The violence of the sun's atmosphere makes it both a difficult subject for study and a compelling one. The strength and energy of solar flares and the winds that flow from them periodically make themselves known on Earth as radio interference and spectacular polar auroras, despite our protective atmosphere and magnetic field. In a study published in Friday's Science, two California researchers shed light on the surface of the sun by examining a body without any protection from solar wind whatsoever, the surface of the moon.
The scientists analyzed samples of lunar soil obtained by Apollo 17 astronauts nearly 30 years ago for the presence and distribution of beryllium-10, an unstable isotope produced in the solar atmosphere and carried by solar wind. The sun produces beryllium-10 by the same mechanism that gives us polar auroras: subatomic particles are trapped in a closed magnetic loop and accelerated to extraordinary velocities. When protons caught this way in the sun's atmosphere collide with the nuclei of carbon, nitrogen and oxygen, they can form a nucleus with four protons and six neutronsone neutron more than the stable form of berylliumfor a total of 10 nuclear particles, giving beryllium-10 its name.
The researchers wanted to know what happens to beryllium-10 produced this way and, by extension, what happens to the outer layers of the solar atmosphere in which it is formed. Two models of the solar surface contend. One posits deep atmospheric mixing, and the other envisions steady outward motion with only shallow mixing. By analyzing the concentration of beryllium-10 on the lunar surface, the team was able to estimate the proportion of the sun's total beryllium-10 production (a known quantity) that is ejected without delay. The isotope under study decomposes rapidly, with a half-life of 1.6 million years. Thus a study of its concentration would not include any deposits dating from the formation of the moon or from a time when beryllium-10 was produced more widely and frequently.
Still, the scientists had to control for other sources of beryllium-10 in their lunar sample. "The moon has no atmosphere and no magnetic field, so the solar wind is not kept in any way, shape or form from hitting the surface of the moon," study co-author Marc Cafee of Lawrence Livermore National Laboratory reports. This circumstance allows cosmic rays to bombard the moon as well, producing some beryllium-10 locally. The researchers used acids to leach the isotope from soil samples taken from the lunar surface and from about 20 centimeters below it. Using the deeper sample as a baseline, they were able to quantify the amount of beryllium-10 that is deposited on the surface of the moon by the wind.
Even though it travels at between 300 and 800 kilometers per second, solar wind cannot drive the isotope any deeper than one nanometer into the surface of a soil grain. Thus the first mild rinse of acid released 14 to 16 percent more beryllium-10 than the average yield of the entire surface sample. The lunar concentration of beryllium-10 of a conclusively solar origin is consistent with a deposition rate of 2.9 (+/-1.2) x 10-6 atoms per square centimeter per year. This rate could only be sustained by constant and direct ejection of the upper atmosphere of sun in the solar wind. If the solar atmosphere were deeply mixed the researchers expect beryllium-10 would be four or five orders of magnitude scarcer.