Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008dps....40.5705b&link_type=abstract
American Astronomical Society, DPS meeting #40, #57.05; Bulletin of the American Astronomical Society, Vol. 40, p.502
Astronomy and Astrophysics
Astronomy
Scientific paper
The earliest bombardment history of the Moon is recorded in the observed lunar basins (i.e., D > 300 km diameter craters). The beginning of the basin-forming epoch is unknown, but the oldest basins cannot be older than the 4.46 Ga lunar crust (here defined as t0). The age of the oldest known basin, South Pole-Aitken, is unknown, but the youngest basins, Imbrium and Orientale, formed 3.8 Ga. Of the remaining basins, only Serenitatis has a undisputed age (3.89 Ga). Taking together with data from smaller craters and lunar samples, many groups have tried to piece together what happened to the Moon between t0 and 3.8 Ga. This has led to two dominant scenarios: (i) the basins were produced by a terminal cataclysm that caused a spike in the impactor flux 3.8-4.0 Ga and (ii) the basins came from the tail end of a monotonically-decreasing impactor population created by planet formation processes. Interestingly, there may be room for both scenarios to work at some level. For (i), consider that planetesimals leftover from terrestrial planet formation were likely scattered up to high eccentricities and inclinations by planetary embryos; these objects were long-lived enough that some should have struck the Moon after t0. In addition, scattering events produced by embryos/planets residing in/near the primordial asteroid and comet populations should have also provided some basin-forming impactors. For (ii), the Nice model provides a plausible dynamical mechanism capable of creating a spike of comets/asteroid impactors 3.8-4.0 Ga. A complicating factor in all of this, however, is the unknown initial and collisionally-evolved size distributions of these populations, which may strongly affect the actual impact signature left on the Moon from (i) and (ii). We have a lot of work to do!
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