The SLAM Impact Experiment: Overview and Preliminary Thoughts

1. The SLAM Impact Experiment: Overview and Preliminary Thoughts Clark R. Chapman Southwest Research Institute Boulder CO SLAM Organizational Meeting SwRI Boulder 14 November 2005

2. Big Issues (for us and for our potential reviewers) Basically, is this (or can we make this) vitally exciting science? • How well do we understand lunar cratering? (scaling, regolith, ejecta, secondary cratering…) • How would the SLAM impact/s compare with other cratering experiments/ simulations? (nuclear explosion craters, Ames gun, Deep Impact…) • How unique and relevant is the experiment? (icy projectile, low-v, multiple impacts…)

3. Some Questions to Ponder…(1) • Where should impact/s be targeted? • Mare vs highlands • Equatorial vs high latitude • Dark side, near limb, near pole (visibility issues) • Near previously characterized site? • Uniformity within error ellipse? • What do we expect to see? • Will there be a flash? Can LRO/HST/Earth-based observers see it? Must it be against dark side? What temperature (hence wavelength) do we expect? • Analogous questions concerning ejecta plume • What range of crater sizes do we expect to see? (~10 m) • What is maximum LRO resolution? (How well can we characterize ejecta fragments, secondaries, crater structure, modification of pre-existing features?)

4. Some Questions to Ponder…(2) • What impact velocities are possible/desirable? • What are requirements of volatile transport experiment? • Are velocities sub-hypervelocity? • What real phenomena will low-velocity impacts simulate? • Will this experiment tell us more about formation of secondary craters than primary craters? • Are velocities too small to generate a visible flash? • Will be be unable to penetrate even thin mare regolith? • Is there any way to increase impact velocity? • What impact angle is desirable? • 45 deg. Is most “typical”; vertical impact has been studied most often; very oblique impacts are interesting • What is trade-off between one large impactor and multiple smaller impactors?

5. Some Questions to Ponder…(3) • What did we learn from the Deep Impact experience that informs the SLAM impact expt.? • How useful is the possibility for varying impactor types (e.g. solid ice, rubble pile, metal)? • What is the potential scope of “SLAM Watch”? • Can amateurs see something, or only the biggest telescopes (I seem to remember that nobody saw the Rangers impact on the Moon, even using large telescopes, though Leonid flashes have been seen) • Earth-orbiting observatories: will we be competing with ourselves? • Potential LRO integration: can we assume it?

6. 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, no impact melt ages prior to 3.9 Ga (prob. Nectaris age) ? (Cumulative) Crater Density Implies: short, 50-100 Myr bombard- ment, but minimal basin formation between crustal formation and LHB LHB

7. “Stonewall” effect (Hartmann 1975, 2003) destroys and pulverizes rocks prior to saturation Grinspoon’s (1989) two-dimensional models concur No impact melts prior to 3.9 Ga (Ryder 1990, 2002) Lunar crust not pene-trated or pulverized (but constrains only top-heavy size distributions) No enrichment in meteoritic/projectile material (not robust: projectile material prefer-entially ejected) Debate over “Cataclysm” A Misconception vs. It Happened! “Tail-end” of accretion Post-crust, pre-spike lull defines LHB (Mostly) uncontroversial sharp decline in bombardment rate from 3.90 Ga to 3.83 Ga Further confusion on LHB decay: >Basin formation decayed in 50 Myr >Rocks degassed over 200 Myr >Impact melts decayed over 1000 Myr[Chapman, Cohen & Grinspoon, 2003] Flux ? Time

8. 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)

9. What Happened Before Nectaris (i.e. before 3.90 - 3.92 [4.1?] 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 the Nectaris Basin, 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 was (or was not) a “lull” in basin formation (and thus a cataclysm). • Weak contraints (listed before): • Lunar crust intact • Minimal meteoritic contamination

10. Conundrum Concerning Impact Melts: Do they Reflect Impact Flux? • No impact melts have been found securely older than Nectaris (3.92 Ga) although 2/3rds of known basins occurred stratigraphically before Nectaris (Wilhelms, 1987). Where are their impact melts? • Cohen et al. (2000) found tiny melt clasts from 4.0 Ga extending all the way to 2.4 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; yet more recent, inefficient melt-production by small craters does yield melts!? LHB There is only one Conclusion: Collected impactmelts are strongly biased to recent events...

11. 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 (Vesta?) Time 3.3 4.4

12. 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

13. A New Look at the “Stonewall” • Saturation by 30-100 km craters would have pulverized/destroyed early melt-rocks (Hartmann, 1975, 2003), 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 [presumably one of his co-authors]) • comminution by a couple generations of large-crater saturation is NOT like modern churning of uppermost meters of regolith • 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 (volumetrically) is much less • However, if melt preferentially veneers surface, as much of it does, and older veneers are covered up, then the 2-D model may approximate the 3-D reality.

