THE LIMITING FACTOR IN ONE'S ABILITY TO FIND ALIENS probably...
When talking about life and space travel, we often talk in terms of fuel, time, water, and biomass as a whole. But we, and I mean myself as well, had never considered the probability of other "small" details posing such difficulty in space travel and exploration, we can simply handwave asteroid mining and be done with it.
Here's how I would approach considering a planet habitable:
Verify if it is in the habitable zone.
Verify mass and size, be certain it is dense enough to have a solid surface.
Verify star and planet for a decent metallicity.
Verify atmosphere mass and composition.
But one can easily fall flat on its face when dealing with item 3. We talk of metallicity for all elements heavier than Helium, when talking about terrestrial planets, it is preferred to have a decent amount of elements heavier than silicon, or else all you have is a world of pure rock, lacking any minerals essential for life. That last part about minerals essential for life is the one we end up overlooking in our process — because we group all of those with the presence of iron, which is not necessarily true at all.
Because iron is pretty heavy in astronomical terms, that is, it's usually the end product when a main-sequence star dies, we tend to assume many of the other elements lighter than iron will also be present in the aftermath of the planetary nebula. That is not true at all. Many elements are created through the collapse process of main-sequence stars of course, but many others are created in abundance or deficiency through other astronomical phenomena as well.
ORIGINS OF PHOSPHORUS
The table above shows that more than half the iron in the universe has been created through the collapse of white dwarfs, elements such as boron and beryllium are created through cosmic ray fission (heavier elements are split into those lighter elements), and basically the heavier half of the periodic table is created in the final moments of low-mass stars or merging neutron stars.
In a previous post, Water-Carbon based Life | Why is it the way to go? — I talked about how carbon-water-oxygen life is chemically superior, and thus more likely to be found if extraterrestrial life is ever to be found. Well, we need to add another vital element to that equation, an element omnipresent in Earth's biology, phosphorus. It's literally part of what gives Life as we know it, its energy, through ATP (Adenosine Tri-phosphate) and ADP (Adenosine Di-phosphate).
Starting with photosynthetic autotrophs (plants and algae), the final product of photosynthesis is turning ADP into higher energy ATP, which in turn can be used to power enzymatic processes within the organism's cells, it is a lucrative process because sunlight does all the work for the organism, that only has to fetch carbon, nitrogen, and phosphorus from the soil or water around it through osmosis or capillary action in its roots. Adding a third phosphorus complex to the molecule greatly increases its instability, thus more energy released once enough energy is added to push it past the tipping point. In non-photosynthetic Life, sugars and oxygen are burned to execute the same process instead, and so all of Earth-life but viruses survive through the phosphate chain processes.
And it is not only energetic, it's also the building block of RNA, the hard helices of the molecule are made of phosphate groups chained in such a structure — Okay, so phosphorus is essential for life, so are the other elements carbon, hydrogen, and oxygen. HOWEVER, we know that not all elements are created equal, including that was one of our main arguments for pushing carbon-water-oxygen Life as whole, it is simply more common.
ELEMENT %MASS %ATOMS
Oxygen 65.0 24.00
Carbon 18.5 12.00
Hydrogen 10.0 62.00
Nitrogen 3.2 1.10
Calcium 1.5 0.22
Phosphorus 1.0 0.22
Potassium 0.4 0.03
Sulfur 0.3 0.038
Sodium 0.2 0.037
Chlorine 0.2 0.024
Magnesium 0.1 0.015
Others <0.1 <0.3
But phosphorus is a rare element, humans are about 1% phosphorus by mass, but the universe contains 10^5.5 phosphorus atoms for 10¹² hydrogen ones, just as rare as chlorine, even given phosphorus is 15x heavier than hydrogen, we are still 35.5 thousand times richer in phosphorus than the rest of the solar system.
Phosphorus in Earth's crust makes up about 0.12% by volume, so the average cubic meter of Earth's crust contains about 2.18 kg of phosphorus, or about 0.04% by mass (25x less than in humans per unit mass). Similar things can be said about other earth-metals, however none of them take such a fundamental role as previously established for phosphorus.
I dare to state that without phosphorus there isn’t Life as we know it, it must be fundamentally necessary for Life to arise, given it is so rare, yet it is the foundation of our metabolism and structure. The abundance and distribution of this element might even be one possible solution, if not part of the solution for the Fermi Paradox.
As the universe ages and generations of stars go by, the overall metallicity of the universe is expected to rise over time. Which means that as we delve further into the past, the less likely it becomes for Life to have arisen somewhere else, because the necessary elements for life had not yet been made by dying stars. We could state that there is a critical point in their saturation where Life becomes possible from that point onwards, and that the solution for the Fermi Paradox lies in the aspect that there hasn't been enough time for most places in the galaxy to mature enough of these elements for Life to develop, and that we are among the first civilizations around.
PHOSPHATE PROBLEM
As we have previously discussed, many heavy elements are created through processes other than supernovae. Phosphorus however is one of those elements created during the brief final moments of a type II supernova (star between 8 and 50 solar masses).
Phosphorus is created during the explosion when Silicon-30 captures a neutron becoming Silicon-31, which is unstable and decays into Phosphorus-31 after 2.6 hours. So not only there is just a 10 second window during which phosphorus can be created, but there is also the problem of Silicon-30 composing only 3.1% of all silicon, so less than 3% of the elemental silicon present in a massive star ends up generating phosphorus as the end result.
I would deem such types of supernovae rather inoffensive compared to other types because the cloud of material around the star when it finally explodes shields surrounding space from most of the radiation, hence why supernovae of this type are often apparently less luminous. If Cassiopeiae A is one typical supernova of this type, then the phosphorus-to-iron ratio in those types supernovae are on the order of 100x that of the average Milky-Way abundance, or 7% phosphorus for each unit mass of iron (log [P/H]~ –3.95).
