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Moon Dirt Mystery Piled Up Over Billions of Years

Cosmicmaterial that has settled on the moon over billions of years could shed lighton where a peculiar type of nitrogen in our solar system came from, a new studysuggests.

The powdery lunarsurface contains isotopes of nitrogen that, for the most part, appear tohave been carried there by still enigmatic sources. Scientists have long knownthere are different types of nitrogen on the moon, but how it all got there isunknown.

"Somehow, we seea substantial amount of nitrogen on themoon, over and above the well-known solar wind, and we don't have a singleclue as to where it comes from," said cosmochemist John Kerridge at the University of California, San Diego in LaJolla, a co-investigator of the study into the lunar conundrum. "It's justbaffling."

Where did it comefrom?

Although the sun'ssolar wind has deposited nitrogen on the moon, this solar nitrogen mostlyconsists of lighter isotopes, as NASA's Genesis spacecraft revealed in samplesit collected of the solar wind. Incontrast, a much heavier combination of nitrogen isotopes outnumbers this solarnitrogen roughly 10 to one on the moon.

"Thenitrogen in the atmosphere we're breathing is not the same that was initiallypresent in the solar system," said study co-investigator cosmochemist KurtMarti also of the University of California, San Diego in La Jolla.

Solving the mysterybehind where this "non-solar" nitrogen comes from could shed light onthe ancient forces that drove the origins and evolution of Earth and the restof the solar system. One alternative regarding theorigin of non-solar nitrogen is that it came from comets.

"It'snot a complete fit, though ? cometsalso are strongly enriched in deuterium, which we don't see as much of on themoon," Marti said.

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

Nitrogenfrom deep space

Anotherpossibility is that this nitrogen came from interstellarspace as our solar system traveled around the galaxy. Telescopes are now powerfulenough to scan interstellar matter for what nitrogen isotopes they possess,Marti noted.

The moon could be anideal place to find out where this non-solar nitrogen came from. For instance,judging from lunar samples collected by Apollo 16, "this non-solarnitrogen 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."

"Thelunar surface is like a history museum," Marti added. "If we canidentify when this nitrogen was implanted in the lunar surface, then we cantrace back what its history was."

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