In the JWST, Extremely Large Telescopes, and LUVOIR era, we expect to characterize a number of potentially habitable Earth-like exoplanets. However, the characterization of these worlds depends crucially on the accuracy of theoretical models. Validating these models against observations of planets with known properties will be key for the future characterization of terrestrial exoplanets. Due to its sensitivity to the micro- and macro-physical properties of an atmosphere, polarimetry will be an important tool that, in tandem with traditional flux-only observations, will enhance the capabilities of characterizing Earth-like planets. In this paper we benchmark two different polarization-enabled radiative-transfer codes against each other and against unique linear spectropolarimetric observations of the earthshine that cover wavelengths from ∼0.4 to ∼2.3 μm. We find that while the results from the two codes generally agree with each other, there is a phase dependency between the compared models. Additionally, with our current assumptions, the models from both codes underestimate the level of polarization of the earthshine. We also report an interesting discrepancy between our models and the observed 1.27 μm O₂ feature in the earthshine, and provide an analysis of potential methods for matching this feature. Our results suggest that only having access to the 1.27 μm O₂ feature coupled with a lack of observations of the O₂ A and B bands could result in a mischaracterization of an Earth-like atmosphere. Providing these assessments is vital to aid the community in the search for life beyond the solar system.

Does life exist outside our Solar System A first step towards searching for life outside our Solar System is detecting life on Earth by using remote sensing applications. One powerful and unambiguous biosignature is the circular polarization resulting from the homochirality of biotic molecules and systems. We aim to investigate the possibility of identifying and characterizing life on Earth by using airborne spectropolarimetric observations from a hot air balloon during our field campaign in Switzerland, May 2022. In this proceeding we present the optical-setup and the data obtained from aerial circular spectropolarimetric measurements of farmland, forests, lakes and urban sites. We make use of the well-calibrated FlyPol instrument that measures the fractionally induced circular polarization (V/I) of (reflected) light with a sensitivity of < 10-4. The instrument operates in the visible spectrum, ranging from 400 to 900 nm. We demonstrate the possibility to distinguish biotic from abiotic features using circular polarization spectra and additional broadband linear polarization information. We review the performance of our optical-setup and discuss potential improvements. This sets the requirements on how to perform future airborne spectropolarimetric measurements of the Earth's surface features from several elevations.

Context. Homochirality is a generic and unique property of life on Earth and is considered a universal and agnostic biosignature. Homochirality induces fractional circular polarization in the incident light that it reflects. Because this circularly polarized light can be sensed remotely, it can be one of the most compelling candidate biosignatures in life detection missions. While there are also other sources of circular polarization, these result in spectrally flat signals with lower magnitude. Additionally, circular polarization can be a valuable tool in Earth remote sensing because the circular polarization signal directly relates to vegetation physiology. Aims. While high-quality circular polarization measurements can be obtained in the laboratory and under semi-static conditions in the field, there has been a significant gap to more realistic remote sensing conditions. Methods. In this study, we present sensitive circular spectropolarimetric measurements of various landscape elements taken from a fast-moving helicopter. Results. We demonstrate that during flight, within mere seconds of measurements, we can differentiate (S∕ N > 5) between grass fields, forests, and abiotic urban areas. Importantly, we show that with only nonzero circular polarization as a discriminant, photosynthetic organisms can even be measured in lakes. Conclusions. Circular spectropolarimetry can be a powerful technique to detect life beyond Earth, and we emphasize the potential of utilizing circular spectropolarimetry as a remote sensing tool to characterize and monitor in detail the vegetation physiology and terrain features of Earth itself.

Are we alone? In our quest to find life beyond Earth, we use our own planet to develop and verify new methods and techniques to remotely detect life. Our Life Signature Detection polarimeter (LSDpol), a snapshot full-Stokes spectropolarimeter to be deployed in the field and in space, looks for signals of life on Earth by sensing the linear and circular polarization states of reflected light. Examples of these biosignatures are linear polarization resulting from O2-A band and vegetation, e.g. the Red edge and the Green bump, as well as circular polarization resulting from the homochirality of biotic molecules. LSDpol is optimized for sensing circular polarization. To this end, LSDpol employs a spatial light modulator in the entrance slit of the spectrograph, a liquid-crystal quarter-wave retarder where the fast axis rotates as a function of slit position. The original design of LSDpol implemented a dual-beam spectropolarimeter by combining a quarter-wave plate with a polarization grating. Unfortunately, this design causes significant linear-to-circular cross-talk. In addition, it revealed spurious polarization modulation effects. Here, we present numerical simulations that illustrate how Fresnel diffraction effects can create these spurious modulations. We verified the simulations with accurate polarization state measurements in the lab using 100% linearly and circularly polarized light.