### Phosphine and the Atmospheric Chemistry of Life

"It has been said that astronomy is a humbling and character-building experience... to me, it underscores our responsibility to deal more kindly with one another and to preserve, and cherish, the pale blue dot; the only home we've ever known
- Pale Blue Dot
Carl Sagan
_______________________________________________________________

Writing Astronomical Returns keeps me up to date with the latest in space exploration, and I can always tell when a new development is really big if the story appears beyond the niche sources like Space.com or Ars Technica that I often immerse myself in, and into mainstream media headlines like CNN and the New York Times. This week the news was abuzz about the discovery of phosphine in Venus' atmosphere, signaling the possibility on life in the surprisingly temperate clouds above Venus' hellish surface. I'll be frank, I'd never heard of the compound phosphine before this week, it doesn't ring a bell from AP Chemistry, 7 years ago. But after reading up on this discovery, I felt that a lot of news articles only scratched the surface of the biochemistry of phosphine, so I thought it'd be worthwhile to try go one level deeper

 Left: Venus in natural light (Mariner 10, 1974) | Right: False-color topological radar mapping (Magellan, 1990-1994)

Phosphine is a colorless gas with the chemical formula PH$_3$, and although pure phosphine has no smell, in most instances it contains trace amounts of P$_2$H$_4$, making it both extremely smelly and also spontaneously combustible with air (here's a good video). But the periodic table provides us with tons of putrid, poisonous gases; why all the riffraff about phosphine all of a sudden? To understand this, some background knowledge on reduction-oxidation (redox) chemical reactions is helpful. If you recall from basic chemistry, a chemical reaction is simply an exchange of electrons between atoms; for example, combusting hydrogen and oxygen to produce water forms a covalent bond in which electrons are shared between the two atoms. But there's an expansive subset of common chemical reactions known as redox reactions, where the oxidation numbers of the individual atoms are changed

So what're oxidation numbers? That's a bit of a tricky definition to provide, but think of it as the electron affinity of the individual atoms of a given molecule. Continuing the example of forming water, prior to the reaction, the hydrogen and oxygen are in their elemental states, so their oxidation number is 0. But after the reaction, the two hydrogens have been oxidized (i.e. lost electrons) and therefore taken an oxidation number of +1, while the one oxygen has been reduced (i.e. gained electrons) and now has an oxidation number of -2. Contrast this to the second example below of hydrochloric acid and sodium hydroxide combining to form table salt and water, a reaction where oxidation numbers remained the same

 To my high school chemistry teachers - Mr. Bunch and Mrs. Brown, if you're reading this, I hope you're proud of me!! :)

To this effect, every high school chemistry student is taught the mnemonic OIL RIG:

• Oxidation Is Losing [electrons]
• Reduction Is Gaining [electrons]

Great, but what does any of this have to do with phosphine and finding little green aliens on Venus? On Earth, phosphine is a rare find because phosphorus doesn't normally exist in such a reduced state in the presence of so much oxygen in our atmosphere. Looking at phosphine's chemical formula, you can determine the phosphorus atom's oxidation number is -3. In contrast, the vast majority of phosphorus on Earth is found in the phosphate ions [PO$_4 \thinspace ^{3-}$] of various compounds, where the phosphorus atom's oxidation number is +5. In other words, phosphine doesn't last long in our atmosphere because it readily reacts with oxygen to form some sort of phosphate compound. To form phosphine naturally takes an enormous amount of energy; some of it is hypothesized to come from lightning strikes, but trace phosphine production has been observed in sewer sludge, where extremely hardy microorganisms deploy some intense enzymes to break it down

 Trust me, this is some nasty stuff. Phosphine compounds are used as fumigants to obliterate rodents and other pests

Phosphine's rarity is to be expected on Venus too; other elements like carbon and sulfur exist in oxidized forms like carbon dioxide and sulfuric acid, so absent some regularly emitting source, phosphine should be nonexistent in the Venusian skies. Therefore, you could imagine the researchers' surprise when they detected phosphine at a concentration of 20 parts per billion - seemingly inconsequential, but orders of magnitude above what should be produced by solely abiotic processes on Venus alone (like lightning and volcanoes). Perhaps they made some sort of measurement error? Possible, but the data was collected by the James Clerk Maxwell telescope in Hawaii and the Atacama Large Millimeter Array in Chile, both of which are basically humanity's most advanced radio wave observatories. So for now, phosphine on Venus remains a mystery, produced either by some yet unknown geological process or an alien microorganism

 Maxwell, Atacama, and BepiColombo: our eyes in the sky towards Venus

By a pure stroke of luck, the BepiColombo spacecraft built by Europe and Japan and launched in 2018 will by flying by Venus next month and again in August 2021 on its way to Mercury. Mission planners will use its instruments to try confirm the phosphine readings just made by our terrestrial telescopes. Still, Venus has long been neglected by planetary science missions in favor of Mars and the outer solar system; hopefully this new discovery will galvanize future missions dedicated to studying Earth's twin

 Artist's impression of a terraformed Venus | Credit: DeviantArt

Finally, the concept of detecting life on other planets due to unexplained atmospheric imbalances is not a new one. To close, I leave you with this excerpt from Carl Sagan's Pale Blue Dot, describing how extraterrestrial observers might detect life on Earth

_______________________________________________________________

"Methane and oxygen together in the same atmosphere is peculiar. The laws of chemistry are clear: In an excess of O$_2$, CH$_4$ should be entirely converted to CO$_2$ and H$_2$O. This process is so efficient that not a single molecule in all the Earth's atmosphere should be methane. Instead, you find that one out of every million molecules is methane, an immense discrepancy. What could it mean?

The only possible explanation is that methane is being injected into the Earth's atmosphere so quickly that its chemical reaction with O$_2$ can't keep pace. Where does all this methane come from? Maybe it seeps out of the deep interior of the Earth - but quantitatively this doesn't seem to work, and Mars and Venus don't have anything like this much methane. The only alternatives are biological, a conclusion that makes no assumptions about the chemistry of life, or what it looks like, but follows merely from how unstable methane is in an oxygen atmosphere. In fact, the methane arises from such sources as bacteria in bogs, the cultivation of rice, the burning of vegetation, natural gas from oil wells, and bovine flatulence. In an oxygen atmosphere, methane is a sign of life

That the intimate intestinal activities of cows should be detectable from interplanetary space is a little disconcerting, especially when so much of what we hold dear is not. But an alien scientist flying by the Earth would, at this point, be unable to deduce bogs, rice, fire, oil, or cows. Just life"