The density and porosity of lunar rocks

Abstract

[1] Accurate lunar rock densities are necessary for constructing gravity models of the Moon's crust and lithosphere. Most Apollo-era density measurements have errors of 2-5% or more and few include porosity measurements. We report new density and porosity measurements using the bead method and helium pycnometry for 6 Apollo samples and 7 lunar meteorites, with typical grain density uncertainties of 10-30 kg m À3 (0.3-0.9%) and porosity uncertainties of 1-3%. Comparison between igneous grain densities and normative mineral densities show that these uncertainties are realistic and that the helium fully penetrates the pore space. Basalt grain densities are a strong function of composition, varying over at least 3270 kg m À3(high aluminum basalt) to 3460 kg m À3 (high titanium basalt). Feldspathic highland crust has a bulk density of 2200-2600 kg m À3 and porosity of 10-20%. Impact basin ejecta has a bulk density of 2350-2600 kg m À3 and porosity of $20%.

Ring faults and ring dikes around the Orientale basin on the Moon

“…Both of these values are in keeping with the measured porosities of lunar samples (Kiefer et al, 2012). However, subsequent GRAIL analyses have shown that the best-fit density and porosity is a function of wavelength, consistent with a density that increases with depth, with an average farside density gradient of 35 kg/m 3 /km .…”

“…This interpretation is supported by the fact that some of the negative gravity gradients are aligned with smooth non-mare material within the depressions adjacent to the Outer Rook, though in other places the negative gravity gradient ring occurs in rough high-standing material. However, this interpretation is at odds with the corrugated and fractured appearance of the Orientale melt in the Maunder formation that is suggestive of a high porosity (Head, 1974), and the measured high porosity of lunar impact melt breccias (Kiefer et al, 2012). Melted mantle material that has been predicted to splash out over the surface during the impact could provide a greater density contrast ), but has not been identified in remote sensing data.…”

“…The best-fit dike widths from these inversions scale with the inverse of the density contrast, assumed to be 500 kg/m 3 here. Density contrasts up to 700 kg/m 3 (Kiefer et al, 2012) would reduce the widths by up to 30%. We note that these widths are much larger than typical dikes observed on Earth, and are more similar to the randomly oriented ancient igneous intrusions revealed by previous analyses of GRAIL data dike width (km) 0 f g h probability probability probability…”

“…4) with an assumed density contrast of 500 kg/m 3 , consistent with the density contrast between the lunar upper crust of ~2550 kg/m 3 and measured bulk densities of Apollo samples of lunar basalts of ~3010-3247 kg/m 3 (Kiefer et al, 2012). In the Monte Carlo model, the width, top depth, and dip of the dikes were left as free parameters.…”

Small-scale density variations in the lunar crust revealed by GRAIL

“…However, the samples obtained from the Apollo missions, as well as lunar meteorites, show that the lunar crust is compositionally heterogeneous, with major rock types including anorthosite, norite, troctolite, and sometimes gabbro (Dymek et al, 1976;Papike et al, 1998;Warren, 1993;Wieczorek et al, 2006). The samples vary widely in both porosity and density (Kiefer et al, 2012a), and the distribution of equivalent rocks in the crust is not constrained. Similar variations in density arising from composition and porosity would be the likely explanation for both the large-scale and small-scale density variations observed in the GRAIL data.…”