Lunar and Planetary Science XLVIII (2017) 1276.pdf

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, Mas- sachusetts Institute of Technology, Cambridge, MA 02139, USA; 4Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA.

Introduction: Establishing the absolute and the gravity data from the Gravity Recovery and Interior relative chronology of ancient lunar events is of Laboratory (GRAIL) mission to re-examine the ages for fundamental importance to our understanding of early the formation of lunar terranes and the chronological Solar System history and the evolution of rocky sequence of major impact basin formation [7-11]. planetary bodies. In this endeavor, the Moon has a Topography (km) (a) 10 unique quantitative role, as it is the only planetary body -9

from which absolute and relative ages can be calibrated 90°000N 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 0010° 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 90002° S 180° 005W 0040° 180003 ° E known to exist. (b) 90°000 N 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 0010° poorly constrained ages of the prominent South Pole- Aitken (SPA) and Nectaris basins [5]. Furthermore, attempts to assign relative ages to the major impact 90002° S basins through the application of traditional crater size- 180° 005W 0040° 180003 ° E frequency analyses are often frustrated by the extensive Figure 1. (a) Craters of diameter greater than 90 km (outlined deposits of dark basaltic plains, or maria, that in black) over a shaded relief map of topography. (b) Surface preferentially flooded and presently obscure the mare deposits (outlined in cyan) and QCMAs (magenta) of diameter greater than 90 km on a morphologic base map. primary surfaces of major impact basins on the lunar nearside [5-6]. To estimate the relative ages of these Methodology: The QCMAs identified in the lunar mare-flooded basins, previous workers used either a gravitational field and shown in Figure 1 have been patchwork of unflooded surfaces of small area [e.g., 6] proposed to represent a population of craters with or made adjustments for the size-frequency surface expressions obscured by the superposition of distributions of mare-covered regions [8], but both of volcanic deposits (maria) or material ejected by those methods inject uncertainty and potential bias, younger impact events. The contribution of the QCMAs especially for heavily flooded basins such as Serenitatis may be assessed with an incremental size-frequency [5-6]. More comprehensive treatments have augmented distribution (SFD) N(D), where N is the number of the traditional crater size-frequency analyses with craters of diameter D (in km) or greater per unit area stratigraphic inferences to establish the relative ages (106 km2). In Figure 2a, we show ratios of incremental and chronologic sequence of lunar basins [e.g., 5], but SFD of impact craters in non-mare regions to those of the uncertain crater density of the pre-mare nearside mare regions binned at 20-km diameter intervals, where surface nonetheless remains an obstacle in establishing each bin includes craters of diameter D ± 20 km. For a reliable chronology. each bin, the crater density is estimated from areal maps Despite the combination of returned samples and constructed with a 500-km-radius moving window the well-preserved state of much of the lunar surface, average, similar to those in Figures 2b and 2c; the there are many unanswered questions surrounding basin errors shown are calculated from the weighted standard and terrane chronology. In this investigation, we jointly error on the mean. use craters with a recognizable surface expression and The incremental SFD in Figure 2a that includes those inferred from quasi-circular mass anomalies QCMAs shows that the apparent deficits of craters in (QCMAs), considered to be buried craters, preserved in mare regions compared with non-mare regions are the lunar gravitational field and revealed by analyses of Lunar and Planetary Science XLVIII (2017) 1276.pdf

nearly eliminated at large diameters, with mare and N(90) value of 12.1±3.2. Since the PKT could have non-mare regions exhibiting similar incremental SFDs formed as late as ~4.3 Ga, on the basis of the youngest for D ≥ 90 km. With QCMAs included in maps of the age for urKREEP crystallization from the lunar magma crater distribution, the crater density of the nearside ocean [12], it then follows that the SPA impact must lunar maria is generally indistinguishable from that of have occurred prior to ~4.3 Ga. To determine the the surroundings at crater diameters greater than 90 km relative ages of lunar basins shown in Figure 3, we use (Fig. 2b), although the density deficits at the centers of N(90) values, inclusive of QCMAs, of the full region major impact basins (e.g., Orientale, Serenitatis, interior to the main rim diameter. As noted by Crisium, Imbrium) can still be readily observed. Given Hartmann and Wood [7], the variations in the derived that mare volcanism had little apparent effect on the crater densities will not be directly proportional to age, SFD and cumulative crater densities of the combined because of a non-uniform cratering rate over time, but set of craters and QCMAs with D ≥ 90 km, we choose instead establish relative (crater retention) ages. this crater and QCMA diameter cutoff to assess the Nonetheless, we find that the N(90) values of those relative ages of lunar geochemical terranes and basins. basins with D ≥ 650 km are in general agreement with (a) 5004 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

