THE LATE HEAVY BOMBARDMENT: WHAT IS THE SOURCE OF THE IMPACTORS?

The concept of a “Terminal Lunar Cataclysm” (Tera et al., 1974, EPSL 22:1) and the later terminology, a “Late Heavy Bombardment” (Wasserberg et al., 1977, Phil. Trans., RSL, 285:7) is a current paradigm in the lunar and planetary sciences. Some of the promoters of this paradigm are attempting to extend it to all parts of the inner part of the Solar System (e.g., Kring and Cohen, 2002, JGR, 107:6) as well as to the outer solar system (Charnoz et al. 2009, Icarus, 109:413; Matter et al., 2009, PSS, 57:816).

Some facts that need to be explained by a successful model. (1) Patterns of lunar rock magnetization associated with lunar mare rocks and lunar breccias strongly suggest that a magnetic field was operating probably within the Moon and in an exponentially decaying mode from ~3.95 to 3.6 Ga (Cisowski et al., 1983, JGR, 88:A691). (2) Many of the dates for either mare basin formation and/or mare basalt fill occurred over a time range of about 3.8-4.0 Ga (Wilhelms 1987, USGS PP-1348). Dates of solidification of many of the older breccias and glasses in breccias occurred over a short period of time in the range of 3.85-3.95 Ga (Dalrymple and Ryder, 1993, JGR, 98:13085 and 1996, JGR, 101:26069). (4) Oxygen isotope information from zircon crystals from Western Australia and other geochemical evidence relating to the recycling of ancient “enriched” crust into the Earth’s mantle indicate that the Earth was cool enough to have ocean water on the surface as early as ~4.4 Ga ago (Valley et al., 2002, Geology, 30:351) and that this ancient crust commenced to be recycled into the mantle of Earth ~3.95 Ga ago (Harrison et al. 2009, Ann. Rev. EPS, 37:479).

Most models for mare basin formation and for the Late Heavy Bombardment can be placed into four categories. (1) Random impact models (e.g., Wilhelms, 1987, USGS PP-1348); (2) Impacts of bodies that were tidally disrupted from nearby planets (e.g., Hartmann, 1977, Icarus, 31:260; Wetherill, 1981, in Schultz and Merrill, eds,, Proc. LPSC 12A:1); (3) Impacts of asteroids caused by rearrangement of the orbits of the outer planets (e.g., Gomes et al., 2005, Nature, 435:466); (4) Impacts of lunar material that was tidally disrupted from the lunar body during a close encounter associated with the capture episode (Malcuit, Springer, Chapter 5).