New Perspectives on the Lunar Cataclysm from Pre-4 Ga Impact Melt Breccia and Cratering Density Populations

摘要: Crystallisation ages of impact melt breccias from the near-side equatorial regions Moon show a pronounced clustering between 3.75 and 3.95 billion years. This age distribution was unexpected produced competing hypotheses for early flux in inner Solar System. In one scenario increased dramatically at ~3.9 Ga. this ‘late heavy bombardment’ several nearside lunar basins formed within relatively brief interval time (200 million years). A late cataclysmic bombardment would have significant implications System dynamics perhaps involving migration outer planets. Alternatively, may declined steadily with small fluctuations since formation Moon. older deposits were destroyed and/or buried by more recent events. Recently, Norman et al. (2007a) measured an absolute 4.20 + 0.07 Ga on Apollo 16 crystalline breccia that they interpret as breccia. is first definitive evidence discrete melt-forming event than 4.0 has been documented sample collection, but significance single cratering history prior to 3.9 difficult assess. genuine gap major events 4.2 constitute strong favouring cataclysm (Turner, 1979) effects megaregolith evolution burial bias sampled rocks needs further clarification (Hartmann, 2003; Chapman al., 2007). As complementary approach we evaluated using crater densities preserved large basins. analysis provides steep stratigraphic sequence changes through depends basins, which are not well established. be strongly supported if South Pole Aitken (SPA) basin, stratigraphically oldest basin Moon, much younger (i.e. ~ 4 Ga). Older assumed SPA (e.g. 4.4 or Ga) produce curves indicating bombardment, weaker cataclysm. The intermediate such Nectaris critical interpreting cataclysm, poorly constrained current data.