12:00 PM - 1:00 PM
The concept of a Late Heavy Bombardment (LHB) – an episode of intense impacts into the Earth-Moon system at ~3.9 Ga – has been a bedrock assumption in planetary science for over 40 years. Emerging from the Apollo-era, the hypothesis found support in histograms of 40Ar/39Ar dates of lunar rocks. Subsequently these age histograms have been used as 1) the basis of a series of dynamical simulations of the solar system (the “Nice” models) in which giant planet migrations unleash massive amounts of asteroidal material into the inner solar system, and 2) a widely held notion that this episode would have sterilized the planet. However, the rocks returned from the Apollo missions sample less than 4% of the Moon’s surface while lunar meteorites (which sample both the near- and farside of the Moon) don’t show any such age spike. Since the lunar crust formed in a relatively brief interval (between about 4.3 to 4.5 Ga), subsequent impacts onto that surface would systematically reduce K-Ar ages potentially yielding apparent spikes where none exist. A numerical model that accounts for the diffusive loss of the weakly held daughter product argon in response to impact heating shows that apparent age peaks are a robust feature of simulations in which impact intensity decreases exponentially over time. That is, the nature of the evidence used to define the Late Heavy Bombardment has an intrinsic tendency to create apparent, but illusory, age spikes that possess an irresistible lure to be interpreted in terms of bombardment history. Does that mean the LHB didn't occur? We actually may have been right for the wrong reason. Several lines of evidence drawn from Hadean terrestrial zircons suggest the interval 3.84-3.92 Ga was a period of unusually intense thermal activity at or near Earth's surface leaving open the possibility that the classically defined LHB may have actually occurred but that we've been justifying its existence using information obtained from the wrong planetary body.