Phase Variation of Earthshine Polarization Spectra

TL;DR

This study analyzes Earthshine polarization spectra across different phase angles and wavelengths, revealing how atmospheric and surface reflections influence polarization, which could help characterize Earth-like exoplanets.

Contribution

It provides the first detailed spectropolarimetric analysis of Earthshine over a range of phase angles, highlighting wavelength-dependent polarization behaviors and their implications.

Findings

Polarization decreases with increasing wavelength at all phases.

Maximum polarization occurs near 90 degrees phase at shorter wavelengths.

Earth's atmosphere shows a unique scattering signature compared to other planets.

Abstract

We present the results of the optical spectropolarimetry of Earthshine on the Moon for Earth phase angles ranging from 49 to 96 degrees. The observations were conducted on 2011 March 9-13 (UT) using the spectropolarimeter HBS installed on the 1.88 m telescope at Okayama Astrophysical Observatory. The wavelength coverage was 450-850 nm with a resolution of 6 nm. The observed Earthshine polarization degree spectra exhibit decreasing polarization degree with increasing wavelength at any phase. The overall degree of polarization increases as the Earth approaches a quadrature phase. The phase dependence differs with the wavelengths; the maximum polarization for the V band occurs at a phase angle of ~90 degrees, whereas that for longer wavelengths is reached at larger phase angles. This is interpreted as indicating that Earthshine polarization at shorter wavelengths is dominated by atmospheric Rayleigh scattering, whereas that at longer wavelengths has an increasingly effective contribution from the Earth surface reflection. The wavelength dependence of the phase angle of the maximum polarization appears to be unique among the terrestrial planetary bodies in the Solar System. Therefore this might constitute important evidence pointing toward a distinctive characteristic of the Earth: the planet has a scattering but transparent atmosphere above its surface.