Venus is often called Earth’s sister planet, a neighboring twin of similar density and size. But the resemblance ends there. As the hottest planet in our solar system, the sweltering Venusian atmosphere is full of heat-trapping carbon dioxide and thick clouds of sulfuric acid that envelop its dry, volcanic terrain.
It is therefore one of the last places where we could think of looking for life beyond our planet.
That’s why it was such a shock last September when a group of scientists, led by Jane Greaves from Cardiff University, announced they had found a possible sign of extraterrestrial life in the Venusian atmosphere. In the study, Posted in Nature astronomy, they reported the detection of a colorless toxic gas called phosphine in the planet’s clouds and concluded that no known chemical or geological process could explain its presence. Phosphine could indicate life, they argued, noting recent work by MIT astrophysicist Clara Sousa-Silva who suggests the gas could be a biosignature. On Earth, phosphine is often found in places that support anaerobic life, including lakes, swamps, rice paddies, and in landfill sludge.
But when the news reached Cornell University astronomer Jonathan Lunine, he and graduate student Ngoc Truong were immediately skeptical. “It’s problematic to invoke phosphine as a biosignature on Venus, just because the environment on Venus is totally different from the environment on Earth,” Truong explains. Even on our own planet, he says, there is some confusion as to whether phosphine is associated with life, and he believes this should be confirmed before extrapolating these observations to environments so different from ours.
Truong and Lunine weren’t the only ones doubting: After the phosphine announcement, the internet exploded with talk of the discovery. Scientists weighed in on Twitter threads, argued over Facebook posts, and flocked to arXiv.org, a preprint server for scientific research, to expose other theories about non-biological processes that might produce phosphine.
Truong, who until then had studied the oceans on Saturn’s moons, convinced Lunin that they needed to further explore one potential source of phosphine in particular: volcanoes. Their research resulted in a new study published Monday in the newspaper Proceedings of the National Academy of Sciences. In it, Truong and Lunine paint a picture of how phosphine might enter the atmosphere of Venus. Traces of phosphides (negatively charged phosphorus ions attached to metals like iron) found deep in Venus’ mantle could have been brought to the surface by volcanic activity. When volcanoes erupt, these phosphides could be blown into the atmosphere and react chemically with sulfuric acid in clouds to form phosphine.
“Our study only suggests a roadmap for assessing the level of volcanic eruptions” on Venus, says Truong. Two conditions are necessary for this explanation to be viable. First, the planet must be volcanically active. (While thousands of volcanoes have been spotted on radar images of Venus, scientists lack data to confirm recent eruptions, because so far, landers can withstand the raging heat and overwhelming pressure of the Venusian surface for only about an hour.) “And not just active in the sense of ‘Hawaiian-style volcanism’,” says Lunine, which typically produces lava flows without much explosiveness. Explosive volcanism is the key, because a mechanism is needed for the phosphides to be ejected into the atmosphere.
Second, scientists should verify that the phosphine is there – and that’s currently a huge point of contention. Without this proof, says Lunine, the theory of volcanoes “becomes an empty postulate rather than a hypothesis.”