KEYNOTE SPEAKER: DEFINING THE HADEAN FROM THE LUNAR IMPACT RECORD
During the formation of the lunar and terrestrial mega-regolith, a few very large basins formed on the Moon. Most workers recognize the youngest and most obvious of these basins, South Pole-Aitken (~2500 km). Evidence exists, however, for older basins of comparable or greater size, the largest being Procellarum (~3200 km). South Pole-Aitken appears to be about 4.2 Gyr as its geometry remains clearly definable and not destroyed by the saturation cratering. The less distinct Procellarum would have formed earlier than South Pole-Aitken, possibly about 4.3 billion years ago, as the on-going saturation cratering, subsequent large basin impacts, and isostatic adjustment have significantly modified its geometry.
The 4.2 ±0.1 Gyr boundary identified above deserves to be a placeholder for the separation of two Hadean periods of great planetary importance and contrast. Prior to ~4.2 Gyr, the seeds of the first continents probably formed by the fractional crystallization of thick, water-rich impact melt sheets formed in very large, continental scale impact basins comparable to South Pole-Aitken and Procellarum. 4.4 Gyr original crystallization of ancient detrital zircon on Earth in the presence of water strongly supports this conclusion.
Subsequent to ~4.2 Gyr, saturation cratering and the formation of continental-scale basins had ceased and a period of smaller but still large basin forming impacts began. The youngest of ~50 such basins (300-1000 km in diameter) formed at 3.8 ±0.1 Gyr. This solidly based boundary on the Moon should be a strong candidate to mark the end of the Hadean on Earth as replicating life made its appearance not long afterwards. Renewed lunar field exploration would add significant definition to the Hadean across the terrestrial planets.