Polarized Transmission Spectrum of Earth as Observed during a Lunar Eclipse

TL;DR

This study presents the first spectropolarimetric observations of Earth's transmission spectrum during a lunar eclipse, revealing polarization features that could inform exoplanet atmospheric characterization through transit polarimetry.

Contribution

It provides the first spectropolarimetric data of Earth's transmission during a lunar eclipse, demonstrating polarization caused by atmospheric double scattering and its potential application to exoplanet studies.

Findings

Detected 2-3% polarization in 500-600 nm range.

Observed enhanced polarization at O₂ A band near 760 nm.

Time variation and wavelength dependence suggest atmospheric inhomogeneity.

Abstract

Polarization during a lunar eclipse is a forgotten mystery. Coyne & Pellicori (1970) reported the detection of significant polarization during the lunar eclipse on 1968 April 13. Multiple scattering during the first transmission through the Earth's atmosphere was suggested as a possible cause of the polarization, but no conclusive determination was made. No further investigations on polarization during a lunar eclipse are known. We revisit this mystery with an interest in possible application to extrasolar planets, if planetary transmitted light is indeed polarized, it may be possible to investigate an exoplanet atmosphere using "transit polarimetry." Here we report results of the first spectropolarimetry for the Moon during a lunar eclipse on 2015 April 4. We observed polarization degrees of 2-3% at wavelengths of 500-600 nm, in addition, an enhanced feature was detected at the O$_2$ A band near 760 nm. The observed time variation and wavelength dependence are consistent with an explanation of polarization caused by double scattering during the first transmission through the Earth's atmosphere, accompanied by latitudinal atmospheric inhomogeneity. Transit polarimetry for exoplanets may be useful to detect O$_2$ gas and to probe the latitudinal atmospheric inhomogeneity, and it is thus worthy of serious consideration.