An attempt has been made to calculate the ages of lunar fines and carb
onaceous chondrites from the contents of Pu-244 fission xenon and uran
ium. The carbonaceous chondrites such as Renazzo and Allende appear to
be the oldest objects in the solar system and they began to retain th
eir xenon about 4,900 million years ago when the ratio of Pu-244 to U-
238 in the solar system was about 0.123 (atom/atom). Lunar fines, whic
h appear to be the second oldest objects in the solar system, began to
retain their xenon 4,568 to 4,857 million years ago, when the ratio o
f Pu-244 to U-238 was 0.0078 to 0.0855 (atom/atom).