We present LOUPE, the Lunar Observatory for Unresolved Polarimetry of the Earth, a compact snapshot spectropolarimeter designed to observe the Earth from the Moon as if it were an exoplanet. Viewing the Earth as it would be seen by a faraway observer will offer novel insight into the spectropolarimetric signatures of planets harboring life, as well as a chance to refine algorithms for the retrieval of exoplanetary properties such as the presence of liquid water, clouds, vegetation, and more. LOUPE boasts a novel solid-state design based on patterned liquid crystal optics built atop the cosine HyperScout®, a flight-proven hyperspectral imager. Uniquely to LOUPE, a microlens array creates a two- dimensional grid of unresolved Earth-images on the detector, resulting in an array of "pale (blue) dots"filtered spectrally along one direction, with polarization modulation applied in the perpendicular direction. The clever use of custom-patterned liquid crystals as a passive modulator thus replaces the need for classical dispersion elements and polarization modulation optics. This pioneering approach enables LOUPE to simultaneously obtain spectral and Stokes measurements for the entire Earth, whilst the position of the Earth-dots also has the benefit of providing input for angle-dependent spectral and polarization calibration. Here we discuss our detailed design process and the challenges involved in creating a unique, space-qualified spectropolarimeter with no moving parts and no bulky optics, whilst maintaining flexibility for different usage scenarios: rovers, landers, orbiters, and more. We present a performance trade-off and optical design informed by ray tracing with polarization effects, to prepare for the demodulation of simulated Earth observation data.

LOUPE, the Lunar Observatory for Unresolved Polarimetry of the Earth, is a small, robust spectro-polarimeter for observing the Earth as an exoplanet. Detecting Earth-like planets in stellar habitable zones is one of the key challenges of modern exoplanetary science. Characterizing such planets and searching for traces of life requires the direct detection of their signals. LOUPE provides unique spectral flux and polarization data of sunlight reflected by Earth, the only planet known to harbour life. These data will be used to test numerical codes to predict signals of Earth-like exoplanets, to test algorithms that retrieve planet properties, and to fine-tune the design and observational strategies of future space observatories. From the Moon, LOUPE will continuously see the entire Earth, enabling it to monitor the signal changes due to the planet's daily rotation, weather patterns and seasons, across all phase angles. Here, we present both the science case and the technology behind LOUPE's instrumental and mission design. This article is part of a discussion meeting issue 'Astronomy from the Moon: the next decades'.

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.

Many biologically produced chiral molecules such as amino acids and sugars show a preference for left or right handedness (homochirality). Light reflected by biological materials such as algae and leaves therefore exhibits a small amount of circular polarization that strongly depends on wavelength. Our Life Signature Detection polarimeter (LSDpol) is optimized to measure these signatures of life. LSDpol is a compact spectropolarimeter concept with no moving parts that instantaneously measures linear and circular polarization averaged over the field of view with a sensitivity of better than 10-4. We expect to launch the instrument into orbit after validating its performance on the ground and from aircraft. LSDpol is based on a spatially varying quarter-wave retarder that is implemented with a patterned liquid-crystal. It is the first optical element to maximize the polarimetric sensitivity. Since this pattern as well as the entrance slit of the spectrograph have to be imaged onto the detector, the slit serves as the aperture, and an internal field stop limits the field of view. The retarder's fast axis angle varies linearly along one spatial dimension. A fixed quarter-wave retarder combined with a polarization grating act as the disperser and the polarizing beam-splitter. Circular and linear polarization are thereby encoded at incompatible modulation frequencies across the spectrum, which minimizes the potential cross-talk from linear into circular polarization.