Quantitative grain size estimation on airless bodies from the negative polarization branch. I. Insights from experiments and lunar observations

This work explores characteristics of the negative polarization branch (NPB), which occurs in scattered light from rough surfaces, with particular focus on the effects of fine particles. Factors such as albedo, compression, roughness, and the refractive index are considered to determine their influence on the NPB. This study compiles experimental data and lunar observations to derive insights from a wide array of literature. Employing our proposed methodology, we estimate the representative grain sizes on the lunar surface to be $D \sim 1 \mathrm{-} 2 \mathrm{\mu m}$, with $D \lesssim 2 \mathrm{-} 4 \mathrm{\mu m}$, consistent with observed grain size frequency distributions in laboratory settings for lunar fines. Considering Mars, we propose that the finest particles are likely lacking ($D\gg 10 \mathrm{\mu m}$), which matches previous estimations. This study highlights the potential of multiwavelength, particularly near-infrared, polarimetry for precisely gauging small particles on airless celestial bodies. The conclusions provided here extend to cross-validation with grain sizes derived from thermal modeling, asteroid taxonomic classification, and regolith evolution studies.