14. Size Distributions: Values of Differential Power-Law Index b

15. Crater Production Function: Areal and Volumetric Implications Standard Function from Neukum & Ivanov (1994) b= -4: equal mass b= -3: equal area, saturation equilibrium   .: ::. :..: • Crater size distribution is not a simple power-law • Areal saturation is dominated by • craters 100 meters to 2 km diameter (surficial regolith) • craters 30 km to 100 km diameter (which penetrate down kilometers) • Volumetric processing is dominated by largest craters/basins • “Steep” size distribution for <1 km craters churns/comminutes upper few meters of lunar soil (particle sizes <100 microns)   .: ::.    .: ::.    .: ::.   .: ::.

16. 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.

17. Conclusions about Lunar Evidence for LHB 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.

18. Non-Lunar Evidence for LHB • Cratered uplands on Mars/Mercury (and even Galilean satellites!) inferred to be due to 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.

19. Remnant Planetesimals: Comets, Asteroids, Trojans, etc. Accretion of planets from planetesimals necessarily results in diverse groups of circumstellar and circumplanetary small bodies, subject to temporary confinement among dynamical resonances Jupiter’s orbit We are here! Trojans Asteroid belt NEOs Sun Comets & OSS planetesimals

20. Proposed Dynamical Origins for LHB Size Distributions *Accretional *Collisional *Tidal disruption • Outer solar system planetesimals from late-forming Uranus/Neptune (Wetherill 1975) • Break-up of large asteroid (but a big enough asteroid is difficult to destroy) • Extended tail-end of accretion; remnants from terrestrial planets region (Morbidelli 2001) • 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) • Late-stage post Moon-formation Earth/Moon-specific LHB (Ryder 1990) More generally: any dynamical readjustment of the planets in a planetary system that “shakes up” (e.g. by changing positions of resonances) remnant small-body populations…could occur late, even very late.

21. Qualitative Features of LHBs (divide by 3 if Nectaris is 4.1 Ga) K-T • On Earth, 1 “Chicxulub” (K-T boundary event, 100 million MT) every 10,000 years. • Each kills virtually every complex lifeform, most fossilizable species go extinct, radiation of many new species • One basin-forming event (10 billion MT!) every 500,000 years. • Each erodes atmosphere, transforms ecosphere, boils oceans • Total LHB: ~100 basins, 1000s of K-T events. The 100 Myr bom-bardment would devastate life. What does it take to sterilize planet Earth???

22. Why Giant Impacts are Especially Lethal • Environmental changes are nearly instantaneous! (Most lethal, global effects occur in a couple of hours to a month or so.) • Very short compared with the lifetime of an individual; most competing mass-extinction theories invoke changes over 1000s to millions of years. • Independent, compoundglobal effects (firestorm, ozone layer destroyed, tsunami, earthquake, oceans poisoned, “impact winter” followed by global warming, etc.) atmosphere surface/ocean crust mantle Impacts dominate or destroy the atmos-phere, dramatically affect the surface and oceans, but their effects may not fully involve the crust and rarely the upper mantle.

23. LHB Issues for Solar System Astrobiology • Lunar evidence on an LHB is less well understood than commonly believed. • It must be re-evaluated: it is our baseline! • How widespread in the solar system was this lunar LHB? • Which small-body reservoirs/dynamical readjustments were responsible? • Were other reservoirs/causes responsible for earlier bombardments, or for the cratered terrains and basins on other planets/satellites/asteroids? • The future: the Earth is likely to suffer another basin-forming impact (not soon!); what else could be in our future? How was early evolving life on Mars or Europa affected? How will Earth’s complex life be affected in the future?

24. LHB Issues for Extra-Solar System Astrobiology • It is plausible that similar, or even much more extreme, LHBs (or VLHBs) would affect planets in other systems. • What planetary system configurations are most likely to result in small-body reservoirs and unstable dynamics that would cause LHBs? • Are LHB/VLHB reservoirs astronomically observable (directly or indirectly)? • What range of bombardment traits foster life (exchanging materials, spurring evolutionary change)? • How frequent would giant impacts have to be to perpetually frustrate the origin or evolutionary progression of life? • How big an LHB surely sterilizes a planet? • What about “attic” storage and reseeding? • How do LHBs compete with other cosmic dangers to life in different stellar/galactic environments?

25. Conclusions • Are Late Heavy Bombardments plausible during the early histories of planets? YES! • Could LHBs profoundly change planetary environments and the origin and evolution of life? YES! • Was there a “cataclysm” in the Earth/Moon system around 3.9 Ga? The bombardment rate was high, but was it a spike? We still DON’T KNOW!