The Variability of Lunar Mare Basalt Properties Inferred from Present-Day Surface Rock Abundance

Abstract

The surface rock abundance map derived from the Lunar Reconnaissance Orbiter (LRO) Diviner Lunar Radiometer Experiment (Diviner) revealed variability in the rock abundance across the surface of the lunar maria [1]. Rocks on the lunar surface break down quickly relative to lunar geologic history [2, 3], so surface rock abundance is likely to be strongly tied to subsurface rock content which could include both coherent layers of mare basalts or large boulders mixed in with regolith. Most of the Moon’s surface is now covered in fine grained regolith, and historically various authors have argued that each surface unit started as a flat coherent layer of rock which gradually broke down into a layer of regolith whose thickness is a function of its bombardment history [e.g. 4, 5]. However, recently Head and Wilson (2020) [6] argued that modern understanding of lunar volcanism suggests substantial variability in post eruption surface conditions (e.g. void space, pyroclastic deposits, etc.) which could affect subsequent regolith development possibly leading to surfaces of the same age having regolith layers of different thicknesses and/or suspended rock populations. We compare the Diviner rock abundance [1] in different maria units defined and dated by Hiesinger et al. (2011) [7] to investigate both the change in surface rock abundance with time, and possible regional variability in rock properties [8]. We find that surface rock abundance does decrease with unit age as expected for a thickening layer of regolith. However, there is significant scatter in this relationship. We calculate the best-fit linear relationship between the median rock abundance and age of the units defined by Hiesinger et al. (2011) [7]. Investigation of the residuals of this fit reveals that they are not random. For example, Mare Australe is similar in age to Mare Tranquillitatis, but nearly all units in Mare Tranquillitatis are rockier than those in Mare Australe. Mare Humorum is notable for being one of the rockiest regions in the maria despite its relatively ancient surface (>3 Ga). These observations support the hypothesis of Head and Wilson (2020) [6], and suggest that further investigation into the properties of present-day surface rocks may provide insight into the initial mare basalts before billions of years of communition. Specifically, future in situ missions across diverse mare locations could offer insights into the variability of basaltic eruption styles that may have formed the lunar maria.   [1] Bandfield+ (2011), JGR, 116, E00H02. [2] Basilevsky+ (2013), PSS, 89, 118. [3] Ghent+ (2014) Geology, 42, 1059. [4] McKay+ (1991) Lunar Sourcebook, Cambridge Press, 285. [5] Hörz (1977), PCE, 10, 3. [6] Head+ (2020) GRL, 47. [7] Hiesinger+ (2011), GSA Special Papers, 477. [8] Elder+ (2023) PSJ, 4:244.