The role of very fine particle sizes in the reflectance spectroscopy of plagioclase-bearing mixtures: New understanding for the interpretation of the finest sizes of the lunar regolith

Highlights

• 5–20% of lunar regolith is very fine grained (<10 µm).

• Very fine sizes increase the albedo but reduce the spectral contrast.

• Very fine plagioclase has featureless spectrum.

• Mafic materials are affected by fine sizes.

Abstract

The lunar surface consists of a regolith layer that covers the underlying bedrocks, and is generally characterized by particulates <1 cm. Lunar soil is the fine fraction of the regolith, and is generally between 60 and 80 µm. Sizes <10 µm, accounting for ca. 5–20% of the soil, were recognized and petrologically classified.

The coarsest sizes of the regolith are chemically and mineralogically similar, while the finest fractions are more feldspathic, probably due to easier fracturing of plagioclase than mafic minerals.

Due to the more feldspathic nature of the very fine lunar soils, in this paper, we quantitatively investigate the influence of very fine (<10 µm) plagioclase on the absorption bands of mafic minerals using the Modified Gaussian Model. We considered two plagioclases with different iron content and two mafic end-members (1) 56% orthopyroxene and 44% clinopyroxene, and (2) 30% orthopyroxene and 70% olivine. We also compared our results with the deconvolution of the same mixtures at coarser sizes. Our results mainly show that:

(1) fine sizes act principally on reflectance and on spectral contrast (with the former increasing and the latter decreasing); (2) very fine plagioclase has a blue slope in the Near Infrared and very shallow 1250 nm band depth, close to zero; (3) consequently, the plagioclase band is always shallower than mafic bands; (4) in mixtures with olivine, the composite band center always shows the typical olivine value, differently from coarser mixtures; and (5) mafic materials have a blue slope in the Short Wavelength Infrared Region, a more V-shaped 1 µm pyroxene absorption and the 1 µm mafic band centers are shifted by ca. 40 nm vs. coarse sizes, reflecting a different weight within the crystal field absorption of the mafic component in very fine size. We also evidenced that a coarse plagioclase could be overestimated, while a very fine one could be underestimated if compared with the 63–125 µm size.

Introduction

Very fine sizes dominate many planetary surfaces and their regolith, e.g., Moon, Mars and Mercury. Different particle size fractions in the regolith (such as soil and dust) affect the optical properties of the surface in different ways. For this reason, very fine particles and their effects on the reflectance spectra of the most common planetary minerals have to be investigated in detail to obtain correct information about the mineralogical composition and the surface texture.

In particular, the lunar surface consists of a regolith layer that covers the underlying bedrocks, with the exception of steep-sided crater walls, central peaks and lava channels (McKay et al., 1991), as shown by the lunar landings and observations. Due to the absence of atmosphere on the Moon, the lunar regolith is the result of different processes, e.g., the impact of meteoroids and bombardments of protons from the sun and the stars, and is generally considered to be characterized by material lower than 1 cm in size (McKay et al., 1991). The fine fraction of the regolith, deriving from mechanical disintegration of lunar rocks, both basaltic and anorthositic, constitutes the lunar soil. The average lunar soil size is generally between 60 and 80 µm (McKay et al., 1991). Lunar dust consists of even finer material than lunar soils (ca. <50 µm). Furthermore, sizes <10 µm, which comprise ca. 5–20% of the soil, have been recognized and petrologically classified (Laul et al., 1978, Laul et al., 1979, Laul and Papike, 1980).

Silicate minerals, such as orthopyroxene (OPX), clinopyroxene (CPX), olivine (OL) and plagioclase (PL), are the most important constituents of the lunar surface, associated with oxides (e.g., Papike et al., 1991), and can be spectrally identified on the basis of their absorption bands. While the iron-richer mafic minerals have always been easily detected (e.g., Tompkins and Pieters, 1999, Spudis et al., 1984), only recently, and thanks to the improvements in spectrometers onboard during the last missions, the absorption due to the low amount of Fe²⁺ in PL could be detected (e.g., Ohtake et al., 2009, Cheek and Pieters, 2012a, Cheek et al., 2012b, Kramer et al., 2013), permitting new evaluation of its modal abundance and composition.

While coarse sizes of the regolith are chemically and mineralogically very similar, the finest fractions, <10 µm, are different and more feldspathic. This may be due to simple comminution processes and easier fracturing of PL than to mafic minerals (Laul et al., 1978, Laul et al., 1979, Laul and Papike, 1980, Devine et al., 1982).

According to the more feldspathic nature of the very fine lunar soils, in this paper we present results for a set of PL-bearing mixtures analyzed at the <10 µm particle size, also proposing a comparison with coarser mixtures analyzed by Serventi et al., 2013, Serventi et al., 2015, to investigate the effects of very fine sizes on the reflectance spectroscopy of lunar-like minerals.