Reviewing “terminal Cataclysm:” What Does It Mean?

Introduction: The idea of an early intense bombardment of the moon can be traced back at least to the 1960s [1], and the more specific concept of a terminal cataclysm dates from ca. 1973 [2,3]. Since then, the term " terminal cataclysm " (or an equivalent term) has been used in many papers as if it were a well-defined, empirically confirmed phenomenon. The meanings attached to this concept, however, range all over the map, from a global metamorphic event, or a 150 Ma-long spike in cratering centered at ~3.92 Ga ago, to still other concepts, such as few impacts before 3.9 Ga ago. Here, we argue that this semantic imprecision has hurt our understanding of the solar system. History of Concepts: Various concepts have been discussed. ** In 1966, Hartmann [1] showed that the average cratering rate before ~3.6 Ga (evidenced in the lunar highlands) had to average ~150-200 times the average rate since then. ** Tera et al. in 1973-4 [2,3] based on Apollo rock samples, proposed a global " terminal cataclysm " met-amorphic event at ~3.9 Ga ago, to explain paucity of earlier rocks. They suggested it might involve either the Imbrium basin impact around 3.9 Ga ago, or a clustering of many impact basins around that time. ** Hartmann, in 1975 [4], argued that a unique cataclysmic event at 3.9 Ga might be a " misconception " and that absence of early samples might involve more uniformitarian cratering, but so intense before ~ 4.0 Ga that earlier rocks were reset in age or lost by pulverization. ** The concept of " late heavy bombardment, " (" LHB ") at 3.9 Ga gained wide acceptance in 1990 when Ryder [5] showed that a huge spike in Apollo impact melt ages centered around 3.85-4.0 Ga ago. He also argued, importantly, that relative lack of impact melts from ~4.4 to 4.0 Ga indicated lack of impacts in that period. ** Circa 1998, Haskin, Korotev, and co-workers [6] argued that prevalence of ~3.9 Ga dates involved KREEP-rich ejecta from the Imbrium impact at that time, present at several Apollo sites. This was controversial , but echoed one of the proposals of Tera et al. in 1974. ** In 2000, Cohen, Swindle, and Kring [7] began reporting dates of impact melt clasts in KREEP-poor lunar meteorites. Their data show no Ryder-like spike at 3.9 Ga. Nonetheless they inferred " Support for the …