We often wonder where all the ancient alien civilizations are, and if they might not exist. But could the reason be something as simple as a vital element being scarce? So we had a big new discovery recently that we found Phosphine gas on Venus, a possible bio signature for life, this article is focused principally on what the scarcity of Phosphorus might mean for looking for alien life, it’s a bit too important and relevant of a discovery not to address and so we’ll be getting to it at the end of the Article. This is the great looming question in our so far search for other lifeforms in the galaxy. Why, when the universe is so vast and old, and life can form spontaneously from base molecules and evolve into sentient beings like we did, have we not detected any clear signs of life anywhere, let alone galactic civilizations. The answer is that so far don’t know why, but we have come up with a number of theories as to what might make it more rare and difficult than we thought for life to form or for intelligence to arise, what forces might just occasionally kill all life off and start the process over, and what errors in judgement might lead civilizations to destroy themselves not very long after they start producing signals we could even detect. And to be honest, I kind of hope that if life really is as rare in the universe as we think it may be, the reason is something amazing and dramatic like supernova storms or cognitive contagions, because then at least we are alone out here because of something interesting. It is frustrating to think that the vast resource rich universe out there might be mostly empty and wasted and that we’re in it alone because of some mundane quirk of chemistry. That is the troubling topic we will be discussing today. The element phosphorus is essential to life as we know it. But phosphorus appears to be rare in our solar system, and more rare in the universe. There are good reasons to suspect that many stellar systems, large swathes of our galaxy and others, where it may be very scarce or at least lack enough of it for life to start. If this is true, the rarity of essential phosphorus might be not only be the reason for the rarity of life, it might also put a hard limit on how much life there can ever be which would really put a crimp in our plans to populate the galaxy with quadrillions of humans. Even if “Life finds a way” to arise and evolve in stellar systems that lack phosphorus, that doesn’t change the fact that we, in our current physical form, absolutely need it. And as we saw in our episode “Non-Carbon Based Life”, the prospects for life not sharing our basic chemistry are not very good. So, let’s take a moment to understand why phosphorus is essential to life as we know it. We describe the lifeforms of Earth as carbon based because so many molecules essential to organic metabolic processes are built around branching chains or loops of carbon atoms. But phosphorus is so ubiquitous in essential organic compounds, it would not be unreasonable to refer to Earth like lifeforms as carbon phosphorus based life. If you ask most science writers why phosphorus is essential, they’ll tell you immediately about Adenosine Tri-Phosphate, ATP, the chemical that all known lifeforms on Earth use to transfer energy. When plants absorb sunlight and produce oxygen in photosynthesis, that oxygen is actually just a byproduct of the first step of photosynthesis. The end result of the last step of photosynthesis is converting ADP, Adenosine Di-Phosphate into higher energy ATP. Pushing the third phosphate group onto the adenosine is somewhat like compressing a spring, storing energy that can be released later and where it’s needed. When enzymes within cells convert the ATP back into ADP, that huge energy release is what powers the enzyme and the basic functions of the cell. Multicellular lifeforms, ourselves included, who don’t do photosynthesis but instead just breathe oxygen to burn sugars to power a different process that also converts ADP into ATP and then uses the ATP to power enzymatic cellular processes in pretty much the same ways plants do. So every known life form on Earth except a few that are just big molecules that strain our definition of the term life form are powered by a cycle of pushing phosphate groups like springs down into ADP and using that stored energy that’s released when they pop back off. But an even better illustration of why phosphorus is essential to Earth-based life is right there in the structure of DNA and its simpler ancestor RNA. Every other link in that vastly long molecular chain is a phosphate group with a phosphorus at the center. So without phosphorus, we lose the very means by which all known life encodes the instructions for becoming alive, remaining alive, and making more stuff that’s alive. Of course you could point to