Accretion to Earth and Moon ∼3.85 Ga

Abstract

The lunar record provides a means of estimating post-accretionary impact influences on the early Earth and a comparison with what we can more directly infer from the Earth's own rocks. In the period from about 3.9 to about 3.8 Ga there was considerable impacting on the Moon with evidence for concurrent marine sedimentation on Earth. This was a time interval in which the impactor flux to the lunar surface was orders of magnitude greater than the present rate; massive impacts as well as an enhanced background flux of extraterrestrial matter should have likewise affected the surface of early Earth. As yet, no unequivocal evidence for a "Late Heavy Bombardment of the Earth" has been established from direct measurement of terrestrial crustal rocks. The lack of a signal of the bombardment on Earth may be due to the fact that: (i) the oldest sediments do not overlap with the bombardment era, (ii) the bombardment was less intense than previously thought, (iii) sedimentation was extremely rapid so as to dilute any extraterrestrial signal, (iv) sample volumes yet analyzed undersample extra-terrestrial input, or (v) windows for sampling elevated fluxes of extraterrestrial bodies were very short between long periods of relative calm. Further extensive studies of the oldest known sediments will be needed to resolve the issues raised in this paper.

Could the Lunar “Late Heavy Bombardment” Have Been Triggered by the Formation of Uranus and Neptune?

“…Although no direct record of early impacts remains on the Earth, the lunar record is better preserved (Sleep et al 1989, Chyba 1991, Zahnle and Sleep 1997, Hartmann et al 2000, Mojzsis and Ryder 2001. We consider three arguments that constrain the mass of material impacting the Moon between the solidification of the lunar crust some 4.4 Gyr ago Lugmair 1979, 1988) and the end of the LHB about 3.8 Gyr ago.…”

Impact bombardment chronology of the terrestrial planets from 4.5 Ga to 3.5 Ga

“…Current estimates for the age of Imbrium basin hover around 3.95 Ga based on interpretations of U-Pb and Ar ages of lunar rocks (e.g. Tera et al 1974;Schäffer & Husain, 1974;Maurer et al, 1978;Mojzsis & Ryder, 2001;Shearer & Borg 2006). Crater counts near Imbrium basin performed by Head et al (2010) show that N 20 =0.032×10 -3 km -2 .…”

“…The decline of the leftover planetesimals is so rapid that >99.5% of the craters on the Moon resulting from the leftover population were created by 3.95 Ga, the suggested date of the formation of the Imbrium basin (e.g. Mojzsis & Ryder, 2001). The impact rate of E-belt objects on the Moon is much slower, however, which occurs with an e-folding time of 176 Myr.…”

Thermal events documented in Hadean zircons by ion microprobe depth profiles

“…Furthermore, it has been long proposed that a lunar cataclysm/late heavy bombardment epoch of the Moon and Earth (Tera et al, 1974;cf. Hartmann, 2003) centered at $3.9 Ga had either deleterious (Maher and Stevenson, 1988) or benign (Cohen et al, 2000;Mojzsis and Harrison, 2000, Mojzsis and Ryder, 2001, Ryder, 2002 consequences for an emergent biosphere (Chyba, 1993;Mojzsis et al, 1996;Cohen et al, 2000;Anbar et al, 2001). It is widely promulgated, though unsubstantiated by data, that the preservation of early crust and the survival of incipient life were precluded by the energetics of the bombardment epoch (Maher and Stevenson, 1988;Hamilton, 1998;Moorbath, 2005).…”