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Phosphine in Atmosphere of Venus Points to Explosive Volcanic Activity

In 2020, planetary scientists detected small amounts of the biosignature gas phosphine (PH3) in the atmosphere of Venus using the James Clerk Maxwell Telescope (JCMT) and Atacama Large Millimeter/submillimeter Array (ALMA) radio telescopes. In a new paper in the Proceedings of the National Academy of Sciences, a duo of scientists from Cornell University argues for the plausibility of volcanically extruded phosphide as an abiotic source of the atmospheric phosphine.

“It is of value to ask why phosphine is in the Venusian atmosphere, if it is there,” said Cornell University’s Jonathan Lunine and doctoral candidate Ngoc Truong.

“Phosphine had been considered and proposed as a potential biosignature in oxidizing terrestrial exoplanets’ atmospheres.”

“However, the specific pathway of biological production of phosphine still remains uncertain with no known direct metabolic pathway.”

The authors hypothesize that trace amounts of phosphides formed in the mantle are a plausible abiotic source of the Venusian phosphine observed.

In their hypothesis, small amounts of phosphides, sourced from a deep mantle, are brought to the surface by volcanism.

These phosphides are then ejected into the atmosphere in the form of volcanic dust by volcanic eruptions.

Sufficiently large explosive eruptions similar to the scale of Earth’s Krakatoa volcanic eruption in Indonesia in 1883 could inject material directly into the Venusian sulfuric acid cloud layer, explosions invoked by other scientists to explain the episodic changes of sulfur dioxide seen in the planet’s atmosphere.

There, these phosphides react with sulfuric acid in the aerosol layer to form phosphine.

“The phosphine is not telling us about the biology of Venus,” Professor Lunine said.

“It’s telling us about the geology. Science is pointing to a planet that has active explosive volcanism today or in the very recent past.”

“Our phosphine model suggests explosive volcanism occurring, while radar images from NASA’s Magellan spacecraft in the 1990s show some geologic features could support this,” he added.

“If Venus has phosphide — a form of phosphorous present in the planet’s deep mantle — and, if it is brought to the surface in an explosive, volcanic way and then injected into the atmosphere, those phosphides react with the Venusian atmosphere’s sulfuric acid to form Phosphine,” Truong said.

“In 1978, on NASA’s Pioneer orbiter mission, scientists uncovered variations of sulfur dioxide in Venus’ upper atmosphere, hinting at the prospect of explosive volcanism, similar to the scale of Krakatoa volcanic eruption.”

“But confirming explosive volcanism on Venus through the gas phosphine was totally unexpected.”