The detection of phosphine in Venus’ atmosphere was one of those quintessential moments in space science. It was an unexpected discovery, and when combined with our incomplete understanding of planetary science, and our wistful hopefulness around the discovery of life, the result was a potent mix that lit up internet headlines.
As always, some of the headlines were a bit of an over-reach. But that’s the way it goes.
At the heart of it all, there is compelling science. And the same, overarching question that keeps popping up: Are we alone?
For people who don’t follow the search for life too closely, finding 20 parts per billion of some obscure chemical that most people have never heard of doesn’t sound much like discovering life. But in the scientific world, this is the reality: the discovery of life likely means finding a strange chemical signature that leads us to single-celled organisms somewhere. Just like we did on Venus.
We’re not likely to discover some type of complex life like the type that populates Earth. Never say never, but the odds are against that.
That’s why the discovery of phosphine (PH3) has generated so much interest in the scientific world. As far as scientists know—and knowledge is incomplete—phosphine is the direct result of living processes, in most cases. Without life, it takes an enormous amount of energy to create, and that energy is absent on Venus and planets like it.
While the discovery has led to a lot of conjecture, some of it over the top, it’s still an intriguing discovery. With further study, we’ll either eventually find an organism somehow creating the phosphine, or we’ll learn something more about Venus that we didn’t know.
A new study proposes an abiotic source for Venus’ phosphine: volcanoes. It’s titled “Hypothesis Perspectives: Might active volcanisms today contribute to the presence of phosphine in Venus’s atmosphere?” The authors are Ngoc Truong and Jonathan I. Lunine. Lunine is a planetary scientist and physicist at Cornell University, and Truong is a grad student at Cornell. The paper is available on pre-press site arxiv.org.
“We propose an abiotic geological mechanism that accounts for the abundance of phosphine detected by Greaves et al., 2020,” the authors write in their paper. “We hypothesize that trace amounts of phosphides formed in the mantle would be brought to the surface by volcanism, and then subsequently ejected into the atmosphere, where they could react with water or sulfuric acid to form phosphine.”
Let’s back up for a minute. A couple weeks ago a team of scientists reported the discovery of phosphine high up in Venus’ atmosphere. A few facts explain why this is an interesting discovery.
Phosphine is a biomarker for life. It’s not direct evidence; it’s just that here on Earth it’s only created through living processes. Without a living source, it takes an awful lot of energy to create it. And Venus lacks that energy. Clara Sousa-Silva is one of the authors of the phosphine study. In a press release accompanying the discovery, she explained that “The reason phosphine is special is, without life it is very difficult to make phosphine on rocky planets. Earth has been the only terrestrial planet where we have found phosphine, because there is life here. Until now.”
Also, phosphine is rapidly destroyed, so finding it means that some process is continuously producing it. It can’t be a relic from the past.
In the original paper presenting the discovery, the authors wrote that “The presence of PH3 is unexplained after exhaustive study of steady-state chemistry and photochemical pathways, with no currently known abiotic production routes in Venus’s atmosphere, clouds, surface and subsurface, or from lightning, volcanic or meteoritic delivery.”
The authors of the new paper think they may have the answer. And despite the initial paper ruling out volcanic activity as the source of Venus’ phosphine, that’s exactly what the new hypothesis states: that phosphides from basaltic lava activity are entering the atmosphere, then reacting with either water or sulphuric acid to form phosphine.
Despite earlier claims that it takes either living processes, or very energetic processes to produce phosphine, the authors point out one other pathway. It stems from impurities in iron and how they react with other substances.
“On Earth, one of the known processes is the production of phosphine gas by aqueous or acid corrosion from phosphorous-containing impurities in iron,” they explain. In a 2010 experiment, “aqueous corrosion produced a
significant amount of phosphine gas comparable to the amount detected in natural terrestrial environments, while sulfuric acid corrosion could produce an amount of phosphine gas three orders of magnitude higher than aqueous corrosion.”
This lines up with their hypothesis for a volcanic source of phosphides ejected into the atmosphere by volcanic activity, and then reacting with water of sulfuric acid. To test their hypothesis, they carried out an order of magnitude calculation.
First hey had to find the volume of phosphine present in Venus’ atmosphere. There are 20 parts per billion in an atmospheric layer 8 km thick, between 53 and 61 km (33 to 38 mi) above the planet’s surface.
In their paper the pair of scientists show their calculations. But the end result of all the calculations shows that Venus’ atmosphere contains 2.7 x 1010 kg of phosphine. That’s 27,000,000,000 kg, or 27 billion kg.
The other part of the picture is the destruction rate for phosphine in Venus’ atmosphere. In the original paper announcing the discovery, Greaves et al examined that issue in depth. The new paper leans on that, and says “We shall assume here that, in the layer 53-61 km, phosphine could be stable for their uppermost value– about a year.”
So each year, Venus would need to produce the same amount of phosphides as the amount of phosphines in the atmosphere at any given time: 27 billion kg. “Based on this assumption, volcanoes would need to produce ~ 2.7 x 1010 kg of new phosphide every year to continuously pump into the middle atmosphere, which then react with the sulfuric acid droplets to produce the observed phosphine.
Then it comes down to lava. The authors calculate that Venus would need to produce 93 cubic kg of lava every year to produce enough phosphides.
The question that all of this rides on is “Does Venus produce this much lava each year? And this is where it gets tricky, if it isn’t already.
There are wide-ranging scientific estimates of Venus’ volcanic activity. Some of that research says yes, Venus can produce that much lava. Others say no. There’s no agreed-upon conclusion in the scientific world, yet. But the estimates are informed by scientific data, particularly from the VIRTIS instrument on the ESA’s Venus Express.
In this paper, the author referenced one study that calculated Venus’ lava production between 23 km3/year and 235 km3/year. They discuss additional detail in lava production estimates before writing “All these estimates are comparable to the 93 km3 /year we calculate as required to produce the phosphide-source of the phosphine.”
Is this the end of the postulation that life in Venus’ clouds could be producing phosphine? Who knows.
“The Greaves et al., 2020 hypothesis that life is producing PH3 in the clouds of Venus requires both the extraordinary claim that life exists in the clouds, and a mechanism to maintain its viability as droplets in the aerosol layer grow and sink,” the authors write. And of course we all know what Carl Sagan said about extraordinary claim.
To the authors, their own hypothesis is more likely. “Our hypothesis,
instead, requires that Venus be currently experiencing a high rate of basaltic volcanism, but one that is consistent with spacecraft observations and laboratory experiments.”
“Rather than pointing to the existence of life in the clouds, we argue that phosphine is pointing to a Venus that is geologically active today—a conclusion perhaps disappointing to biologists but surely
intriguing to planetary scientists.”
To be fair to the authors of the original paper announcing the discovery of phosphine at Venus, they never claimed it was proof of life. They themselves were circumspect about that conclusion. “If this is not life, then our understanding of rocky planets is severely lacking,” said co-author Janusz Petkowski.
And co-author Clara Sousa-Silva said, “Now, astronomers will think of all the ways to justify phosphine without life, and I welcome that. Please do, because we are at the end of our possibilities to show abiotic processes that can make phosphine.”
Sounds like it’s time for a mission to Venus to sort this all out.
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