NASA is engaged in planning for a Habitable Worlds Observatory (HabWorlds), a coronagraphic space mission to detect rocky planets in habitable zones and establish their habitability. Surface liquid water is central to the definition of planetary habitability. Photometric and polarimetric phase curves of starlight reflected by an exoplanet can reveal ocean glint, rainbows, and other phenomena caused by scattering by clouds or atmospheric gas. Direct imaging missions are optimized for planets near quadrature, but HabWorlds’ coronagraph may obscure the phase angles where such optical features are strongest. The range of accessible phase angles for a given exoplanet will depend on the planet’s orbital inclination and/or the coronagraph’s inner working angle (IWA). We use a recently created catalog relevant to HabWorlds of 164 stars to estimate the number of exo-Earths that could be searched for ocean glint, rainbows, and polarization effects due to Rayleigh scattering. We find that the polarimetric Rayleigh scattering peak is accessible in most of the exo-Earth planetary systems. The rainbow due to water clouds at phase angles of ∼20^◦ − 60^◦ would be accessible with HabWorlds for a planet with an Earth equivalent instellation in ∼46 systems, while the ocean glint signature at phase angles of ∼130^◦ − 170^◦ would be accessible in ∼16 systems, assuming an IWA = 62 mas (3λ/D). Improving the IWA = 41 mas (2λ/D) increases accessibility to rainbows and glints by factors of approximately 2 and 3, respectively. By observing these scattering features, HabWorlds could detect a surface ocean and water cycle, key indicators of habitability.

We present the performance characterization of the Life Signature Detection polarimeter (LSDpol), a prototype instrument designed to identify life on Earth and derive the integrated signal of Earth-as-an-exoplanet through global polarization measurements from the Airbus Bartolomeo platform on the International Space Station (ISS). LSDpol is optimized for the measurement of an unambiguous biomarker exhibited by chlorophyll and other bio-pigments: homochirality. The instrument is very sensitive to small signals in circular polarization induced by this preference in handedness found in biological molecules. LSDpol has the capability of measuring full Stokes parameters as a function of wavelength while containing no moving parts and a compact design suitable for SmallSats. The point-and-shoot configuration of this instrument uses a patterned liquid crystal spatial polarization modulator at the slit followed by a quarter wave retarder and a liquid crystal polarization grating. This combination decouples the faint circular and strong linear polarization signals through spatial modulation making it insensitive to cross-talk. In this paper we present detailed simulations and results from the performance characterization of LSDpol. We discuss the current design and the impact of instrumental artefacts such as distortions, flat field, and retardation errors in the quarter-waveplate based on simulations of the spatial modulation. Our study looks at the instruments’ capabilities in the laboratory and outdoors. Abiotic data from artificial vegetation and concrete are used as a control against the chlorophyll measurements of interest. Preliminary results from beetles, leaves and grass demonstrate the current capabilities of LSDpol. This versatile instrument concept will be ideally suited for remote sensing of homochirality, enabling vegetation health monitoring on Earth and detection of possible biotic signatures on icy moons.