Aims. We present a multiband photometric study of the lunar mare surface based on in situ measurements using the Panoramic Camera on board the Chang'E-3 mission. We aim to obtain photometric properties and to investigate the differences in measuring sites in various conditions. Methods. Data were collected with a vast range of phase angles from 0.5° to 160°, probing 0–5° phases to constrain the opposition effect (OE) and phases above 120° to constrain the phase function. Stray light and spectral corrections were conducted to calibrate the regolith ’s bidirectional reflectance distribution function. We employed the Hapke photometric model to describe the regolith’s photometric properties. Results. Phase functions and Hapke parameters at three measuring sites were retrieved. The mare regolith at the measuring sites had a single-scattering albedo (SSA) of 0.17-0.21 for the green channel and exhibited backscattering and a strong OE. The coherent backscatter opposition effect (CBOE) represented the majority of OE, with a proportion of around 80% for pristine regolith. The phase ratio curves were arch-shaped, and the arch positions of the curves were associated with grain size.

Theoretical modelling has suggested that the deposition of charged solar energetic particles in the lunar regolith can generate electric fields strong enough to cause millimeter-scale dielectric breakdown, and that these violent electric discharges are responsible for such space weathering effects as submicron iron particles, glass, low albedo and reddish color. It is shown that, when secondary electron emission and the particulate, porous, reentrant structure of the regolith are taken into account, electric fields sufficient to initiate breakdown are unlikely to occur there.

The opposition effect is the sharp, narrow surge observed in the reflectance of a scattering medium near zero phase angle. Numerous observations and experiments have shown that the primary cause of the phenomenon in particulate media is coherent backscattering, in which wavelets traveling in opposite directions along chains of scatterers interfere constructively and generate the peak. A broader opposition surge caused by shadow hiding and preferential escape is also present, but is entangled with the incoherent continuum reflectance on which the coherent peak is superposed, making it difficult to identify and isolate. Theoretical models of media of independent scatterers predict that the angular width and shape of the coherent backscatter peak depend on the wavelength, porosity and particle size. It was hoped that remote measurements of the opposition effect would give information on the latter two quantities in planetary regoliths. However, observations and laboratory studies of media of large particles in contact with one another find little dependence on any of these quantities. Instead, these studies imply that the opposition effect in regolith-like media comes from reflection by short chains only a few scatterers long located on the surfaces of the particles of the medium, and that the lengths of these chains are proportional to the wavelength. Since the angular width of the peak is controlled by the ratio of the wavelength to the mean scattering chain length, the width is independent of wavelength. Because the wavelets never enter a particle, low albedo media can exhibit a strong coherent backscatter peak. Opposition effect peaks less than a degree wide on solar system bodies can imply an immature regolith; peaks several degrees wide imply a mature regolith.

OBSERVATIONS. Brett W. Denevi1, Hiroyuki Sato2,3, Anna C. Martin1, Aaron K. Boyd3, Bruce W. Hapke4, Mallory J. Kinczyk5, and Mark S. Robinson3, 1Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA. 2Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Tyuo-ku, Sagamihara, Kanagawa, Japan. 3Arizona State University, Tempe, AZ 85287, USA. 4University of Pittsburgh, Pittsburgh, PA 15260, USA. 5North Carolina State University, Raleigh, NC 27695, USA.

A critical typographical error and the value of a parameter in the new Lunar Reconnaissance Orbiter Wide Angle Camera TiO2 algorithm are corrected.

A critical typographical error and the value of a parameter in the new Lunar Reconnaissance Orbiter Wide Angle Camera TiO2 algorithm are corrected.

