Biosignatures & Paleobiology

Fundamental Challenges To Remote Sensing Of Exo-Earths

By Keith Cowing
June 2, 2021
Filed under
Fundamental Challenges To Remote Sensing Of Exo-Earths
High-resolution spectra for each of our 1,874 models. The top panel shows the radiance of each model from 350 nm to 80 ????m. The middle panel shows the color-coded ratio of the planet’s radiance to the incident stellar flux, as a function of wavelength and fractional habitability, ????hab, such that the ‘most-habitable’ planets are at the top of the plot, and the ‘least-habitable’ at the bottom. Wavelengths at which ????hab varies with normalized radiance the most are particularly interesting for observational characterization. This is highlighted in the bottom panel, where the square of the Pearson’s ???? correlation coefficient between ????hab and the log normalized radiance is shown for each wavelength. The midpoint of the middle panel’s colorbar indicates the point at which thermal emission outweighs reflected light. Models with ????hab=0 or ????hab=1 are included in the middle panel as small extensions above and below the 0–1 range, sorted by mean annual surface temperature. We have indicated the wavelengths of the 4 strongest local maxima in the correlation coefficient with vertical dashed lines in the bottom panel, and local maxima of the covariance are shown with orange dash-dotted lines. Note that despite obvious trends, there are clear degeneracies in the observables represented by a given climate state. Note that we have not included any measurement errors, which would only exacerbate the problem.

Inferring the climate and surface conditions of terrestrial exoplanets in the habitable zone is a major goal for the field of exoplanet science. This pursuit will require both statistical analyses of the population of habitable planets as well as in-depth analyses of the climates of individual planets.

Given the close relationship between habitability and surface liquid water, it is important to ask whether the fraction of a planet’s surface where water can be a liquid, χhab, can be inferred from observations. We have produced a diverse bank of 1,874 3D climate models and computed the full-phase reflectance and emission spectrum for each model to investigate whether surface climate inference is feasible with high-quality direct imaging or secondary eclipse spectroscopy. These models represent the outcome of approximately 200,000 total simulated years of climate and over 50,000 CPU-hours, and the roughly-100 GB model bank and its associated spectra are being made publicly-available for community use. We find that there are correlations between spectra and χhab that will permit statistical approaches.

However, spectral degeneracies in the climate observables produced by our model bank indicate that inference of individual climates is likely to be model-dependent, and inference will likely be impossible without exhaustive explorations of the climate parameter space. The diversity of potential climates on habitable planets therefore poses fundamental challenges to remote sensing efforts targeting exo-Earths.

Adiv Paradise, Kristen Menou, Christopher Lee, Bo Lin Fan

Comments: 10 pages, 5 figures, 1 table. Submitted to MNRAS. Comments are welcomed
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)
Cite as: arXiv:2106.00079 [astro-ph.EP] (or arXiv:2106.00079v1 [astro-ph.EP] for this version)
Submission history
From: Adiv Paradise
[v1] Mon, 31 May 2021 19:55:00 UTC (17,308 KB)

SpaceRef co-founder, Explorers Club Fellow, ex-NASA, Away Teams, Journalist, Space & Astrobiology, Lapsed climber.