Are the Moon's Nearside‐Farside Asymmetries the Result of a Giant Impact?

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 182W 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.

Topographic Evolution of Von Kármán Crater Revealed by the Lunar Rover Yutu‐2

“…In order to investigate whether the topography at the CE‐4 landing site is dominated by the ejecta of neighboring craters of Von Kármán crater, we used the multimaterial, multirheology iSALE‐2D (Dellen version) shock physics code (Collins et al, ; Wünnemann et al, ) to model the impact cratering process and ejecta distribution of Finsen and Alder craters. The iSALE is based on the SALE (simplified arbitrary Lagrangian Eulerian) hydrocode (Amsden et al, ) and has been used to simulate both small‐scale laboratory experiments (Wünnemann et al, ) and large‐scale lunar basin formation (Miljković et al, ; Potter et al, ; Potter et al, ; Yue et al, ; Zhu et al, , , ).…”

“…The three 360°image mosaics (left is north, clockwise) produced from the Pancam's left-eye images taken at the locations marked as black dots in Figure 2 labeled by S1, S2, and S3. tracked the ejecta trajectories during the impact cratering process and estimated the ejecta thickness at any given distance from the impact site (see Wünnemann et al, 2016;Zhu et al, 2015Zhu et al, , 2017Zhu et al, , 2019.…”

Studying the Global Spatial Randomness of Impact Craters on Mercury, Venus, and the Moon With Geodesic Neighborhood Relationships

“…On the farside, the lunar maria are sparsely distributed and cover much smaller areas, mostly within large craters and basins. The asymmetric distribution of lunar maria has been attributed to increased volcanic activity on the lunar nearside due to a thinner crust and a higher abundance of heat producing elements (e.g., Head & Wilson, 1992; Joliff et al., 2000; Miljković et al., 2013; Wieczorek et al., 2012; Zhu et al., 2019). This in turn, resulted in higher subsurface temperatures compared to the lunar farside and the formation of larger craters and basins due to differences in target properties (Miljković et al., 2013).…”

Constraining the Evolutionary History of the Moon and the Inner Solar System: A Case for New Returned Lunar Samples

“…Remote sensing data have failed to detect the typical gravity anomaly associated with impact basins around the Oceanus Procellarum (Andrews- . Nevertheless, such a large impact event would not only provide a way to explain observed asymmetries between the lunar nearside and farside (Zhu et al 2019), but could also provide explanation for the ∼4.35-4.4 Ga model ages.…”

“…10-15 km thicker compared to the nearside crust . Several scenarios have been proposed to explain such dichotomy, including (i) asymmetric convection during LMO crystallisation and crustal growth (Loper and Werner 2002;Ohtake et al 2012), (ii) asymmetric impact cratering (Wood 1973), (iii) ejecta deposition from SPA (Zuber et al 1994), (iv) inhomogeneous early tidal heating (Garrick-Bethell et al 2010), (v) accretion of a companion moon (Jutzi and Asphaug 2011), or (vi) giant impact of a ∼500-800 km diameter impactor onto the lunar nearside soon after the Moon's formation (e.g., Zhu et al 2019). Returning samples from the farside crust would certainly help in disentangling between these different scenarios, in addition to providing opportunities to evaluate the possible chemical differences between farside magnesian anorthosites and nearside ferroan anorthosites (see Sect.…”