Colors of an Earth-like exoplanet -- Temporal flux and polarization signals of the Earth

Abstract

Understanding the total flux and polarization signals of Earth-like planets and their spectral and temporal variability is essential for the future characterization of such exoplanets. We provide computed total (F) and linearly (Q and U) and circularly (V) polarized fluxes, and the degree of polarization P of sunlight that is reflected by a model Earth, to be used for instrument designs, optimizing observational strategies, and/or developing retrieval algorithms. We modeled a realistic Earth-like planet using one year of daily Earth-observation data: cloud parameters (distribution, optical thickness, top pressure, and particle effective radius), and surface parameters (distribution, surface type, and albedo). The Stokes vector of the disk-averaged reflected sunlight was computed for phase angles alpha from 0 to 180 degrees, and for wavelengths lambda from 350 to 865 nm. The total flux F is one order of magnitude higher than the polarized flux Q, and Q is two and four orders of magnitude higher than U and V, respectively. Without clouds, the peak-to-peak daily variations due to the planetary rotation increase with increasing lambda for F, Q, and P, while they decrease for U and V. Clouds modify but do not completely suppress the variations that are due to rotating surface features. With clouds, the variation in F increases with increasing lambda, while in Q, it decreases with increasing lambda, except at the largest phase angles. In earlier work, it was shown that with oceans, Q changes color from blue through white to red. The alpha where the color changes increases with increasing cloud coverage. Here, we show that this unique color change in Q also occurs when the oceans are partly replaced by continents, with or without clouds. The degree of polarization P shows a similar color change. Our computed fluxes and degree of polarization will be made publicly available.

Accepted for publication in Astronomy & Astrophysics

References in corpus (13)

• Vegetation's Red Edge: A Possible Spectroscopic Biosignature of Extraterrestrial Plants
• Spectral signatures of photosynthesis I: Review of Earth organisms
• Spectral signatures of photosynthesis II: coevolution with other stars and the atmosphere on extrasolar worlds
• Detecting the Glint of Starlight on the Oceans of Distant Planets
• Spectropolarimetric signatures of Earth-like extrasolar planets
• Identifying the rotation rate and the presence of dynamic weather on extrasolar Earth-like planets from photometric observations
• Vegetation signature in the observed globally-integrated spectrum of Earth: Modeling the red edge strength using simultaneous cloud data and application for extrasolar planets
• The Effects of Refraction on Transit Transmission Spectroscopy: Application to Earth-like Exoplanets
• Blue, white, and red ocean planets - Simulations of orbital variations in flux and polarization colors
• Polarization in exoplanetary systems caused by transits, grazing transits, and starspots
• Towards a comprehensive model of Earth's disk-integrated Stokes vector
• Global Mapping of an Exo-Earth using Sparse Modeling
• Fourier spectra from exoplanets with polar caps and ocean glint

Cited by in corpus (4)

• Biosignatures of the Earth I. Airborne spectropolarimetric detection of photosynthetic life
• Polarized Signatures of a Habitable World: Comparing Models of an Exoplanet Earth with Visible and Near-infrared Earthshine Spectra
• Directional aspects of vegetation linear and circular polarization biosignatures
• Effect of multiple scattering on the Transmission spectra and the Polarization phase curves for Earth-like Exoplanets