An Inquiry about Evidence for the Late Heavy Bombardment

1. An Inquiry about Evidence for the Late Heavy Bombardment Clark R. Chapman & David H. Grinspoon SwRI, Boulder CO Barbara A. Cohen HIGP, Univ. Hawaii, Honolulu 65th Meteoritical Society Meeting (2002) UCLA, Los Angeles, CA Thurs. p.m., 25 July 2002

2. Late Heavy Bombardment… or “terminal cataclysm” After Wilhelms (1987) • Proposed in 1973 by Tera et al. who noted a peak in radiometric ages of lunar samples ~4.0 - 3.8 Ga • Sharply declining basin-formation rate between Imbrium (3.85 Ga) and final basin, Orientale (3.82 Ga) • Few rock ages, and no impact melt ages prior to 3.9 Ga (Nectaris age) ? Implies: short, 50-100 Myr bombard- ment, but minimal basin formation between crustal formation and LHB LHB

3. Proposed Dynamical Origins for LHB • Outer solar system planetesimals from late-forming Uranus/Neptune (Wetherill 1975) • Break-up of large asteroid • Extended tail-end of accretion; remnants from terrestrial planets region • Expulsion of a 5th terrestrial planet (Chambers & Lissauer 2002; Levison 2002) • OSS planetesimals and asteroids perturbed by sudden expulsion of Uranus & Neptune from between Jupiter & Saturn (Levison et al. 2001)

4. Relevance of Impact Melts(Graham Ryder, 1990) • Basin formation produces copious melts (~10% of involved materials) • Smaller craters contribute few melts • Melt formation efficiency increases with crater size • Basins dominate involved materials because of shallow size-distribution • Impact melts are produced more efficiently than rock ages are reset Therefore, age-distribution of impact melts should be robust evidence of basin formation history (given unbiased sampling)

5. What Happened Before Nectaris (i.e. prior to 3.90 to 3.92 Ga)? • Fragmentary geology remains from earlier times. • But 50% of Wilhelms’ “definite” basins pre-date Nectaris (and 70% of all “definite”+“probable”+“possible” ones). • Surprisingly, no impact melts pre-date Nectaris, so none of the earlier basins formed melts… or those melts are somehow “hidden” from being collected! (Even though some pre-Nectarian rocks exist.) • During the long period from crustal solidification until the oldest known basins, there may (or may not) have been a “lull” in basin formation (and thus a cataclysm). • Weak contraints: • Lunar crust is “intact” (depends on size-distribution) • Impactor “contamination” (projectile retention efficiency)

6. “Stonewall” effect (Hartmann, 1975) destroys and pulverizes rocks prior to saturation Grinspoon’s (1989) 2-dimensional models concur No impact melts prior to Nectaris (Ryder, 1990) Lunar crust not pene-trated or pulverized (but constrains only top-heavy size distributions) No enrichment in meteoritic/projectile material (not robust) Debate over “Cataclysm” A Misconception... It Happened! Flux Time

7. Conundrum concerning Impact Melts • No impact melts have been found older than the Nectaris Basin (3.92 Ga) despite the fact that 2/3rds of known basins occurred stratigraphically before Nectaris (Wilhelms, 1987). Where are their impact melts? • Cohen et al. (2000) found melt clasts from 3.9 Ga extending all the way to 2.8 Ga (only 2 of 7 melt-producing “events” occurred back during the LHB). Thus, many impact melts are found dating from more recent times when we know that basins weren’t forming. • Numerous early basins yield no melts; recent, in-efficient melt-production by small craters yields melts!? There is only one Conclusion: Collected impactmelts are strongly biased to recent events...

8. Lunar, HED Rock Degassing Ages The LHB, as defined by basin ages, is a narrow range (100 Myr LHB shown by pink box). Predominant lunar rock ages range from 3.6 to 4.2 Ga. (Impact melts are restricted to <3.92 Ga.) So rock ages correlate poorly with basin ages. (HED meteorite ages range from 3.2 to 4.3 Ga. So bombardment in the asteroid belt extended ~300 Myr after end of lunar rock degassings.) [Data summarized by Bogard (1995)] Moon HED Parent Body Time 3.3 4.4

9. Non-Lunar Evidence for LHB • Cratered uplands on Mars/Mercury (and even Galilean satellites!) inferred to be same LHB… but absolute chronology is poorly known or unknown. • ALH84001 has a ~4 Ga resetting age… but that is “statistics of one”. • Peaks in resetting ages noted for some types of meteorites (HEDs, ordinary chondrites)… but age distributions differ from lunar case.

10. Asteroidal vs. Lunar LHB LHB Lunar rock de-gassing ages • Kring & Cohen (2002) summary of meteorite de-gassing ages • Very “spread out” compared with lunar LHB • Somewhat “spread out” compared with lunar rock impact degassing ages • Evidence is dissimilar! • Different impact histories, or • Different selection biases

11. A New Look at the “Stonewall” • Saturation of megaregolith would have pulverized/destroyed early rocks (Hartmann, 1975), creating artificial rock-age spike. • but “it is patently not the case” that all rocks would have been reset or “pulverized to fine powder” (Hartmann et al., 2000) • Grinspoon’s (1989) mathematical model seemed to verify the stonewall effect. • but it is a 2-D model; he converts 100% of crater floor to melt while the real percent is much less • However, if melt preferentially veneers surface, as is generally expected to be true, then the 2-D model may approximate the 3-D reality.

12. We Need to Model the 3-D Emplacement/Collection of Melts • Model needs: (building on work by L. Haskin and students) • %-tage melt production as function of diameter • 3-D mapping of emplacement of melts and other ejecta • time-history of megaregolith excavation, deposition, and “churning”, varying the impactor size-distribution • gardening/impact destruction near surface over last ~3.5 Gyr • analysis of collection/selection criteria and biases • Some qualitative sampling biases are clear: • if each new basin distributes its melts uniformly throughout the volume of the megaregolith, and churns earlier melts uniformly, then impact melts collected at the surface should sample the basin formation history in an unbiased fashion. • If each new basin distributes melts in a surface veneer, and older melts are covered by ejecta blankets, then surface sampling will be dominated by most recent basin.

13. LHB Conclusions But how robust is THIS chronology??? • If lunar basin formation sharply declined from 3.85 Ga (Imbrium) to ~3.82 Ga (Orientale, the very last one), then dynamics of LHB source bodies are strongly constrained. • Until the processes that cause sampling bias for impact melts are understood (3-D models), absence of melts from ancient times provides a minimal constraint on the pre-Nectaris bombardment rate. • Hence, whether LHB was a “cataclysm” or just an inflection in a declining flux remains unknown. • Mismatch in lunar/asteroidal age histograms means (a) different LHBs or (b) different sampling biases. We can’t conclude anything about (a) until (b) is understood.