Lost planets on the moon

Natural History, Nov, 2004 by Dave Forest

Since the mid-1960s, geologists have known that lunar chemistry is controlled, in part, by the Sun. Our star constantly spews streams of ionized elements, known as solar wind, which bombard the surface of the Moon. Those bombardments explain why certain elements occur in near-surface lunar rocks at roughly the same proportions as occur in the Sun.

One notable exception is the element xenon: lunar concentrations of some of its isotopes are well above the accepted solar estimates. the prevailing explanation has been that orphan, or excess, xenon gas formed deep within the Moon and then slowly escaped to the surface. But that didn't smell right to Minoru Ozima, a geochemist at the University of Tokyo, and his colleagues. All the isotopes of xenon occur on the Moon, but some of them--the ones that form through the decay of relatively short-lived iodine and plutonium isotopes--are present in greater-than-normal concentrations. If those xenon isotopes escaped from within the Moon, they must have remained inside for several hundred million years before being evenly released to the lunar surface. To Ozima and his co-workers, that explanation for the excess xenon seems highly unlikely:

Xenon produced from plutonium and iodine occurs at greater relative concentrations inside the planets than it does inside the Sun. The investigators speculate that if, early in the tumultuous history of the solar system, numerous rocky planetesimals were absorbed by the Sun, the Sung outer layers would have been enriched with that kind of xenon. The xenon could then have blown onto the Moon, along with the solar wind. ("Orphan radiogenic noble gases in lunar breccias: Evidence for planet pollution of the Sun?" Icarus 170:17-23, July 2004)