In an era of deep space exploration, a tantalizing and surprising discovery has raised the possibility of life on Earth’s nearest, scorching-hot planetary neighbor Venus.
Scientists observing with the James Clerk Maxwell Telescope in Hawaii detected the spectroscopic signature of the chemical compound phosphine in the Venusian clouds, about 35 miles above the surface. Follow-up observations with the Atacama Large Millimeter/submillimeter Array in Chile confirmed the discovery.
Phosphine, or PH3, a molecule composed of one phosphorus and three hydrogen atoms, is a “biomarker“ chemical that scientists hope to find in the atmospheres of distant Earth-like extrasolar planets to indicate possible biological activity.
On Earth, besides human industrial activity, the only known generator of phosphine is anaerobic life (which does not require oxygen to grow), either from microbial organisms or the decomposition of organic matter. And though there are nonbiological processes that produce phosphine deep in the hydrogen atmospheres of giant planets like Jupiter and Saturn, those conditions are not found on small rocky worlds like Earth and Venus.
Follow the Phosphine?
Astrobiologists have focused their search for extraterrestrial life on places that harbor liquid water. NASA’s life-seeking motto is “Follow the water,” since life as we understand it on Earth requires water to thrive, let alone originate.
Orbital spacecraft like NASA’s Mars Odyssey and Mars Reconnaissance Orbiter, and rovers like Spirit, Opportunity and Curiosity, have raked the dry surface of Mars to find and analyze mineral residues from its extinct seas. Soon, Perseverance will dig for signs of past Martian life that may have thrived in those waters. The Galileo spacecraft and Hubble Space Telescope have revealed signs of an ocean hidden under the icy crust of Jupiter’s moon Europa, and the Cassini probe sampled plumes of mineral-laden water erupting from Saturn’s moon Enceladus, believed to originate from a subsurface sea.
But one thing we have learned about life on Earth is that it keeps showing up in places where we least expect to find it. “Extremophiles,” earthly organisms that thrive in some of the hottest, coldest and most toxic watery environments on our planet, have been found deep within frigid Antarctic and alpine lakes, around superheated hydrothermal vents at the bottom of the ocean, and at the fringes of toxic geothermal hot springs. These highly resilient and adaptable life forms give us hope of finding signs of life in the waters of completely alien worlds.
With very little water vapor in its atmosphere, Venus is not a place where scientists expect to find signs of life, but the discovery of the biomarker phosphine is fueling further investigation and possible future missions to Venus to explore the question.
If This Is Life, Where Did It Come From?
If the phosphine in Venus’ atmosphere is produced by non-oxygen-using microbial life and not abiotic chemical processes (processes not derived from living organisms) that we simply don’t yet understand, where did the critters come from?
On Venus’ surface, temperatures soar to around 900 degrees Fahrenheit, and the atmospheric pressure is 90 times greater than sea level on Earth, equivalent to the water pressure half a mile deep in Earth’s oceans. It is challenging to imagine even a hardy form of Venusian life existing there.
But at the altitude where Venus’ phosphine was detected, between 30 and 40 miles above the surface, the atmospheric pressure and temperature are similar to Earth’s surface, though the chemistry is very different, dominated by carbon dioxide gas and liquid droplets of sulfuric acid.
Some scientists believe that conditions on Venus were very different in the past, and that billions of years ago Venus might have had a surface ocean of water and atmospheric conditions closer to that of the young Earth. And if life could arise in Earth’s primordial oceans and atmosphere, why not on Venus?
As the environment on Venus’ surface changed from more clement conditions to the hellish planetary pressure cooker it is today, some theorize that extremophile life forms could have fled skyward to survive, adapting to a less harsh environmental niche at higher altitude.
What’s Next in the Hunt for Life on Venus?
Researchers stress that the detection of phosphine in the clouds above Venus does not mean the certain presence of life there, and that there may be a nonbiological explanation that we have yet to understand.
Further telescopic observations will be made to learn more, but verifying what’s going on in Venus’ atmosphere will require sending a spacecraft to investigate. NASA is currently considering mission proposals to do just that.
If there is life thriving in Venus’ clouds, it raises even more questions: Did life begin independently on Earth and Venus, and if so, which came first? Or could life on both planets share a common origin? And, if life sprung up readily on both sister planets, and possibly on neighboring Mars as well, what does that say about how common life may be in the universe?
Personally, I’m keeping my fingers crossed that a future robotic probe floating through the Venusian atmosphere will send us evidence of microscopic life floating in the clouds of our closest neighboring planet.