any atom in RNA or DNA the carbon, nitrogen, oxygen, or hydrogen and declare that without it life as we know it couldn’t exist. And you would be correct, but all of those elements are quite abundant throughout the universe. We are only a little over 1% Phosphorus by mass, but it is even rarer on Earth, making up about 0.1% of the crust and not even in the top ten most common elements here. It’s even rarer than that in our Solar System, coming in at seventeenth place for abundance, making up a paltry 0.0007% of the mass of the solar system. We aren’t sure how common it is Universe wide and there’s some debate if it’s more abundant in our solar system or not, but planets and stars older than Earth would tend to have lower concentrations as fewer supernovae would have occurred in the distant past to add heavy elements to the galaxy at large. And for the Fermi Paradox, a critical concept is why we don’t see older civilizations around and perhaps it is from Phosphorus scarcity preventing life from emerging in the first place. Earth’s amount and the general reactivity issues of phosphorus already are low enough that for decades the “Phosphate Problem”has stumped scientists trying to figure out the origin of life on Earth, and we’ll discuss that in a bit, if you go to some planet with an even lower concentration of Phosphorus that problem would only be worse. The most ten common elements in the Universe, in descending order, are hydrogen, helium, oxygen, carbon, neon, iron, nitrogen, silicon, magnesium, and sulfur. Only 1 in 2000 atoms are not one of those ten, and Phosphorus only makes up 3 out of every ten million atoms. Silicon atoms are 100 times more common. So why is phosphorus so much less abundant than the other elements essential to life? To answer, it helps to look at how and where the various elements are produced. It’s a common misconception that most heavier elements are produced in supernovae, but this is not correct. Almost all elements with more than 40 protons in them like gold, platinum, or uranium come from the collision of neutron stars. In contrast, the lighter elements mostly come from the death pangs of smaller stars, the final stages of fusion. For example, silicon is the last element for fusion fuel and one that only the most massive stars only burn for about a day before they explode. Big stars are very inefficient users of fusion fuel and burn in layers, so when they detonate plenty of unfused fuel of various types gets scattered. Many of the elements said to come from Supernovae are actually produced in this way, as fusion products in the star’s old age, and just get scattered when it explodes. And not all supernovae are Type II, giant exploding stars. The other common variety, Type 1a, are white dwarfs stellar remnants that explode after pulling mass off some close binary partner. But phosphorus seems to mostly be produced in Type II supernovae, and only during the supernova event itself. The most likely mechanism is when the Silicon 30 isotope captures a neutron during the explosion, briefly becomes unstable Silicon 31, then quickly emits an electron and decays into phosphorus-31. But silicon-30 is not the most common silicon isotope, and its capture cross section for neutrons is very small, meaning the neutron has to hit just right to get absorbed, so it doesn’t happen very often. We also know that Type II supernovae are not distributed evenly in the galaxy, and supernova remnants appear to have significantly varying concentrations of phosphorus. Shock waves from supernovae carry matter into space and are thought to be what compresses the interstellar material and triggers it to begin coalescing into proto stellar systems. If phosphorus were lower in concentration in those shock waves, then those new star systems and their planets will also have less phosphorus than other systems like our own. So the problem isn’t just that phosphorus is rare, it might also be unevenly distributed, with huge swathes of our galaxy almost lacking it. And the mere existence of phosphorus in as tellar system doesn’t necessarily mean it’s available to facilitate the formation of life. The abundance of elements in the crust of our planet doesn’t match up too well with the solar system at large, even ignoring the hydrogen and helium differences as they’re super abundant but only in places like the Sun and Gas Giants that can hold onto the ultra light particles. Our crust is not our planet and some materials sank in toward the core when the place was molten. Especially those prone to forming up big dense molecules, while Silicon for instance tends to float on magma well and stay near the surface,and thus is right behind oxygen in it’s crustal abundance even though carbon, nitrogen,and neon are more common in the solar system. Earth has a lot of phosphide, relatively