Tuesday, August 28, 2012

A Sawtooth-like Timeline for the First Billion Years of Lunar Bombardment

A revised timeline of lunar bombardment, featuring greater granularity and matching new assumptions. (Figure 3, Morbidelli, Marchi, Bottke & Kring, 2012) [ arXiv:1208.4624v1 ].
Morbidelli, Marchi, Bottke & Kring
OCA, Nice
Center for Lunar Origin & Evolution, SwRI, Boulder
Center for Lunar Science & Exploration, USRA -
Lunar and Planetary Institute, Houston

We revisit the early evolution of the Moon's bombardment.

Our work combines modeling (based on plausible projectile sources and their dynamical decay rates) with constraints from the lunar crater record, radiometric ages of the youngest lunar basins and the abundance of highly siderophile elements in the lunar crust and mantle. 

The new profile for lunar bombardment
matches a population of 150 basins since
lunar formation
[Frey (2009)].
We deduce that the evolution of the impact flux did not decline exponentially over the first billion years of lunar history, and no prominent and narrow impact spike 3.9 billion years ago, unlike that typically envisioned in the lunar cataclysm scenario. Instead, we show the timeline of the lunar bombardment has a sawtooth-like profile with an uptick in the impact  flux near 4.1 billion years ago. The impact flux at the beginning of this weaker cataclysm was 5-10 times higher than during the immediately preceding period.

The Nectaris basin should have been one of the first basins formed at the sawtooth. We predict the bombardment rate since 4.1 billion years ago declined slowly and adhered relatively close to classic crater chronology models (
Neukum and Ivanov (1994)). Overall we expect the sawtooth event accounted for about 1/4 the total bombardment suffered by the Moon since its formation. Consequently, considering that 12-14 basins formed during the sawtooth event, we expect the net number of basins formed on the Moon was 45-50.

From our expected bombardment timeline, we derived a new and improved lunar chronology suitable for use on Pre-Nectarian surface units. According to this chronology, a significant portion of the oldest lunar cratered terrains have an age of 4.38 - 4.42 billion years. Moreover, the largest lunar basin, South Pole-Aitken, is older than 4.3 billion years and was therefore not produced during the lunar cataclysm.

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