The Moon is believed to have formed from a collision between a Mars-sized planetoid and primordial Earth. This event threw hot material into orbit, which eventually coalesced into our only natural satellite, the Moon. A brand-new study now suggests that things might not be as simple as that.

The study, published in Science Advances, looked at the global emissions of carbon from the Moon. The collision theory relies on the notion the Moon was "dry", depleted of carbon due to its hot formation. But analysis of data from the KAGUYA orbiter shows that this is not the case.

The Japanese spacecraft observed carbon ions being emitted from across the Moon's surface. There was even a regional difference of emissions; about 51,000 carbon ions per square centimeter every second from the lunar “seas”, or maria as they are called (Latin for seas), and slightly lower at about 45,000 from the highlands. To rule out an external source of carbon emissions, such as the solar wind or collisions with meteoroids, the researchers calculated the carbon emissions from both. They estimated that to be around 42,000 atoms per square centimeter per second, less than what they were detecting.

“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 researchers write in the study.

This data doesn’t mean that the impact hypothesis is wrong but needs to be expanded or revised to include the idea that the Moon may in fact be “wet”, richer in lighter substances such a carbon or water, which also appears to be more present than expected.

New models have approached this question, suggesting that while the disk of materials from which the Moon formed could have reached temperatures as high as the surface of the Sun, the depletion of light elements might not have happened efficiently so those elements became trapped within the solidifying Moon.

If the carbon was not there from the very beginning, the team suggests that it might have got there early in the Moon's formation, long before the maria set between 3-3.5 billion years ago. Further research is necessary to better understand the origin of this carbon supply, but researchers are excited to employ the same observation technique in upcoming Japanese missions like the Martian Moons eXploration missions and the BepiColombo Mercury Magnetospheric Orbiter.

The spacecraft KAGUYA is officially called SELENE (Selenological and Engineering Explorer), like the Greek goddess of the Moon, but is better known by its nickname KAGUYA after the lunar princess in one of the oldest Japanese folktales. It was deorbited and impacted the Moon in 2009.