The Effects of Basin Formation on the Lunar Geochemical Terranes

Introduction: The early history of the Moon, namely the first ~700 My following its formation, was an era marked by the formation of 43 basins (impact craters >300 km in diameter) [1,2,3]. Basin formation during this period had significant effects on the entire lunar crust; material from depth was excavated and redistributed across the entire surface [4]. The material redistributed by basins carried the geochemical signature of its source region and thus basin-forming impacts distributed that signature across the lunar surface [5,6]. In this study, we show that modeling the distribution of total accumulated ejecta from basins indicates that the distribution of geochemical terranes on the lunar surface may be a result of the redistribution of material during the heavy bombardment. Images of the farside of the Moon reveal a surface almost entirely lacking the large regions of mare basalt found on the nearside. The origin of this distinction between the near and farside of the Moon remains unknown. Other large-scale differences have been found on the Moon as well. Geophysical measurements from lunar orbiters indicate that there is a difference between the thickness of the nearside and farside crust [7,8]. A number of recent orbital missions have measured elemental abundances across the lunar surface [9,10]. Analysis of such data has shown that at least three geochemical terranes, each with their own distinct properties, cover large continuous regions of the lunar surface and represent an additional nearside-farside dichotomy [11]. Here we investigate an additional nearside-farside dichotomy: the cumulative amount of basin ejecta. Comparisons between the observed geochemical terranes and our estimated abundances of basin ejecta across the Moon are made. First we examine geochemical and geophysical differences across the Moon and how these may be related in part to the accumulation of basin material. This is followed by a discussion of the distribution of basin material across the entire Moon and how this distribution may give rise to the present location of the terranes. Geochemical Observations of the Lunar Surface: Jolliff et al. [11] analyzed both Clementine and Lunar Prospector data in order to assess the intricate geochemistry of the lunar surface. Using iron estimates derived from the multispectral Clementine UVVIS data [12] as well as Lunar Prospector thorium data, Jolliff et al. divided the Moon into three terranes based on their FeO and Th characteristics, each presumably having unique geologic histories. The three primary terranes are the Procellarum KREEP Terrane (PKT), the South Pole-Aitken Basin Terrane (SPAT), and the Feldspathic Highlands Terrane (FHT). The location of these terranes is identified in Figure 1a.