One of the important outstanding goals of lunar science is understanding the bombardment history of the Moon and calibrating the impact flux curve for extrapolation to the Earth and other terrestrial planets. The "terminal lunar cataclysm," a brief but intense period of bombardment about 3.9 billion years ago, is of particular scientific interest. Radiometric dating of lunar impact-melt rocks forms the backbone of the lunar cataclysm hypothesis. A histogram of precise age determinations of impact-melt rocks shows the characteristics of the classic formulation of the lunar cataclysm hypothesis: a sharp peak at 3.9 Ga, a steep decline after 3.9 Ga perhaps only 20-200 Myr long, and few rocks of impact origin prior to 4.0 Ga

Cohen, Barbara A.

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THE LUNAR CATACLYSM AND HOW LRO CAN HELP TEST IT. B. A. Cohen, NASA Marshall Space
Flight Center, Huntsville AL 35812 (Barbara.A.Cohen@nasa.gov )
Introduction: One of the important outstanding
goals of lunar science is understanding the bombard-
ment history of the Moon and calibrating the impact
flux curve for extrapolation to the Earth and other ter-
restrial planets. The "terminal lunar cataclysm," a
brief but intense period of bombardment about 3.9
billion years ago, is of particular scientific interest.
Radiometric dating of lunar impact-melt rocks forms
the backbone of the lunar cataclysm hypothesis. A
histogram of precise age determinations of impact-
melt rocks shows the characteristics of the classic
formulation of the lunar cataclysm hypothesis: a
sharp peak at 3.9 Ga, a steep decline after 3.9 Ga
perhaps only 20-200 Myr long, and few rocks of im-
pact origin prior to —4.0 Ga [1-3]. The paucity of im-
pact-melt rocks older than 3.9 Ga has been be vari-
ously interpreted as evidence that there was a low
flux of impactors prior to —3.9 Ga, creating few im-
pact melt rocks [2], that the early impactor flux was
so high that the lunar crust was reset and/or destroyed
(the “stone wall” effect) [4], or that the dated samples
may all be related to a single basin-forming event,
Imbrium [5]. If the early lunar flux were much lower,
then the apparent spike in impact rate would be a true
"cataclysm.” If the flux were much higher, than the
increased flux could be viewed as simply a bump or
inflection on a generally declining post-accretionary
impact rate.
Mass constraints on the amount of material
needed to create the large lunar basins appears to
support a low pre -Nectarian flux [6], and the popula-
tion of planetesimals remaining from planetary accre-
tion would have been insufficient to produce as many
basins as late as Imbrium, Serenitatis, and Nectaris
[7]. Instead, insights gleaned from our improved un-
derstanding of giant planet formation and migration
in planetesimal disks suggest that the Jovian planets
experienced a late, sudden instability as they crossed
some mutual resonance. This triggered a rapid deple-
tion of the trans-Neptunian planetesimal disk and
caused an acceleration of the migration of Jupiter and
Saturn, which in turn destabilized the majority of the
asteroids in the main belt. The “Nice model” built on
this idea not only explains the main characteristics of
the impact spike in terms of delay, intensity and dura-
tion, but also the current orbital architecture of the
giant planets, the existence and the orbital distribu-
tion of many populations of small bodies (trojans,
KBOs, satellites) [8-10]. This class of dynamical
models, that invokes secular sweeping of the asteroid
main belt via resonances, has been bolstered by work
on the size-frequency distribution of lunar craters
mirrors that of the main belt [11]. The responsible
impactor population, and the dynamics of its deliv-
ery, plays an important role in determining whether
the impact flux history of the Moon should map onto
the other terrestrial planets.
The lunar cataclysm hypothesis continues to be
tested. Indeed, the top three science goals articulated
in The Scientific Context for the Exploration of the
Moon (SCEM) [12] relate to placing better con-
straints on the lunar impact flux. Because of the fine
detail gleaned in terrestrial labs from existing sam-
ples, the level of precision needed to address some of
the outstanding questions related to the cataclysm de-
pends on sample return. However LRO could assist
in meeting these goals in important ways:
(SCEM 1a) Test the cataclysm hypothesis by de-
termining the spacing in time of the lunar basins.
There is little recognizable pre-Nectarian terrain on
the Moon for crater counting, stratigraphy, or asso-
ciation with rock ages. However, the timing of the
large Imbrian-era basins can be constrained by using
LRO high-resolution images to provide targeted cra-
ter counts of undisturbed ejecta surfaces from Orien-
tale, Imbrium, Serenitatis, and Nectaris, as well as
Imbrian-era farside basins. Identification and map-
ping of extant melt sheets in nearside basins such as
Nectaris and in farside basins would be important in
guiding future missions to sample such lithologies.
