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- Title
Grain Size Dependence of Brightness Phase Curves of the Lunar Surface.
- Authors
Jeong, Minsup; Choi, Young‐Jun; Kim, Sungsoo S.; Shkuratov, Yuriy G.
- Abstract
The lunar opposition effect is the increase of brightness on the lunar surface when the phase angle approaches 0°, which is caused mainly by shadow‐hiding and coherent backscattering. However, the contribution of coherent backscattering is not yet well established. This study analyzes the contribution of coherent backscattering to the opposition effect using the correlation between the contribution of increased brightness of the theoretical coherent backscattering and the grain size of the reflecting surface. We analyze the correlation between the two using data from the Lunar Reconnaissance Orbiter Wide Angle Camera and median grain size d calculated by linear polarization observations. The size of effective scatterers of coherent backscattering is similar to the observation wavelength. In this case, the effective scatterer size for coherent backscattering is 0.6 μm. The results of our analyses show that there is no correlation between d and reflectance phase curve in the phase angle range from 0° to 6°. In addition, we analyze the distribution of grain size of lunar samples collected during the Apollo and Luna missions. We find an inversely proportional correlation between mean grain sizes and submicron grain content in the lunar samples. Therefore, d seems to indicate the content of effective scatterer. The strength of the coherent backscattering opposition effect seems to be affected by the content of the submicron grains. Thus, the lack of correlation between d and the opposition effect seems to imply minimal contribution of coherent backscattering to the lunar opposition effect. Plain Language Summary: Brightness at the lunar surface is changed by the angle between the Sun‐Moon‐observer, which is called the phase angle. In particular, the brightness dramatically increases at a phase angle less 6°in a phenomenon called the opposition effect. This effect is mainly due to shadow‐hiding effect and coherent backscattering. The shadow‐hiding effect occurs because reflecting grains hide under the shadows created by individual grains in the regolith at a phase angle near zero. Coherent backscattering occurs when multiple scattered light waves travel in opposite directions along the same path when the phase angle approaches zero. However, the contributions of shadow hiding and coherent backscattering are not well‐known. To understand the contribution of coherent backscattering, we analyze the strength of brightness enhancement when grain sizes are varied, because coherent backscattering has a correlation with grain size of the reflecting surface. Moreover, we analyze the grain size distribution of lunar sample data from Graf (1993, https://ntrs.nasa.gov/search.jsp?R=19930012474). In conclusion, we find a negligible correlation between strength of brightness enhancement and grain size, implying that coherent backscattering between discrete particles in lunar regolith seems to have a weak influence. Key Points: The opposition effect is known to be mainly from shadow hiding effect and coherent backscatteringHowever, the contribution of coherent backscattering are not well‐known yetWe find that the coherent backscattering between discrete particles seems to have a weak influence on the lunar surface even when the phase angle is small enough
- Subjects
LUNAR surface; GRAIN size; LUNAR Reconnaissance Orbiter (Spacecraft); COHERENT backscattering; REGOLITH; REFLECTANCE
- Publication
Journal of Geophysical Research. Planets, 2020, Vol 125, Issue 7, p1
- ISSN
2169-9097
- Publication type
Article
- DOI
10.1029/2019JE006164