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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
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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 3.5-4.1 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 4.05-4.2 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 *Ofcourse, 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: 76535 • McCallum et al. (2006) argue excavated from 40-50 km
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?