Introduction: Over nine years of mission operations since September 2009, the Lunar Reconnaissance Orbiter Camera (LROC) Wide Angle Camera (WAC) has acquired ~490,000 multispectral observations, resulting in a significant number of repeated observations. Especially in the northern high latitudes (>50°N), the WAC acquired three times of forwardpitch (up to 45°) observations [1] that extend the available phase angle range (to the lower values), resulting in more accurate photometric solutions. From this large WAC observation database, we derived a 7-color (321-689 nm) mosaic of the north pole as well as the 7 band polar Hapke parameter maps [2]. In this new mosaic, a systematically decreasing reflectance was found on the equator-facing slopes [2]. Due to the lower incidence angles on these slopes relative to the horizontal surfaces, however, the photometric artifact may cause an apparent darkening rather than the albedo of the surface materials. Here we examined the photometry on the slopes and revealed that the decreasing trend should be real. We also examined the north-polar trend of reflectance as a function of slope angle and slope azimuth. Methodology: We used all the WAC observations acquired above 60°N from January 2010 to August 2017. The EDR 8-bit DN values of all the images were radiometrically calibrated [3] to 32-bit radiance factor (I/F) [4]. The latitude, longitude, incidence (i), emission (e), and phase (g) angles were computed using the digital terrain model (DTM) from the Lunar Orbiter Laser Altimeter (LOLA) and the Kaguya Terrain Camera DTM [5,6] for each image pixel. To keep similar pixel scale across the wavelengths (~400 m/pixel in UV bands; ~100 m/pixel in visible bands), we downsampled visible bands to match the pixel scale of the UV bands. We also used the WAC north-polar color mosaic (>60°N) [2] to examine the correlations between the photometrically normalized I/F (nI/F) and the slopes. We selected the sampling box areas on the slopes darker than the surroundings and the neighboring flat surfaces for several sampling sites. For each sampling box area, we calculated the Hapke parameters (single scattering albedo w, phase function parameter b, and roughness parameter θ) [4] using the same calculation scheme as described in [7]. We note that the added angle variations by the pitch observations enabled this method. Results: The plot of I/F vs i for the site 1 (inside Froelich crater located at 80.0°N, 248.4°E; 57 km in diameter) shows a clear offset between the slopes and the crater floor (Fig.1). The offset in high i range (>60°) indicates the difference in albedo or macroscopic roughness. The derived Hapke parameters with several calculation settings (Table.1) indicate that the slope must have lower albedo to explain the I/F offset. All the sampling sites show a similar result, suggesting that the darker slopes are due to their lower albedo rather than the photometric artifact.

A new algorithm is proposed for estimating TiO2 abundance on the moon using lunar reflectance values measured by the Wide Angle Camera on the Lunar Reconnaissance Orbiter spacecraft. The algorithm provides useful values for mature regoliths on the entire lunar surface including highlands and low titanium maria. However, it underestimates the abundances of immature regoliths, so that the algorithm returns a lower limit for such features as young craters and rays.

DATA IN TERRESTRIAL CONTEXT. R.M. Nelson1 , M. D. Boryta2, B.W. Hapke3, K.S. Manatt4, Y.G. Shkuratov5, V.A. Psarev5, K. Vandervoort6, D. Kroner2,7,A. Nebedum2, C. Vides2,6, J. Quinones2,8 and Y. Wu8; 1Planetary Science Institute, 775 North Mentor Avenue, Pasadena, CA, rnelson@psi.edu, 2Mt. San Antonio College, Walnut, CA, 3University of Pittsburgh, Pittsburgh, PA, 4Jet Propulsion Laboratory, Pasadena CA, 5Karazin University, Kharkiv, Ukraine, 6California Polytechnic State University at Pomona, Pomona, CA. 7University of California at Los Angeles, Los Angeles, 8California State University at Los Angeles, Los Angeles, CA. 8Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, China

