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

*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: 76535

– McCallum et al. (2006) argue excavated from 40-50 km

SPA samples?

63503 (6 fragments)

Dhofar 489

Yamato-86032

76535

60025

67955

78155

78235

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)