Deep Structure of the Lunar South Pole‐Aitken Basin

Abstract

The South Pole‐Aitken basin is a gigantic impact structure on the far side of the Moon, with an inner rim extending approximately 2,000 km in the long axis dimension. The structure and history of this basin are illuminated by gravity and topography data, which constrain the subsurface distribution of mass. These data point to the existence of a large excess of mass in the Moon's mantle under the South Pole‐Aitken basin. This anomaly has a minimum mass of 2.18 × 1018 kg and likely extends to depths of more than 300 km. Plausible sources for this anomaly include metal from the core of a differentiated impactor or oxides from the last stage of magma ocean crystallization. Although the basin‐forming impact event likely excavated the vast majority of the preexisting crust, the present‐day crust of the basin interior is at least 16 km thick in undisturbed regions.

Evidence for a Stratified Upper Mantle Preserved Within the South Pole‐Aitken Basin

“…The SPA interior exhibits distinctive geochemical properties (e.g., Jolliff et al., 2000), including broad iron and thorium anomalies and a pyroxene‐dominated mineralogical signature (Borst et al., 2012; Hu et al., 2019; Huang et al., 2020; Lawrence et al., 2000; Lucey et al., 1998; Moriarty & Pieters, 2018; Ohtake et al., 2014; Pieters et al., 2001). Additionally, the basin exhibits unusual volcanic (Chen et al., 2020; Hagerty et al., 2011; Ivanov et al., 2018; Moriarty & Pieters, 2015, 2018; Pasckert et al., 2018; Pieters et al., 2001; Whitten & Head, 2015; Yingst and Head, 1999) and geophysical (James et al., 2019) properties. While SPA exhibits a lower degree of mare volcanism than nearside basins, features such as Mons Marguerite (formerly Mafic Mound) and the SPA Compositional Anomaly (SPACA) appear to be volcanic resurfacing deposits with mineralogies distinct from typical mare basalts (Moriarty & Pieters, 2015, 2018), suggesting a unique and localized thermal and magmatic history.…”

An Object‐Oriented Bayesian Gravity Inversion Scheme for Inferring Density Anomalies in Planetary Interiors

“…Evidence for a deep mass anomaly below SPA has been advanced by James et al. (2019) but it is in a different location than we consider here.…”

“…There is no evidence that this deep feature exists within the Moon but we include it in the target model to investigate the potential of the THeBOOGIe to detect deep mantle features. Evidence for a deep mass anomaly below SPA has been advanced by James et al (2019) but it is in a different location than we consider here.…”

Inferring the Shallow Layered Structure at the Chang’E‐4 Landing Site: A Novel Interpretation Approach Using Lunar Penetrating Radar

“…The Chinese lunar probe Chang’E‐4, carrying the Yutu‐2 rover, was the first human‐made object that landed on the far‐side of the Moon on 3rd of January 2019 (Li et al., 2019; Tang et al., 2020). The landing site is located at the South Pole‐Aitkens (SPA) basin—the oldest and biggest crater on the Moon (Huang et al., 2018; Hu et al., 2019; James et al., 2019). The SPA basin is pre‐Nectarian in age and has an elliptical shape (Garrick‐Bethell & Zuber, 2009) with an approximate diameter of 2,100–2,500 km (Moriarty et al., 2013).…”