Investigating the influence of clouds on the reflectance spectra of exo-Earths using the HWO mission concept

A primary goal of the upcoming Habitable Worlds Observatory (HWO) is to identify and characterize Earth-like exoplanets through reflected light spectroscopy. However, a planet’s spectrum is dynamic and represents a time-dependent snapshot of its properties. Changing atmospheric conditions due to climate and weather patterns, particularly variation in cloud cover can significantly affect the spectrum in ways that complicate an understanding of a planet’s baseline atmospheric properties. Variable cloud patchiness and cloud properties affect the detectability of atmospheric constituents, which also greatly influence the radiative transfer that determines a planet’s spectrum. This has considerable implications for observations of potentially habitable exoplanets and thus it is critical to study and characterize the effects of clouds on their spectra. In this work, we construct accurate models of exo-Earths using NASA’s MERRA-2 dataset which assimilates data from a variety of satellites orbiting Earth. Utilizing the Planetary Spectrum Generator (PSG), we simulate observations of these exo-Earths at different orbital and rotational phases and inclinations using the HWO mission concepts. We identify periods of significantly high/low cloud coverage on Earth, construct exo-Earth atmospheres with a similar cloud distribution, and quantify the variations in the simulated spectra for these periods. Our findings also quantify the effects of clouds on the detectability of atmospheric constituents, specifically biomarkers like O2, O3, H2O, etc. Through this work, we are developing a comprehensive suite of ‘cloudy’ spectra that will be vital in making accurate atmospheric retrievals and determining the optimal observation strategies for the HWO mission.