Understanding the Lunar Nearside–Farside Dichotomy via In Situ Trace Element Measurements: The Scientific Framework of a Prospective Landed Mission

Abstract

Trace elements, distinguished by their low abundances (parts per million by weight (ppmw)), track local, regional, and planetary-scale processes in samples sourced from throughout the solar system. Such analyses of lunar samples have provided insights on its surface rocks and interpretations of its deep interior. However, returned samples, sourced from the lunar nearside, cannot be used to address processes responsible for the morphological dichotomy between the lunar nearside and farside. The hemispherical dichotomy points to distinct evolutionary histories of these two domains, rendering our understanding of lunar history incomplete. We outline the scientific justification for a landed, in situ investigation of lunar farside lithologies, focusing on chemical analyses that will constrain the Moon’s bi-hemispherical chemical evolution. Newly developed and heritage spaceflight instruments, capable of measuring low element abundances (limits of detection <10 ppmw ± 20%), can be deployed on the lunar farside and provide constraints on (1) the temperature and pressure of mare basalt crystallization, (2) depth-dependent mineralogical and compositional changes in the lunar mantle, (3) the chronology of major geologic events, and (4) abundances and distributions of refractory and heat-producing elements of the lunar farside mantle. The science return and logistical challenges of targeting four specific landing sites on the lunar farside are identified: Moscoviense, Apollo, Von Kármán, and Leibnitz craters. These sites maximize impact melt basin lithologies and later mare magmatism, and they minimize terrain hazards.