The topmost layer of the Moon's surface is coated with trace materials from the Sun, and these are providing some insight into processes in the solar atmosphere. Unlike the Earth, the Moon isn't protected by a particle-deflecting magnetosphere and atmosphere. Thus, the lunar surface is subject to the full force of the solar wind and high-energy cosmic rays from elsewhere in the galaxy.

Kunihiko Nishiizumi (University of California, Berkeley) and Marc W. Caffee (Lawrence Livermore National Laboratory) examined specimens of lunar soil collected by Apollo 17 astronauts Eugene A. Cernan and Harrison H. Schmitt. The Moon-walkers collected one sample within a centimeter of the surface and dug out the other from a 25-cm-deep trench. Nishiizumi and Caffee explain in the October 12, 2001, issue of Science how they used several baths of acid to liberate radioactive isotopes of beryllium, aluminum, and chlorine from the soil particles and then measured the quantities of the atoms. In the case of beryllium-10 — which is unstable because it has an extra, sixth neutron in its nucleus — they identified about 15 percent more in the shallow soil sample than would be expected if the atoms came from cosmic rays.

Furthermore, beryllium-10 has a half-life of 1.5 million years, far younger than the Moon, so there must be a source of continual replenishment. Nishiizumi and Caffee conclude that the beryllium is being carried to the Moon from the Sun. Energetic protons bombarding carbon, nitrogen, and oxygen atoms in the solar atmosphere create the isotope. The solar wind then carries it away.

The researchers note that the quantity of beryllium-10 suggests that the atoms are flung out soon after they are formed. Calculations revealed that if they mixed more deeply into the Sun's surface layers first, there would be only about a ten-thousandth as much beryllium in the lunar soil. Consequently, much less of the solar atmosphere seems to churn back into deeper layers within the Sun than solar researchers have believed.