Two researchers at Rutgers University might have found the answer. The physicists created a tiny lunar sample in ultra-high vacuum chamber and discovered a particular wavelength of ultraviolet light liberates sodium from the simulated soil.

Boris Yakshinskiy and Theodore Madey, both of whom work at the universitys Laboratory for Surface Modification, used bits of silicon dioxide film to serve as the moons surface, then deposited sodium onto the samples. Silicon dioxide is the most prevalent component in moon rocks. On the moon, sodium in the atmosphere is being continually replenished, although exactly how this is occurring is unknown.

The researchers bombarded their sample, which measures less than a square centimeter, with photons, electrons and ions to test the effects of light, heat, micrometeorite impacts and other processes that occur in space. Measurements of the amount of sodium released were taken and analyzed.

"Its a world in a thumbnail," said Madey.

The experiment showed that some of the leading theories on how the sodium was getting into the atmosphere were wrong, including ionization from the solar wind, micrometeorite impacts and heating of the lunar surface by the sun.

Instead, the researchers found that photons of light in wavelengths of about 300 nanometers -- between the colors of deep blue and ultraviolet accounted for the sodiums release from the simulated soil sample and replenishment in the surrounding atmosphere.

"Its been a basic puzzle of science," said Madey, who usually is studying more terrestrial aspects of whats happening on the surfaces of solids.

Madey would like to continue the research, which was funded under a three-year $160,000 NASA grant, and hopes to get his hands on an actual fragment of moon rock for additional studies.