Editor's note: Newer evidence does not support the existence of phosphine in the Venusian atmosphere. This article explains the state of research at the original time of publication.
You can find the molecule phosphine in some interesting places: insecticides, penguin tummies, and possibly the clouds of Venus.
Phosphine can be a byproduct of living things, so finding it in Venus’ clouds could possibly indicate something alive floating around the planet. Venus’ surface is inhospitable, but there are regions in the cloud decks where temperatures and pressures are similar to those found on Earth.
Astronomer and astrobiologist Jane Greaves led the team that announced the discovery of phosphine in the Venusian atmosphere in 2020. She began including a picture of a penguin in her talks to illustrate one of the places you can find phosphine, and it caught on as a meme.
“I’m afraid we did confuse people, rather, with this penguin idea,” she said during an American Astronomical Society (AAS) meeting presentation in June. “We did not in fact suggest there are penguins flying desperately, flapping above the surface of Venus.”
What she does suggest is that there is solid evidence for phosphine in the Venusian clouds. In the two years since her original phosphine paper garnered great interest from the public and scientific community, many other scientists studied the paper and did their own research. Some scientists supported while others questioned the validity of the detection of phosphine.
In her AAS talk, Greaves outlined the story thus far and presented new findings in support of her phosphine in the clouds of Venus.
JCMT sees the phosphine signal again
In the original phosphine detection paper, Greaves and her 18 co-authors included results from two telescopes. The James Clerk Maxwell Telescope (JCMT) in Maunakea, Hawaiʻi observed Venus in 2017, while the Atacama Large Millimeter/submillimeter Array (ALMA) radio telescope system in northern Chile collected data in 2019.
Not included in the paper were a second round of JCMT observations from summer 2020 that have now been processed. Greaves reports that not only did JCMT see phosphine again, it did so using a different science instrument on the telescope. That means phosphine has been seen at three different points in time using three different telescope instruments at two different observatories.
What comes next? Greaves’ colleagues have been awarded 200 hours of JCMT time to observe Venus in 2022 and 2023, which will allow them to gather even more data.
NASA’s SOFIA telescope makes a potential detection
Another new potential detection of phosphine comes from NASA’s flying SOFIA telescope.
SOFIA, which is scheduled to be shut down due to budget cuts on Sept. 30, searched for phosphine on Venus late last year. Greaves processed publicly available data from the flight and made a potential discovery of phosphine at 3 parts per billion, at an altitude of 65 kilometers (40 miles) or higher. This includes Venus’ highest-most clouds.
“I’d say there’s a hint of a detection there,” she said. “But it’s really tentative.”
Why is it so hard to detect phosphine?
Venus is the brightest object in the sky after the Sun and the Moon. Searching for a faint signal like phosphine is like looking for a firefly in front of the Sun. This caused a brief snag for the original phosphine findings: The ALMA data had to be recalibrated because the telescope is not used to looking at such bright objects.
Complicating things further is the fact that Venus and Earth are always moving with respect to each other, creating doppler shifts in the signal that must be filtered out.
Only with careful data processing does a phosphine signal emerge, and scientists must be cautious to verify that what they’re seeing is real. Greaves is working on a simpler way to crunch her observation data using a mathematical technique called a Fourier transform. She hopes the process will reduce user bias and make it easier for others to replicate her work.
Another potential issue is that some molecules, like sulfur dioxide, have signals that look a lot like phosphine. Greaves and her team have re-analyzed their data and found that although sulfur dioxide could amplify a phosphine signal, the effect would be small compared to what they actually observed.
Venus space missions
The best way to search for phosphine would be to study Venus’ atmosphere up close.
NASA’s Pioneer Venus Multiprobe mission dropped probes into the planet’s atmosphere in 1978. A team led by Rakesh Mogul, a professor of biological chemistry at Cal Poly University in Pomona, California, examined old data from the mission and found evidence of phosphine.
New Venus missions have until recently been far and few between. That’s about to change, with at least six space probes scheduled to visit the planet Venus in the next ten years:
- The launch vehicle firm Rocket Lab plans to send the first private mission to Venus in 2023 to search directly for phosphine.
- India’s Shukrayaan-1 is scheduled to launch as soon as 2024 and will include an instrument that will be able to detect phosphine.
- NASA's DAVINCI, launching as early as 2028, will explore Venus' atmosphere. Greaves told The Planetary Society that “we’re about to make the case to the DAVINCI team” that the spacecraft should have the capability to search for phosphine.
- NASA's VERITAS, also launching as early as 2028, will use radar to map Venus’ surface. While it isn’t expected to search for phosphine directly, the spacecraft will help scientists understand the planet’s volcanoes — a potential non-life source for phosphine.
- The UAE’s multiple-asteroid mission, launching in 2028, includes a Venus flyby. It’s unclear if the spacecraft could detect the presence of phosphine.
- The European Space Agency’s EnVision mission launches no earlier than 2031 to investigate the atmosphere of Venus and the planet’s overall structure. It could help the phosphine search by mapping volcanoes.
NASA’s newly christened James Webb Space Telescope may also be able to help. Venus is too close to the Sun for JWST to observe it — the telescope has to stay cool at all times by keeping its back towards the inner solar system.
However, it’s possible that JWST could detect phosphine gas on exoplanets. The observatory would have to stare at a distant world for “tens of hours,” according to a paper that makes the case. Fortunately, JWST could scan for water and methane at the same time it looks for phosphine, allowing it to look for two other potential biosignatures.
If we confirm the existence of phosphine on Venus, does that mean we’ve found signs of life?
Unfortunately not. Until we are able to observe any possible Venus microbes directly — perhaps through sample return — the next step would be to rule out all non-life sources of the phosphine.
One possibility is volcanoes. We know Venus has volcanoes, but we don’t know how many of them are active. Our best maps of the surface come from NASA’s Magellan spacecraft, which orbited Venus three decades ago from 1990 to 1994. Papers published on the topic have argued for and against the notion that volcanoes could explain the amount of phosphine detected.
It would also be helpful to know whether Venus’ clouds are habitable to life as we know it. Although some regions have Earth-like temperatures and pressures, the clouds, as a whole, are low in water and high in sulfuric acid. However, data from Japan’s Akatsuki spacecraft show the clouds appear to be non-uniform and change over time. This could mean that life might be able to find a safe harbor in niche habitable environments.
For now, the question of whether there is life above Venus remains unanswered.
Greaves told The Planetary Society that she is more confident that her team found phosphine than she was two years ago, thanks to additional detections, the Pioneer Venus Multiprobe findings, and independent analyses of her work. After her AAS talk, an audience member asked her if she’d like to take a guess on where the phosphine is coming from.
“I essentially wanted the flying penguins, which no one will let me have,” she joked. “I’d love for it to turn out to be life.”