HOUSTON—A British-led research effort has caught a possible low probability hint of biological activity in the high altitude cloud layers of neighboring Venus.
The planet has long been considered much too hot with hyperacidic atmospheric conditions that have had most astrobiologists looking the other way.
The hint is phosphine gas, or PH3, a combination of phosphorus and hydrogen, which on Earth is associated primarily with biological activity in oxygen-free environments.
The curious observation and assessment was led by Jane Greaves, of Cardiff University and the University of Cambridge, in the U.K., and 18 other researchers from Great Britain, Japan and the U.S. It was made with the James Clerk Maxwell Telescope at the Mauna Kea Observatory in Hawaii in 2017 and the Atacama Large Millimeter/submillimeter Array in northern Chile in 2019.
Their efforts detected the spectral signature of phosphine, at concentrations of 20 parts per billion, in cloud layers at altitudes between 53 and 62 km (33 and 38.5 mi.) in the planet’s dense atmosphere and in conditions that would quickly destroy phosphine. The gas they detected was present in cloud regions considered temperate, about 30C (80F), but not over either pole.
The researchers’ 20-page paper is called “Planetary Science: Phosphine detected in the clouds of Venus.” It was published Sept. 14 in the journal Nature Astronomy. In the paper, Greaves and her colleagues explain that they investigated how the colorless but toxic and flammable gas might somehow be traced to the surface of Venus through meteorites, lightning or chemical processes within the clouds. But they could not pin down a source.
“An ideal biosignature gas would be unambiguous,” the study concludes in part.
Greaves, motivated by theories that suggest Venus once hosted water oceans and temperate conditions before undergoing a significant climate change, spoke during a virtual Sept. 14 news briefing hosted by the Royal Astronomical Society.
“The reason for our excitement is that phosphine gas on Earth is made by microorganisms that live in oxygen-free environments. So, there is a chance we have detected some kind of living organisms in the clouds of Venus,” Greaves explained. “Conditions today are really hostile, with temperatures hot enough to melt landers. But much earlier in the history of Venus the surface was much cooler and wetter and life could have possibly originated. There is a longstanding theory that some of the smallest forms of life, microorganisms, might have been able to evolve upward into the high clouds.”
At altitudes of 50 km (31 mil.) and higher on Venus, the temperatures and pressures are Earthlike.
“It’s been hypothesized this is a living habitat today,” Greaves said.
If no known chemical process can explain PH3 within the upper atmosphere of Venus, then it must be produced by a process not previously considered plausible for Venusian conditions, the research team as a whole concludes.
The options include yet-to-be unraveled photochemical or geochemical processes. But the possibility of life cannot be ruled out.
“We are not claiming we have found life on Venus,” co-author Sara Seager, a Massachusetts Institute of Technology (MIT) planetary scientist, told the news briefing. “We are claiming the confident detection of phosphine gas, whose existence is a mystery.”
On Earth, Seager noted, microbes in water droplets gather in clouds and travel between continents before raining down. On Venus, the cloud layers are more vast, deep and long lived, though chemically harsh.
She offered the possibility that bacteria-like life forms could find protection within cloud-borne droplets of hydrosulphuric solution that collide and coalesce for long periods until they are heavy enough to fall like rain. During the fall, the droplets would evaporate, leaving dried-out, spore-like life forms so light they stopped falling and perhaps occupied a haze-like layer in the planet’s lower atmosphere.
After months to years, some of the spore-like life forms would be updrafted once again to encounter temperate conditions to rehydrate, enabling their life cycle to continue, Seager explained.
Recent missions and studies have raised excitement over past, and perhaps current, biological activity on Mars, Jupiter’s moon Europa and Saturn’s moon’s Enceladus and Titan.
“The phosphine gas discovery has raised Venus higher up on the ladder,” Seager contends.
Further exploration of Venus is warranted, the researchers believe.
Two Russian Vega missions that launched to Venus in December 1984 included balloons and landers for atmospheric descent. They detected atmospheric phosphorus as an element, but their sensors were not equipped to assess additional chemical speciation.
Earlier this year, NASA selected four possible future Discovery class missions, two of them focused on Venus, for further concept studies and development;
The Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging Plus (Davinci+) mission would analyze Venus’ atmosphere to understand how it formed and evolved, and determine whether Venus ever had an ocean.
In the Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy (Veritas) mission, an orbiter equipped with synthetic aperture radar would topographically map the planet’s surface to better understand its geologic history and why it evolved so differently than Earth.
Among four candidate missions, each received $3 million for future study. Two of the four could be fully funded through further evaluation.
