  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.
