Lunar and Planetary Science XLVIII (2017)
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INSIGHTS INTO EARLY LUNAR CHRONOLOGY FROM GRAIL DATA. Alexander J. Evans1,2, Jeffrey C.
Andrews-Hanna1,2, Jason M. Soderblom3, Sean C. Solomon4, and Maria T. Zuber3. 1Planetary Science Directorate,
Southwest Research Institute, Boulder, CO 80302, USA, [email protected]; 2Lunar and Planetary Laboratory,
University of Arizona, Tucson, AZ 85721, USA; 3Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA; 4Lamont-Doherty Earth Observatory, Columbia
University, Palisades, NY 10964, USA.
Introduction:
Establishing the absolute and
relative chronology of ancient lunar events is of
fundamental importance to our understanding of early
Solar System history and the evolution of rocky
planetary bodies. In this endeavor, the Moon has a
unique quantitative role, as it is the only planetary body
from which absolute and relative ages can be calibrated
to one another, by way of radiometric dating of returned
samples and observable surface crater densities,
respectively [1-4]. In contrast to the relatively young
and heavily modified surfaces of some other planetary
bodies, a majority of the lunar surface has been well
preserved since antiquity and thus retains the most
comprehensive surface cratering record presently
known to exist.
For some lunar deposits associated with impact
basins, such as Imbrium, absolute ages have been
relatively well constrained [5]. For others, substantial
uncertainty remains in their ages, as exemplified by the
poorly constrained ages of the prominent South PoleAitken (SPA) and Nectaris basins [5]. Furthermore,
attempts to assign relative ages to the major impact
basins through the application of traditional crater sizefrequency analyses are often frustrated by the extensive
deposits of dark basaltic plains, or maria, that
preferentially flooded and presently obscure the
primary surfaces of major impact basins on the lunar
nearside [5-6]. To estimate the relative ages of these
mare-flooded basins, previous workers used either a
patchwork of unflooded surfaces of small area [e.g., 6]
or made adjustments for the size-frequency
distributions of mare-covered regions [8], but both of
those methods inject uncertainty and potential bias,
especially for heavily flooded basins such as Serenitatis
[5-6]. More comprehensive treatments have augmented
the traditional crater size-frequency analyses with
stratigraphic inferences to establish the relative ages
and chronologic sequence of lunar basins [e.g., 5], but
the uncertain crater density of the pre-mare nearside
surface nonetheless remains an obstacle in establishing
a reliable chronology.
Despite the combination of returned samples and
the well-preserved state of much of the lunar surface,
there are many unanswered questions surrounding basin
and terrane chronology. In this investigation, we jointly
use craters with a recognizable surface expression and
those inferred from quasi-circular mass anomalies
(QCMAs), considered to be buried craters, preserved in
the lunar gravitational field and revealed by analyses of
the gravity data from the Gravity Recovery and Interior
Laboratory (GRAIL) mission to re-examine the ages for
the formation of lunar terranes and the chronological
sequence of major impact basin formation [7-11].
(a)
-9
Topography (km)
10
90°000
N
0°
001
(b)
90°002
S
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W
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004
0°
003 E
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0°
003 E
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Figure 1. (a) Craters of diameter greater than 90 km (outlined
in black) over a shaded relief map of topography. (b) Surface
mare deposits (outlined in cyan) and QCMAs (magenta) of
diameter greater than 90 km on a morphologic base map.
Methodology: The QCMAs identified in the lunar
gravitational field and shown in Figure 1 have been
proposed to represent a population of craters with
surface expressions obscured by the superposition of
volcanic deposits (maria) or material ejected by
younger impact events. The contribution of the QCMAs
may be assessed with an incremental size-frequency
distribution (SFD) N(D), where N is the number of
craters of diameter D (in km) or greater per unit area
(106 km2). In Figure 2a, we show ratios of incremental
SFD of impact craters in non-mare regions to those of
mare regions binned at 20-km diameter intervals, where
each bin includes craters of diameter D ± 20 km. For
each bin, the crater density is estimated from areal maps
constructed with a 500-km-radius moving window
average, similar to those in Figures 2b and 2c; the
errors shown are calculated from the weighted standard
error on the mean.
The incremental SFD in Figure 2a that includes
QCMAs shows that the apparent deficits of craters in
mare regions compared with non-mare regions are
Lunar and Planetary Science XLVIII (2017)
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nearly eliminated at large diameters, with mare and
non-mare regions exhibiting similar incremental SFDs
for D ≥ 90 km. With QCMAs included in maps of the
crater distribution, the crater density of the nearside
lunar maria is generally indistinguishable from that of
the surroundings at crater diameters greater than 90 km
(Fig. 2b), although the density deficits at the centers of
major impact basins (e.g., Orientale, Serenitatis,
Crisium, Imbrium) can still be readily observed. Given
that mare volcanism had little apparent effect on the
SFD and cumulative crater densities of the combined
set of craters and QCMAs with D ≥ 90 km, we choose
this crater and QCMA diameter cutoff to assess the
relative ages of lunar geochemical terranes and basins.