Another possibility for putting age constraints on
ancient surfaces may be improved crater counting on
the oldest basalt flows. In turn, stratigraphic relation-
ships between such ancient basalt flows and basin
ejecta may help bound basin formation ages. Some of
these flows have been identified on the eastern limb
by crater counting [13, 14]. Others may be identified
based on their mineralogical or elemental affiliation
with ancient basalt samples in our collection, such as
the high-Al basalts and lunar meteorite Kalahari 009
[15-17]. In particular, farside flows may hold impor-
tant clues. Model ages of mare deposits on the lunar
farside using crater frequency distributions in 10
m/px images obtained by Kaguya’s Terrain Camera
identified an ancient basalt flow in Mare Nishina, at
—3.85 Ga.[18].
(SCEM 1b) Anchor the early Earth-Moon impact
flux curve by determining the age of the oldest lunar
basin, the South Pole-Aitken (SPA) basin. The SPA
basin, with a diameter of 2000 km, is the stratigraphi-
cally oldest lunar basin and probably created more
impact melt than all other lunar craters combined.
https://ntrs.nasa.gov/search.jsp?R=20090029325 2019-08-30T07:39:52+00:00Z
Endogenous impact melt probably still resides on the
basin floor and could be directly sampled by a robotic
mission. Before that time, however, higher-resolution
images to provide targeted crater counts on ejecta of
basins within SPA would help bound the SPA forma-
tion age and provide constraints on the impact history
provided by a scoop sample. Remote sensing of pos-
sible sample collection sites is also crucial to help de-
termine the regional geologic context of future re-
turned samples.
(SCEM 1c) Establish a precise absolute chronol-
ogy. It is important to understand the inflections and
changes in of the lunar flux throughout time so that
we can judge whether a period, such as the Cata-
clysm, is truly anomalous. Are age-correlated
changes in the apparent lunar crater size-frequency
distribution due to of erasure of small craters or due
to evolution of the production function? How do
changes in the lunar crater size-frequency distribu-
tions reflect the impactor populations responsible for
creating them? Higher-resolution images providing
targeted crater counts on selected ejecta facies, such
as Copernicus and Tycho, will be able to be corre-
lated with radiometric ages. The very young end of
the lunar flux curve can be examined by comparing
new remote sensing data sets with Apollo-era data
sets to detect formation of new craters.
References: [1] F. Tera et al. (1974) Earth Planet
Sci Lett 22, 1. [2] G. Ryder (1990) Eos 71, 313. [3]
B.A. Cohen et al. (2005) Met Planet Sci 40, 755. [4]
W.K. Hartmann (2003) Met Planet Sci 38, 579. [5]
L.A. Haskin et al. (1998), Met Planet Sci 33, 959. [6]
G. Ryder (2002) J Geophys Res 107, DOI
10.1029/2001JE001583. [7] W.F. Bottke et al. (2007)
Icarus 190, 203. [8] R. Gomes et al. (2005) Nature
435, 466. [9] A. Morbidelli et al. (2005) Nature 435,
462. [10] K. Tsiganis et al. (2005) Nature 435, 459.
[11] R.G. Strom et al. (2005) Science 309, 1847. [12]
National Research Council (2007) The Scientific
Context for the Exploration of the Moon, 120 pp, Na-
tional Academies Press, Washington, DC. [13] H.
Hiesinger et al. (2003) J Geophys Res 108, 5065. [14]
H. Hiesinger et al. (2000) J Geophys Res 105, 29239.
[15] C.R. Neal and G.Y. Kramer (2006) Am Min 91,
1521. [16] G.Y. Kramer et al. (2008) J Geophys Res
113, 01002. [17] A.K. Sokol et al. (2008) Geochim
Cosmochim Acta 72, 4845. [18] J. Haruyama et al.
(2009) Science 323, 905.
• Many Apollo 14, 16, 17 rocks crystallized at 4.5 Ga but
experienced Pb loss at 3.9 Ga Tera et al (1974)
• Subsequent Rb-Sr and Ar-Ar ages corroborate the large
number of ~3.9 resetor disturbance ages
• Impact-melt sample groups tied to individual impact
events based on major, minor, and trace elements, r +rgeologic interpretation 	 4,;;^nor
• Elements of theclassic cataclysm: 	 nl	 +	
•
Ð Widespread lunar metamorphism by impact
ÐCreated at multiple large basins in <0.2 Gyr 	 aaa...++++^^
~ÐResurfaced 80% of the lunar surface
• An important time in Earth-Moon system 	• :+-
Jy	 r.^i'r .
t	 1a. Test the cataclysm hypothesisby determining the spacing in time of the
creation of the lunarbasins.
