Marisa Iati and Joel Achenbach
An international team of astronomers has detected a rare molecule in the atmosphere of Venus that could be produced by living organisms, according to a study published Monday.
The discovery instantly puts the brightest planet in the night sky back into the conversation about where to search for extraterrestrial life.
The researchers made clear this is not a direct detection of life on Venus.
But the astronomical observations confirmed the highly intriguing presence of the chemical phosphine near the top of the acidic clouds that blanket the planet.
Phosphine is a simple molecule produced on Earth by bacteria and through industrial processes. As a result, it is on the list of molecules - oxygen being another - considered by scientists to be potential "biosignatures" of life on Earth-sized planets whose atmospheres can be viewed through telescopes.
The researchers said they know of no non-biological explanation for the relatively high abundance of the molecule in the Venusian atmosphere.
"We did our very best to show what else would be causing phosphine in the abundance we found on Venus. And we found nothing. We found nothing close," said Clara Sousa-Silva, a molecular astrophysicist at the Massachusetts Institute of Technology and a co-author of the paper published Monday in the journal Nature Astronomy.
Venus is broiling at the surface, but there are layers of the atmosphere where temperatures and pressures are moderate and where solar radiation isn't too intense. For decades, some planetary scientists have speculated that microbes could be circulating in the atmosphere, which is dominated by sulfuric acid and carbon dioxide and has only small traces of water vapour.
Venus has long been overshadowed by Mars as a potential abode of life, because the planet's dense atmosphere and proximity to the sun has led to a runaway greenhouse effect, resulting in hellish surface temperatures and crushing atmospheric pressures. Robotic probes have revealed a landscape that appears inhospitable to any imaginable life form.
Mars has always appeared more congenial to life and potential human exploration, and has been targeted by multiple robotic missions, including most recently Nasa's Mars 2020 rover, Perseverance. Nasa is pondering proposals for two relatively low-cost robotic missions to Venus, but they have not been approved. Monday's announcement could push Nasa and other space agencies to take a closer look at Venus.
"For something this big, we need follow-up confirmations, we need to have strong scientific debate," said Casey Dreier, senior space policy adviser at the Planetary Society, a nonprofit pro-space organisation that was not involved in the new research. "Ultimately, we're going to need missions to Venus, and maybe even bringing samples back to Earth."
Sarah Stewart Johnson, a planetary scientist at Georgetown University who also was not involved with the new study, echoed that sentiment in an email: "This is exactly the kind of anomalous finding we should be following up on. There may be things we're missing photochemically - that we simply don't understand - but it's possible that the phosphine is the result of a biotic process, and its detection surely increases the chances for life."
Sousa-Silva said she had spent years studying phosphine as a potential biosignature in the atmospheres of exoplanets - the planets that orbit distant stars. But she had not considered searching for phosphine within our own solar system.
Then, a bit more than a year ago, she received an email from Jane Greaves, an astronomer at Cardiff University and lead author of the new paper, who said her telescopic observations within the solar system had turned up signs of phosphine in Venus.
Follow-up observations with another telescope confirmed that initial detection.
Phosphine is a toxic, malodorous gas that is extremely poisonous and has been used in chemical weapons. The molecule is composed of one phosphorus atom and three hydrogen atoms, and is shaped like a pyramid with the phosphorus on top. These atoms do not favour one another, and thus the molecule is generally rare in nature. To create a phosphine molecule, Sousa-Silva said, requires an unusual force or mechanism.
Living things can create the molecule through a metabolic pathway not fully understood, she said. And extreme forces in nature can make the molecule. For example, phosphine has been detected in the atmospheres of Jupiter and Saturn, the solar system's two gas giants, where gravity creates extreme environments.
But Venus, like Earth, is a relatively small, rocky planet. The researchers pondered many possible mechanisms for the creation of phosphine, including volcanoes and meteor impacts, but could not get the numbers to come out right.
"On Venus, there are no mechanisms we know of that can spontaneously make phosphine, because it is so hard to produce," Sousa-Silva said.
The scientists involved in this new detection were careful not to overstate their findings. For example, although a non-biological source of the phosphine in Venus is not known, that doesn't mean there isn't one, Sousa-Silva stipulated.
Any claim of a detection of life beyond Earth carries with it a heavy burden of proof. The search for extraterrestrial life has had a long history of thrilling hypotheses, rancorous debates and crushing disappointments. To date, no alien life has been found - anywhere.
Mars periodically has generated great excitement, only to have claims erode under the harsh light of further investigation. In several high-profile cases, something that looked irrefutably biological turned out on closer scrutiny to be potentially explicable through more prosaic processes.
"My first reaction, as always, is skepticism," said Bruce Jakosky, a planetary scientist at the University of Colorado, when asked about the new report. "One of the things I've seen is that when people discover new, cool things, their first thought is life, and then they're able to come up with alternative, plausible explanations for what they saw."
Even so, he said, the phosphine discovery is "intriguing."
A similar situation has popped up over on Mars, where methane has been detected in the atmosphere. That incited speculation that it was produced by Martian organisms. But this remains unresolved, because there are non-biological explanations for the presence of the gas, according to Nasa. One international mission designed specifically to look for it, the ExoMars Trace Gas Orbiter, couldn't find it at all.
Monday's announcement suggests that maybe scientists should spend more time looking elsewhere - back toward the second rock from the Sun.