New measurements of the topography of the Moon from the Lunar Orbiter Laser Altimeter (LOLA)[1] provide an excellent base-map for analyzing the large crater population (D.20 km)of the lunar surface [2, 3]. We have recently used this data to calculate crater size-frequency distributions (CSFD) for 30 lunar impact basins, which have implications for their stratigraphy and sequence. These data provide an avenue for assessing the timing of the transitions between distinct crater populations characteristic of ancient and young lunar terrains, which has been linked to the late heavy bombardment (LHB). We also use LOLA data to re-examine relative stratigraphic relationships between key lunar basins

Fassett, C. I.

Head, J. W.

Kadish, S. J.

Mazarico, E.

Neumann, G. A.

Smith, D. E.

Zuber, M. T.

NASA Technical Reports Server

STRATIGRAPHY, SEQUENCE, AND CRATER POPULATIONS OF LUNAR IMPACT BASINS FROM 
LUNAR ORBITER LASER ALTIMETER (LOLA) DATA: IMPLICATIONS FOR THE LATE HEAVY 
BOMBARDMENT.  C.I. Fassett1,2, J.W. Head2, S.J. Kadish2, E. Mazarico3, G.A. Neumann3, D.E. Smith3,4, M.T. 
Zuber4.  1Dept. of Astronomy, Mt. Holyoke College, South Hadley, MA 01075, 2Dept. of Geological Sciences, 
Brown University, Providence, RI 02912, 3Solar System Exploration Division, NASA/GSFC, Greenbelt, MD 
20771, 4Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cam-
bridge, MA 02139. (cfassett@mtholyoke.edu). 
 
