Directional aspects of vegetation linear and circular polarization biosignatures

TL;DR

This study investigates how the phase angle influences circular polarization in vegetation, highlighting its potential as an agnostic biosignature for remote life detection, distinct from linear polarization.

Contribution

It demonstrates that circular polarization phase angle dependency in vegetation causes minor spectral shape changes, supporting its use as a complementary biosignature.

Findings

Circular polarization magnitude varies with phase angle.

Spectral shape remains relatively stable across phase angles.

Supports circular spectropolarimetry as a promising biosignature.

Abstract

Homochirality is a generic and unique property of all biochemical life and is considered a universal and agnostic biosignature. Upon interaction with unpolarized light, homochirality induces fractional circular polarization in the light that is scattered from it, which can be sensed remotely. As such, it can be a prime candidate biosignature in the context of future life-detection missions and observatories. The linear polarizance of vegetation is also sometimes envisaged as a biosignature, although it does not share the molecular origin as circular polarization. It is known that the linear polarization of surfaces is strongly dependent on the phase angle. The relation between the phase angle and circular polarization stemming from macromolecular assemblies, such as in vegetation, however, remained unclear. We demonstrate in this study using the average of 27 different species that the circular polarization phase angle dependency of vegetation induces relatively small changes in spectral shape and mostly affects the signal magnitude. With these results we underline the use of circular spectropolarimetry as a promising agnostic biosignature complementary to the use of linear spectropolarimetry and scalar reflectance.