Projectile remnants in central peaks of lunar impact craters

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Abstract

The projectiles responsible for the formation of large impact craters are often assumed to melt or vaporize during the impact, so that only geochemical traces1,2 or small fragments3,4 remain in the final crater. In high-speed oblique impacts, some projectile material may survive5,6,7, but this material is scattered far down-range from the impact site. Unusual minerals, such as magnesium-rich spinel8,9 and olivine10,11, observed in the central peaks of many lunar craters are therefore attributed to the excavation of layers below the lunar surface. Yet these minerals are abundant in many asteroids, meteorites and chondrules12,13,14,15. Here we use a numerical model to simulate the formation of impact craters and to trace the fate of the projectile material. We find that for vertical impact velocities below about 12 km s−1, the projectile may both survive the impact and be swept back into the central peak of the final crater as it collapses, although it would be fragmented and strongly deformed. We conclude that some unusual minerals observed in the central peaks of many lunar impact craters could be exogenic in origin and may not be indigenous to the Moon.

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Figure 1: Formation of Copernicus crater by a low-speed impact.
Figure 2: Formation of Copernicus crater by a high-speed projectile.
Figure 3: Impact velocity distribution on the Moon.

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Acknowledgements

We gratefully acknowledge the developers of iSALE2D, the simulation code used in our research, including G. Collins, K. Wünnemann, B. Ivanov and D. Elbeshausen. We thank B. Bottke for pointing out the further implications of Mg-spinel and helpful discussion. The research was supported by the National Natural Science Foundation of China (Grant No. 41002120 and 41171355) and NASA PGG grant NNX10AU88G.

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Z.Y. conceived the research and implemented the Copernicus simulations with the help of B.C.J., who refined the hydrocode models to better fit the observations and created the crater profile from LOLA data; H.J.M. first realized the possibility that the olivine in Copernicus central peaks might be from the projectile; D.A.M. proved the presumption of low impact velocity on the lunar surface, and Z.Y. confirmed the similarity of mafic minerals between the central peaks and the asteroid. K.D., W.H. and Y.L. obtained and processed data used to support our research.

Corresponding author

Correspondence to H. J. Melosh.

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The authors declare no competing financial interests.

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Yue, Z., Johnson, B., Minton, D. et al. Projectile remnants in central peaks of lunar impact craters. Nature Geosci 6, 435–437 (2013). https://doi.org/10.1038/ngeo1828

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