speaking, but most of it is down in the planetary core. What’s more, phosphide isn’t very useful for life, unlike phosphate, which is phosphorus linked up to oxygen atoms which might be a bit problematic on Earth for the purpose of life forming since there was not an oxygen atmosphere on this planet until long after life had formed. But Phosphorus also tends to bind into molecules that are insoluble in water, which is very problematic for life, since it is pretty much predicated on water solubility. Indeed of all the bio geochemical cycles for life, many of which are quite quick and often involve the atmosphere, the phosphorus cycle does not involve the atmosphere and has one of the slowest cycles. It slowly grows rarer on land as it washes out to sea and sinks, and only gets refreshed by tectonic activity. If you have some hypothetical chemical solution from which the first building blocks of life needed to arise, then we have an issue with there being virtually no phosphorus around in that solution. If the stuff is rare, in forms that aren’t water soluble, and prone to sinking, then you’ve got a bit of a problem. Just to take a simplified example, we said there were six key elements for life, Oxygen, Carbon, Hydrogen, Nitrogen, Calcium, and Phosphorus in that order, and that is same number as there are sides on a dice. Let’s say we were rolling a bunch of dice,10 of them. The odds of any specific combination getting rolled is 1 in 6¹⁰, about 1 in 60 million. On fair dice anyway. So our odds of rolling 10 6’s are pretty low. Now if our dice are badly balanced so that instead of a six coming up 1 in 6 times, 17% of the time, it only came up 1% of the time, or 1 in 100, then the odds of rolling 10 6’s isn’t 1 in 6¹⁰ but rather 1 in 100¹⁰, not 1 in 60 million but rather 1 in 100 billion. For perspective, if we had some solution that was forming molecules once a second on that 1 in 60 million odds, and dropped the concentration so it was now 1 in 100 billion, that formation reaction goes from once a second to once every 50,000 years. And if we dropped the odds from 1% even more, down to .1%, that would now happen on average once every 500 trillion years, 40,000 times longer than the Age of the Universe. That gives you an idea how important having the right concentration of Phosphorus in the primordial soup of life is. Even just halving the chance of rolling a six in our previous example would have lowered the odds a thousandfold, requiring either a thousand times as much of the solution or a thousand times as long, so even a relatively minor variation in the abundance of phosphorus in a planet’s crust is a big deal. Using our 10 dice example, a thousand planets with half the concentration would have the same odds of generating life on just one of them as one planet with double the concentration has, and for the ones with a tenth the concentration,you’d need 10 billion of those planets to equal the odds that one lone planet had. As you can see, even an entire galaxy of planets doesn’t need much of a drop in availability of phosphorus to be very unlikely to have a single planet have life form on it. And as I mentioned earlier, Earth has unusually high concentrations of Phosphorus in the crust, but they’re still troublingly low for models of life forming here. So how did we get a high enough Phosphorus concentration here on Earth? Well, we generally assume life formed in one of 3 ways, in tidal pools, or around deep sea thermal vents, or with it originating in space and coming in on a comet or meteorite a concept called Panspermia. And indeed we suspect that a lot of the phosphorus available to early life came in by meteorites after the planet formed. For life to originate near geothermal vents or tidal pools, the problem is the phosphorus concentration. Phosphorus in seawater nowadays is 0.1 parts per million, quite low and not really very conducive to life, but we see it higher near thermal vents, if not really high enough, and tidal pools tend accumulate runoff, get stirred up and muddy, and evaporate to higher concentrations, in the absence of something better, but still don’t exhibit anything like the concentration of phosphates that would make the odds good and we always assume life started in or near an ocean given that life was around for billions of years in the ocean before land life emerged. And this is the “Phosphate Problem” I mentioned near the beginning. There just doesn’t seem to be any good options for anything in the sea to have had a decent concentration of phosphates to make a primordial soup which life could emerge in. However, recently it’s been suggested that carbonate rich lakes, those in dry environments where runoff water flows in but evaporation keeps them salty and alkaline might be a better candidate. While they vary in concentration a lot, we have found