PRIMORDIAL SURFACE FORMATION AND THE ORIGINS OF LIFE

During the first 600 My of Earth-moon history, >30 times as many impact craters formed, at all scales, as formed in the 3.9 Ga since. This is known from dating of lunar Apollo and Luna rock samples. According to accretion theory, the highest impact rates would have been at the close of planet formation. Another "cataclysmic" burst of solar-system-wide cratering has been postulated at 3.9 Gy ago, though the lunar and asteroidal meteorite data are not consistent on this point.

These data constrain geomorphologic and environmental conditions on the early Earth. Earth’s early solid crust, as on the moon, was probably heavily brecciated and partly pulverized into "mega-regolith" to depths of several kilometers until plate tectonic crustal processing was established. Multi-ring impact basins of some 600-1000 km diameter were forming on timescales < the 50-100 Ma timescale for current plate tectonic crustal disruption. Thus, dominant early "mountain" landforms may not have been upthrust chains at plate collision sites, but arcuate, shocked rims of giant crater structures. Some of the initial low density "continental" blocks of crustal material may have been stochastic pile-ups of ejected rim material, rather than products of plate tectonic crumpling. The earliest ocean basins were probably impact basins.

These factors impacted, literally, life’s origins. Here, the controversy over cataclysm 3.9 is crucial. Ryder (1990, EOS 71, 312), Stöffler and Ryder (2001, Spa. Sci. Rev. 96, 9), and others hypothesized that most lunar multi-ring basins formed in a ~150 Ma period at 3.9 Ga, with low impact rates before that, in which case the early crust structure and any pre-3.9 Ga biology would likely have been virtually wiped out and "reset" 3.9 Ga ago, due to ocean vaporization and other profound impact effects. If the lunar 3.9 Ga peak in impact melts is primarily due to the Imbrium/Orientale impacts, however, then big impacts were more spread out in time from 4.4 to 3.9 Ga ago, and life on Earth probably underwent a number of global "resets," if not complete restarts.

Until we understand the solar system impact environment in the first 600 Ma, we don’t fully understand the origin of life or the evolution of early planetary surfaces and crusts.