Polarized Signatures of Variable Worlds: Modeling Heterogeneous Habitable Earth- and Early Mars-like (Exo)planets

Abstract Determining the habitability of terrestrial exoplanets is a complex problem that represents the next major step for the astrophysical community. The majority of current models treat these planets as homogeneous or contain heterogeneity that is constant in time. In reality, habitable exoplanets are expected to contain atmospheric and surface heterogeneities similar to Earth, with diurnal rotation, seasonal changes, and weather patterns resulting in complex, time-dependent signatures. Due to its ability to measure light as a vector, polarimetry provides an important tool that will enhance the characterizations of heterogeneous worlds. Here we model the visible to near-infrared linear spectropolarimetric signatures, as functions of wavelength and planetary phase angle, of various heterogeneous Earth scenarios as well as the first signals of an early wet and potentially habitable Mars. The contributions from the different atmospheric and surface properties result in asymmetric phase curves and variable spectra, with the polarization appearing to be more sensitive than flux to heterogeneities such as patchy clouds and continents moving into and out of view. Our models provide important predictions of expected polarized and unpolarized signatures of heterogeneous exoplanets that will help guide the designs and observing plans of future polarimeters, including those proposed for the upcoming Habitable Worlds Observatory.