What Are the Real Constraints on the Existence and Magnitude of the Late Heavy Bombardment?
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This article was originally published in a journal published by Elsevier, and the attached copy is provided by Elsevier for the author’s benefit and for the benefit of the author’s institution, for non-commercial research and educational use including without limitation use in instruction at your institution, sending it to specific colleagues that you know, and providing a copy to your institution’s administrator. All other uses, reproduction and distribution, including without limitation commercial reprints, selling or licensing copies or access, or posting on open internet sites, your personal or institution’s website or repository, are prohibited. For exceptions, permission may be sought for such use through Elsevier’s permissions site at: http://www.elsevier.com/locate/permissionusematerial Icarus 189 (2007) 233–245 www.elsevier.com/locate/icarus What are the real constraints on the existence and magnitude of the late heavy bombardment? Clark R. Chapman a,∗, Barbara A. Cohen b, David H. Grinspoon c a Southwest Research Institute, Suite 400, 1050 Walnut St., Boulder, CO 80302, USA b Institute of Meteoritics, University of New Mexico, Albuquerque, NM 87131, USA c Denver Museum of Nature and Science, 2001 Colorado Blvd., Denver, CO 80205, USA Received 30 December 2003; revised 14 December 2006 Available online 24 January 2007 Abstract Geochronological evidence for and against a late heavy bombardment (LHB) spike in impact rates on the Moon and meteorite parent bodies is re-examined. In particular, we find that the sampling of impact melts on the Moon is strongly biased against older examples, possibly due to preferential surface deposition of such melts and/or blanketing and burial by basin ejecta (arguments that the bias might be due to pulverization of old impact melts at depth are incorrect). The apparent absence of melts older than 4 Ga thus provides much weaker evidence for a pre-nectarian lull in bombardment (which would define a post-nectarian cataclysm) than has been argued, although their absence compared with other rocks from depth may remain a weak constraint. Differences in rock-age histograms between meteorite parent bodies and the Moon may imply that different sampling biases exist for bodies in the asteroid belt; otherwise, the more straightforward interpretation is that the bombardment that affected the asteroids was more long-lasting than the lunar LHB. Since the strongest constraint on the dynamics of source populations for the LHB remains the very rapid decline in rate of basin formation from 3.90 to 3.85 Ga, we must re-establish that the associations of dated samples with particular basins are robust. Further studies of processes that can bias sampling are warranted, in particular more quantitative modeling of regolith and megaregolith evolution. In summary, we find that constraints either for or against the lunar cataclysm as a spike in the bombardment rate that commenced shortly before the formation of Nectaris are very weak. There may or may not have been a lull in bombardment before 3.9 Ga. Only the rapid decline in bombardment rate after Imbrium is fairly secure and can be adopted as a constraint to be matched by various dynamical scenarios for the impact history of the Moon. © 2007 Elsevier Inc. All rights reserved. Keywords: Cosmochemistry; Cratering; Geological processes; Impact processes; Moon, surface 1. Introduction “new set a” in their Table VI] from Nectaris (3.90 ± 0.03 Ga; conceivably as old as ∼4.1 Ga) to Imbrium (3.85 ± 0.02 Ga), Tera et al. (1973, 1974) first proposed, on the basis of a indicate that bombardment ended suddenly; only Orientale spike in resetting ages of Apollo samples, that a “terminal cat- formed ∼10–80 Myr later, defining an especially abrupt post- aclysm” or late heavy bombardment (LHB) had occurred on spike decline or cessation of bombardment by large projectiles the Moon about 4 Ga. The LHB has more recently been ad- (half-life <50 Myr). The basin ages are derived from resetting vocated on the basis of a spike in ages of lunar impact melts, ages of rocks, which are inferred to have been derived from, or at least by an absence of secure impact melt ages prior to or affected by, the particular basin-forming events (reviewed by 4Ga(Ryder, 1990, 2002;Author's Bogard, 1995; Dalrymple et al., 2001;personalStöffler and Ryder, 2001 ). Ascopy we will discuss, the evidence con- Cohen et al., 2000). Dates for lunar impact basins [Wilhelms cerning bombardment history prior to Nectaris is quite incom- (1987), updated by Stöffler and Ryder (2001); we adopt their plete; there is much debate about whether the post-accretionary bombardment rate ∼4.4–4.0 Ga was light, in which case the * Corresponding author. Fax: +1 303 546 9687. LHB was a comparatively large spike in bombardment rate, or E-mail address: [email protected] (C.R. Chapman). instead the rate was heavy, so that the most salient feature of the 0019-1035/$ – see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.icarus.2006.12.020 234 C.R. Chapman et al. / Icarus 189 (2007) 233–245 LHB is that its end marks a sudden decline after a long history its explanation of the LHB is argument in reverse: they accept of bombardment. that there was an LHB spike at 3.9 Ga, and tune their simula- The LHB has played a significant role in planetary science tions to be compatible with that. Other choices for parameters since it was proposed to have happened on the Moon. When in their model could have an LHB-like spike occurring as early craters and basins were found on Mercury by Mariner 10, it as 4.3 Ga or as late as 3.4 Ga. While the likelihood that such an was argued that many of them dated from the same epoch as event occurred at some point may be regarded as high because the lunar LHB (Murray et al., 1975;butseeChapman, 1976). other features of the Solar System are explained, and while the Indeed, since heliocentric impactors that strike the Moon are lunar cratering record precludes it having happened later than scattered around the inner Solar System, it has generally been 3.8 Ga, the available lunar evidence cannot tell us about an presumed that all terrestrial planets—including Earth—were event as early as 4.3 Ga (or even 4.1 Ga). So a bombardment heavily bombarded at the same time, which has important im- spike could have happened early on, while different cratering plications for the early geological and biological evolution of scenarios would have to account for whatever happened subse- Earth. The presumption also is the basis, in the absence of dat- quently. able returned samples, for some commonly adopted absolute Our goal in this paper is not to evaluate what might have stratigraphic chronologies for both Mercury and Mars (Venus caused a cataclysm, or disprove such hypotheses. Rather, we fo- observable geology occurred much later). Indeed, when Voy- cus on the observational and laboratory data, mainly concerning ager discovered the heavily cratered surfaces of many moons the Moon but also the asteroids and other bodies, that constrain of Jupiter and Saturn, the LHB concept was extrapolated to the the actual evidence for the LHB. There are some fledgling argu- outer Solar System (cf. Smith et al., 1981) and it still under- ments involving zircons that may support a cataclysm affecting pins a minority view of Ganymede’s chronology (Wagner et al., the Earth around 3.95 Ga (Trail et al., 2006), near or before 1999). Bogard (1995), Swindle and Kring (2006), and others the time that Nectaris formed. On the other hand, there remain have argued that the LHB may be observed in some classes of long-standing arguments against an LHB. For example, Bald- meteorites and thus it affected the asteroid belt. win (1987, 2006) has long argued that the degradation states There were early attempts (cf. Wetherill, 1975, 1977)to of lunar basins require a multi-hundred-million-year time span provide a dynamical explanation for the LHB. Recently sev- between the formation of Nectaris and Imbrium, much longer eral new dynamical hypotheses for the LHB have been pub- than is inferred from lunar sample ages. His plausible argu- lished, based on remarkable developments in dynamical sim- ment against a cataclysm depends on the assumption that the ulations (cf. Morbidelli et al., 2001; Levison et al., 2001; degradation of lunar basins was due to viscosity of the lunar Chambers and Lissauer, 2002; Gomes et al., 2005); all of these crust. However, if the degradation was instead due to super- involve dynamical evolution or disturbance of heliocentrically imposed cratering and ejecta blankets, then his arguments are orbiting populations of small bodies, whose subsequent im- undermined. pacts might have affected all of the terrestrial planets or even In this paper, we critically analyze the chief sample-age evi- all solid-surfaced bodies in the Solar System. A very different dence for the cataclysm on the Moon, which has dominated the perspective, that the LHB might have been due to a population pro-and-con debates since the hypothesis was first proposed. of impactors trapped in geocentric orbits left over from the for- The evidence mainly consists of “resetting ages” of lunar rocks; mation of the Moon (Ryder, 1990), would have very different these ages include crystallization ages for impact melt rocks implications: the timing of the LHB on the Moon could not then and resetting ages for the less complete modification of other be extrapolated to other planets, except the Earth. Perhaps the rocks due to metamorphism in hot basin ejecta blankets. We new proposal of Cuk et al.