Imaging & Spectroscopy

Polarized Signatures of a Habitable World: Comparing Models of an Exoplanet Earth with Visible and Near-infrared Earthshine Spectra

By Keith Cowing
Status Report
January 17, 2023
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Polarized Signatures of a Habitable World: Comparing Models of an Exoplanet Earth with Visible and Near-infrared Earthshine Spectra
Comparison between the full-blown exoplanet-Earth model generated by DAP (solid blue line) and a crude exoplanet-Earth model created from the four simple VSTAR models from Figure 10 (dashed gold line) with the original earthshine observations (dotted black line) and the earthshine observations corrected for lunar depolarization (dashed-dotted red line). The DAP and VSTAR models are at α = 106◦ to match the observations. The DAP model includes all five surface types and clouds with varying bc and pc, while the VSTAR model includes only forest and ocean surfaces and clouds with a set bc and pc. The DAP and VSTAR model clouds are composed of type B particles. The VSTAR cloud particles used the wavelength-dependent nw of Hale & Querry (1973), while the DAP cloud particles used a wavelength-independent nw due to computational time restrictions. Also included in this plot are data points from two additional crude exoplanet-Earth models created by DAP, but now using the type C (purple squares) and type D (green triangles) cloud particles with the same wavelength-independent nw as the type B clouds of the DAP model. — astro-ph.EP

In the JWST, Extremely Large Telescopes, and LUVOIR era, we expect to characterize a number of potentially habitable Earth-like exoplanets.

However, the characterization of these worlds depends crucially on the accuracy of theoretical models. Validating these models against observations of planets with known properties will be key for the future characterization of terrestrial exoplanets.

Due to its sensitivity to the micro- and macro-physical properties of an atmosphere, polarimetry will be an important tool that, in tandem with traditional flux-only observations, will enhance the capabilities of characterizing Earth-like planets.

In this paper we benchmark two different polarization-enabled radiative-transfer codes against each other and against unique linear spectropolarimetric observations of the earthshine that cover wavelengths from ∼0.4 to ∼2.3 μm.

We find that while the results from the two codes generally agree with each other, there is a phase dependency between the compared models. Additionally, with our current assumptions, the models from both codes underestimate the level of polarization of the earthshine.

We also report an interesting discrepancy between our models and the observed 1.27 μm O2 feature in the earthshine, and provide an analysis of potential methods for matching this feature. Our results suggest that only having access to the 1.27 μm O2 feature coupled with a lack of observations of the O2 A and B bands could result in a mischaracterization of an Earth-like atmosphere. Providing these assessments is vital to aid the community in the search for life beyond the solar system.

Kenneth E. Gordon, Theodora Karalidi, Kimberly M. Bott, Paulo A. Miles-Páez, Willeke Mulder, Daphne M. Stam

Comments: 24 pages, 18 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2301.05734 [astro-ph.EP] (or arXiv:2301.05734v1 [astro-ph.EP] for this version)
Submission history
From: Kenneth Gordon
[v1] Fri, 13 Jan 2023 19:28:49 UTC (17,795 KB)

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