Exploring and Validating Exoplanet Atmospheric Retrievals with Solar System Analog Observations

TL;DR

The paper addresses the need to validate exoplanet atmospheric retrievals by using Solar System analogs with ground-truth data. It introduces rfast, a fast, open-source retrieval suite capable of handling reflected-light, thermal-emission, and transit-spectrum data to support mission concept studies. Through validations against analytic solutions and high-fidelity models, and by applying the tool to Earth and Titan analogs (including EPOXI, MGS-TES, and Cassini/VIMS data), the work demonstrates reliable gas-concentration and cloud-property inferences and clarifies data-quality trade-offs. The results show that Solar System observations can effectively validate and inform exoplanet inference pipelines, guiding instrument design and observational strategies for future missions like HabEx, LUVOIR, and JWST-era studies.

Abstract

Solar System observations that serve as analogs for exoplanet remote sensing data can provide important opportunities to validate ideas and models related to exoplanet environments. Critically, and unlike true exoplanet observations, Solar System analog data benefit from available high-quality ground- or orbiter-derived "truth" constraints that enable strong validations of exoplanet data interpretation tools. In this work, we first present a versatile atmospheric retrieval suite, capable of application to reflected light, thermal emission, and transmission observations spanning a broad range of wavelengths and thermochemical conditions. The tool -- dubbed rfast -- is designed, in part, to enable exoplanet mission concept feasibility studies. Following model validation, the retrieval tool is applied to a range of Solar System analog observations for exoplanet environments. Retrieval studies using Earth reflected light observations from NASA's EPOXI mission provide a key proof-of-concept for under-development exo-Earth direct imaging concept missions. Inverse modeling applied to an infrared spectrum of Earth from the Mars Global Surveyor Thermal Emission Spectrometer achieves good constraints on atmospheric gases, including many biosignature gases. Finally, retrieval analysis applied to a transit spectrum of Titan derived from the Cassini Visual and Infrared Mapping Spectrometer provides a proof-of-concept for interpreting more feature-rich transiting exoplanet observations from NASA's James Webb Space Telescope (JWST). In the future, Solar System analog observations for exoplanets could be used to verify exoplanet models and parameterizations, and future exoplanet analog observations of any Solar System worlds from planetary science missions should be encouraged.

Figures (23)

• Figure 1: Layer reflectivity (top left), transmissivity (top right), and emissivity (bottom) as a function of layer extinction optical depth in the rfast model (grey lines) versus detailed calculations from hunt1973 (yellow lines). Limiting cases of pure absorption and forward scattering are considered, where layers have no reflectivity in the pure absorption case.
• Figure 2: Phase-dependent planetary reflectivity for infinitely deep Rayleigh and isotropic scattering atmospheres from madhusudhanburrows2012 as compared to rfast.
• Figure 3: Planetary geometric ($A_{\rm g}$; solid lines) and spherical ($A_{\rm s}$; dashed lines) albedo as a function of single scattering albedo from hengetal2021 and rfast for an infinitely deep atmosphere whose medium obeys a Henyey-Greenstein scattering phase function of asymmetry parameter 0.508 hengetal2021.
• Figure 4: Reflection spectra for a partially-clouded (50% cloud coverage) Earth-like case from the rfast (grey) and SMART (yellow) models. Left figure is for a single-scene, plane-parallel setup. Right figure is for a three-dimensional treatment where plane-parallel calculations are run for pixels on the planetary disk and then spatially integrated.
• Figure 5: Thermal emission spectra for a clearsky Earth-like case from the rfast (grey) and SMART (yellow) models.