Rethinking Solar System Bombardment: New Views on the Timing and Delivery of Lunar Impactors

Nicolle ZellnerAlbion College

@astrodivanzellner@albion.edu

Why the Moon?• Samples and surface are (mostly) undisturbed• Samples can be dated (40Ar/39Ar, 87Rb/87Sr, etc.)• Craters can be counted

Lunar cratering rate anchors the impact chronology for the entire (inner) Solar System

Lunar Impact Flux Models

No Impacts

Cataclysm = LHB

Early IntenseBombardment

Monotonic

Decline

Cool early Earth First life First fossils?(~3.5 Ga)

Sawtooth

Modified from Zellner (PhD thesis); Hartmann (1965, 1966, 2000); Tera et al. (1974); Morbidelli et al. (2012)

The Impact FluxWays to interpret the time-varying impact flux:

Samples:▪ crystalline melts in Apollo samples▪ crystalline melt clasts in meteorites▪ zircons▪ lunar impact glass

Other:▪ crater counting and stratigraphy

~200 µm

10s µm

The “Cataclysm”

Wilhelms (1987), Hartmann (2000), Ryder et al. (2000), Koeberl (2006)

U-Pb agesStratigraphy

Crater countingA15, A17 breccia

40Ar/39Ar ages(Dalyrymple & Ryder,

1993, 1996)

4.3ish – 4.05 GaSPA

3.85 ± 0.02GaImbrium 3.893 ± 0.009 Ga

Serenitatis

~3.89 GaCrisium

3.92 – 3.90 Ga (?)Nectaris3.84 – 3.80 Ga (?)

Orientale

~800 MaCopernicus

~100 MaTycho

Other EvidenceMeteorites:impact, shock

Zircons:episodes ofheating

14 Lunar Meteorites

Age (Ga)4.5

2.03.9

# of

impa

ct e

vent

s

Prob

abilit

y of

a s

ampl

e ha

ving

a g

iven

age

4

1

2

3100s µm

from Figure 6c in Cohen et al. 2005

Nice ModelDescribed the Impact Mechanism

Objects scatter intothe inner Solar System after the orbital shift of Neptune (dark blue) and Uranus (lt. blue)

Current-ish Solar System, after

ejection of objects by planets

(JSUN)

Early configuration, before Jupiter and

Saturn reach a 2:1 resonance

(JSNU)

Gomes et al. (2005); Tsiganis et al. (2005); Morbidelli et al. (2005)

Changing Views

Artist Conception (NASA)

Orbital Data:LRO: LOLA, LROC

Sample Data:New interpretations

More dataImproved Techniques

LRO/LCROSS

Isotope re-CalibrationsSimilar ages insamples with similar compsfrom multiple

Apollo landing sites

All represent 1 event:Imbrium (~3.9 Ga)

Liu et al. (2012), Merle et al. (2014), Mercer et al. (2015)

Grange et al. (2010)

?

Crater Age (today)

SPA 4.2 Ga (?)

Serenitatis >4.1 – 3.87 Ga

Nectaris 4.1 Ga (?)

Crisium ~3.9 Ga (?)

Imbrium 3.77-3.90 Ga+

Norman (2008); Grange et al. (2010); Spudis et al. (2011)

+ Imbrium’s age is based on Apollo 14 and Apollo 15 samples, whose geologic provenance is not well-established

Age (before)

4.3ish – 4.05 Ga

3.893 ± 0.009 Ga

3.92 – 3.90 Ga (?)

~3.89 Ga

3.85 ± 0.02 Ga

(based on new calibrations and superpositioning of ejecta blankets from orbital data)

Updated Basin Ages

Neumann et al. 2015Multiple new basins w/ >300 km diameters6 known basins with D >200 km larger than previously measured

GRAIL Data

Result → CSFDs need to be recalibrated and theimpact flux changes

Byerly and Lowe (2010, 2015)Lowe et al. (2014)

Result: LHB lasted longer than we thought

Barberton (SA):Multiple impact spherule layers from large distal impacts between3.5 and 3.2 Ga

Archean Impacts on Earth

Old Lunar Sample AgesApollo 16 impact breccia U-Pb age:

large event at 4.22 ± 0.01Ga

Apollo 16 melt 40Ar–39Ar ages:4.21 ± 0.05 Ga and 4.29 ± 0.04 Ga

Fernandes et al. (2013)

Lunar zircon heating events w/ U–Pb ages: 4.3 ± 0.01, 4.2 ± 0.01, and 3.9 ± 0.01 Ga

Hopkins and Mojzsis (2015)

Norman et al. (2016)Norman and Nemchin (2014)

10s µm

ImbriumSamples

O2 Whiffs @3200 Ma, 2600 Ma

GOE

Lunar Flux

Early Life on EarthC isotope evidence for life:

>4.0, zircons (Bell et al. 2015)

3.95 Ga, Canada (Tashiro et al. 2017)

3.85 Ga, Akilia, Greenland (Mojzsis et al. 1996)(though highly contested in the literature)

3.8 Ga, Isua Greenland (Schidlowski, 1988)

Fossil evidence for life: 3.77 Ga, marine hematite

tubes (Dodd et al. 2017)

3.48 Ga, terrestrial palisadefabric (Djokic et al. 2017)

Impacts were not very frequent but were prolonged (~4.1 to 3.2 (or younger) Ga)

No impact frustration (Maher and Stevenson, 1988)

No impact sterilization (Sleep et al. 1989; Nisbet and Sleep, 2001)

A cool early Earth (Wilde et al., 2001; Watson and Harrison, 2005)

Delivery of CHONPS (e.g., amino acids, sugars)

Impacts did affect life butstill it persisted

Early Life on Earth

Suggested ReadingZellner N. E. B. (2017) Cataclysm no more: New views on the timing and delivery of lunar impactors, Origins of Life and Evolution of Biospheres, 47(3), 261-280,doi:10.1007/s11084-017-9536-3

“Bashing holes in the tale of Earth’s troubled youth”:https://www.nature.com/articles/d41586-018-01074-6

“Fossil Discoveries Challenge Ideas About Earth’s Start”: https://www.quantamagazine.org/fossil-discoveries-

challenge-ideas-about-earths-start-20180122/

ReferencesHartmann W. K. (1965) Terrestrial and lunar flux of

large meteorites in the last two billion years. Icarus, 4, 157–165.

Hartmann W. K. (1966) Early lunar cratering. Icarus, 5, 406–418.

Hartmann W. K., Ryder G., Dones L., and Grinspoon D. (2000) The time-dependent intense bombardment of the primordial Earth/Moon system, in Origin of the Earth and Moon (R. Canupand K. Righter, eds.), 493-512, University of Arizona Press, Tucson, 555 pp.

ReferencesTera F., Papanastassious D. A., and Wasserburg G.

J. (1974) Isotopic evidence for a terminal lunar cataclysm. Earth Planet. Sci. Lett. 22, 1-21.

Gomes R., Levison H. F., Tsiganis K., Morbidelli A. (2005). "Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets". Nature 435 (7041): 466–9.

Ryder, G. (2002) Mass flux in the ancient Earth-Moon system and benign implications for the origin of life on Earth, J. Geophys. Res., 107 (E4), doi: 10.1029/2001JE001583.