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Lunar compositional asymmetry explained by mantle overturn following the South Pole–Aitken impact

Abstract

The spatial distribution of mare basalts, titanium and KREEP (potassium, rare earth elements and phosphorus) on the Moon is asymmetrical between the nearside and farside. These asymmetries cannot be readily explained by solidification of a global magma ocean and subsequent mantle overturn, which should result in a layered and spherically symmetric lunar interior. Alternative scenarios have been proposed to explain the observed compositional asymmetry, but its origin remains enigmatic. Here, we present hydro- and mantle convection numerical simulations of the giant impact event that formed the South Pole–Aitken basin—the largest impact basin on the Moon—and the subsequent impact-induced convection with the assistance of gravitational instability. We find that the impact induces thermochemical instabilities that drive the dense KREEP-rich ilmenite-bearing cumulate to migrate towards the nearside following lunar magma ocean solidification. This results in the formation of a chemical reservoir under the nearside crust that could explain the observed geochemical asymmetries. We suggest that enrichments of ilmenite and KREEP in the nearside hemisphere following the South Pole–Aitken impact event provide a viable explanation for the wide composition range of mare basalts observed on the lunar surface.

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Fig. 1: Comparison between observed and modelled compositional asymmetries.
Fig. 2: Modelled thermochemical structures at six evolutionary stages during and after the SPA impact.
Fig. 3: Modelled IBC temporal evolution and spatial distribution.

Data availability

The data for the iSALE and CitcomS models required to reproduce key figures in this Article are provided in a Zenodo repository (SPA Overturn v2, https://doi.org/10.5281/zenodo.5652899). The element abundance data were retrieved from the Geosciences Node of NASA’s Planetary Data System (https://pds-geosciences.wustl.edu/lunar/lp-l-grs-5-elem-abundance-v1/lp_9001/data/), the albedo map from the Annex of PDS Cartography & Imaging Sciences Node (https://astrogeology.usgs.gov/search/map/Moon/Clementine/UVVIS/Lunar_Clementine_UVVIS_750nm_Global_Mosaic_118m_v2) and the mare boundaries from a digitized database (http://wms.lroc.asu.edu/lroc/view_rdr/SHAPEFILE_LROC_GLOBAL_MARE). Source data are provided with this paper.

Code availability

At present, iSALE is not a fully open-source. It is distributed on a case by case basis to academic users in the impact community, strictly for non-commercial use. Scientists interested in using or developing iSALE should see http://www.isale-code.de for a description of application requirements. CitcomS is an open-source software available at Computational Infrastructure for Geodynamics (https://geodynamics.org/cig/software/citcoms/).

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Acknowledgements

We are grateful for discussions with Y. Liang, E. M. Parmentier and S. Zhong. We acknowledge the developers of iSALE-3D and thank the Computational Infrastructure for Geodynamics for distributing CitcomS software. This work is supported by the National Natural Science Foundation of China via grant no. 41674098 and pre-research project of the Civil Aerospace Technologies of China National Space Administration grant no. D020205 (to N.Z.), Macau Science and Technology Development Fund grant no. 0020/2021/A1, National Natural Science Foundation of China grant no. 12173106 and pre-research project on Civil Aerospace Technologies of China National Space Administration grant no. D020303 (to M.D.), Macau Science and Technology Development Fund 0079/2018/A2 and pre-research project on Civil Aerospace Technologies of China National Space Administration grant no. D020202 (to M.-H.Z.) and Strategic Priority Program of the Chinese Academy of Sciences grant no. XDB41000000 (to Z.Y.). The geodynamics computational work was supported by resources provided by the High-performance Computing Platform of Peking University and the Pawsey Supercomputing Center with funding from the Australian Government. The impact simulations were conducted on the High-Performance Computer at Macau University of Science and Technology, supported by the Macau Science and Technology Development Fund.

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Authors and Affiliations

Authors

Contributions

N.Z. and M.D. developed the concept and M.-H.Z. framed the work. M.-H.Z. conducted the 3D impact modelling. N.Z. and Haoyuan Li performed the convection experiments. N.Z., M.D. and M.-H.Z. analysed the results and wrote the manuscript. Huacheng Li and Z.Y. joined the initial discussions.

Corresponding author

Correspondence to Min Ding.

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Nature Geoscience thanks Ian Garrick-Bethell and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editors: Simon Harold, Stefan Lachowycz.

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Extended data

Extended Data Fig. 1 SPA-induced mantle uplift and thermochemical structure.

(a-d) Modeled region with uplifted mantle (grey dots with black boundaries) for four cases with varied impactor size (420 and 315 km) and impact angle (30, 45, and 60°). See Extended Data Table 1 for the labelling scheme of the cases. Bouguer gravity anomaly is shown for comparison, together with the inner (blue ellipse) and outer (red ellipse) rims of the SPA basin28. The plots are presented in Azimuthal projection centred at (169°W, 53.2°S), the center of SPA basin28. (e-h) Corresponding impact-induced temperature anomaly and IBC re-distribution, similar to Fig. 2b. The chemical structures are plotted in the 2D cross section that cut through the SPA center (corresponding to the white dashed great-circle in Fig. 1c) with greenish IBC materials. The thermal structures are plotted as 3D contours of the 1,800 °C isotherm (red contours). The SPA spatial extent is marked by grey double-headed arrows (e-h).

