KEYNOTE SPEAKER: THE TERMINAL LUNAR CATACLYSM & THE LATE HEAVY BOMBARDMENT OF THE INNER SOLAR SYSTEM

The time scale for cratering was established from Apollo 11 at ~3.5 Gy for Imbrium flooding. All Lunar basalts gave ages ranging from 3.0 to 3.8 Gy. Highly recrystallized lunar breccias & impact melts gave ages of 3.8-4.0 Gy, slightly greater (~10%) than the oldest basalts. ⁸⁷Rb-⁸⁷Sr & ⁴⁰Ar/³⁹Ar studies provided most of the data base . Ancient lunar igneous rocks were found which gave reliable ages ~4.5Gy. None were found with ages between 4.4 -4.0 Gy. All breccias & impact melts were found to be extensively isotopically equilibrated between 4.0 -3.8 Gy. Rb-Sr ‘internal isochrons” showed that the cores of crystals preserved a primitive Sr & implied an age of ~4.5 Gy for the breccia. ⁴⁰Ar/³⁹Ar ages on these same crystals yielded two plateaus one at 3.8-4.0 Gy & one at ~4.5 Gy. There was abundant evidence of an early lunar crust that had then been impacted in the interval 3.8-4.0 Gy. “Parentless initial lead” was discovered that impregnated samples with ²⁰⁷Pb^*/²⁰⁶Pb^*=1.45 dating the time of this “paint job” at 3.9 Gy ago. There was thus a barrier of 4.0 Gy for impact ages of breccias & a widespread lead isotopic signature giving the same age. This would require an intense period of closely spaced major events ~0.5 Gy after planet formation, too long for planetary accretion processes. We proposed a “Terminal Lunar Cataclysm”- a late heavy bombardment of the whole inner solar system. Opponents of the LHB hypothesis argued that it was due to inadequate sampling. Subsequent evidence from lunar meteorites, intense studies of many lunar impact melts has shown that the interval of late heavy bombardment was indeed between 3.8-4.0 Gy Studies of highland crater size distributions indicate that the projectiles that struck Mars, Mercury and Moon during earlier epochs were comparable in size to the population in the current asteroidal belt & that the younger craters sampled a population of much smaller projectiles (Strom et al, Science 2005). Dynamical studies show that migration of the major planets would perturb the Asteroid belt. A computer simulation of an initial distribution of planets and an outer planetesimal disk, shows that the system is quiescent , but when Jupiter and Saturn cross a resonance this triggers a dramatic orbital instability occurs. It thus appears that plausible mechanisms exist & the LHB hypothesis may be theoretically justified.