FIGURE 2.3 Competing models of meteorite-impact rate for the first 2 billion years (Ga) of Earth and Moon history. Note that Earth is believed to have formed about 4.55 Ga before present. Two hypotheses are shown: exponential decay of impact rate (dashes; data from W.K. Hartmann, G. Ryder, L. Dones, and D. Grinspoon, The time-dependent intense bombardment of the primordial Earth/Moon system, pp. 493-512 in Origin of the Earth and Moon (R.M. Canup and K. Righter, eds.), University of Arizona Press, Tucson, 2000), and cool early Earth–late heavy bombardment (solid curve). The approximate half-life is given in million years (m.y.) for periods of exponential decline. The cool early Earth hypothesis (solid curve) suggests that impact rates had dropped precipitously by 4.4 to 4.3 Ga, consistent with clement conditions that were hospitable for life. SOURCE: Courtesy of John Valley; adapted from J.W. Valley, W.H. Peck, E.M. King, and S.A. Wilde, A cool early Earth, Geology 30:351-354, 2002.

exploited the well-documented and curated collection of lunar samples and meteorites, (3) the increase in computational capability, (4) the recognition of meteorites from the Moon, and (5) remote sensing space missions.

The first factor, time, has enabled decades of intense scrutiny of lunar data and materials by a small but dedicated cadre of lunar scientists. This group has explored in great detail the available data and developed the currently known characteristics of the Moon, from its atmosphere to its core. A highly important revelation during this period of contemplation was the formulation of the giant impact hypothesis for lunar origin. The fundamental question of lunar origin persisted beyond Apollo, and prevailing hypotheses all suffered from one or more serious shortcomings. The giant impact hypothesis overcame these problems and meets all known constraints. The new insight was prompted less by working with new data, than by integrating the available data and engaging in

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