Moon Dirt Mystery Piled Up Over Billions of Years

Cosmic material that has settled on the moon over billions of years could shed light on where a peculiar type of nitrogen in our solar system came from, a new study suggests.

The powdery lunar surface contains isotopes of nitrogen that, for the most part, appear to have been carried there by still enigmatic sources. Scientists have long known there are different types of nitrogen on the moon, but how it all got there is unknown.

"Somehow, we see a substantial amount of nitrogen on the moon, over and above the well-known solar wind, and we don't have a single clue as to where it comes from," said cosmochemist John Kerridge at the University of California, San Diego in La Jolla, a co-investigator of the study into the lunar conundrum. "It's just baffling."

Where did it come from?

Although the sun's solar wind has deposited nitrogen on the moon, this solar nitrogen mostly consists of lighter isotopes, as NASA's Genesis spacecraft revealed in samples it collected of the solar wind. In contrast, a much heavier combination of nitrogen isotopes outnumbers this solar nitrogen roughly 10 to one on the moon.

"The nitrogen in the atmosphere we're breathing is not the same that was initially present in the solar system," said study co-investigator cosmochemist Kurt Marti also of the University of California, San Diego in La Jolla.

Solving the mystery behind where this "non-solar" nitrogen comes from could shed light on the ancient forces that drove the origins and evolution of Earth and the rest of the solar system. One alternative regarding the origin of non-solar nitrogen is that it came from comets.

"It's not a complete fit, though ? comets also are strongly enriched in deuterium, which we don't see as much of on the moon," Marti said.

Marti and Kerridge detailed this research in the May 28 issue of the journal Science.

Nitrogen from deep space

Another possibility is that this nitrogen came from interstellar space as our solar system traveled around the galaxy. Telescopes are now powerful enough to scan interstellar matter for what nitrogen isotopes they possess, Marti noted.

The moon could be an ideal place to find out where this non-solar nitrogen came from. For instance, judging from lunar samples collected by Apollo 16, "this non-solar nitrogen is still coming in from unknown sources," Marti explained. "In the 2-million-year-old South Ray Crater, we see this nitrogen there, and also in another crater made 50 million years ago."

"The lunar surface is like a history museum," Marti added. "If we can identify when this nitrogen was implanted in the lunar surface, then we can trace back what its history was."

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