A spacecraft orbiting the Moon has detected something really peculiar. Our satellite is emitting a steady stream of carbon ions from almost its entire surface, contrary to the long-standing thinking that the Moon is depleted of carbon and other volatile elements.

In fact, it seems that carbon has been there since the Moon's formation, or very shortly after, 4.5 billion years or so ago. This means that the details of the impact theory of the Moon's formation - which largely relies on a lack of volatiles - may need to be reconsidered.

The results come from the Japanese Aerospace Exploration Agency's (JAXA) Selenological and Engineering Explorer spacecraft, better known by its nickname, Kaguya. A decade ago, it spent roughly a year and a half in orbit around the Moon, collecting global-mapping observations.

One of its instruments was an ion mass spectrometer, which detected and mapped lunar ions, including carbon - something that had not turned up in Apollo data, leading to the belief that the Moon had very little of them.

But recent analyses have found traces of carbon and volatile water in volcanic lunar glasses, prompting adjustments to the impact formation model, so a team of researchers revisited the Kaguya data to try to figure out where the carbon came from.

"These emissions were distributed over almost the total lunar surface, but amounts were differed with respect to lunar geographical areas," the researchers wrote in their paper.

"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 readings found a concentration of carbon ions that could not be explained by the deposition of carbon by the solar wind, nor the delivery of carbon on micrometeorites - both mechanisms known to supply the Moon with small amounts of carbon.

In addition, the concentrations varied. The younger volcanic basalt plains on the lunar near side emitted more carbon ions than the older highlands. This suggests that the carbon is embedded in the Moon.

The reason that's a problem for the lunar impact formation model - in which a large body we call Theia collided with Earth sometime in the early years of the Solar System, breaking off a chunk and sending it into Earth orbit - is because volatiles have a low boiling point.

But the Theia collision would have generated pretty intense temperatures - 4,000-6,000 Kelvin - which should have partially vaporised the debris, and boiled away the volatiles, producing what is known as a volatile-depleted 'dry' Moon.

That the detections instead revealed a volatile-rich 'wet' Moon suggests that the temperatures generated by the impact could have been much less intense than we previously thought. Or perhaps the impact model will need other revisions.

At the very least, this result shows that further investigation into lunar volatiles could be very telling; and it could inform the instrumentation aboard future lunar orbiters, or NASA's planned crewed mission to the Moon.

"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.

The research has been published in Science Advances.