Latitudinal variation in spectral properties of the lunar maria and implications for space weathering

Highlights

• The spectral properties of the lunar maria vary systematically with latitude.

• Lunar swirls and high latitude maria are both bright due to reduced solar wind flux.

• We distinguish between the effects of solar wind and micrometeoroid weathering.

Abstract

Space weathering alters the optical properties of exposed surfaces over time, complicating the interpretation of spectroscopic observations of airless bodies like asteroids, Mercury, and the Moon. Solar wind and micrometeoroids are likely the dominant agents of space weathering, but their relative contributions are not yet well understood. Based primarily on Clementine mosaics, we report a previously unrecognized systematic latitudinal variation in the near-infrared spectral properties of the lunar maria and show that the characteristics of this latitudinal trend match those observed at ‘lunar swirls’, where magnetic fields alter local solar wind flux without affecting the flux of micrometeoroids. We show that the observed latitudinal color variations are not artifacts of phase angle effects and cannot be accounted for by compositional variation alone. We propose that reduced solar wind flux, which should occur both at swirls and toward higher latitudes, is the common mechanism behind these color variations. This model helps us quantify the distinct effects of solar wind and micrometeoroid weathering and could aid in interpreting the spectra of airless bodies throughout the Solar System.

Introduction

‘Space weathering’ refers to the processes by which the optical properties of airless bodies change due to exposure to solar wind and micrometeoroid impacts. However, the difficulties of reproducing space-weathering conditions in the laboratory, or returning weathered samples to Earth, make it challenging to determine precisely how space weathering operates (Pieters et al., 2000, Pieters et al., 2012, Hapke, 2001, Vernazza et al., 2009, Domingue et al., 2014). Remote sensing measurements, studies of lunar samples, and laboratory experiments have established that solar wind ion and micrometeoroid bombardment weaken spectral absorption features and cause the lunar surface to darken and redden (increase in spectral continuum slope in the visible and near-infrared) with time. These changes appear to be due to some combination of the formation of impact glasses and agglutinates (Adams and McCord, 1971), the regolith’s disintegration into increasingly finer soils (Pieters et al., 1993), and the accumulation of nanophase iron (Hapke, 2001, Sasaki et al., 2001, Noble et al., 2007). Larger impacts also expose fresh material, which then gradually matures until the reflectance spectrum reaches a steady state, which we call ‘equilibrium color’ for simplicity.

The equilibrium color varies considerably across the lunar surface, due primarily to differences in mineralogy. This is most obvious in the dichotomy between the bright, anorthositic highlands and the darker basaltic maria. However, as we will argue, the presence of ‘lunar swirls’ suggests that equilibrium color may also be influenced by the flux of weathering agents, rather than just their total accumulation (see Sections 3.1 Color variation at lunar swirls, 4 Discussion). If this is the case, then equilibrium color may also vary with latitude. Both solar wind and micrometeoroids originate primarily from within the ecliptic plane, which is inclined from the Moon’s equator by just 1.5°. Hence maximum flux of these weathering agents occurs near the equator, with flux decreasing as incidence angle increases toward the poles.

This paper’s central observation is that, when we examine imagery from across the lunar surface, we find that the equilibrium color does vary systematically with latitude. In Section 3.2, we show that this latitudinal color trend persists across a range of distinct compositions and that it is not an artifact of phase angle biases in the Clementine mosaics. Interestingly, the spectral properties of the latitudinal color trend match the characteristic color variation found at lunar swirls. In Section 3.1, we quantify the characteristics of the swirl-related color variation and, in Section 3.2, we show that it is statistically equivalent to the observed latitudinal color trends, with a transition toward higher latitudes being attended by the same color change that occurs toward brighter parts of swirls. Finally, in Section 4, we argue that the best candidate for a common mechanism behind these color variations is altered solar wind flux. We present a qualitative model illustrating how this hypothesis comports with the observations and we discuss the possible implications with respect to the interpretation of spectral data, particularly at high latitudes.