What’s new with the Lunar Cataclysm? Tim Swindle Lunar and Planetary Lab University of Arizona Background: Kaguya image of the central part of SPA

mg-sized Apollo 16 basin samples Norman, Duncan & Huard (2006) GCA 71, 6032 Apollo 16 impact melts Fall in four distinct clusters, based on petrology –All four within ~100 Ma –Single outlier at >4 Ga Agrees with Ryder & Dalrymple results of 1990s, but with smaller samples –Pre-4.0 Ga impact samples not just broken to bits Apollo 16 impact melts, from Norman et al. (2006). Single samples at 4.19 and 3.68 Ga not plotted.

U-Pb on coexisting apatites and zircons in lunar breccias Nemchin & Pidgeon (2008) LPSC XXXIX, #1558; Pidgeon et al. (2006) GCA 71, 1370 Apatites all ~3.9 Ga Zircons mostly >4 Ga –Argue at least two are secondary, with ages of 4.333(7) Ga and 4.187(11) Ga –Implies at least two pre-4 Ga impacts

H chondrite ages Several with ages Ga No spike One poorly-defined age between 4.1 and 4.4 Ga Similar to HED meteorites (Bogard), lunar meteorite impact melt clasts (Cohen), lunar glasses (Culler, Levine, Zellner) From Swindle et al. (submitted)

Terrestrial cataclysm Trail, Mojzsis & Harrison (2007) GCA 71, 4044 SIMS U-Th-P depth profiling in Jack Hills (W. Australia) zircons 4 zircons (of 6 analyzed) have plateaus of Pb- Pb ages between 3.93 and 3.97 Ga –Cores Ga Argue based on U-Th-Pb systematics (U/Th, concordance) that it could be impact

Terrestrial cataclysm Zircon depth profiles (Trail et al., 2007)

Nice model For the first time, there’s a dynamical paradigm* which predicts a cataclysm –Morbidelli, Levison, et al. –Gomes (2005) Nature 435, 466 Central premise: Jovian planets migrate –Idea originally developed by Malhotra –Cause: Interactions with proto-Kuiper Belt *Of course, that doesn’t mean it’s right

Implications of, for Nice model Nice model  cometary impactors –Storm et al. (2005) have argued they’re asteroidal based on size-frequency distribution –PGEs in SPA impact melts Bottke: Might have two heavy bombardment epochs, ~4.5 Ga, ~3.9 Ga –SPA could be very old and still have Cataclysm Nice model predicts spike (like Ryder, Norman results on basin materials), not extended bombardment (like lunar glasses, lunar meteorite melts, asteroidal meteorites) –Bottke thinks it might be “comets” injected into unstable orbits in outer Main Belt

Samples from SPA? 1. Ages Garrick-Bethell, Fernandes, Weiss, Shuster, & Becker (2008) –NLSI conference, Early Impact Bombardment workshop Many samples with ages ~4.2 Ga SPA is the most likely source (ejecta models) No chemistry discussed Key sample: –McCallum et al. (2006) argue excavated from km SPA samples? (6 fragments) Dhofar 489 Yamato

Samples from SPA? 2. Chemistry Jolliff et al. (2008) LPSC XXXIX, #2519 Impact melt from Dhofar 961 Argue that it matches SPA orbital data No ages (yet) “It may not be possible to determine unambiguously if Dhofar 961 comes from SPA basin until a sample of basin floor material has been collected…”

What’s new on the cataclysm? Small Apollo 16 samples just like big ones Coexisting zircons, apatites  pre 4 Ga impacts? Asteroidal meteorites look like lunar meteorites Terrestrial evidence for cataclysm Nice model provides dynamical mechanism Apollo samples, lunar meteorites from SPA?

Pre-Nectarian basins (Wilhelms, 1987)