the researchers wrote in a paper published in the journal Science Advances.