The O₂ A-Band in the Fluxes and Polarization of Starlight Reflected by Earth-Like Exoplanets

Abstract

Earth-like, potentially habitable exoplanets are prime targets in the search for extraterrestrial life. Information about their atmospheres and surfaces can be derived by analyzing thelight of the parent star reflected by the planet. We investigate the influence of the surface albedo A s , the optical thickness b cloud ,thealtitude of water clouds, and the mixing ratio of biosignature O 2 on the strength of the O 2 A-band (around 760 nm) in theflux and polarization spectra of starlight reflected by Earth-like exoplanets. Our computations for horizontally homogeneous planets show that small mixing ratios (η<0.4) will yield moderately deep bands in flux and moderate-to-small band strengths in polarization, and that clouds will usually decrease the band depth in flux and the band strength in polarization. However, cloud influence will be strongly dependent onproperties such as optical thickness, top altitude, particle phase, coverage fraction, andhorizontal distribution. Depending on the surface albedoand cloud properties, different O 2 mixing ratios η can give similar absorption-band depths in flux and band strengths in polarization, especially if the clouds have moderate-to-high optical thicknesses. Measuring both the flux and the polarization is essential to reduce the degeneracies, although it will not solve them,especiallynot for horizontally inhomogeneous planets. Observations at a wide range of phase angles and with a high temporal resolution could help to derive cloud properties and, once those are known, the mixing ratio of O 2 or any other absorbing gas.

High-resolution spectroscopy and spectropolarimetry of the total lunar eclipse January 2019

“…Additionally, linear line polarization could be introduced into the transmission spectrum by coherent scattering at electrons in the Earth's ionosphere, just like the Hanleeffect on the Sun, and then adds to the total polarization signal. Fauchez et al (2017) showed that gaseous absorption bands like the O 2 bands not only show up in flux spectra of light reflected by (exo)planets but also appear in polarization spectra. However, our QUV data do not show any polarization above 0.2-0.4%.…”

Spectral and temporal variability of Earth observed in polarization

“…The new observational contribution in this work, however, is the large variation of this line independent of phase and not directly correlated to "A" or "P" sceneries. Fauchez et al (2017) investigate the influence of the planet surface albedo, cloud optical thickness, altitude of the cloud deck and the O 2 mixing ratio on the polarization in the O 2 -A band, and find that these parameters may not be easily discriminated in the higher optical depth regimes that can be probed with high spectral resolution. The cloud deck altitude and horizontal distribution across the region of the Earth that contributes strongest to the observed signal may be decisive factors for the appearance of the band, but more detailed simulations of Earth polarization measurements are needed to confirm this.…”

PYMIEDAP: a Python–Fortran tool for computing fluxes and polarization signals of (exo)planets

“…An example of the variability computed using PyMieDAP can be found in Rossi & Stam (2017) where it was used to generate the disk-integrated signals of Earth-like exoplanets with varying types and amount of coverage by liquid water clouds. Thanks to the use of Fourier coefficients, only a limited number of model computations were necessary: clear sky and cloudy case with different cloud-top altitudes.…”

“…Firstly, because light is fully described by a vector and scattering processes by matrices, ignoring polarization can induce errors up to several percent in computed total fluxes both locally and disk-integrated (Mishchenko et al 1994;. In particular, in gaseous absorption bands, where the linear polarization usually differs from the polarization in the continuum (Fauchez et al 2017;Boesche et al 2009;Stammes et al 1994;Aben et al 1997), such errors will lead to errors in derived gas mixing ratios and e.g. cloud top altitudes .…”