The Moon's reflectance spectrum records many of its important properties. However, prior to Chang'E-3 (CE-3), no spectra had previously been measured on the lunar surface. Here we show the in situ reflectance spectra of the Moon acquired on the lunar surface by the Visible-Near Infrared Spectrometer (VNIS) onboard the CE-3 rover. The VNIS detected thermal radiation from the lunar regolith, though with much shorter wavelength range than typical thermal radiometer. The measured temperatures are higher than expected from theoretical model, indicating low thermal inertia of the lunar soil and the effects of grain facet on soil temperature in submillimeter scale. The in situ spectra also reveal that 1) brightness changes visible from orbit are related to the reduction in maturity due to the removal of the fine and weathered particles by the lander's rocket exhaust, not the smoothing of the surface and 2) the spectra of the uppermost soil detected by remote sensing exhibit substantial differences with that immediately beneath, which has important implications for the remote compositional analysis. The reflectance spectra measured by VNIS not only reveal the thermal, compositional, and space-weathering properties of the Moon but also provide a means for the calibration of optical instruments that view the surface remotely.

We present reflectance and polarization phase curve measurements of highly reflective planetary regolith analogues having physical characteristics expected on atmosphereless solar system bodies (ASSBs) such as a eucritic asteroids or icy satellites. We used a goniometric photopolarimeter (GPP) of novel design to study thirteen well-sorted particle size fractions of aluminum oxide (Al2O3). The sample suite included particle sizes larger than, approximately equal to, and smaller than the wavelength of the incident monochromatic radiation (lambda = 635 nm). The observed phase angle, alpha, was 0.056 < alpha < 15 degrees. These Al2O3 particulate samples have very high normal reflectance (> similar to 95%). The incident radiation has a very high probability of being multiply scattered before being backscattered toward the incident direction or ultimately absorbed. The five smallest particle sizes exhibited extremely high void space (> similar to 95%). The reflectance phase curves for all particle size fractions show a pronounced non-linear reflectance increase with decreasing phase angle at alpha similar to <3 degrees. Our earlier studies suggest that the cause of this nonlinear reflectance increase is constructive interference of counter-propagating waves in the medium by coherent backscattering (CB), a photonic analog of Anderson localization of electrons in solid state media. The polarization phase curves for particle size fractions with size parameter (particle radius/wavelength) r/lambda < similar to 1, show that the linear polarization rapidly decreases as a increases from 0; it reaches a minimum near alpha = similar to 2 degrees. Longward of similar to 2 degrees, the negative polarization decreases as phase angle increases, becoming positive between 12 degrees and at least 15 degrees, (probably similar to 20 degrees) depending on particle size. For size parameters r/lambda > similar to 1 we detect no polarization. This polarization behavior is distinct from that observed in low albedo solar system objects such as the Moon and asteroids and for absorbing materials in the laboratory. We suggest this behavior arises because photons that are backscattered have a high probability of having interacted with two or more particles, thus giving rise to the CB process. These results may explain the unusual negative polarization behavior observed near small phase angles reported for several decades on highly reflective ASSBs such as the asteroids 44 Nysa, 64 Angelina and the Galilean satellites lo, Europa and Ganymede. Our results suggest these ASSB regoliths scatter electromagnetic radiation as if they were extremely fine grained with void space > similar to 95%, and grain sizes of the order < = lambda. This portends consequences for efforts to deploy landers on high ASSBs such as Europa. These results are also germane to the field of terrestrial geo-engineering, particularly to suggestions that earth's radiation balance can be modified by injecting Al2O3 particulates into the stratosphere thereby offsetting the effect of anthropogenic greenhouse gas emissions. The GPP used in this study was modified from our previous design so that the sample is presented with light that is alternatingly polarized perpendicular to and parallel to the scattering plane. There are no analyzers before the detector. This optical arrangement, following the Helmholtz Reciprocity Principle (HRP), produces a physically identical result to the traditional laboratory reflectance polarization measurements in which the incident light is unpolarized and the analyzers are placed before the detector. The results are identical in samples measured by both methods. We believe that ours is the first experimental demonstration of the HRP for polarized light, first proposed by Helmholtz in 1856. (C) 2017 Elsevier Inc. All rights reserved.

