Global characteristics of porosity and density stratification within the lunar crust from GRAIL gravity and Lunar Orbiter Laser Altimeter topography data

Abstract

The Gravity Recovery and Interior Laboratory (GRAIL) mission is providing unprecedentedly high-resolution gravity data. The gravity signal in relation to topography decreases from 100 km to 30 km wavelength, equivalent to a uniform crustal density of 2450 kg/m 3 that is 100 kg/m 3 smaller than the density required at 100 km. To explain such frequency-dependent behavior, we introduce rock compaction models under lithostatic pressure that yield radially stratified porosity (and thus density) and examine the depth extent of porosity. Our modeling and analysis support the assertion that the crustal density must vary from surface to deep crust by up to 500 kg/m 3 . We found that the surface density of megaregolith is around 2400 kg/m 3 with an initial porosity of 10-20%, and this porosity is eliminated at 10-20 km depth due to lithostatic overburden pressure. Our stratified density models provide improved fits to both GRAIL primary and extended mission data.

GRAIL gravity constraints on the vertical and lateral density structure of the lunar crust

“…The typical high mare surface density (>2600 kg m −3 ) presumably reflects a combination of compositional (the mare have higher grain densities) and porosity (the mare are relatively uncratered) effects. The general density increase with depth that characterizes the farside highlands presumably reflects a decrease in porosity with depth [ Wieczorek et al , ; Han et al , ], though a more mafic lower crust might contribute to this general gradient.…”

“…The resulting global effective density spectrum shows a decrease in density with increasing ℓ (see below). Since smaller degrees sample greater depths, this implies that density generally increases with depth [ Wieczorek et al , ; Han et al , ]. Note that ρ eff ( ℓ ) would be constant if the crustal density was constant with depth.…”

Impact cutoff frequency – momentum scaling law inverted from Apollo seismic data

“…GRAIL gravity and Lunar Orbiter Laser Altimeter topography data analysis (Han et al, 2014) reveals radially stratified porosity within 10-20 km thick (porous) layer. The latest porosity map derived from the GRAIL gravity measurements (Wieczorek et al, 2013) provided estimates of porosity in the farside highlands but not in the nearside maria regions.…”

“…During the early impact bombardment, when the largest impact events on the Moon had occurred, the lunar mega-regolith layer (e.g. Hartmann et al, 2000) was formed. Subsequently, megaregolith has been continuously bombarded by smaller impact events, which contributed to the formation of the upper-most finely grained regolith layer.…”

High‐Resolution Gravity Field Models from GRAIL Data and Implications for Models of the Density Structure of the Moon's Crust

“…Following Besserer et al () and Han et al (), we can fit theoretical models to measure effective density with equations and or equation and determine the parameters of interest: ρ s,lin , ρ s,exp , a , and d . Lateral density variations in the surface densities and gradient or e‐fold depth are obtained by means of localization.…”