A Signal in the Clouds
In September 2020, astronomers announced the detection of a rare molecule, phosphine (PH3), in the atmosphere of Venus. A team led by Jane Greaves of Cardiff University first spotted a hint of the gas using the James Clerk Maxwell Telescope (JCMT) in Hawaii. To confirm the finding, they used the more sensitive Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. Both telescopes observed Venus at a wavelength of about 1 millimeter, detecting a faint absorption pattern that matched the spectral signature of phosphine.
The signal indicated a concentration of about 20 molecules for every billion in the atmosphere. This discovery was made not at the planet's surface—where temperatures reach 465°C—but high up in the temperate cloud decks, between 50 and 60 kilometers in altitude. In this atmospheric layer, conditions are less extreme, with temperatures ranging from 30 to 70 degrees Celsius and pressures similar to Earth's at sea level. These more moderate conditions have led some scientists to speculate for decades that microbial life could potentially survive there, floating amid clouds made of concentrated sulfuric acid.
A Controversial Biosignature
The detection of phosphine immediately drew widespread attention because on rocky planets like Earth, the gas is associated with life. It is produced by microbes that live in oxygen-free environments, such as swamps, lake sediments, and even the guts of some animals. While the gas is also found in the high-pressure interiors of gas giants like Jupiter and Saturn, producing it through non-biological processes on a planet like Venus is chemically difficult.
The original research team investigated known abiotic mechanisms for producing phosphine on Venus, including sunlight, lightning, meteorites, and minerals blown up from the surface. Their calculations suggested that these sources could produce at most one ten-thousandth of the amount of phosphine they had detected. This left a significant, unexplained surplus, leading to speculation about unknown geological processes or even a biological origin.
The scientific community responded with intense scrutiny. Several independent research groups re-analyzed the original ALMA data and failed to confirm the phosphine signal, suggesting the initial observation might have been an artifact of data processing or a misidentification of sulfur dioxide, which is abundant on Venus. Other observation campaigns using different instruments, such as the Stratospheric Observatory for Infrared Astronomy (SOFIA), failed to find evidence of phosphine, placing a strict upper limit on its possible concentration at just 0.8 parts per billion. Some scientists have proposed that active, explosive volcanism could eject phosphorus compounds from the mantle that then react with sulfuric acid in the clouds to form phosphine, providing a geological explanation. The original discovery team has stood by its findings, presenting new data from JCMT that appears to show the signal again.
The debate is ongoing, and the question of whether phosphine exists in the Venusian clouds remains unresolved. Final answers will likely require direct sampling of the atmosphere. Future missions, including NASA's DAVINCI and VERITAS probes and Europe's EnVision orbiter, are planned for the coming years and are equipped to analyze the planet's atmospheric chemistry in unprecedented detail, potentially solving this mystery.