Research Associate Boris Yakshinskiy of Highland Park and Professor Theodore E. Madey of Bound Brook, both of Rutgers' Laboratory for Surface Modification, the department of physics and astronomy of the Faculty of Arts and Sciences-New Brunswick, describe in their paper the creation of laboratory simulations of the lunar surface and then demonstrate how the resupply of sodium to the lunar atmosphere from the surface occurs.

The relatively thin atmosphere of the Moon, as well as that of the planet Mercury, has been shown to contain measurable amounts of atomic sodium and potassium vapor.

Because these atoms remain in the atmosphere for only a few hours, they must constantly be resupplied. The Rutgers physicists undertook experiments to uncover the mechanisms required for the continuing presence of one of these elements (sodium) in the atmosphere, as observed over time by other scientists.

"Several mechanisms have been proposed to explain the resupply of atmospheric components from the lunar surface," said Madey. "Other scientists have considered the effects of the solar wind (a loosely packed stream of particle radiation from the sun), micrometeorite impacts, release of elements caused by surface heating, and release stimulated by the effects of light, but there is little data to support any one explanation and no general agreement about which processes are the most important."

"We simply place a sample of the surface we intend to investigate in a good vacuum, we start throwing photons, electrons, and ions at it, and then we look at what comes back from the surface and how," said Yakshinskiy, describing the use of highly sophisticated ultra-high vacuum apparatus, X-ray photoelectron spectroscopy, photon-stimulated desorption, temperature programmed desorption and low-energy ion scattering in laboratory analysis.

As silicon-dioxide (SiO2) is the major component of the Moon's surface, SiO2 films were prepared in vacuum to simulate it and then sodium was deposited upon the films.

The simulated lunar surfaces were bombarded with the photons, electrons and ions to test the effects of light, heat, meteoritic and other radiation mechanisms. Measurements of the liberated sodium were taken and analyzed.

Based on their experimental results, Yakshinskiy and Madey conclude that the electron flux or flow from the solar wind is too small to expel sodium from the lunar surface, "...but the solar photon flux (the light itself) is more than sufficient. These measurements provide strong scientific support and rationale for the arguments ... that a photon-stimulated desorption (replenishment) process plays a major role in production of the lunar sodium atmosphere."

Some observable short-term, local sodium concentrations may also be attributed, they believe, to associated meteor showers. With respect to the planet Mercury, they believe that heat contributes to the surface release of sodium, given the high daytime temperatures, but the photon flux is still likely to play a major role, as on the Moon.

"As we move out beyond the confines of our home planet, we must look to new scientific approaches to deal with the questions posed in this new era of planetary exploration," said Madey. "The study of surface modification is one such approach, already providing some of the answers we seek and helping us to understand the environments we will encounter on our solar system neighbors."

The Laboratory for Surface Modification provides a focus for research into atomic-level phenomena that occur on the surface of solids. It involves the overlapping disciplines of physics, chemistry, materials science and engineering with applications to semiconductor devices and catalysis, as well as surface processes in outer space.

Yakshinskiy and Madey's paper in Nature is titled "Photon-stimulated Desorption as a Substantial Source of Sodium in the Lunar Atmosphere."