Source data

Extended Data Fig. 2 Final evolution states of modeled thermochemical structures.

The final state is reached after the disappearance of pushing effect of impact-induced hot anomalies at ~ 300 Myr (a-g) and ~ 200 Myr (h) after the impact. Similar to Fig. 2f but for different test cases: (a) I420_deg60_v-2D50 (that is, the reference case, same with Fig. 2f); (b) I315_deg60_v-2D50; (c) I420_deg45_v-2D50; (d) I420_deg30_v-2D50; (e) I420_deg60_v-3D50; (f) I420_deg60_v-1D50; (g) v-2D50 (the base case without the SPA impact); (h) I420_deg60_v-2D50_den0 (no IBC density contrast with respect to the surrounding mantle). Mantle flow vectors are included in (h) to demonstrate that the impact-induced hot anomalies cannot maintain the global degree-one convection without the assistance of mantle overturn.

Source data

Extended Data Fig. 3 Modelled final distribution of the IBC thickness.

Similar to Fig. 1c but without vertical integration. The cases shown for comparison are the same with those in Extended Data Fig. 3: (a) I420_deg60_v-2D50 (that is, the reference case); (b) I315_deg60_v-2D50; (c) I420_deg45_v-2D50; (d) I420_deg30_v-2D50; (e) I420_deg60_v-3D50; (f) I420_deg60_v-1D50; (g) v-2D50 (the base case without the SPA impact); (h) I420_deg60_v-2D50_den0 (no IBC density contrast with respect to the surrounding mantle). The last two cases (using a different colour bar from other cases) demonstrate that neither the impact-induced hot anomalies nor the IBC gravitational instability alone can lead to a hemispherical azimuthal migration of IBC. The plots are presented in the Mollweide equal‐area projection centred at 90°W longitude, with the nearside on the right side of the map and the farside on the left side.

Source data

Extended Data Fig. 4 Comparison between observed and modelled degree-one TiO2 distribution.

Degree-one component of (a) observed TiO2 distribution (corresponding to Fig. 1a) and (b) modeled TiO2 distribution for the reference case (corresponding to Fig. 1c). Mare boundaries and the SPA boundary are shown by black contours and white curve, respectively. Data are presented in a Mollweide equal‐area projection centred at 90°W longitude, with the nearside on the right side of the map and the farside on the left side.

Source data

Extended Data Table 1 Input and output parameters for tested models

Supplementary information

Supplementary Video 1

Modelled SPA-induced thermodynamic evolution. The impactor strikes the Moon from the top of this cross-section at an impact angle of 60°. This cross-section is the same as in Fig. 2, but rotated by 180° and horizontally flipped. Ejecta at locations distant from the SPA basin are numerical artefacts caused by a limited resolution of the simulation. This video is for the reference case and lasts for 8 h.

Supplementary Video 2

Modelled post-impact thermochemical evolution with mantle overturn. Top: evolution of thermochemical structure with red 3D contours representing the 1,800 °C isotherm and green colour representing the IBC fraction, similar to Fig. 2. Bottom: corresponding evolution of the IBC thickness, similar to Extended Data Fig. 4. The video is generated for the reference case and lasts for 270 Myr.

Source data

Source Data Fig. 1

Source data for the modeled TiO2 and Th content of our reference model in Fig. 1c,d.

Source Data Fig. 2

Source data for Fig. 2 (that is, our reference case). It is 3D ASCII data compressed by the software 7-zip.

Source Data Fig. 3

Source data for the compositional evolution of our reference model in Fig. 3a, and source data for the IBC shape in Fig. 3b.

Source Data Extended Data Fig. 1

Modeled tracer points with mantle uplift from iSALE-3D models in Extended Data Fig. 1. Locations of these points (currently centred at 52° S, 180° E) should be shifted and rotated to be consistent with the location of SPA.

Source Data Extended Data Fig. 2

Source data for plotting Extended Data Fig. 2b-h. The data source for Extended Data Fig. 2a is the same as that for Fig. 2. It is 3D ASCII data compressed by the software 7-zip.

Source Data Extended Data Fig. 3

Modeled IBC distribution from CitcomS shown in Extended Data Fig. 3.

Source Data Extended Data Fig. 4

Source data for degree-1 components of the observed and modeled TiO2 content in Extended Data Fig. 4.

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Zhang, N., Ding, M., Zhu, MH. et al. Lunar compositional asymmetry explained by mantle overturn following the South Pole–Aitken impact. Nat. Geosci. 15, 37–41 (2022). https://doi.org/10.1038/s41561-021-00872-4

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