Abstract
This study examines the history of the paradigm concerning a lunar (or solar-systemwide)terminal cataclysm (also called “Late Heavy Bombardment” or LHB), a putative, brief spikein impacts at ~3.9 Ga ago, preceded by low impact rates. We examine origin of the ideas, why theywere accepted, and why the ideas are currently being seriously revised, if not abandoned. Thepaper 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 discussionsof how science moves forward through self-correction of hypotheses.
Highlights
The author hopes that this open-access discussion may prove useful for classroom discussions of how science moves forward through self-correction of hypotheses
Lineweaver (2010 [146]), under the title “Crater-counting Evidence against the Late Heavy Bombardment Hypothesis” concluded “Our analysis does not support the LHB hypothesis as articulated by Ryder, nor do we find the data from impact breccias, glass spherules or lunar meteorites supportive of LHB”
By 2012, Bottke et al [140] extended the duration of the LHB even further, arguing that Archaean and early Proterozoic spherule beds on Earth mark additional, post-Orientale multi-ring-basin-scale impacts caused by E-belt objects, scattered by planet migration, producing an “LHB” during which basin-scale impacts occurred in the Earth–Moon system for at least 1600 Ma: “Here we report that the LHB lasted much longer than previously thought, with most late impactors coming from the E belt [producing ] 10 lunar basins between 3.7 and 4.1 Gyr ago [and] 15 terrestrial basins between 2.5 and 3.7 Gyr ago . . . (Bottke et al (2012 [140] p. 78)).”
Summary
In the field of planetary science, a “classic” paradigm (Kuhn, 1962 [1]) of terminal cataclysm was initiated in 1973–1974. We will use the term “classic” to refer to the long-accepted version of lunar (or solar-system-wide) evolution, involving ~400–500 Ma of relatively low-impact cratering, followed by ~170 Ma-long “late heavy bombardment” that included formation of many of the large lunar basins, around 3.9 Ga ago, followed by a declining rate ever since. A stronger, simultaneous suggestion along the same lines was made in 1973 by a second group in England (Turner and Cadogan 1973 [5]): The 40Ar–39Ar ages of samples from the lunar highlands clustered in the interval 3.88 to 4.05 aeons and were taken to indicate that at least three and probably six of the major lunar basins were formed by impact in this period Whether these impacts represent the final stages of a continuous accretion process for the Moon or whether they represent an episodic burst of bombardment is not yet clear. Turner (1977 [27]), for example, correctly noted that, “What is not yet clear is whether this ‘cataclysm’ represents the effective termination of an approximately monotonic decrease in the early cratering or . . . a period of increased influx to close to 4.0 Ga.”
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