Elsevier

Planetary and Space Science

Volume 59, Issue 14, November 2011, Pages 1804-1814
Planetary and Space Science

Light scattering by complex particles in the Moon's exosphere: Toward a taxonomy of models for the realistic simulation of the scattering behavior of lunar dust

https://doi.org/10.1016/j.pss.2011.01.003Get rights and content

Abstract

It is suspected that the lunar exosphere has a dusty component dispersed above the surface by various physical mechanisms. Most of the evidence for this phenomenon comes from observations of “lunar horizon glow” (LHG), which is thought to be produced by the scattering of sunlight by this exospheric dust. The characterization of exospheric dust populations at the Moon is key to furthering our understanding of fundamental surface processes, as well as a necessary requirement for the planning of future robotic and human exploration.

We present a model to simulate the scattering of sunlight by complex lunar dust grains (i.e. grains that are non-spherical and can be inhomogeneous in composition) to be used in the interpretation of remote sensing data from current and future lunar missions. We numerically model lunar dust grains with several different morphologies and compositions and compute their individual scattering signatures using the Discrete Dipole Approximation (DDA). These scattering properties are then used in a radiative transfer code to simulate the light scattering due to a dust size distribution, as would likely be observed in the lunar exosphere at high altitudes 10's of km. We demonstrate the usefulness and relevance of our model by examining mode: irregular grains, aggregate of spherical monomers and spherical grains with nano-phase iron inclusions. We subsequently simulate the scattering by two grain size distributions (0.1 and 0.3μm radius), and show the results normalized per-grain. A similar methodology can also be applied to the analysis of the LHG observations, which are believed to be produced by scattering from larger dust grains within about a meter of the surface.

As expected, significant differences in scattering properties are shown between the analyses employing the widely used Mie theory and our more realistic grain geometries. These differences include large variations in intensity as well as a positive polarization of scattered sunlight caused by non-spherical grains. Positive polarization occurs even when the grain size is small compared to the wavelength of incident sunlight, thus confirming that the interpretation of LHG based on Mie theory could lead to large errors in estimating the distribution and abundances of exospheric dust.

Research highlights

► Complex lunar dust grain modeled and their scattering properties computed. ► The scattering by a lunar exosphere composed of such is simulated. ► Comparison is made with the standard model for spherical grains. ► Significant variations in scattering are observed between the two models. ► Interpretation of observations based on spherical grains would lead to large errors.

Introduction

It has been suggested that the observations of “lunar horizon glow” (LHG) during the Apollo era could have been caused by sunlight scattering from micron and sub-micron populations of dust in the Moon's exosphere (Rennilson and Criswell, 1974, McCoy and Criswell, 1974, McCoy, 1976, Zook and McCoy, 1991). Since the inferred exospheric dust abundances were orders of magnitude greater than the expected production rate from meteoritic ejecta alone (e.g. Rennilson and Criswell, 1974), it has been suggested that these dust populations were electrostatically transported by various mechanisms (see Colwell et al., 2007, and references therein). Both the lunar surface and exospheric dust are electrostatically charged by the photoemission of electrons from solar UV, as well as the collection of charged particles from the surrounding space plasma environment (Freeman and Ibrahim, 1975, Halekas et al., 2002, Colwell et al., 2007, Stubbs et al., 2007a). If the lunar surface and an exospheric dust grain are like-charged, then the dust grain is electrostatically repelled from the surface, which could result in it being transported about the surface of the Moon (Sickafoose et al., 2002, Colwell et al., 2007, Nitter et al., 1998, Stubbs et al., 2006, Stubbs et al., 2007b). The processes by which a dust grain scatters incident sunlight and becomes electrostatically charged are both highly dependent on the grain shape and composition (Richard and Davis, 2008, Whipple, 1981). Therefore, the accurate inversion of grain shapes and composition from LHG measurements is critical to characterize and understand exospheric dust abundances, dynamics and sources. Here we describe a method for constraining the size and geometry of exospheric dust from measurements of LHG intensities and polarization as a function of scattering angle.

The first LHG observations were acquired by the TV cameras aboard some of the Surveyor landers while looking toward the local horizon just after sunset (Rennilson and Criswell, 1974). This low altitude LHG occurred within about a meter of the surface and persisted upto 2.5 h. It appeared to be produced by dust grains with radii of 5–6μm with line-of-sight column concentrations of 50 grains cm−2. Arguably, the most compelling evidence for the presence of electrostatically transported dust came from the Lunar Ejecta and Meteorites (LEAM) experiment that was deployed on the surface by Apollo 17 to measure the interplanetary dust flux (a few impacts per day) with velocities from ∼1 to 25 km s−1. Instead, LEAM mostly appeared to detect highly charged lunar dust grains moving at speeds of order 100 m s−1, with count rates of up to hundreds of impacts per day that peaked around the sunrise and sunset terminators (Berg et al., 1976, Colwell et al., 2007). These measurements were not consistent with expectations for meteoritic ejecta on ballistic trajectories, and the peak in dust activity around the terminator (where strong electric fields are expected to occur) strongly suggests a connection with LHG.

