WHAT DO APOLLO IMPACT GLASSES TELL US ABOUT POST-COPERNICAN IMPACT FLUX? (Invited Presentation)

Standard lunar crater chronology systems assume there has been little change in the impact flux over the past three billion years. At least two independent data sets of lunar impact glasses (spherules and shards) suggest that their ⁴⁰Ar/³⁹Ar-derived age distributions may be in disagreement with this constant flux model.

The first data set focuses on the accumulation of lunar impact glass spherules over the last three billion years. The age distribution of lunar impact glasses derived from ⁴⁰Ar/³⁹Ar dating shows an excess of impact glasses with ages from the last 0.4-0.5 Ga. Previous studies, including the size-dependent argon diffusion correction and the evolution of glass spherules at shallow depths, suggest a potential sampling bias may lead to the excess number of impact glasses with young ages. Because of this sampling bias, a scenario that the lunar impact rate increased in the last 0.5 Ga is therefore not required to match the observed age distribution of lunar impact spherules.

The second data set looked into geochemically-distinct impact glasses. Although a sampling bias may have influenced the observed age distribution of lunar impact glasses, geochemically-distinct glass spherules and fragments appear to reflect a potential impact record for large cratering events. These so-called "exotic" glasses most likely originated in geochemically-distinct regions that are few hundred kilometers away from where astronauts collected the glasses in the regolith samples. The "exotic" glass populations show a higher concentration of ages between 700 and 900 Ma that overlap the formation of Copernicus Crater at ~800 Ma.

Whether or not the lunar impact rate spikes at ~800 Ma, we quantified the provenances of those "exotic" glasses using elemental abundance map data acquired by the Lunar Prospector Gamma Ray Spectrometer. The possibility of Copernicus Crater-forming debris affecting the impact record represented by "exotic" glasses is examined.

Overall, studying lunar impact glasses provides insight into the lunar impact rate and the spherule formation mechanism. Our study may be helpful in planning and interpreting data from future sample returned missions: we found a correlation between the age distributions of lunar impact glasses and sampling depths, as well as quantified the provenance of "exotic" glasses.