Biosignatures of the Earth

I. Airborne spectropolarimetric detection of photosynthetic life

¹ Physikalisches Institut, Universität Bern, 3012 Bern, Switzerland
e-mail: lucas.patty@space.unibe.ch; chlucaspatty@gmail.com
² Center for Space and Habitability, Universität Bern, 3012 Bern, Switzerland
³ Département d’Astronomie, Université de Genève, 1290 Versoix, Switzerland
⁴ Institute of Plant Biology, Biological Research Centre Szeged, Temesvári krt. 62, 6726 Szeged, Hungary
⁵ Lund Observatory, Lund University, PO Box 43, 22100 Lund, Sweden
⁶ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁷ Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, The Netherlands
⁸ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France

Received: 22 March 2021
Accepted: 18 May 2021

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.