Surface and Temporal Biosignatures

TL;DR

This paper reviews surface and temporal biosignatures, such as spectral edges and fluorescence, as promising indicators of life on exoplanets, complementing atmospheric biosignatures and offering new avenues for remote life detection.

Contribution

It provides a comprehensive overview of surface and temporal biosignatures, highlighting recent advances, potential detection methods, and their significance in exoplanet life searches.

Findings

Vegetation red-edge (VRE) is the most studied biosignature.

Spectral edges from pigments could indicate life.

Temporal signals like fluorescence offer additional evidence.

Abstract

Recent discoveries of potentially habitable exoplanets have ignited the prospect of spectroscopic investigations of exoplanet surfaces and atmospheres for signs of life. This chapter provides an overview of potential surface and temporal exoplanet biosignatures, reviewing Earth analogues and proposed applications based on observations and models. The vegetation red-edge (VRE) remains the most well-studied surface biosignature. Extensions of the VRE, spectral "edges" produced in part by photosynthetic or nonphotosynthetic pigments, may likewise present potential evidence of life. Polarization signatures have the capacity to discriminate between biotic and abiotic "edge" features in the face of false positives from band-gap generating material. Temporal biosignatures -- modulations in measurable quantities such as gas abundances (e.g., CO2), surface features, or emission of light (e.g., fluorescence, bioluminescence) that can be directly linked to the actions of a biosphere -- are in general less well studied than surface or gaseous biosignatures. However, remote observations of Earth's biosphere nonetheless provide proofs of concept for these techniques and are reviewed here. Surface and temporal biosignatures provide complementary information to gaseous biosignatures, and while likely more challenging to observe, would contribute information inaccessible from study of the time-averaged atmospheric composition alone.