1. What are the Astrobiological Constraints from What is Known about the Late Heavy Bombardment? What are the Astrobiological Constraints from What is Known about the Late Heavy Bombardment? Clark R. Chapman Southwest Research Institute Boulder CO NAI General Meeting 2003 Tempe, Arizona 12 February 2003

2. Late Heavy Bombardment… or “terminal cataclysm” 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 After Wilhelms (1987) ? LHB

3. Debate over “Cataclysm” “Stonewall” effect (Hartmann 1975) destroys and pulverizes rocks prior to saturation Grinspoon’s (1989) two- 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) A Misconception vs.It Happened! Time Flux “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, 2002] ?

4. 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.

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

6. Proposed Dynamical Origins for LHB Outer solar system planetesimals from late-forming Uranus/Neptune (Wetherill 1975) Break-up of large asteroid (but big enough asteroids 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 & 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.

7. Qualitative Features of LHBs 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. Life would be deva- stated at the end of the 100 Myr. What does it take to sterilize planet Earth??? K-T

8. 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, compound global 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.

9. LHB Issues for Solar System Astrobiology Lunar evidence on LHB is less well understood than commonly believed. It must be re-evaluated: it is our baseline! How widespread 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: Earth is likely to suffer another basin-forming impact (not soon!); what else could be in our future? How would early evolving life on Mars or Europa have been affected? Earth’s complex life in the future?

10. 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 bombardments 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? How do LHBs compete with other cosmic dangers to life in different stellar/galactic environments?