Are the Moon's Nearside‐Farside Asymmetries the Result of a Giant Impact?. Issue 8 (12th August 2019)

• Record Type: Journal Article
• Authors: Zhu, Meng‐Hua; Wünnemann, Kai; Potter, Ross W.K.; Kleine, Thorsten; Morbidelli, Alessandro
• Is Part Of: Journal of geophysical research. Volume 124:Issue 8(2019)
• Page Start: 2117
• Page End: 2140
• Publication Date: 2019-08-12
• Subjects: Moon's asymmetries -- Giant impact -- KREEP -- Nearside's lowlands; Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2018JE005826
• Languages: English
• ISSNs: 2169-9097

Abstract

Abstract: The Moon exhibits striking geological asymmetries in elevation, crustal thickness, and composition between its nearside and farside. Although several scenarios have been proposed to explain these asymmetries, their origin remains debated. Recent remote sensing observations suggest that (1) the crust on the farside highlands consists of two layers: a primary anorthositic layer with thickness of ~30‐50 km and on top a more mafic‐rich layer ~10 km thick and (2) the nearside exhibits a large area of low‐Ca pyroxene that has been interpreted to have an impact origin. These observations support the idea that the lunar nearside‐farside asymmetries may be the result of a giant impact. Here using quantitative numerical modeling, we test the hypothesis that a giant impact on the early Moon can explain the striking differences in elevation, crustal thickness, and composition between the nearside and farside of the Moon. We find that a large impactor, impacting the current nearside with a low velocity, can form a mega‐basin and reproduce the characteristics of the crustal asymmetry and structures comparable to those observed on the current Moon, including the nearside lowlands and the farside's mafic‐rich layer on top of a primordial anorthositic crust. Our model shows that the excavated deep‐seated KREEP (potassium, rare earth elements, and phosphorus) material, deposited close to the basin rim, slumps back into the basin and covers the entire basin floor; subsequent large impacts can transport the shallow KREEP material to the surface, resulting in its observed distribution. In addition, our model suggests that prior to the asymmetry‐forming impact, the Moon may have had an 182 W anomaly compared to the immediate post‐giant impact Earth's mantle, as predicted if the Moon was created through a giant collision with the proto‐Earth. Plain Language Summary: Beginning with the Apollo era, spacecraft observations have shown that the Moon has striking asymmetries between its nearside and farside in topography, crustal thickness, and composition. These asymmetries are likely a product of very early geological processes on the Moon. Understanding their formation mechanism may help to constrain models of global lunar evolution and magma‐ocean crystallization. Several hypotheses have been suggested, though none can explain the observations satisfactorily. Recent spacecraft observations from the Gravity Recovery and Interior Laboratory (GRAIL) mission indicate that the farside crust is ~20‐km thicker than the nearside crust, and remote sensing data show that this extra crust is composed of a mafic‐rich layer covering the primary crust. This layered crustal structure on the farside, together with a large area of low‐Ca pyroxene on the nearside observed by Kaguya mission that was explained to be formed via impact through melting a mixture of crust and mantle materials, means that a giant impact on the nearside may explain the nearside‐farside asymmetries. To investigate this possibility, we quantitatively studied the giant impact theory using numerical modeling. Our models confirm that a giant impact on the nearside can explain nearside‐farside asymmetries. We demonstrate that a large body slowly impacting the nearside of the Moon can reproduce the observed crustal thickness asymmetry and form both the farside highlands and the nearside lowlands. Additionally, the model shows that the resulting impact ejecta would cover the primordial anorthositic crust to form a two‐layer crust on the farside, as observed. Overall, the modeling results are generally in agreement with assumptions that are based observations and provide credible explanations for the observed asymmetries in crustal thickness and elevation. This work also provides a plausible explanation for the existence of KREEP (potassium, rare earth element, and phosphorus) on the lunar surface. A very important implication of this work is that it can explain the conundrum about isotopic differences between the Earth and Moon, particularly the significant anomaly of 182 W in the Moon, as this anomaly would occur if this giant impact added material to the Moon after the initial Moon‐forming. Our model can thus explain this isotope anomaly in the context of the giant impact scenario of the Moon's origin. In summary, this work quantitatively supports the long‐standing hypothesis that a giant impact resulted in the Moon's nearside‐farside asymmetries and the Procellarum KREEP terrain was formed as a consequence of such an impact event. In addition, this work also provides a reference for reconstructing the early history of planetary bodies with similar asymmetries, such as Mars. Key Points: Giant impact on the nearside of the Moon can reproduce the crustal thickness asymmetry between the nearside and farside The nearside lowlands and KREEP materials on the lunar surface can be explained by the giant impact scenario Giant impact predicts that the Moon has a significant 182 W anomaly after the Moon formation, as expected by the collision formation of the Moon