 
Introduction:  New measurements of the topogra-
phy of the Moon from the Lunar Orbiter Laser Altime-
ter (LOLA)[1] provide an excellent basemap for ana-
lyzing the large crater population (D≥20 km)of the 
lunar surface [2, 3].  We have recently used this data to 
calculate crater size-frequency distributions (CSFD) 
for 30 lunar impact basins, which have implications for 
their stratigraphy and sequence. These data provide an 
avenue for assessing the timing of the transitions be-
tween distinct crater populations characteristic of an-
cient and young lunar terrains, which has been linked 
to the late heavy bombardment (LHB).  We also use 
LOLA data to re-examine relative stratigraphic rela-
tionships between key lunar basins. 
Method:  We derive the CSFD for each basin by 
mapping preserved basin-related materials (the region 
proximal to and within the basin rim that has not been 
resurfaced by volcanism or other processes).  We use a 
buffered area correction and include craters which are 
superposed on the basin but which are centered outside 
the count region [e.g., 4]  This technique slightly ex-
pands our effective count area, since large craters sub-
tend more area than small ones.  It also allows exclu-
sion of resurfaced regions from the mapped count area 
without losing information about craters superposed on 
the edge of basin material.   
For crater data, we start with the catalog of lunar 
craters ≥20 km in diameter from LOLA data [2,3]. We 
then re-examined each basin using the 64 ppd LOLA 
DTM and shaded relief to systematically search for 
additional craters beyond the global database.  In total, 
a modest number of additional craters were found 
(12%); these are generally small (<40 km) and degrad-
ed.  Crater measurement are made with CraterTools 
[5]; all areas and diameters are computed using equal 
area map projections.   
Stratigraphy:  Our measurements provide a test of 
the widely-used Wilhelms [6] sequence of lunar ba-
sins.  There is strong qualitative agreement between 
this earlier sequence and our new measurements.  
However, there are substantial quantitative differences 
in the crater densities we observe, particularly for old-
er basins.  For example, the N(20) (# of craters ≥20 km 
normalized to an area of 106 km2) we derive for the 
Nectaris basin is 135±14 compared to Wilhelms’s 
79±14. This difference has implications for the crater-
ing history of the Moon and the crater frequency of the 
Pre-Nectarian/Nectarian boundary: Wilhelms’s data 
[6] would imply the Moon had ~1900 impacts ≥20 km 
between Nectaris and Imbrium, whereas our data 
would suggest that ~4000 such craters formed.   
Impactor Populations and the LHB:  There has 
been a long debate in lunar science about whether 
heavily-cratered highlands records a distinct popula-
tion of impactors from the lunar maria [7,8,9,10,11]. 
Measurements of the CSFDs of the highlands and 
mare imply that the highlands have a lower ratio of 
~20-40 km craters to ~80-100 km craters than the mare 
[9; see also 2].  This difference is statistically signifi-
cant at 96% confidence when applying the two-sample 
Kolmogorov-Smirnov (K-S) test to their CSFDs. 
This observation does not guarantee that the differ-
ence in CSFDs is a result of a shift in the impactor 
populations.  An alternative is geologic processes such 
as volcanism or repeated cratering preferentially re-
moved small craters on ancient surfaces in a manner 
that resulted in the observed change [10].  Although 
possible, this hypothesis seems less likely given that a 
similar shift in CSFDs is observed on Mercury [9, 12] 
and Mars [9] to what is observed on the Moon.  Given 
their distinct geologic histories, there is no reason that 
crater removal on each planet should be similar.   
Strom et al. [9] propose that the shift in crater pop-
ulations is related to the LHB.  They note that the early 
population (Pop. 1) is similar to what would be ex-
pected from direct delivery of a collisionally-evolved 
population like the Main Asteroid Belt, and the young-
er population (Pop. 2) matches well with the Near-
Earth Objects, which have a ‘flatter’ shape on an R-
plot than Population 1 and are delivered via a more 
size-selective process [13]. 
Our measurements of basin CSFDs allows us to 
examine when the hypothesized transition between 
populations occurred.  We examine this change by 
aggregating the statistics of basins from a given peri-
od, since counting statistics of individual basins alone 
are insufficient to assess population differences.  We 
combine the crater counts and areas for the Imbrian 
basins (including Imbrium; aggregate N(20) of 22±3), 
the Nectarian basins (including Nectaris; aggregate 
4023.pdfEarly Solar System Impact Bombardment II (2012)
https://ntrs.nasa.gov/search.jsp?R=20120009937 2019-08-30T20:43:52+00:00Z
N(20) of 110±5), and Pre-Nectarian basins (excluding 
SPA to avoid it dominating the statistics; 