N(90) value of 12.1±3.2. Since the PKT could have
formed as late as ~4.3 Ga, on the basis of the youngest
age for urKREEP crystallization from the lunar magma
ocean [12], it then follows that the SPA impact must
have occurred prior to ~4.3 Ga. To determine the
relative ages of lunar basins shown in Figure 3, we use
N(90) values, inclusive of QCMAs, of the full region
interior to the main rim diameter. As noted by
Hartmann and Wood [7], the variations in the derived
crater densities will not be directly proportional to age,
because of a non-uniform cratering rate over time, but
instead establish relative (crater retention) ages.
Nonetheless, we find that the N(90) values of those
basins with D ≥ 650 km are in general agreement with
the lunar chronology of Wilhelms [5]. Furthermore, the
N(90) values for investigated pre-Nectarian basins
(shown in Figure 3) vary between 16.6 and 19.8, and
these basins have a relative age that is greater than that
of the PKT (when using their combined area),
determined at the 99% confidence level, indicative that
such basins impacted the Moon prior to ~4.3 Ga. In
contrast, the N(90) of Serenitatis is statistically
indistinguishable from that of Imbrium.
Our results show that by using craters and QCMAs
with D ≥ 90 km, new constraints on the ages of basins
and geochemical terranes can be established. In
particular, QCMAs can be used to establish a relative
age for the PKT and, once anchored to an absolute age
from urKREEP crystallization times, can further
constrain the pre-Nectarian basins that formed prior to
Smythii and Coulomb-Sarton to be older than ~4.3 Ga.
(a) 5
Incremental
005 SFD Ratio
004
0
003
000
(b)
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(km)
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N(90)/1066 km
km22
N(90)/10
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015
015 N(90)
014
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30
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180
Figure 2. (a) Ratio of incremental SFD for craters in nonmare regions to that in mare regions both with (blue) and
without (black) QCMAs. Ratios of incremental SFDs are
determined at intervals of 20 km in diameter and for 40-kmdiameter bin sizes. Errors follow from the weighted standard
error on the mean. The red line denotes a 1:1 ratio. Eckert IV
projections of N(90) (b) with QCMAs and (c) without
QCMAs (averaged over a circular window of 500-km radius) are also shown. Major basins are outlined and labeled.
Results:
The use of N(90) for crater age
assessments provides ages generally unbiased by
volcanism. The elimination of this bias is particularly
important for the nearside major impact basins that
were substantially flooded by maria. For the lunar
geochemical terranes, although we find that the N(90)
values of the SPA basin and Feldspathic Highlands
Terrane (FHT) are indistinguishable, 17.9±2.0 and
17.2±3.1, respectively, both terranes are significantly
older (99% confidence level) than that of the
Procellarum KREEP Terrane (PKT), which has an
Nectarian
Pre-Nectarian
011
011
20
010
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Fecunditatis (3)
Australe (3)
Nubium (3)
Imbrian
Coulomb-Sarton (4)
Smythii (4)
SPA interior (1)
SPA (1)
Nectaris (10)
Mendel-Rydberg (10)
Serenitatis (11)
Humorum (11)
Imbrium(12)
Crisium (11)
Orientale (12)
001
001
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002
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003
003
004
6
8
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Basin Groups
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12
Figure 3. Cumulative crater density N(90) clustered by
stratigraphic group, according to Wilhelms [5].
References: [1] Neukum G. et al. (1975) Moon, 12, 201229. [2] Marchi S. et al. (2009) Astron. J., 137, 4936-4948.
[3] Hiesinger H. et al. (2011) GSA Special Paper 477, 1-51.
[4] Le Feuvre M. L. and Wieczorek M. A. (2011) Icarus, 214,
1-20. [5] Wilhelms D. E. (1987) USGS Prof. Paper 1348. [6]
Fassett C. I. et al. (2012) JGR, 117, E00H06. [7] Hartmann
W. K. and Wood C. A. (1971) Moon, 3, 3-78. [8] Head J. W.
et al. (2010) Science, 329, 1504-1507. [9] Zuber M. T. et al.
(2013) Science, 339, 668-671. [10] Neumann G. A. et al.
(2015) Science Advances, 1, e150002852. [11] Evans A. J. et
al. (2016) GRL, 43, 2445-2455. [12] Borg L. E. et al. (2015)
Meteorit. Planet. Sci., 50, 715-732.