2022ApJ...933....6H

Biosignature gas research has been growing in recent years thanks to next-generation space- and ground-based telescopes. Methanol (CH₃OH) has many advantages as a biosignature gas candidate. First, CH₃OH's hydroxyl group (OH) has a unique spectral feature not present in other anticipated gases in the atmospheres of rocky exoplanets. Second, there are no significant known abiotic CH₃OH sources on terrestrial planets in the solar system. Third, life on Earth produces CH₃OH in large quantities. However, despite CH₃OH's advantages, we consider it a poor biosignature gas in the atmospheres of terrestrial exoplanets due to the enormous production flux required to reach its detection limit. CH₃OH's high water solubility makes it very difficult to accumulate in the atmosphere. For the highly favorable planetary scenario of an exoplanet with an H₂-dominated atmosphere orbiting an M5V dwarf star, we find that only when the column-averaged mixing ratio of CH₃OH reaches at least 10 ppm can we detect it with the James Webb Space Telescope (JWST). The CH₃OH bioproduction flux required to reach the JWST detection threshold of 10 ppm must be of the order of 10¹⁴ molecules cm^–2 s^–1, which is roughly three times the annual O₂ production on Earth. Considering that such an enormous flux of CH₃OH is essentially a massive waste of organic carbon-a major building block of life, we think this flux, while mathematically possible, is likely biologically unattainable. Although CH₃OH can theoretically accumulate on exoplanets with CO₂- or N₂-dominated atmospheres, such planets' small atmospheric scale heights and weak atmospheric signals put them out of reach for near-term observations.