Quick Q: Er, why is the Moon emitting carbon? And does this mean it wasn't formed from Theia hitting Earth?

Decades-old theory may require a rethink thanks to Japanese probe

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The Moon bathed in solar wind, emitting carbon ... Illustration credit: S. Yokata

The Moon is believed to have formed from the leftovers of a proto-Earth smashing into a Mars-sized Theia nearly 4.5 billion years ago.

Now, that has been called into question after a spacecraft detected carbon ions emitted from the natural satellite’s surface.

Kaguya, also known as the Selenological and Engineering Explorer (SELENE), operated by Japan's space agency JAXA, circled the Moon from 2007 until 2009 when it crashed into the regolith. Boffins led by Osaka University, Japan, analyzed data from that trip, and found the Moon emits carbon – a volatile element – contrary to previous measurements taken during NASA’s Apollo missions.

That's interesting because it suggests the Moon wasn't formed from the two worlds violently colliding because the high-temperature prang should have boiled off all the volatile element. And yet, it's detected all over the surface. If the Moon wasn't formed from Earth and Theia whacking into each other, this carbon-coated wonder must have come from somewhere or something else.

“We present observations by the lunar orbiter Kaguya of carbon ions emitted from the Moon,” stated a paper documenting the findings, which was published in Science Advances on Wednesday. “The estimated emission fluxes to space were ~5.0 × 104 per square centimetre per second, which is greater than possible ongoing supplies from the solar wind and micrometeoroids.”

The scientists concluded "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."

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Lunar samples collected during the Apollo missions supported the idea that the Moon was “dry,” depleted of any volatiles. The latest measurements from Kaguya, however, show that the Moon is actually “wet,” in that it emits carbon ions from its surface. Some regions released a higher concentration of carbon than others, due to those regions simply having more carbon than others.

“The possible explanations for regional differences are not extraneous but internal factors, such as the distributions of [ethylene] (C2H4 and [carbon dioxide] CO2 stored in remnants of ancient lunar volcanism,” the academics explained. Carbon is brought to the Moon by the solar wind and tiny bits of floating space debris, but the estimated number of atoms transported this way isn’t enough to influence Kaguya’s observations.

The latest measurements may contradict the giant-impact hypothesis on one account, though they fail to explain other evidence that the Moon did form from a collision between an early Earth and another body. For example, the Moon does seem to have a similar abundances of oxygen isotopes to Earth suggesting a common origin.

Kaguya also helped astronomers and geologists produce the most detailed geology map of the Moon's surface to date. ®

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