Biosignatures

A very nice overview of biosignatures:

Exoplanet Biosignatures: A Review of Remotely Detectable Signs of LifeIn the coming years and decades, advanced space- and ground-based observatories will allow an unprecedented opportunity to probe the atmospheres and surfaces of potentially habitable exoplanets for signatures of life. Life on Earth, through its gaseous products and reflectance and scattering properties, has left its fingerprint on the spectrum of our planet. Aided by the universality of the laws of physics and chemistry, we turn to Earth's biosphere, both in the present and through geologic time, for analog signatures that will aid in the search for life elsewhere. Considering the insights gained from modern and ancient Earth, and the broader array of hypothetical exoplanet possibilities, we have compiled a state-of-the-art overview of our current understanding of potential exoplanet biosignatures including gaseous, surface, and temporal biosignatures. We additionally survey biogenic spectral features that are well-known in the specialist literature but have not yet been robustly vetted in the context of exoplanet biosignatures. We briefly review advances in assessing biosignature plausibility, including novel methods for determining chemical disequilibrium from remotely obtainable data and assessment tools for determining the minimum biomass required for a given atmospheric signature. We focus particularly on advances made since the seminal review by Des Marais et al. (2002). The purpose of this work is not to propose new biosignatures strategies, a goal left to companion papers in this series, but to review the current literature, draw meaningful connections between seemingly disparate areas, and clear the way for a path forward.

Overview

A biosignature denotes the presence of a substance or phenomenon that is indicative of life, past or present. Biosignature is a term defined in the context of the field of astrobiology and it represents evidence of life in the context of the system where it is being detected. More specifically, if this system is a planet or a moon, the biosignature represents the detection of chemical compounds on the surface or in the atmosphere (if it has an atmosphere) that are indicative of life on that planet or moon. For example, biosignatures can be chemical compounds, particularly organic compounds, visible macroscopic patterns, atmospheric gases.

The past couple of decades have seen an explosion in biosignature science, but currently only the Viking mission has been equipped to search for biosignatures in our solar system. Upcoming missions target the moons Europa and Titan for searches. One of Earth’s biosignatures is the atmospheric composition of gases, particularly oxygen and methane in strong thermodynamic equilibrium, the surface reflectance of the vegetation in color red, and narrow-band, pulse modulated radio signs. These “signatures” together suggest an inhabited planet, but each signature by itself might be a “false positive” in confirming the presence of life.

Additional to how we understand the presence of life on Earth as a starting point for biosignature searches on exoplanets, there has been considerable work done in understanding “agnostic biosignature”, or biosignatures that have very little in common with life on Earth. The agnostic biosignatures are based on a broader definition of life, based on processes and activities and not on specific molecular structures.

Biosig.pdf
Cartoon adapted from Victoria Meadows' 2018 oxygen as a biosignature study.

Relationship to human societies and planet Earth

According to Merriam-Webster dictionary, this term was been first used in 1995 and for a while was synonymous with the term “biomarker”. It was first used in connection with NASA’s Strategy for Mars Exploration. While “biomarker” remains a term used in medicine and biology to identify specific substances and molecules, the term “biosignatures” has been mostly adopted by astrobiology and space sciences.

Aside from its’ scientific implications, the term is important for our culture and society. Biosignatures research has a fundamental role in our understanding of the origin of life, in our quest to understand the universality or uniqueness of life on Earth, it has deep roots in our propensity for exploration beyond our planet and, most of all, in answering a deeply important question: Are we alone? The implications of finding biosignatures off Earth, in deep space, are staggering – they would expand our collective, social nature beyond our planetary home in Cosmos. 

"The probability is one, but don't count on it". (SELF-ANNIHILATING SENTENCES - Saul Gorn's Compendium Of Rarely Used Cliches, 1985)