Cohen, Barbara

NASA Technical Reports Server

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Barbara Cohen 
NASA Marshall Space Flight Center 
Barbara.A.Cohen@nasa.gov 
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1a. Test the cataclysm hypothesis by determining the spacing in time of the 
creation of the lunar basins.  
1b. Anchor the early Earth-Moon impact flux curve by determining the age 
of the oldest lunar basin (South Pole-Aitken Basin).  
1c. Establish a precise absolute chronology.  
4a. Determine the compositional state (elemental, isotopic, mineralogic) 
and compositional distribution (lateral and depth) of the volatile 
component in 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 
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.  
3b. Inventory the variety, age, distribution, and origin of lunar 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.  
•  LRO Targeting Workshop (2009) - How LRO can help address 
issues in impact cratering 
•  The Moon is a lab for understanding the impact process 
–  Craters are preserved better than on any other planet 
–  Craters are numerous and range over size and terrain 
–  The Moon is airless and volatile free, so some variables are simpler 
•  The Moon preserves a temporal record of the Earth-Moon 
bombardment flux 
–  Relative crater densities can be tied to absolute sample ages 
–  Stratigraphic relationships can be recognized 
•  Craters will be targets for future robotic and human exploration 
•  What progress has been made since then? 
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•  Lunar Reconnaissance Orbiter 
Camera (LROC) and Mini-RF 
radar images reveal that impact 
melt deposits have complex 
morphologies on m- to km- scales 
(Bray et al., 2010; Carter et al., 
2010; Robinson et al., 2010) 
•  New LROC WAC images show 
that the Sculptured Hills (A17 
poikilitic impact-melt rocks) are not 
localized around the Apollo 17 
site, but are widespread 
throughout the Taurus Mountains 
– are these rocks Serenitatis or 
Imbrium? (Spudis et al. 2011) 
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Melt distribution map of lunar crater Moore F. Impact melt units 
include: hummocky floor melt (pink stripes) to the NE, smooth melt 
ponds (magenta), and channeled melt flows (orange). From Bray et 
al. (2010). 
•  Pre-3.9 Ga ages of non-impact-melt samples 
–  Apollo 16 and 17 granulitic breccias have peak 
metamorphic ages of 3.9 Ga OR 4.1 Ga (Hudgins et al. 
2008) 
–  A14 and A17 zircon overgrowths are 4.33 and 4.2 Ga 
(Grange et al. 2009, 2011) 
–  Seven Apollo 16 regolith samples (feldspathic breccias 
and anorthosites) have plateau ages 3.9 Ga OR 4.2 Ga 
(Shuster et al. 2010) 
–  Unique mafic-mineral rich sample of FAN 60025 has a 
multiple-isotopic age of 4360 ± 3 Ma (Borg et al. 2011) 
•  Are these recording ancient basin-forming 
impact events or lunar igneous activity? 
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•  A16 anorthosites experienced 
partial loss by a heating event 
3.3 Ga, likely an additional 
impact event (Shuster et al. 
2010) 
•  Basaltic meteorites crystallized 
3.7-3.8 Ga but show partial loss 
corresponding to impact event 
~200 and ~600 Myr ago 
(Fernandes et al. 2010) 
•  Polycrystalline zircon in lunar 
meteorite Dhofar 458 
recrystallized at 3.4 Ga (Zhang 
et al. 2011) 
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Duration–temperature constraints on a thermal excursion 
experienced by 63503 at 3.3 Ga derived from seven samples in 
regolith sample 63503 (Shuster et al. 2010). 
https://ntrs.nasa.gov/search.jsp?R=20120016723 2019-08-30T23:06:35+00:00Z
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•  New crater counts to small 
sizes using LROC & LO 
images indicate that lunar 
surfaces have more secondary 
craters than previously 
anticipated (Hiesinger et al. 
2010, Kirchoff et al. 2010, 
Plescia and Robinson 2011) 
•  New small crater frequencies 
give model ages greater than 
the known radiometric age 
•  The calibration between 
absolute age and crater 
frequency for young ages has 
significant uncertainties 
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Cumulative crater size-frequency data for ejecta surfaces 
of Cone, North Ray and South Ray craters from LROC 
images (Plescia and Robinson 2011). 
•  Lunar highlands have SFD similar 
to main belt asteroids, while 
“morphologically fresh” Class 1 
craters have SFD similar to NEAs 
(Strom et al. 2005)  
•  Cuk et al. (2010) argue that Class 
1 craters have the same density on 
the whole moon as on basin ejecta 
blankets – therefore must be 
remnants of the LHB 
•  How can crater counts from 
different units be combined? Does 
crater morphology correlate with 
age? How are secondaries 
accounted for? 
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Size-frequency distributions of different crater classes 
(Cuk et al. 2010) 
•  New lunar crater production 
functions derived from impact 
modeling differ significantly from 
those based on measurements 
(Marchi et al. 2009, Le Fevre and 
Wieczorek 2011)  
–  reconcile measured lunar SFD with 
near-Earth asteroid population by 
assuming craters < few km form in a 
porous megaregolith, suggested by 
Ivanov et al. (2007) 
–  quantify spatial cratering asymmetries 
that may bias crater density ages – 
worse for smaller bodies than larger 
–  Estimate both Orientale and Caloris 
basins to be 3.73 Ga 
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Empirical fit from crater counting (Neukum et al. 2001) 
Analytical fit (Marchi et al. 2009) 
Analytical fit (Le Favre and Wieczorek 2011) 
•  Dynamical modeling of larger main 
belt asteroids shows that these are 
subject to loss; predicted rate of 
impacts declines by a factor of 3 
over the last 3 Gyr (Minton and 
Malhotra 2010) 
•  “E-belt” source of main-belt 
asteroids predicts production of 
large lunar basins, long tailoff at 
Earth, and later siderophile veneer 
(Bottke et al. 2010, 2011) 
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Impact rates of D > 10 km asteroids on the Earth and Moon from 
model of Minton and Malhotra (2010) compared with the Neukum 
Production Function (shaded regions). 
•  Terrestrial spherule beds continue to be discovered: 3.47-3.23 Ga 
(7) 2.63-2.49 Ga (4) 2.1-1.6 Ga (2) (Glikson 2010) 
•  Isua metasedimentary rocks are enriched (150 ppt) in iridium 
compared to present-day ocean crust (20 ppt) – argues to be 
evidence of cometary input rather than asteroidal (Jørgensen et al. 
2009) 
•  New (U, Th)-He technique for terrestrial impact-generated zircons 
(van Soest et al. 2010, Wartho et al. 2011)  
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•  Selenochronology is getting more complicated: new results question 
meaning of sample ages, crater counts, crater production functions, 
and the solar system itself 
•  But there is hope! 
–  Improved geological mapping of lunar geologic units and boundaries using 
multiple remotes-sensing datasets 
–  High-resolution image-based crater counting of discrete geologic units and 
relating them to location 
–  Improved understanding of the regolith thickness and its global variation 
(GRAIL) 
–  Tying the sampling of impact-melt rocks to the lunar impact flux 
–  Using improved techniques (magnetic fields, diffusion studies, isotopic analysis) 
on existing samples 
–  New sample return from benchmark craters, particularly SPA, which appears in 
2013 Decadal Survey 
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Recent Improvements to Selenochronology by Samples and Missions and Models

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