N(20)=188±7).   
These aggregated data imply that both the Imbrian-
aged basins and the Nectarian-aged basins are con-
sistent with the flatter Population 2 (more mare-like) 
shapes (Fig. 1).  The K-S test suggests that the Pre-
Nectarian and Nectarian-aged basins are distinct from 
each other at the 94% confidence level.   
These observations are surprising, since the transi-
tion from Population 1 to 2 has previously been linked 
to the transition from the LHB population to the mod-
ern population [9], and the basins formed during the 
Nectarian are commonly assumed to be part of the 
Late Heavy Bombardment.  Moreover, because of the 
high crater flux during this time period, 65-75% of the 
craters that we measure on the Nectarian basins actual-
ly formed during the Nectarian period.  If Population 1 
dominated at that time, we would expect to see its sig-
nature in the Nectarian basin curve.  Instead, the im-
pactor size-frequency distribution by the mid-
Nectarian is consistent with that of the lunar mare, 
despite the high flux during this period, rather than 
with the size-frequency distribution characteristic of 
the lunar highlands.  These data would suggest that the 
transition observed in the lunar impact crater popula-
tion occurred earlier than has been previously suggest-
ed.  It is unknown whether the transition between the 
two impactor populations was gradual or abrupt, but 
Population 1 cannot have remained the predominant 
source of lunar impacts as late as Imbrium. 
Relative Stratigraphy of  Lunar Basins:  Crisium 
and Humboldtianum:  The relationship between 
Crisium and Humboldtianum is uncertain despite their 
close proximity and good preservation state.  LOLA 
data (Fig. 2a) suggest that secondary craters and sculp-
ture from Humboldtianum reach to, or across, the outer 
ring of Crisium. These stratigraphic relationships, 
which agree with crater statistics, support the interpre-
tation that Humboldtianum is younger than Crisium. 
Serenitatis and Nectaris: The relationship of Se-
renitatis to its surrounding basins is a long-standing 
problem in lunar science and is closely tied to the in-
terpretation of samples from Apollo 17. In general 
early crater counting preferred an interpretation where 
Serenitatis was Pre-Nectarian [e.g., 14], a view that 
has been advocated anew based on analyses of the 
sculptured hills of the Taurus-Littrow region with Lu-
nar Reconnaissance Orbiter Camera data [15].  LOLA 
topography provides support for the interpretation that 
Serenitatis  is older than Nectaris as well:  (1) crater 
counting, which finds a crater density in the Taurus 
Mountains at a factor of two times that of Crisium or 
Nectaris, with more than sufficient counting statistics, 
and (2) evidence in LOLA topography for sculpturing 
from Nectaris on the south-eastern rim of Serenitatis 
near the Apollo 17 landing site (Fig. 2b-c).   
References: [1] Smith, D.E. et al. (2010), Space Sci. 
Rev., 150, 209-241. [2] Head, J.W. et al. (2010) Science, 
329, 1504-1507. [3] Kadish, S.J. et al., LPSC 42, 1006. [4] 
Fassett, C.I., Head, J.W. (2008), Icarus, 195, 61-89. [5] 
Kneissl, T. et al. (2010), PSS, 59, 1243–1254. [6] Wilhelms, 
D.E. (1987), The Geologic History of the Moon, USGS PP 
no. 1348. [7] Whitaker, E.A., Strom, R.G. (1976),  Abs. LPS, 
7, 933–934. [8] Wilhelms, D.E. et al. (1978), Proc. Lunar 
Plan Sci. Conf., 9, 3735–3762. [9] Strom, R.G. et al. (2005),  
Science, 309, 1847–1850. [10] Hartmann, W.K. (1995), Me-
teoritics, 30, 451–467. [11] Neukum, G. et al. (2001), Space 
Sci. Rev., 96, 55–86.  [12]  Fassett, C.I. et al. (2011), GRL, 
38, L10202. [13] Morbidelli, A., Vokrouhlický, D.  (2003), 
Icarus, 163, 120-134. [14] Hartmann, W.K., Wood, C.A. 
(1971), Moon, 3, 3–78.  [15]  Spudis, P.D. et al. (2011), JGR, 
in press, doi:10.1029/2011JE003903.  
 Fig. 1.  R-plot showing the CSFDs for Pre-Nectarian, Nectarian, & Imbrian basins.  Nectarian basins have a distribution consistent with 
the mare-like, Population 2.  The Pre-Nectarian basins are more similar to Population 1.  This suggests the transition from predominantly 
Pop. 1 to 2 happened by the mid-Nectarian, as the Nectarian basins primarily accumulated craters from Population 2. 
 
Fig. 2. (a) LOLA shaded-relief overlaid by topography, showing sculptured ejecta from Hum-
boldtianum superposed on Crisium. (b) Context and (c) closeup views of the Serenitatis basin 
and Taurus mountains, showing potential sculpture from Nectaris superposed on Serenitatis. 
4023.pdfEarly Solar System Impact Bombardment II (2012)


Stratigraphy, Sequence, and Crater Populations of Lunar Impact Basins from Lunar Orbiter Laser Altimeter (LOLA) Data: Implications for the Late Heavy Bombardment

Stratigraphy, Sequence, and Crater Populations of Lunar Impact Basins from Lunar Orbiter Laser Altimeter (LOLA) Data: Implications for the Late Heavy Bombardment

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