Lunar Accretion Research Articles

Extralunar materials in Apollo 16 soils and the decay rate of the extralunar flux 4.0 Gy ago

The concentration of extralunar materials in the Apollo 16 regolith is about 3.5%, about 50% higher than that observed at the Apollo 14 site, and three times higher than values at mare landing sites. The integrated flux of extralunar materials is 2.5 times higher at the Apollo 16 than at the Apollo 14 site. These data support the hypothesis that the flux of extralunar materials decreased rapidly between 4.0 and 3.7 Gy ago, and demonstrate the presence of appreciable unaccreted materials near 1 AU 600 my after the formation of the solar system. Assumed ages of 3.91 and 4.00 Gy for the Apollo 14 and 16 sites yield a half-life of this short-lived population of about45 ± 15 my. The lunar accretion rate 4.00 Gy ago was about 2.4 × 10 −6 g cm −2 yr −1, about 700 times greater than the current accretion rate of 3.5 × 10 −9 g cm −2 yr −1.