Evidence for high altitude exospheric dust, up to altitudes above 100 km, came from Apollo astronaut observations and coronal photography from orbit while within lunar shadow (McCoy and Criswell, 1974, McCoy, 1976). Apollo astronaut sketches showed a narrow glow spreading along the lunar horizon that first appeared approximately 3 minutes before orbital sunrise when the spacecraft was about 28° from the terminator (solar zenith angle SZA 118°). Analyses of the Apollo 15 and 17 coronal photographs revealed an “excess brightness” that decayed rapidly in intensity with altitude and distance from the terminator. These LHG observations were interpreted as being caused by a dust population with grain radius of 0.1μm and scale height of 10–20 km (Zook and McCoy, 1991). The astrophotometer aboard the Lunokhod-2 rover also detected a brighter than expected lunar twilight sky (Severnyi et al., 1975), while Zook et al. (1995) suggested that LHG may have been observed by the Clementine star tracker cameras. We point out that horizon glow observations near the terminator appear to be a highly variable phenomenon. For example, a search for LHG was undertaken during Apollo 16, but it was not observed (McCoy and Criswell, 1974).

In this study we focus on characterizing the properties of the high altitude exospheric dust population, which will be most readily observed from orbit by instruments such as the Ultraviolet Spectrometer (UVS) aboard the Lunar Atmosphere and Dust Environment Explorer (LADEE) mission. LADEE is currently scheduled for launch in 2013. However, we recognize that the methodology described here could also be applied to interpreting the low altitude LHG detected by some of the Surveyor landers, which is anticipated to be observed by future landers, such as those planned by the Russian and European space agencies.

The upper few centimeters of the lunar regolith, from which this exospheric dust originates, is a layer of fine grains, the product of comminution by micrometeoroid impacts. The mean grain size of soil samples returned from the Apollo and Luna programs averages between 60 and 80μm, with a sub-micron population that has been difficult to characterize until recently. Scanning electron photomicrographs of Apollo regolith samples revealed the presence of a variety of grain morphologies, from agglutinates with irregular and sharp edges to smooth glass droplets of volcanic origins (for a review of the lunar regolith properties, see Heiken et al., 1991.) Using scanning electron micrographs of Apollo 17 samples Park et al. (2006) identified four prominent shapes for micron-scale particles: spherical, angular blocks, glass shards and irregular.

The ultra-fine particle content of the regolith (100nm) has been characterized by Greenberg et al. (2007) and Liu et al. (2008). These particles exhibit compact ellipsoid shapes with small aspect ratios. Based on these results, one is tempted to assume that exospheric dust will be composed of nearly spherical grains at the sub-micron scale and contain more complex shapes only at the micron scale and above. However, this would be neglecting the propensity for the cohesiveness of particulates to increase as their mean size decreases, a result of the scaling of surface forces with particle surface and mass, making finer powders far more cohesive. Partly for this reason, the sub-micron dust population in lunar regolith samples has only recently been quantitatively characterized—the difficulty residing in the separation and counting techniques that need to be used for sub-micron particles. Standard geological techniques are unable to separate ultra-fine powders (sifting for example is ineffective for particle sizes less than about 20μm). Other separation methods such as aerodynamic shearing technique used in aerosol science appear to be more appropriate (Greenberg et al., 2007, Liu et al., 2008). Powder flow-ability also decreases in lower gravity, a fact confirmed on lunar regolith by comparisons between in-situ and earth-based cohesive properties. Non-intuitively, hard vacuum also increases particulate cohesiveness; when particles are coated with adsorbed gases, the inter-particle distance increases, leading to a decrease in atomic and molecular forces, while the added capillary adhesion force due to the adsorbed species is smaller (See Walton et al., 2007 for a review of factors influencing lunar dust adhesive and cohesive properties). In fact, even after the separation process, Greenberg et al. (2007) observed aggregates consisting of these ultra-fine dust grains. Thus far, isolated grains smaller than ∼20 nm have not been confidently detected. Whether this is due to the aggregative properties of the lunar regolith or a bias of the laboratory measurements is unclear. The storage history of the samples might also play a role in the depletion of the smallest particles, as they tend to preferentially stick to container walls and any other surface they come into contact with. Therefore, it is likely that a significant fraction of ultra-fine dust grains is transported either as aggregates, or as so-called “parasites” adhering onto larger “host” grains. These arguments do not preclude the movement of ultra-fine grains, since one can not rule out mechanisms that could lead to grain separation. For example, Glenar et al. (this issue) have pointed out a possible connection between meteoritic flux and the presence of sub-micron populations of exospheric dust. It is possible that meteoritic impacts, perhaps mostly by micro-meteoroids, could serve as a mechanism for liberating sub-micron dust from the strong cohesive forces at the surface. The relative abundances of ultra-fine compact, near-spherical individual grains, grain clumps and complex aggregates in the naturally transported dust distribution depend on the processes of release and transport, which remain uncertain at the present time. Therefore, it would seem wise to include not only spherical grains but also observed grain shapes in radiative transfer models for the lunar exospheric dust at all altitudes.