some of these carbonate rich soda lakes with 50,000 times the phosphorus levels of seawater. Such lakes might have been a good deal more common in the past too, given that a Primordial Earth with no plants and roots holding the soil in place would have a lot of runoff and erosion. Those lakes being carbonate rich helps too. Normally if you have a lot of calcium present,and there’s more of it than phosphorus, it will bind with phosphorus into calcium phosphate which life can’t access, but carbonate can bond to the calcium as calcium carbonate and leave some phosphorus free. Primordial Earth is thought to have had an atmosphere principally of nitrogen and carbon dioxide, and also far more volcanic activity, which may have allowed even higher concentrations of phosphorus than these modern lakes we have investigated. Phosphate levels might have been able to climb in some cases to a million times the concentration in seawater, potentially 1 in 10 atoms rather than 1 in 10 million, vastly higher than any suggested in normal tidal pools or deep sea thermal vents. So, okay, maybe phosphorus is rare, and biologically accessible phosphorus even rarer, and key biological processes require phosphorus. But didn’t Ian Malcolm from Jurassic Park teach us that “Life finds a way”? Surely, life and evolution being as inventive as they are could concoct some alternative molecular mechanism for energy transfer that doesn’t involve phosphorus, right? Well, that’s why I keep using the phrase,“Life as we know it”. Of course we can’t know for sure if life needs to run on water and carbon and phosphorus like it does here and in the same way it does here. However, the very fact that life on Earth evolved to make such ubiquitous use of phosphorus when phosphorus is not all that common tells us one of two things, possibly both. Life based on phosphorus was easier to form than a non phosphorus mechanism, and/or life based on phosphorus was far more successful once it did form. And that strongly implies that whatever life could form in a primordial ooze lacking phosphorus won’t form easily and/or might not be nearly as adaptable or successful. If you’ve got a creature on an alien planet than can eat iron ore and another that can only eat gold nuggets, all things being equal that iron eater is going to be wildly more successful, so if you find a world dominated by gold eaters but plentiful in iron ore, it tells you iron eating life is either wildly improbable to develop or that gold eating life is way more likely to prosper. Same concept for phosphorus, if life had been able to develop using something less scarce, Occam’s Razor says it would have. So this is a very plausible solution to the Fermi Paradox. We might very well be alone because we are on a very rare planet that has enough of a rare element that life doesn’t form easily or work well without. And the rest of the universe is just out of luck. Or alternatively, such planets are pretty rare and were even rarer in the Earlier periods of the Universe, which is almost as good for the Fermi Paradox, because as we saw in the Great Filters series, there’s plenty of other things that can lower the odds of life forming too, and lower the odds further of it getting intelligent and building spaceships. Remember, the Fermi Paradox isn’t about if other civilizations exist equal to modern day humanity, but rather if older ones exist who could have gone out and colonized the galaxy and be noticeable to us with our current technology. So if worlds older than Earth tended to be scarcer in Phosphorus, you might have more worlds with life but just not that developed yet. We might just be the first to be building spaceships, as we looked at recently in our episode Fermi Paradox: Firstborn. Time is a factor too. Keep in mind I gave concentrations of Phosphorus in the modern Universe, not what it was 4 or 5 billion years ago when our young planet got it’s allotment. The Universal average would have been much lower at that time than now because there hadn’t been as many stars that became Type II supernovae. The younger a planet or solar system is, the older the universe was when it formed, and the better its odds of having higher concentrations of Phosphorus. So there’s an optimistic angle on this story;as the Universe gets older, it might be becoming more fertile for life to form. But the most annoying part of the Phosphorus Problem for futurists is that, since we humans require phosphorus, we might not be able to just rush out and populate that vast universe full of resources awaiting us, because how much phosphorus we find or make sets a hard limit on how many of us there can ever be. Humans and our food are mostly made of water, which by mass is mostly made of oxygen, but we’re also about 1% phosphorus. So let’s say we send a colony ship to a stellar system abundant