1 b. Anchor the early Earth-Moon impact flux curve by determining the age
of the oldest lunar basin (South Pole-Aitken Basin).
1c. Establish aprecise absolute chronology.
4a. Determine thecompositional state (elemental,isotopic, mineralogic)
and compositionaldistribution (lateral and depth) of the volatile
componentin lunar polar regions.
3a. Determine the lateral extent and composition of the primary feldspathic
crust, KREEP layer, and other products of planetary differentiation.
2a. Determine the thickness of the lunar crust (upper and lower) and	 .!J
characterize its lateral variability on regional and global scales.
2b. Characterize the chemical/physical stratification in the mantle, particularly the nature of the putative 500-km
discontinuity and the composition of the lower mantle.
8a. Determine the global density, composition, and time variability of the fragile lunar atmosphere before it is
perturbed by further human activity.
2c. Determine the size, composition, and state (solid/liquid) of the core of the Moon. 	
—7 •
3b. Inventory the variety, age, distribution, and origin oflunar rock types.
8b. Determine the size, charge, and spatial distribution of electrostatically transported dust grains and assess
their likely effects on lunar exploration and lunar-based astronomy.
• ALA
Naomi
• Little recognizable pre-Nectarian terrain on the Moon for
crater counting, stratigraphy, or association with rock 	 ,r
ages
• LRO high-resolution images could provide targeted crater
counts of undisturbed ejecta surfaces from Orientale,
Imbrium, Serenitatis, and Nectaris, as well as Imbrian-era
farside basins
• Identificationand mapping of extant melt sheets in •
nearside basins such as Nectaris and in farside basins r
will be important in guiding future missions to sample
	
) r
such lithologies
	 +^	 - ,*	 •y^,
U	 . 'r
• Stratigraphic relationships between ancient basalt flows "
and basin ejecta may help bound basin formation ages
• Targets = improved crater counting on oldest basalt flows
Ðeastern limb by crater counting (Hiesinger et al. 2000, 2003)
Ðhigh-Al basalts possibly TBD by M3
Ðancient basalt flow in Mare Nishina dated to ~3.85 Ga using
10 m/px images from Kaguya’s Terrain Camera (Haruyama
et al. 2009)	
•y _	 ..
^:.'r	 1.
• The SPA basin is the stratigraphically oldest lunar basin
and probably created more impact melt than all other
lunar craters combined.
	 '
• Targeted crater counts on ejecta of basins within SPA IL
(e.g. Apollo, Ingenii, Poincaré) help bound SPA age and
provide constraints on the impact history provided by a
scoop sample. 	 , *: •	 ^^
• Imagingofpossible sample collection sitesis also crucial
to help determine the regional geologic contextoffuture
returnedsamples. .
	 i d6 1'
- rt^ - ^  ^; i r	 S `
• Understand the inflections and changes in lunar flux to be
able to judge whether a period, such as the Cataclysm, is
truly anomalous
" ÐAreage-correlated changes in the apparent lunar crater
R1 size-frequency distribution due to of erasure of small craters
4'0 or evolution of the production function?
I^ Ð How do changes in the lunar size-frequency distributions
reflect the impactor populations that created them?
• Target: crater counts on benchmark crater ejecta
Jthe
(Copernicus,Tycho)to correlate with absolute ages
Target: Change detection since Apollo to understand
very young end of the lunar flux curve
• OSEWG and other groups have been planning candidate
science scenarios to help derive CxP architecture
requirements and capabilities	 .eye
• Several scenariosintended to address Cataclysm issues
(Schrodinger, Nectarissortie)
• Strategy: Target areas that will aid traverse planning to '+r
more accurately plan routes (=distance), realistic goals
(steep slopes, boulders), maximize success
• Issues: These areas are large to immense; impossible to
completely cover. But small areas would validate/inform s
interpretation of lower-resimaging
 of adjacent areas.
• Determining Nectaris Basin age, establishes age/
structural relationship between basin formation and
volcanic activity.	 rxy
• Samples portions ofexposed highland basin massif, ^+•
basin floor, and mare basalt fill.	
rip
• Supports Goals A-1, 2, 3, 4, 5, 6, 8, B-1
• Petro etal LPSC2009,
 Bleacher et al LPSC 2009 . • +-i
2
0 .
• How LRO can help test the Cataclysm: targeted RS used
for improved crater counts, identification/context of
sampling areas, and support for human activities
– Nectaris and Schrodinger Basin sortie areas
– Interior/edge of farside basins
– In-SPA basin ejecta (suggest Apollo, Ingenii, Poincare)
– Possible sample collection sites within SPA if teams provide
them
–Oldest basalt flows (Hiesinger)
– Benchmark crater ejecta (Copernicus, Tycho) ''.^ •	 i
–Change detection (Robinson)
3


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