Research Projects
∙ Early evolution of terrestrial planets
Crustal Annulus of Lunar Impact Basins Controlled by Regional Thermal State
The Moon reveals striking asymmetries in the crustal structure and chemical composition between its nearside and farside. With the gravity-based crustal thickness model, we find that most large impact basins with rim diameters greater than 450 km on the nearside have no crustal annulus, which is instead common for basins on the farside. Previous impact hydrodynamic simulations show that the thermal state has significant influences on the impact cratering process. However, the effects of the thermal state on the long-term post-impact relaxation, not well studied, may be equally important for explaining the different crustal annulus structures between the nearside and farside basins. In this work, we export crustal structure and impact-induced thermal anomaly from impact hydrodynamic simulations to subsequent post-impact viscoelastic relaxation models, and vary the near-surface temperature gradient and radiogenic element content (represented by Th content as a proxy) to explore influences of the crustal thermal state on the post-impact relaxation of the annulus. Our modeling results indicate that as the basin structure starts with a relatively hot state and cools afterwards, a prolonged high-temperature (>1,200 K) duration of the crustal annulus is required for its effective relaxation, which corresponds to a high near-surface temperature gradient or Th content of the crust. More specifically, a near-surface temperature gradient greater than 30 K/km in combination with a Th content of 4 ppm for the bulk crust (or a Th content of 10 ppm for a 10 km-thick KREEP layer underlying the crust), likely representing the early thermal state of the nearside, can produce the complete annulus relaxation. In contrast, a near-surface temperature gradient lower than 20 K/km with a crustal Th content of 1 ppm, possibly representing the thermal state of the lunar farside, produces limited relaxation. Our modeling results show that the thermal state of the lunar crust controls the final crustal annulus structure of impact basins, which can explain the different crustal annuli of large impact basins between the nearside and farside of the Moon.
Publication: Ding, M., & Zhu, M.-H. (2022). Effects of Regional Thermal State on the Crustal Annulus Relaxation of Lunar Large Impact Basins. Journal of Geophysical Research: Planets, 127(3), e2021JE007132. https://doi.org/10.1029/2021JE007132
EOS Editor's Highlight: An Impact Basin Thermometer for the Moon. https://eos.org/editor-highlights/an-impact-basin-thermometer-for-the-moon
Lunar Compositional Asymmetry Caused by SPA-induced Mantle Convection
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.
Publication: Zhang, N., Ding, M.*, Zhu, M.-H., Li, H., Li, H. & Yue, Z (2021). Lunar compositional asymmetry explained by mantle overturn following the South Pole–Aitken impact. Nature Geoscience. DOI: 10.1038/s41561-021-00872-4.
Nature Research Highlight: Cosmic crash explains a mystery on the Moon. https://www.nature.com/articles/d41586-022-00064-z
Phys.org News: Model suggests differences between near and far side of moon due to cosmic impact millions of years ago. https://phys.org/news/2022-01-differences-side-moon-due-cosmic.html
∙ GRAIL Data: Using Impact Craters as Probes
Publication: Ding, M., & Zhu, M.-H. (2022). Effects of Regional Thermal State on the Crustal Annulus Relaxation of Lunar Large Impact Basins. Journal of Geophysical Research: Planets, 127(3), e2021JE007132. https://doi.org/10.1029/2021JE007132
Ding, M., Soderblom, J. M., Bierson, C. J., & Zuber, M. T (2021). Investigating the Influences of Crustal Thickness and Temperature on the Uplift of Mantle Material Beneath Large Impact Craters on the Moon. J. Geophys. Res. Planets. DOI:10.1029/2020JE006533.
Ding, M., Soderblom, J. M., Bierson, C. J., Nimmo, F., Milbury, C., & Zuber, M. T. (2018). Constraints on lunar crustal porosity from the gravitational signature of impact craters. J. Geophys. Res. Planets, 123. https://doi.org/10.1029/2018JE005654
∙ Mars Lithospheric Flexure
Publication: Ding, M., Lin, J., Gu, C., Huang, Q., & Zuber, M. T. (2019). Variations in Martian Lithospheric Strength Based on Gravity/Topography Analysis. Journal of Geophysical Research: Planets, 124, 3095–3118. https://doi.org/10.1029/2019JE005937
∙ Seamount Subduction
Publication: Ding, M., & Lin, J. (2016). Deformation and faulting of subduction overriding plate caused by a subducted seamount. Geophys. Res. Lett., 2016GL069785. https://doi.org/10.1002/2016GL069785
