The origin of life on Earth is commonly considered to have been negatively affected by intense impacting in the Hadean, with the potential for the repeated evaporation and sterilization of any ocean. The impact flux is based on scaling from the lunar crater density record, but that record has no tie to any absolute age determination for any identified stratigraphic unit older than ~3.9 Ga (Nectaris basin). The flux can be described in terms of mass accretion [Hartmann, 1980], and various independent means can be used to estimate the mass flux in different intervals. The critical interval is that between the end of essential crustal formation (~4.4 Ga) and the oldest mare times (~3.8 Ga). The masses of the basin-forming projectiles during Nectarian and early Imbrian times, when the last 15 of the ~45 identified impact basins formed, can be reasonably estimated as minima. These in sum provide a minimum of 2 × 1021 g for the mass flux to the Moon during those times. If the interval was 80 million years (Nectaris 3.90 Ga, Orientale 3.82 Ga), then the flux was ~2 × 1013 g/yr over this period. This is higher by more than an order of magnitude than a flux curve that declines continuously and uniformly from lunar accretion to the rate inferred for the older mare plains. This rate cannot be extrapolated back increasingly into pre-Nectarian times, because the Moon would have added masses far in excess of itself in post-crust-formation time. Thus this episode was a distinct and cataclysmic set of events. There are ~30 pre-Nectarian basins, and they were probably part of the same cataclysm (starting at ~4.0 Ga?) because the crust is fairly intact, the meteoritic contamination of the pre-Nectarian crust is very low, impact melt rocks older than 3.92 Ga are virtually unknown, and ancient volcanic and plutonic rocks have survived this interval. The accretionary flux from ~4.4 to ~4.0 Ga was comparatively benign. When scaled to Earth, even the late cataclysm does not produce ocean-evaporating, globally sterilizing events. The rooted concept that such events took place is based on the extrapolation of a nonexistent lunar record to the Hadean. The Earth from ~4.4 to ~3.8 Ga was comparatively peaceful, and the impacting itself could have been thermally and hydrothermally beneficial. The origin of life could have taken place at any time between 4.4 and 3.85 Ga, given the current impact constraints, and there is no justification for the claim that life originated (or reoriginated) as late as 3.85 Ga in response to the end of hostile impact conditions.
Journal of Geophysical Research (Planets)
- Pub Date:
- April 2002
- Planetary Sciences: Impact phenomena (includes cratering);
- Planetary Sciences: Origin and evolution;
- Planetary Sciences: Surface materials and properties;
- Planetary Sciences: General or miscellaneous