005 these basins have a relative age that is greater than that of the PKT (when using their combined area),

Incremental SFD Ratio determined at the 99% confidence level, indicative that

003 0 000 001 002 such basins impacted the Moon prior to ~4.3 Ga. In 20 006 200 Diameter (km) contrast, the N(90) of Serenitatis is statistically

(b) ◦ 90°◦ N 20 90 N N(90) 20 indistinguishable from that of Imbrium. CSCS Our results show that by using craters and QCMAs II 2 SS 2 C with D ≥ 90 km, new constraints on the ages of basins C km km 6 0◦°◦ 6 0 FF SmH and geochemical terranes can be established. In N O Nu N O HSm Nu particular, QCMAs can be used to establish a relative N(90)/10 AA N(90)/10 MR SPASPA MR age for the PKT and, once anchored to an absolute age ◦ ◦ 90 S 00 9090° SS ◦ ◦ ◦ from urKREEP crystallization times, can further 180180180° W◦WW 00°◦ 180180180◦E°EE (c) ◦ 90° N N(90) 20 constrain the pre-Nectarian basins that formed prior to CS Smythii and Coulomb-Sarton to be older than ~4.3 Ga. I 015 N(90)015 2 S 2 C km km 6 6 014 014 0°◦ 0 F SmH Pre-Nectarian Nectarian Imbrian N O Nu 013 01330 Fecunditatis (3) HSm Australe (3) N(90)/10 A N(90)/10 Coulomb-Sarton (4) A Nubium (3) Smythii (4) SPA MR 012 012 ◦ SPA interior (1) 90° S 0 SPA (1) 180 ◦ W 0◦ 180◦ E 011 01120 Nectaris (10) 180180° W W 0° 180180E° E Mendel-Rydberg (10) Figure 2. (a) Ratio of incremental SFD for craters in non- 010 010 Serenitatis (11) mare regions to that in mare regions both with (blue) and Humorum (11) 009 Imbrium(12) without (black) QCMAs. Ratios of incremental SFDs are 009 10 Crisium (11) Orientale (12) determined at intervals of 20 km in diameter and for 40-km- 008 008 diameter bin sizes. Errors follow from the weighted standard 007 0070 error on the mean. The red line denotes a 1:1 ratio. Eckert IV 000000 001 001 2 002 002 4 003 003 6 004 004 8 005 005 10 006 006 12 Basin Groups projections of N(90) (b) with QCMAs and (c) without Figure 3. Cumulative crater density N(90) clustered by QCMAs (averaged over a circular window of 500-km radi- stratigraphic group, according to Wilhelms [5]. us) are also shown. Major basins are outlined and labeled. References: [1] Neukum G. et al. (1975) Moon, 12, 201- Results: The use of N(90) for crater age 229. [2] Marchi S. et al. (2009) Astron. J., 137, 4936-4948. assessments provides ages generally unbiased by [3] Hiesinger H. et al. (2011) GSA Special Paper 477, 1-51. volcanism. The elimination of this bias is particularly [4] Le Feuvre M. L. and Wieczorek M. A. (2011) Icarus, 214, important for the nearside major impact basins that 1-20. [5] Wilhelms D. E. (1987) USGS Prof. Paper 1348. [6] were substantially flooded by maria. For the lunar Fassett C. I. et al. (2012) JGR, 117, E00H06. [7] Hartmann geochemical terranes, although we find that the N(90) 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. values of the SPA basin and Feldspathic Highlands (2013) Science, 339, 668-671. [10] Neumann G. A. et al. Terrane (FHT) are indistinguishable, 17.9±2.0 and (2015) Science Advances, 1, e150002852. [11] Evans A. J. et 17.2±3.1, respectively, both terranes are significantly al. (2016) GRL, 43, 2445-2455. [12] Borg L. E. et al. (2015) older (99% confidence level) than that of the Meteorit. Planet. Sci., 50, 715-732. Procellarum KREEP Terrane (PKT), which has an