Untitled Document

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

Marc Norman1,3 , Larry Nyquist4, and Charles H. Lineweaver2

1Research School of Earth Sciences, Australian National University, Canberra ACT 0200 Australia  
2Planetary Science Institute of the Research School of Earth Sciences and the Research School of Astronomy and Astrophysics, Australian National University, Canberra ACT 0200 Australia
3 Lunar and Planetary Science Institute, 3600 Bay Area Boulevard, Houston TX 77058 USA
4NASA Johnson Space Center, Houston TX 77058 USA

Crystallisation ages of impact melt breccias from the near-side equatorial regions of the Moon show a pronounced clustering between 3.75 and 3.95 billion years. This age distribution was unexpected and produced competing hypotheses for the early impact flux in the inner Solar System. In one scenario the impact flux spiked dramatically about 4 billion years ago. In this 'late heavy bombardment' scenario several of the nearside lunar basins formed within a relatively brief interval of time. A late cataclysmic bombardment would have significant implications for Solar System dynamics perhaps involving migration of the outer planets. Alternatively, the impact flux may have declined steadily with relatively small fluctuations since formation of the Moon.  In this scenario older impact deposits were destroyed and/or buried by more recent events.

Recently, we measured an absolute age of 4.20 + 0.07 Ga on the Apollo 16 crystalline breccia 67955, which we interpret as an impact melt breccia (Figs. 1, 2). This is the first definitive evidence for a discrete melt-forming impact event older than 4.0 Ga so far documented from the Apollo lunar sample collection, but the significance of a single sample for defining the lunar impact cratering history prior to 3.9 Ga is difficult to assess. Monte Carlo models suggest that a genuine gap in major impact events between 4.2 and 3.9 Ga would constitute strong evidence favouring a late cataclysm (Turner, 1979) but the effects of megaregolith evolution and burial bias on the age distribution of sampled impact melt rocks needs further clarification (Hartmann, 2003; Chapman et al., 2007).

As a complementary approach we evaluated the long-term lunar impact flux using crater densities preserved within large basins. This analysis provides strong evidence for a steep cratering flux early in the stratigraphic sequence of lunar basins but the implications for changes in the cratering flux through time depends on the absolute ages of lunar basins, which are not well established. A late cataclysm would be strongly supported if the South Pole-Aitken (SPA) basin, stratigraphically the oldest basin on the Moon, has an absolute age not much older than the younger basins (i.e. ~ 4 Ga).  Older assumed ages for SPA (e.g. 4.4 Ga or 4.2 Ga) produce cratering flux curves indicating an early heavy bombardment, and weaker evidence for a late cataclysm. However, the absolute age of stratigraphically intermediate basins such as Nectaris play a dominant role in interpretions of the cratering density curves for a late cataclysm.

Figure 1.  Thin section photomicrograph of lunar sample 67955.

Figure 2.  Sm-Nd mineral isochron age of lunar sample 67955 (from Norman et al., 2007)

 


Chapman C.R., Cohen B.A., Grinspoon D.H. (2007) What are the real constraints on the existence and magnitude of the late heavy bombardment? Icarus 189:233-245
Hartmann W.K. (2003) Megaregolith evolution and cratering cataclysm models - Lunar cataclysm as a misconception (28 years later).  Meteoritics & Planetary Science 38:579-593
Norman M.D., Shih C.-Y., Nyquist L.E., Bogard,D.D, and Taylor, L.A. (2007) Early impacts on the Moon: crystallization ages of Apollo 16 melt breccias. Lunar and Planetary Science Conference 38, abstract #1991. The Lunar and Planetary Institute, Houston
Turner G. (1979) A Monte Carlo fragmentation model for the production of meteorites: implications for gas retention ages. Proceedings of the 10th Lunar and Planetary Science Conference:1917-1941