Measurement of the lunar impact record for the past 3.5 b.y. and implications for the Nemesis theory

Measurements of the ages of 155 lunar spherules from the Apollo 14 site suggest that the solar system impact rate over the past 3.5 b.y. first gradually declined, and then increased starting at 0.4 Ga, back to the level it had been 3 Ga. A possible explanation is offered in terms of the Nemesis theory, which postulated a solar companion star. A sudden change in the orbit of that star at 0.4 Ga transformed a circular orbit (which does not trigger comet showers) into an eccentric orbit (which does). The Nemesis theory is speculative but viable; contrary to prior assertions, the orbit is sufficiently stable to account for the data. Muller, R.A., 2002, Measurement of the lunar impact record for the past 3.5 b.y. and implications for the Nemesis theory, in Koeberl, C., and MacLeod, K.G., eds., Catastrophic Events and Mass Extinctions: Impacts and Beyond: Boulder, Colorado, Geological Society of America Special Paper 356, p. 659–665. *E-mail: muller@physics.berkeley.edu; web site: http://muller.lbl.gov INTRODUCTION A new approach to estimating past impact rates has been developed by our group at Berkeley: rather than measuring the ages of identified craters, the ages of small glass droplets called spherules that are produced from unidentified impact craters are measured (Muller, 1993). Most lunar spherules are formed in impacts; others are primarily from pyroclastic eruptions. From analysis of the spherule ages, it was concluded that most of the spherules come from separate craters (Culler et al., 2000; Muller et al., 2000a). Even though we do not know the crater from which a spherule originates, the distribution of spherule ages gives us information about the rate at which craters were formed. Even 1 g of lunar soil, collected from the Apollo missions, typically contains more than 100 spherules in the size range of 150–250 lm diameter. At the Apollo 14 landing site, the spherules contain sufficient potassium to allow Ar/Ar measurement of their age to a mean accuracy of 221 m.y.; half of the spherules had age uncertainties 132 m.y. Larger spherules, which have higher total potassium contents, yielded the most accurate ages. The original goal of the experiment was to test the Nemesis theory (Davis et al., 1984), which predicts that the impact rate will have a large component from periodic comet showers, events in which multiple impacts occur within short (1–2 m.y.) time spans. Moreover, the theory predicts that the comet showers will recur whenever the cause (Nemesis, the hypothetical companion star to the Sun) reaches perihelion, which is once per orbit. The spacing between comet showers would be regular, with a 26–28 m.y. spacing, matching the period seen in paleontological extinctions; however, due to perturbations in the orbit from passing stars, there are expected variations of several million years in the timing of the showers, as pointed out by Davis et al. (1984). Herein I review the status of the Nemesis theory, including the critical issue of the stability of the Nemesis orbit. In March 2000, Culler et al. (2000) reported the measurement of the chemical composition and ages of 155 spherules from the Apollo 14 site. Based on the belief that the soil sample at our site was well mixed, we (Culler et al., 2000) argued that the observed spherule age distribution reflects the cratering rate on the surface of the Moon and the Earth. Additional aspects of our study of these issues are discussed in this chapter. The spherule age distribution is shown in Figure 1. At the bottom of the plot are 155 Gaussian curves, one for each