A biosignature is any substance, structure, or pattern that provides scientific evidence of past or present life, or that could only have been created through biological processes. This concept forms the central bridge between biology and astronomy, providing the measurable evidence scientists use to hunt for life beyond Earth. The search for extraterrestrial life is fundamentally a search for these unique biological fingerprints. Researchers are developing strategies to look for a combination of clues across vast distances. Correctly identifying these chemical, physical, and spectral markers determines humanity’s success in answering one of its oldest questions: are we alone?
What Scientists Look For
Scientists categorize biosignatures into three main groups based on the nature of the evidence. Gaseous or atmospheric signatures involve volatile molecules produced by metabolic processes that accumulate in a planet’s atmosphere. Gases like oxygen ($\text{O}_2$) and methane ($\text{CH}_4$) are strong candidates because life on Earth heavily produces them. Since the mere presence of a gas is insufficient, scientists look for chemical disequilibrium, where two or more gases coexist despite thermodynamics dictating they should quickly react and disappear.
Morphological signatures represent the physical evidence of life, often preserved in rocks or sediments. These include microfossils, the microscopic remains of organisms, and macroscopic structures like stromatolites. Stromatolites are layered, dome-like formations created when microbial mats trap and bind sediment particles. Both microfossils and stromatolites offer a window into ancient life and are primary targets in searches on rocky bodies like Mars.
Spectral signatures focus on how biological materials interact with light. Photosynthetic pigments, such as chlorophyll on Earth, absorb specific wavelengths of light to generate energy. If life on an exoplanet uses a similar mechanism, its surface would display a distinct change in reflectivity, sometimes called the “red edge.” This light absorption provides a unique spectral feature that can be remotely detected.
Analyzing Distant Worlds
The search for biosignatures on planets outside our solar system, known as exoplanets, relies almost entirely on remote sensing techniques, primarily transit spectroscopy. This method observes a star’s light as an orbiting exoplanet passes directly in front of it, causing a slight dimming. Light passing through the planet’s atmosphere is selectively absorbed by the gases present, leaving a chemical fingerprint in the spectrum that reaches our telescopes.
The James Webb Space Telescope (JWST) performs this analysis with high sensitivity, particularly in the infrared spectrum where many biogenic molecules absorb light. JWST searches for gas pairs that suggest biological activity, such as methane ($\text{CH}_4$) and carbon dioxide ($\text{CO}_2$) existing without significant amounts of carbon monoxide ($\text{CO}$). This specific disequilibrium would be difficult to maintain without a constant biological source actively consuming the $\text{CO}$.
Detecting oxygen ($\text{O}_2$) is challenging because its strongest spectral features are at wavelengths less accessible to JWST. Researchers often target biosignature pairs like $\text{CH}_4$ and $\text{O}_2$, which are produced simultaneously by life on Earth and are highly out of chemical equilibrium. Detecting this strong chemical imbalance would signal a planet where natural chemical reactions are overwhelmed by a massive global biological process.
Searching Within Our Solar System
The search for life within our solar system focuses on in-situ (on-site) examination of planetary bodies where liquid water either existed in the past or may still exist today. Mars is a prime target for searching for ancient life, with rovers like Curiosity and Perseverance collecting and analyzing rock samples. The goal on Mars is to find preserved organic molecules or microfossils left behind from when the planet was a warmer, wetter world billions of years ago. Perseverance is collecting samples planned for eventual return to Earth, where they can be analyzed in sophisticated laboratories for definitive biosignatures.
The icy moons of the outer solar system, particularly Europa (Jupiter) and Enceladus (Saturn), are the most promising locations for extant life. Both moons harbor vast, subsurface saltwater oceans that are kept liquid by tidal heating from their parent planets. On Enceladus, dramatic plumes of water vapor and ice particles erupt from the surface, allowing spacecraft to sample the ocean’s chemical composition without landing. The Cassini mission detected organic molecules in these plumes, though not definitive biosignatures.
Searching for biosignatures on Europa is more challenging due to its harsher radiation environment and thicker ice shell. Proposed lander missions would need to drill or sample the shallow subsurface, likely around 20 centimeters deep, to find organic molecules protected from Jupiter’s radiation. The strategy for these icy moons is direct chemical analysis of the ocean material to identify complex organic molecules, such as amino acids, that are the building blocks of life.
The Challenge of Abiotic Mimics
The greatest hurdle in confirming an extraterrestrial biosignature is the problem of the abiotic mimic, or false positive. An abiotic mimic is a non-biological geological or chemical process that produces the exact same substance as a biosignature. For instance, oxygen ($\text{O}_2$), which is overwhelmingly produced by photosynthesis on Earth, can be generated abiotically through the photolysis of water vapor in a runaway greenhouse scenario. On a planet with a massive ocean and specific atmospheric conditions, this non-biological process could build up an $\text{O}_2$-rich atmosphere, mimicking the signature of life.
Methane ($\text{CH}_4$) also has well-known abiotic sources, most notably through water-rock reactions like serpentinization. This process occurs when water reacts with iron-rich minerals, generating hydrogen that can then react to form methane. Therefore, detecting methane alone does not prove the existence of life, as it could simply be a sign of active geology.
To overcome these false positives, scientists must rely on a contextual approach, requiring multiple, independent lines of evidence. A single molecule is insufficient; a confirmed biosignature must be part of a larger chemical system that is demonstrably out of equilibrium and cannot be explained by any known non-biological process. The strategy is to not only find a potential biosignature but also to search for “anti-biosignatures,” such as the presence of abundant carbon monoxide ($\text{CO}$), which would argue against life because life on Earth readily consumes this molecule.