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The Moon Polarizes and It Matters: New Lunar Spectropolarimetric Datasets, Effects and Models

Presentation #101.05 in the session Moon & Earth I (Oral Presentation)

Published onOct 23, 2023
The Moon Polarizes and It Matters: New Lunar Spectropolarimetric Datasets, Effects and Models

This study aims at an analysis of the intricate small-scale structure of the lunar regolith by examining the linear polarization characteristics of light reflected from the lunar surface. Our investigation relies on telescopic imaging to obtain multispectral UBGRI polarimetric data acquired through our setup described in [1].

We coregistered our telescopic polarimetric data with the Chandrayaan-1 Moon Mineralogy Mapper (M³) reflectance data, using a procedure similar to [2].

We generated three distinct types of data products:

1. Grain size: Using the method of [2], our multiband telescopic data, and the M³ reflectance data, we constructed relative grain size maps of the target region for each wavelength at different phase angles.

2. Umov exponent E: Assuming a power-law dependence between the wavelength-dependent degree of linear polarization P and spectral reflectance R normalized to 30° incidence angle and 0° emission angle of P~RE (“Umov law” [2]), we conducted a pixel-wise estimation of E by a linear fit to the respective log P(λ) vs. log R(λ) relation. Since M³ data are not available in the U band, our estimation of E relies on the BGRI bands only. Our E maps appear to exhibit a correlation with the npFe0 maps of [3].

3. Clustering: We employ a self-organizing map technique to derive prototypical spectra of the degree P(λ) and angle W(λ) of linear polarization. This technique also allows for the automatic identification of spectropolarimetric surface units. We found that both P(λ) and W(λ) correlate with lunar terrain structures.

A preliminary version of our dataset is available at https://agbv-epsc2023-arnaut.streamlit.app/. Its final version will be made accessible in an upcoming publication.

The current understanding of spectropolarimetric data of regolith surfaces still needs to be refined, and only few physical models exist that incorporate them. We hope that our dataset and also the polarimetric data to be acquired during the Danuri mission with the PolCAM instrument [4] will stimulate new research into this field leading to an improved understanding of the fine-structure of the lunar surface.

[1] Wöhler, C. et al. (2023) LPSC LIV, 1609

[2] Shkuratov, Y. and Opanasenko, N. (1992) Icarus 99(2), 468-484

[3] Trang, D and Lucey, P. G. (2019) Icarus 321, 307-323

[4] Sim, C. K. et al. (2020) Astronomical Society of the Pacific 132, 1007

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