Polarized Transmission Spectrum of Earth as Observed during a Lunar Eclipse

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.