Reflectance spectroscopy is an important method of measuring the absorption spectra and other properties of solids. Because this type of measurement can be made without contacting the sample, it is a powerful technique for the remote determination of the composition of soils and regoliths on the surfaces of bodies of the solar system, as well as powders in the laboratory. Analysis of the specular reflection from the polished surface of a solid can give the complex index of refraction using the Kramers-Kronig method. The exact solution of Maxwell's equations for the scattering of light by realistic powders and soils is well beyond present computational ability. However, approximate models of diffuse reflectance based on the equation of radiative transfer are capable of quantitatively predicting the angular scattering behavior and measuring the spectral absorption coefficient of particulate media.

The visible (400-700 nm) and near-infrared (700-2800 nm) reflectance of the lunar regolith is dominantly controlled by variations in the abundance of plagioclase, iron-bearing silicate minerals, opaque minerals (e.g., ilmenite), and maturation products (e.g., agglutinate glass, radiation-produced rims on soil grains, and Fe-metal). The same materials control reflectance into the near-UV (250-400 nm) with varying degrees of importance. A key difference is that while ilmenite is spectrally neutral in the visible to near-infrared, it exhibits a diagnostic upturn in reflectance in the near-UV, at wavelengths shorter than about 450 nm. The Lunar Reconnaissance Orbiter Wide Angle Camera (WAC) filters were specifically designed to take advantage of this spectral feature to enable more accurate mapping of ilmenite within mare soils than previously possible. Using the reflectance measured at 321 and 415 nm during 62 months of repeated near-global WAC observations, first we found a linear correlation between the TiO2 contents of the lunar soil samples and the 321/415 nm ratio of each sample return site. We then used the coefficients from the linear regression and the near-global WAC multispectral mosaic to derive a new TiO2 map. The average TiO2 content is 3.9 wt% for the 17 major maria. The highest TiO2 values were found in Mare Tranquillitatis (similar to 12.6 wt%) and Oceanus Procellarum (similar to 11.6 wt%). Regions contaminated by highland ejecta, lunar swirls, and the low-TiO2 maria (e.g., Mare Frigoris, the northeastern units of Mare Imbrium) exhibit very low TiO2 values (<2 wt%). We find that the Clementine visible to near-infrared based TiO2 maps (Lucey et al., 2000) have systematically higher values relative to the WAC estimates. The Lunar Prospector Gamma-Ray Spectrometer (GRS) TiO2 map is consistent with the WAC TiO2 map, although there are local offsets possibly due to the different depth sensitivities and large pixel scale of the GRS relative to the WAC. We find a wide variation of TiO2 abundances (from 0 to 10 wt%) for early mare volcanism (>2.6 Ga), whereas only medium to high TiO2 values (average = 6.8 wt%, minimum = 4.5 wt%) are found for younger mare units (<2.6 Ga). (C) 2017 Published by Elsevier Inc.

The porosity of the upper centimeter or so of the lunar regolith strongly affects several properties that are commonly studied remotely. Hence, it is important to determine its value. We have reanalyzed the data of Ohtake et al. (Ohtake et al. [2010]. Space Sci. Rev., 154, 57-77), who used spacecraft and laboratory reflectance measurements of the Moon by Kaguya Multiband Imager instruments and an Apollo sample to infer a lunar regolith porosity of 74-87%. Our analysis was augmented by using Lunar Reconnaissance Orbiter Wide and Narrow Angle Camera images. We confirm the Ohtake et al. (Ohtake et al. [2010]. Space Sci. Rev., 154, 57-77) estimate and refine it to 83 +/- 3%. However, depending on the validity of key assumptions, this value could be a lower limit, so that the actual porosity could be somewhat higher. Even though the magnetic resonance index of the sample indicates that it is mature, it is appears to be optically less mature than a standard photometric site near the sample collection site. (C) 2015 Elsevier Inc. All rights reserved.