The Problem of The Proposed Late Cratering Cataclysm

Details The Problem of The Proposed Late Cratering Cataclysm The Problem of The Proposed Late Cratering Cataclysm

Sep 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009dps....41.5808h&link_type=abstract

American Astronomical Society, DPS meeting #41, #58.08

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Scientific paper

The concept of a "late heavy bombardment" or "terminal cataclysm" is fairly widely accepted in the planetary science community. It originated in the early 1970s, when radiometric dating of Apollo samples showed a cutoff in rocks prior to 3.9 or 4.0 Gy ago. It is usually presented as a period of intense cratering at 3.9 Gy ago, lasting about 150 My, during which many or most of the large multi-ring basins formed. It was strongly supported when Graham Ryder, ca. 1990, found an extreme spike in lunar impact melt rocks at 3.9 Gy ago. Since 2005, it has been tied to the Nice model of planet formation, in which outer solar system planetesimals undergo a cataclysmic scattering throughout the inner solar system.
These concepts have several serious, unresolved problems.
(1) The time profile of impact related dates from asteroidal meteorites does not show the extreme, Ryder-type spike, but a much softer maximum from about 4.1 to 3.6 Gy ago.
(2) The time profile of impact related clasts in KREEP-poor lunar meteorites shows no spike at 3.9 Gy.
(3) There are suggestions that the sharp spike in Apollo impact melt ages at 4.0 to 3.9 Gy ago is associated largely with the Imbrium multi-ring impact basin (and possibly the Orientale basin as well).
(4) The dating of most other lunar multi-ring basins is uncertain, contrary to common opinion.
(5) The Nice model does not predict the date of the scattering event. Parameters can be adjusted so that scattering peaks at pre-chosen times. The fascinating model does not in itself establish the existence of a 150-My-long cataclysm 3.9 Gy ago.
Other impact chronologies may be consistent with the radiometric and dynamical data.
This work was supported in part by NASA Mars Data Analysis grant NNX08AK91G.

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