in every imaginable resource except it’s sorely lacking in phosphorus that’s reasonably easy to get to. But say we send those colonists off with a generous stock of a million metric tonnes of phosphorus from Earth, a billion kilograms. And let us say that they don’t waste any precious phosphorus on pleasant grasslands or forests or pets, and they strictly recycle every precious atom of it. And let’s further assume that every human has a mass of about 70 kg and that at any given moment there’s another 30 kg of feed stock on hand, which makes a nice round number of 100 kg of biomass and hence 1 kg of phosphorus required per human. That means that the billion kilograms of phosphorus they brought will build one billion humans and their food, and that’s it. Perhaps you could double that by making people smaller and keeping less food in reserve, but clearly there’s a hard limit where they can’t make any more food or babies until they find some more phosphorus. And that’s assuming there are nothing but humans, when in practice, our planet has about 3 trillion tons of life on it, 18% of that is carbon and we often give biomass in tons of carbon, and 1% is Phosphorus, or 3 billion tons. That means for terra forming and colonizing a world to our current population and ecology, you’d have about 400 kilograms of Phosphorus per person, not one kilogram, only including the Phosphorus tied up at any given moment in something alive, and if you’re terra forming planets or building big space habitats, you are going to need values like that. Again you might lower that by having a higher percentage of biomass be people, or it might be lower if you like garden parks and rural space habitats and planets, and of course you need a lot sitting in soil not just lifeforms. Let us ballpark it at one ton per person for simplicity’s sake. Earth is the most massive rocky planet in the solar system and indeed masses around as much as all the other rocky planets, moons, and asteroids combined do. However only about 1% of its mass is in the crust and only about .1% of that is Phosphorus, so if we extracted every last bit of it from our crust we would have about 10 million billion tons, enough for 10 million billion people,a million times our current population. Since the Sun gives of 2 billion times more light than reaches Earth, we generally just multiply our population by 2 billion to guess as Dyson Swarm populations, and that would be 16 billion people, 1600 times the 10 million billion figure for a ton of phosphorus per person and still a bit larger than the one kilogram of phosphorus per person we could have if humans were the only life period. For Dyson swarms, we don’t assume that collection of objects orbiting the sun and using all its light would all be space habitats, but we are often worried about finding enough mass for all those and one solution to come up with the raw material is to engage in star lifting, as stars form from the same stuff as the planets around them so are heavy in metals, just way heavier in hydrogen and helium. And indeed most of the Phosphorus in our solar system is in our Sun, it doesn’t make Phosphorus, mind you, and never will, but again it formed from the same nebula Earth did. If you sucked all the Phosphorus out of our Sun, which is presumably where 99.8% of our solar system’s stockpile is since it is 99.8% of the solar system’s mass, then you would have a lot more. Our Solar system masses 2 billion tons. Again though, Phosphorus is less common off Earth, making up only 7 parts per million of the mass or 14 billion trillion tons. So that’s your hard limit, if you suck out every drop from the Sun, excluding the Sun it drops a lot, and leaving in the Gas Giants, which have most of the remainder, leaves you 28 billion billion tons. So there is enough for space habitats, barely, but it turns out to be a major control factor on building them if you want life on them. And this is only if you’re dismantling solar systems, not if you’re just mining asteroids, and we can’t assume at the moment that phosphorus will be decently abundant in other solar systems, especially on the crust of big planets, so folks going out and colonizing distant planets around other stars are going to need to find some phosphorus. Or make some. We never want to limit ourselves to assuming natural sources when dealing with advanced civilizations. After all we’ve barely reached our own Moon but we already make elements that don’t occur naturally in the Universe. So a civilization might avoid a Phosphorus bottleneck by just getting it out of stars not simply by star lifitng, but by using the star’s power supply to run a ton of super colliders or atom smashers to make some. Silicon-30 is fairly common, the heaviest of the 3 stable silicon isotopes and about 3.1% of natural silicon, you whack that with