biosignatures

History

The idea of searching for life-related gases in planetary atmospheres took shape in the 1960s. Astrobiologist Carl Sagan demonstrated that Earth's atmosphere with oxygen and methane would be a clear sign of life to a distant observer.

How it works

Just as the smell of exhaust fumes indicates a car, a combination of gases in a planet's air can reveal its inhabitants. If oxygen and methane—gases that quickly react and vanish—are seen simultaneously in the spectrum, it means there is a source constantly replenishing them, possibly living organisms.

History

The idea of searching for life-related gases in planetary atmospheres took shape in the 1960s. Astrobiologist Carl Sagan demonstrated that Earth's atmosphere with oxygen and methane would be a clear sign of life to a distant observer.

How it works

Just as the smell of exhaust fumes indicates a car, a combination of gases in a planet's air can reveal its inhabitants. If oxygen and methane—gases that quickly react and vanish—are seen simultaneously in the spectrum, it means there is a source constantly replenishing them, possibly living organisms.

History

The first ideas about possible signs of life on Mars were voiced back in the 19th century (Schiaparelli's canals). A systematic approach emerged in the 1960s, when James Lovelock analyzed Earth's atmosphere as a product of the biosphere. Carl Sagan and others developed the concept of thermodynamic disequilibrium. With the discovery of exoplanets in the 1990s, the search for biosignatures became a central goal of astrobiology.

How it works

During a transit of an exoplanet in front of its star, part of the starlight is filtered by its atmosphere, and absorption lines of molecules appear in the spectrum. Each molecule has a unique spectral 'fingerprint'. By measuring the depth of lines at different wavelengths, the gas composition is determined. A biosignature is considered reliable if a combination of gases is detected that cannot be maintained without a biological source, and all known abiotic pathways under the given conditions (temperature, radiation, geology) are ruled out. Principles of spectroscopy and photochemical modeling are used.

Open problems & fun facts

Which gases could serve as a universal biosignature for life based on different chemistry?; How to distinguish oxygen of biological origin from oxygen accumulating due to water photodissociation and hydrogen escape?; Are there reliable surface biosignatures that do not require atmospheric measurements?; Can cloudy worlds like Venus retain biosignatures in their upper atmospheres?

💡 The spectrum of Earth, taken during a lunar eclipse, showed the simultaneous presence of oxygen, methane, and water—the very 'triumvirate' sought after on exoplanets.