It is well-known that the shape of the particle scattering phase function conveys important “1st order” information on the size of aerosol particles (both dust and volatiles). This simple fact is the motivation for measurements of scattered light intensity as a function of solar phase angle for planetary and cometary atmospheres as well as for the lunar exosphere. In the latter case, the treatment of the radiative transfer is simplified by the assumption of single scattering due to the very small scattering optical depths expected for exospheric dust (Stubbs et al., 2010). Likewise, the polarization state of this scattered light offers both size and shape information.

The scattering behavior of complex individual particles is a rapidly evolving area of study (cf. Mishchenko, 2009), but it is still mostly limited by computing power and experimental difficulties. Because of the wide diversity of plausible size, shape and environment of origin, there is no general theory to predict it, so models and experimental measurements are mostly limited to specific applications. Lasue and Levasseur-Regourd (2006) and Hadamcik et al., 2003, Hadamcik et al., 2006, Hadamcik et al., 2007 modeled cometary observations of light scattered by porous irregular aggregates of sub-micron grains of silica and organics, a structure relevant not only for comets and the study of soot particles (Liu and Mishchenko, 2007) but also for lunar regolith (Richard and Davis, 2008). Other recent work addresses various aspects of scattering by non-spherical particles such as the effect of surface roughness (Nousiainen and Muinonen, 2007, Zubko et al., 2007), and of the edge sharpness of aggregates (Moreno et al., 2007). Progress in our understanding of scattering and polarization by non-spherical grains is summarized in recent several books and review papers by Mishchenko et al., 2000, Mishchenko et al., 2002 and Mishchenko (2009).

Despite these advances and their obvious importance for studies of terrestrial clouds and aerosols, non-terrestrial observations are still mostly interpreted using the analytically convenient Mie theory. Scattering properties depend on the size parameter X=2πaeff/λ; i.e. the ratio of the effective target size to the wavelength, where the effective target radius aeff is the spherical radius. The accuracy of Mie predictions can be acceptable if the particles are small compared to the wavelength of the incident light, i.e. for X1, or if the particles are near spherical, but it does not apply for larger X>1 and more complex particles. Several NASA missions have made, or will make, optical measurements of lunar dust, transported either by natural phenomenon or by anthropogenic mechanisms. In order to interpret these measurements, a description is needed of the scattering properties exhibited by the fine-fraction in Apollo samples, and thus expected in the lunar environment. Mie-based modeling is relevant in some situations, but only realistic models can allow us to investigate the circumstances under which Mie can be used to interpret dust properties and abundances.

The objective of this current study is to examine the scattering properties of some representative grain shapes observed in the fine-fraction of Apollo samples, on the assumption that one or more are responsible for the observed scattering in the lunar exosphere. We illustrate the scattering properties of “virtual” lunar dust grains using a robust numerical technique, the Discrete Dipole Approximation (DDA), which should allow more confident interpretation of both existing and future measurements. We expect that these results will lead to more accurate interpretations of dust scattering measurements. We describe the techniques used to compute individual scatterer signatures, and present some illustrative results based on our chosen morphologies. We then describe how these models can be applied to predict the scattering signature of dust in terms of size-distribution averaged particle scattering behavior. We restrict our focus to the smaller high altitude dust grains in this paper, as current and near-term lunar missions will acquire measurements from lunar orbit. We discuss the differences between our model and Mie-based simulations and the consequences for the interpretation of lunar horizon-glow observations.

Section snippets

Grain modeling

A central objective of our project is the development of a “virtual lunar simulant”, a collection of numerical models of dust grains which possess scattering properties of the fine and ultra-fine fractions of the lunar regolith. This virtual simulant will be useable in radiative transfer models analogous to the way physical simulants are used in engineering model testing.

The present paper demonstrates the effect of taking into account complex particle shapes in radiative transfer models of the

Scattering behavior in the lunar environment

In the previous sections, we have compared the optical scattering parameters (efficiencies, scattering and polarization phase functions) of measured grain shapes with those of a homogeneous spherical target. We now apply these results to spectral simulations of forward scattering by dust grains, as would be observed by an orbiting spectrometer. We incorporate the DDA scattering properties of the previous section (as well as those of comparison Mie spheres) to a radiative transfer model, using

Summary and conclusions

We modeled three families of lunar dust grain structure: aggregates of spherical monomers, irregular grains and spherical grains with inclusions of nano-phasic iron. We numerically computed their light scattering properties both as individual grains and as distributions of mixed particle types, through a combination of discrete dipole approximation and radiative transfer models. The DDA single grain scattering modeling demonstrates significant deviation from the standard idealized homogeneous

Acknowledgments

This work was funded in part by NASA Grants: NNX09AO79G (LASER), NNX09AG78A (NLSI/DREAM), and NNX08AM76G (LROPS). Resources supporting this work were provided by the NASA High-End Computing (HEC) Program through the NASA Advanced Supercomputing (NAS) Division at Ames Research Center. DDA computations have been conducted using the DDSCAT software developed by B.T. Draine and P.J. Flatau. Figures have been created in part with the Grace plotting software, the R software environment for

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