The effect of phase angle on biosignature detection with the Habitable Worlds Observatory

Characterizing the atmospheres of Earth-like planets is a key goal of NASA’s next flagship-class astrophysics mission concept, the Habitable Worlds Observatory (HWO). We simulate HWO observations of an Earth-like exoplanet at varying orbital configurations and perform spectral retrievals to constrain the surface and atmospheric composition. The phase angle, like cloud coverage and surface features, acts as a normalizing factor for the spectra. We explore how including the phase angle as a free parameter affects atmospheric and surface retrievals. We discuss how phase angle, surface spectra, and water clouds can harbor significant degeneracies. This will hinder placing potential biosignature detections into the full context of the planet. Further development of retrieval algorithms is necessary before robust statistical constraints can be achieved for terrestrial planets.