Experts conclude the intriguing phosphine signal in Venus' atmosphere was likely just common sulfur dioxide—delaying hopes for a historic biosignature discovery.
In mid-September, researchers announced the detection of phosphine in Venus' upper atmosphere, sparking global headlines. On Earth, phosphine is linked to anaerobic bacteria, fueling speculation about extraterrestrial life. However, subsequent analyses have challenged this finding. A new study from the University of Washington and NASA scientists provides compelling evidence that the signal wasn't phosphine at all.
Chemical compounds absorb distinct wavelengths in the electromagnetic spectrum, allowing astronomers to decode planetary atmospheres. The initial team reported a spectral signal at 266.94 gigahertz in Venus' atmosphere.
Both phosphine and sulfur dioxide—a far more plausible Venusian compound—absorb radio waves near this frequency, prompting scrutiny.
Follow-up observations with the Atacama Large Millimeter/submillimeter Array (ALMA) focused on sulfur dioxide frequencies. The team deemed SO2 levels too low to explain the signal, attributing it to phosphine.
This latest research modeled Venus' atmospheric conditions using decades of observational data, then reanalyzed ALMA datasets for both observed and undetected features.
By simulating signals from phosphine and SO2 at various altitudes, researchers assessed ALMA's detection capabilities. The signal's shape indicated it originated not from Venus' cloud tops, as first claimed, but from the mesosphere, about 80 km above the surface.
At that height, harsh chemicals and UV radiation would destroy phosphine molecules in seconds, ruling it out.
Crucially, ALMA data had severely underestimated sulfur dioxide abundance. "ALMA's 2019 antenna configuration caused spectral line dilution, where widespread gases like SO2 appear weaker than localized ones," explains Alex Akins of NASA's Jet Propulsion Laboratory.
Sulfur dioxide fully accounts for the initial observations and aligns with Venus' known atmosphere, rich in sulfuric acid clouds. As the third most abundant compound there, SO2 is not indicative of life.
