Remember the rocks that astronauts brought back from the Apollo moon expeditions? They've been a scientific investment whose latest dividend is new insight into the sun.
Physicist Marc Caffee notes, "Surprisingly, there are lots of things we still don't know about the sun." What happens to stuff made in the sun's outer atmosphere is one of them. Is it gulped back into the solar interior? Or spit out through the solar system?
Dr. Caffee, at Purdue University in West Lafayette, Ind., and Kuni Nishiizumi, at the University of California, Berkeley, now think they know. They have found enough of a relatively short-lived element made in the solar atmosphere and embedded in lunar soil to conclude that the sun rids itself of such material as fast as it can.
The element is beryllium-10. It decays so fast that most of a sample produced today would be gone in a few million years. Drs. Nishiizumi and Caffee explained recently in the journal Science that the abundance of this evanescent material in lunar soil indicates it is being constantly replenished. Caffee observes, "The sun is constantly shedding pieces of itself."
That's one small step forward for astrophysicists trying to understand the sun. It also shows how moon rocks help scientists make giant strides in understanding the evolution of the solar system. Much of that history is recorded in lunar geology. Nishiizumi and Caffee worked with samples taken in 1972 by Eugene Cernan and Harrison Schmidt, the last humans to walk on the moon. These and other lunar samples provide what scientists call "ground truth," which can be compared with data taken remotely.
Reviewing this possibility last month in Science, Paul Spudis of the Lunar and Planetary Institute in Houston explained that such "ground truth" helped scientists interpret what the instruments saw when the Clementine mission in 1994 and the Lunar Prospector mission in 1998 mapped the moon remotely.
"The moon is a touchstone for our understanding of the terrestrial planets," Dr. Spudis observed. "Once the data are fully assimilated, many ideas and concepts in planetary science may require substantial revision."