Nonphotosynthetic Pigments as Potential Biosignatures

TL;DR

This study explores the potential of nonphotosynthetic pigments as biosignatures on Earth-like planets, analyzing their spectral properties and detectability, and highlighting challenges in distinguishing them from other surface features using broadband colors.

Contribution

It provides an interdisciplinary analysis of nonphotosynthetic pigments as biosignatures, including environmental contexts, spectral diversity, and modeling of their detectability on exoplanets.

Findings

Nonphotosynthetic pigments can significantly affect planetary spectra with sufficient coverage.

Broadband colors alone are insufficient to reliably identify nonphotosynthetic biosignatures.

Spectrally resolved data are likely necessary for accurate detection of nonphotosynthetic pigments.

Abstract

Previous work on possible surface reflectance biosignatures for Earth-like planets has typically focused on analogues to spectral features produced by photosynthetic organisms on Earth, such as the vegetation red edge. Although oxygenic photosynthesis, facilitated by pigments evolved to capture photons, is the dominant metabolism on our planet, pigmentation has evolved for multiple purposes to adapt organisms to their environment. We present an interdisciplinary study of the diversity and detectability of nonphotosynthetic pigments as biosignatures, which includes a description of environments that host nonphotosynthetic biologically pigmented surfaces, and a lab-based experimental analysis of the spectral and broadband color diversity of pigmented organisms on Earth. We test the utility of broadband color to distinguish between Earth-like planets with significant coverage of nonphotosynthetic pigments and those with photosynthetic or nonbiological surfaces, using both 1-D and 3-D spectral models. We demonstrate that, given sufficient surface coverage, nonphotosynthetic pigments could significantly impact the disk-averaged spectrum of a planet. However, we find that due to the possible diversity of organisms and environments, and the confounding effects of the atmosphere and clouds, determination of substantial coverage by biologically produced pigments would be difficult with broadband colors alone and would likely require spectrally resolved data.