Biosignatures of the Earth

Abstract

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.

Negative polarization properties of regolith simulants

“…The polarization measurements were carried out with the PO-Larimeter for ICE Samples (POLICES) at University of Bern (see also Poch et al (2018) and Patty et al (2022) in revision). POLICES is a full Stokes polarimeter (Dual PEM II/FS42-47, Hinds Instruments) that allows us to measure the polarization state of the light scattered by a sample at different phase angles.…”

Spectropolarimetry of life: airborne measurements from a hot air balloon

“…Data acquired through remote-sensing offers crucial information for preparation for the design and development of the next generation of space-based spectropolarimeters. We follow up upon the results of Patty et al (2021) 11 . They used a helicopter to perform airborne measurements.…”

“…Using the NDVI values and the reference photos for the individual scenes, we were able to differentiate between five surface types; grass, soil, trees, urban, and water. The spectra presented in Figure 6 2021) 11 flew over the lake Lac des Taillères and measured a circular polarization signal of V /I = 1.1 × 10 −3 , which indicates a possible presence of photosynthetic organisms like algae. We pointed FlyPol several times at the Murtzensee to try and detect a visible red edge as well.…”

“…Unlike the grass spectra, those from tree canopies show vary both from shape and sign. The forest spectra, as presented in Patty et al (2021) 11 , have both a positive and negative band that exhibit a larger variation in shape and magnitude than the grass spectra. The difference between the two is illustrated in Figure 9.…”

“…FlyPol was oriented using a calibrated parallax-free telescope pointer 11 . A GoPro HERO7 reference camera and a broadband Thorlabs' Polarization-Sensitive Kiralux ® camera were aligned with the pointer and mounted on top of FlyPol's instrument casing, see Figure 2.…”