geosciences Article History of the Terminal Cataclysm Paradigm: Epistemology of a Planetary Bombardment That Never (?) Happened William K. Hartmann Planetary Science Institute, 1700 East Fort Lowell Road, Suite 106, Tucson, AZ 85719, USA;
[email protected] Received: 30 November 2018; Accepted: 30 May 2019; Published: 28 June 2019 Abstract: This study examines the history of the paradigm concerning a lunar (or solar-system-wide) terminal cataclysm (also called “Late Heavy Bombardment” or LHB), a putative, brief spike in impacts at ~3.9 Ga ago, preceded by low impact rates. We examine origin of the ideas, why they were accepted, and why the ideas are currently being seriously revised, if not abandoned. The paper is divided into the following sections: 1. Overview of paradigm. 2. Pre-Apollo views (1949–1969). 3. Initial suggestions of cataclysm (ca. 1974). 4. Ironies. 5. Alternative suggestions, megaregolith evolution (1970s). 6. Impact melt rocks “establish” cataclysm (1990). 7. Imbrium redux (ca. 1998). 8. Impact melt clasts (early 2000s). 9. Dating of front-side lunar basins? 10. Dynamical models “explain” the cataclysm (c. 2000s). 11. Asteroids as a test case. 12. Impact melts predating 4.0 Ga ago (ca. 2008–present.). 13. Biological issues. 14. Growing doubts (ca. 1994–2014). 15. Evolving Dynamical Models (ca. 2001–present). 16. Connections to lunar origin. 17. Dismantling the paradigm (2015–2018). 18. “Megaregolith Evolution Model” for explaining the data. 19. Conclusions and new directions for future work. The author hopes that this open-access discussion may prove useful for classroom discussions of how science moves forward through self-correction of hypotheses.