a neutron at a speed that captures it and you’ve got silicon-31, with a half life of a couple hours before it decays into regular old stable phosphorus. As we mentioned last week in the Future of Fission, we do this type of transmutation in labs all the time via breeder reactors, it’s how we make plutonium. Transmutation is rather expensive, but if phosphorus turns out to be a bottleneck for the growth of our civilization, I’m sure we will develop the means to get a lot better at it. But even if we do develop such industrial processes for mass synthesizing phosphorus, synthesizing just about anything is a lot more expensive than simply finding it. Any native deposits of phosphorus will be prized, mined quickly and maybe even become a critical commodity folks have wars over, perhaps even more prized than the precious metals in the asteroids since, literally, we can live without precious metals. There might be life out there somewhere that eats gold, but we don’t, we need phosphorus, and the odds are high that life out there does too. So as I mentioned at the beginning, we had a big piece of news come in while we were working on this episode, the discovery of clouds of Phosphine gas on Venus, and I didn’t think we could run this episode without giving that a mention. The phosphine molecule consists of one phosphorus atom bonded to three hydrogen atoms, forming a trigonal pyramid. It’s highly flammable in oxygen and highly toxic to life on Earth. In fact, it’s a substance we normally only encounter on earth in life that’s falling over dead. It’s literally rat poison, so phosphine gas actually kills things, at least things that breathe oxygen. Importantly, though, it is only poisonous to aerobic, oxygen breathing organisms. Anaerobic life is a completely different consideration. On Earth, it’s produced by some an aerobic microbes in water low in Oxygen as anaerobic life depends on low Oxygen levels. Venus has very little molecular oxygen inits atmosphere, so any airborne organisms there would be anaerobic and might emit phosphine like their Earthly cousins. To understand the excitement around discovering phosphine in Venus’ atmosphere, we need to talk about atmospheric bio signatures. We usually mean by this that a given molecule either shouldn’t exist in the atmosphere or should be far lower in concentration without some biological process in place to replenish it. High concentration of molecular oxygen is the big one, because we know of photosynthesis and not many other processes that produce it, and because it’s so reactive that plenty of processes would consume it all if it wasn’t being replenished by life. Something similar might apply to phosphine gas. As I said, some anaerobic Earth microbes do produce the stuff, and it breaks down very quickly in our oxygen rich atmosphere. Earth microbes produce it from environmental phosphorus or indirectly by fermentation of organic matter to get energy. It also acts as a defense mechanism for them, both to poison oxygen breathing competitor microbes and to remove deadly oxygen from the water around them. So if you find any concentration of it on a planet with an oxygen atmosphere, it would be a strong bio signature. But it’s not such a definite life sign in Venus’s atmosphere, which has abundant clouds of acid to react with any metal phosphides present to release phosphine, and where the phosphine would be broken down reacting with other gases that break it down more slowly than oxygen does. So the phosphine could be being created and replenished by life on Venus, but the evidence isn’t strong. It’s really just a could be. We also shouldn’t discount the differences in chemistry between Earth and Venus. Venus has some truly hellish environments,both on the planet’s surface and in its thick atmosphere and I’m not only talking about high temperatures and pressures, but also the highly acidic and chemically reactive environments too. Phosphine is a very simple chemical and we know that even comparatively more complex amino acids that we have detected in comets and asteroids appear to be a purely chemical byproduct without the need for life to produce them. We are far from fully understanding the very different chemical melting pot that is Venus and the phosphine could simply be a product of that chemical stew. As to Phosphorus scarcity, Venus certainly has a decent amount of Phosphorus but don’t take this as an indicator it represents a great stockpile, the amount of Phosphine found in the atmosphere is quite small compared to what we might expect for a bio signature. The shock of this discovery was mostly just that we found any phosphine gas at all. Still, it can’t be ruled out that there might be life in the clouds of Venus, and strange life at that.
This is all for today, Until next time, thanks for watching, and have a great day!