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Change in the Earth–Moon impactor population at about 3.5 billion years ago

Abstract

The bombardment of impactors (leftover planetesimals, asteroids and comets) created numerous impact craters on the Moon. The giant planets in the outer Solar System are believed to have experienced a dynamical instability, in which the migration of giant planets delivers impactors to the inner Solar System bodies1,2. The difference between the population of large (diameter more than ~5 km) impact craters observed on heavily cratered lunar highlands and that on the lunar maria3 was thought to support the lunar Late Heavy Bombardment, which started ~0.6 billion years after planet formation and could have been caused by the late instability of giant planets4,5,6. However, large craters on various-aged lunar surfaces have similar size–frequency distributions when considering the preferential erasure of small craters7,8. In addition, dynamical and geochemical evidence favour an early instability of giant planets at ~4.5 Gyr ago2,5,9. Here, we report the evolution at geological scales of regolith thickness on the Moon, which is a proxy for the change of the size–frequency distribution slope for Earth–Moon impactors with diameters less than ~50 m (which generate craters with diameters less than ~1 km (ref. 10)). We found an abnormally slow growth of regolith thickness per unit of impact flux before \(3.5_{ - 0.6}^{ + 0.3}\) Gyr ago (3σ uncertainty), which can best be explained by a population of craters of less than ~1 km whose size–frequency distribution had a shallower power-law slope (\(- 2.57_{ - 0.16}^{ + 0.30}\)) than that afterward (\(- 3.24_{ - 0.06}^{ + 0.03}\)). The transition time at ~3.5 Gyr ago supports the early instability of giant planets, in which dominant Earth–Moon impactors changed from leftover planetesimals to asteroids5. The value of −3.24 is consistent with the preferential delivery of small asteroids via Yarkovsky–YORP effects11.

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Fig. 1: Regolith thicknesses at Apollo landing sites.
Fig. 2: Crater production populations deciphered from the evolution of regolith thickness on the Moon.
Fig. 3: Locations of studied geological units and examples of fresh crater measurements.

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Data availability

The Lunar Reconnaissance Orbiter Camera narrow-angle camera images used in this work were downloaded from the Space Exploration Resources Web Map Service System (http://wms.lroc.asu.edu/lroc/search) and processed using the US Geological Survey Integrated Software for Imagers and Spectrometers, which is available at https://isis.astrogeology.usgs.gov/. Measured fresh craters are given in the Supplementary Information. Source data are provided with this paper.

Code availability

The code used to produce the figures and calculate the results is available from M.X. upon reasonable request.

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Acknowledgements

This work was supported by the B-type Strategic Priority Program of the Chinese Academy of Sciences (grant no. XDB41000000), the pre-research Project on Civil Aerospace Technologies (nos. D020202 and D020101) funded by the Chinese National Space Administration, the Science and Technology Development Fund of Macau (0042/2018/A2, 043/2016/A2) and the National Natural Science Foundation of China (41830214, 11573005). We thank W. F. Bottke for the helpful discussion, and the NASA Lunar Reconnaissance Orbiter Camera team for providing high-resolution optical images.

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Contributions

Regolith evolution model and regolith thickness measurements were performed by M.X. in collaboration with Z.X., L.X., W.F. and A.X.; all authors contributed to writing the manuscript.

Corresponding authors

Correspondence to Minggang Xie or Zhiyong Xiao.

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The authors declare no competing interests.

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Peer review information Nature Astronomy thanks Takashi Ito and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Examples of production and equilibrium functions described as equations (1) and (2).

The intersection between the production and equilibrium populations gives Deq. PF shape keeps constant under aac.

Source data

Extended Data Fig. 2 Measurements of fresh craters.

Identified fresh normal (the red circles), flat-bottomed and central-mound (the green circles), and concentric craters (the blue circles) on (a) Apollo 11, (b) Apollo 12, (c) Apollo 15, (d) Apollo 17, (e) Luna 16, (f) Luna 24, (g) Serenitatis S2, (h) Serenitatis S16, (i) impact melt of the Copernicus crater, (j) a region near Chang’e-3 landing site (k) a region at the candidate landing region of Chang’e-5, (l) Sinus Medii SM2, and (m) Ibn Yunus Ma1. The landing sites are shown in the green stars. Secondary crater fields are excluded (light yellow). All the NAC images are shown in Mercator projections. The detailed information of NAC images used for the study is listed in the Extended Data Fig. 5, and measured diameters are given in the Supplementary Information.

Extended Data Fig. 3 Crater distributions at the Apollo 11 landing site.

(a) The SFDs of differential crater number (light color gives 1σ uncertainty) versus geometric mean diameter. (b) The fraction of craters in each diameter bin. The squares represent the data used to derive the crater size-frequency distributions. We use a Gaussian kernel in the kernel density estimator with standard deviation of 0.1D, where D is the diameter of a measured fresh crater.

Source data

Extended Data Fig. 4 The thickness distributions of regolith thicker than T (that is, W(>T)) derived from the SFDs of normal (red) and concentric (blue) craters.

Regolith thicknesses determined by the Apollo in situ measurements (black dashed line) are shown for comparison.

Source data

Extended Data Fig. 5 Summary of NAC image information, cratering frequency and regolith thickness.

1Both left and right NAC images are used in this study. 2The total number of measured fresh craters.

Extended Data Fig. 6 Craters (red circles) on the 7 studied units used to derive N(>1).

Craters (red circles) on the 7 studied units used to derive N(>1). Craters mapped by Robbins46 on (a) the impact melt of crater Copernicus, (b) Chang’e-5 (unit boundary taken from Zhang, et al.65), (c) Serenitatis S2 (unit name and boundary taken from Hiesinger, et al.66), (d) Chang’e-3, (e) Serenitatis S16 (unit name and boundary taken from Cai, et al.67), (f) Sinus Medii SM2 (unit name and boundary taken from Hiesinger, et al.66), and (g) Ibn Yunus Ma1 (unit name and boundary taken from Hiesinger, et al.66). Crater mapping areas are shown in blue polygons. The density of D>1 km craters on the above locations are given in the Extended Data Fig. 5. Craters marked in yellow are excluded, because they are prior to the formation of the units. The craters mapped by Robbins46 are almost complete, except only one crater (the green circle in (c)). Obvious secondary crater fields (light green) are excluded.

Source data

Supplementary information

Supplementary Data 1

Measured fresh craters.

Source data

Source Data Fig. 1

Data used to produce Fig. 1.

Source Data Fig. 2

Numerical data in ASCII format.

Source Data Fig. 3

Numerical data in ASCII format.

Source Data Extended Data Fig. 1

Numerical data in ASCII format.

Source Data Extended Data Fig. 3

Data used to derive the crater distributions.

Source Data Extended Data Fig. 4

Numerical data in ASCII format.

Source Data Extended Data Fig. 6

Craters at studied units used to derive N(>1).

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Xie, M., Xiao, Z., Xu, L. et al. Change in the Earth–Moon impactor population at about 3.5 billion years ago. Nat Astron 5, 128–133 (2021). https://doi.org/10.1038/s41550-020-01241-8

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