So the average type II supernova releases about 1000 Earth-masses of phosphorus into space. And considering this material spreads like a bubble, a supernova remnant 10ly across has 3.18 Earth-masses of phosphorus per square light-year. Needless to say that you'd need thousands of supernovae to seed a region of space enough to reach solar system levels, given that the further away you are from such an explosion the less material you receive. Given that a supernova echo is also what you need to start a star system's formation, there are few opportunities for seeding to take place.
Because star formation is more prominent in a galaxy’s spiral arms, this is visible when observing galaxies at high frequency wavelengths such as UV and X-rays emitted by very hot massive stars, most massive blue stars that could generate this phosphorus are concentrated together and thus we have another caveat other than its rarity.
PHOSPHORUS SOURCES ARE NOT EVENLY DISTRIBUTED
The very center of the spiral arms are generally deemed uninhabitable, since the very thing that creates phosphorus also has the potential to destroy the biosphere in the blink of an eye. A typical type II supernova peaks in brightness at 10 billion solar luminosities for a few days, decaying to a billion solar luminosities after 3 months, and 10 million solar luminosities after a year. Given that the average safe distance is at least 50 ly, the irradiance at the observer should be less than 0.001x that of the Sun on Earth.
According to the following map, the closest open cluster (rich in O-stars) is located at least 225 light-years from the Sun, it could be more depending if it is located above/below the Sun’s orbital plane.
The Sun rarely passed through star-forming regions where it was sprayed by supernovae through its lifetime so far, because the stars around the Milky-Way orbit alike a rigid disk, the relative velocity of stars around the Milky-Way is pretty low, and so the Sun and other stars of the galaxy could complete a couple of orbits without crossing paths. If the Sun orbits the Milky Way every 230 million years, its velocity would be around 214 km/s, a star-forming region with a relative velocity of ±15 km/s at 1000 ly further out than the Sun, would take between 223 and 258 million years depending on eccentricity, the encounters would happen every 7.3 billion years (0.14x/Gyr) and 2.1 billion years (0.48x/Gyr) respectively, just to give a sense of scale.
The abundance of phosphorus and other heavy elements are expected to decrease with the square of the distance from those star-forming regions, while the inverse is rather true for safety. However, the abundance of phosphorus does not follow linearly with the probability of life arising.
Like futurist Isaac Arthur puts in one of his videos as a simple example, if getting the right amount of phosphorus is as easy as getting ten six-sided die to roll 6, then the odds of spawning life are 1 in 60.5 million, but by rising the number of faces to 100 instead of 6, the possibility decreases to 1 in 100 million trillion.
1 in 6¹⁰ ≈ 60.46 million
1 in 12¹⁰ ≈ 62 billion
1 in 100¹⁰ ≈ 100 million × 10¹²
So even by halving the possibility of scoring a 6 on these dice, the probability shrinks a thousand fold. In the first case it takes 1 second to spawn one molecule, on the second one it takes 17 minutes, and on the third one it takes 52 thousand years.
And there are only a few hundred to a few thousand ways those essential atoms can bond according to the rules of chemistry and even less possibilities under the right conditions such as temperature, acidity, and saturation.
Current sea concentrations of phosphorus tend to 1 in 10 million parts, but tide pools can lose water to evaporation and concentrate those to about 1 in 200 parts, and that's under present day atmosphere, where phosphorus binds with calcium to form calcium phosphate, which is useless for Life — in Earth's primitive atmosphere with nearly +5% Carbon dioxide, that calcium would have reacted instead with carbon monoxide to form calcium carbonate, leaving elemental phosphorus to react with Life and other chemicals, peaking possibly around 1 in 10 parts.
The carbonates in the atmosphere are not the only accused of increasing phosphorus levels early on, salts and phosphates delivered by meteor and meteorite strikes during the Late Heavy Bombardment would have greatly increased the ocean saturation creating hotspots and oceanic plumes of dissolving material, mainly schreibersite, it so happens that phosphorus is pretty siderophilic, and so metallic asteroids and planetesimals are great candidates to hoard phosphorus in an easily accessible form for the surface of Earth, since most of Earth’s phosphorus would be in its insides, so much so that up to 10% of Earth’s crustal phosphorus is believed to have been delivered like so.
Because the Earth’s crust is constantly being recycled in the tectonic cycle, that is, the average crust is no older than 2 Gyr, the actual meteorite-delivered phosphorus would then amount from 10¹⁷ to 10¹⁹ kg, compatible with the Late Heavy Bombardment model for around 10²² kg of material delivered (average 0.1% by mass), though because of vaporization in the atmosphere, the actual amount to reach the surface may be close to the lower end.
Schreibersite is extremely reactive even below 100ºC, at usual temperatures found in alkaline hot springs, between 60 and 80ºC, it can phosphorylate with an efficiency up to 6%, that is, about 6% of the rock does react to form nucleotides, these would have been essential to boost the potential of simple Life 4 billion years ago, however it would had to develop for half a billion years prior without such help.
Because LUCA’s estimated time frame must be clear for us to have any real estimate, we have to consider not only the current total phosphates delivered from space, but its natural abundance, consider Earth’s natural ability to spawn life as such. The thing here is that we are talking about a ≤10% difference, and so the question remains had LUCA appeared 4.4 billion years ago, the phosphate delivered from space was only a boost in Life’s ability to produce energy and thus develop, but if LUCA’s origin is really displaced by 400-500 million years later, since it is generally accepted that the Earth’s surface was pretty much molten around the period of the Moon-forming impact, thus rendered impossible for Life to have its origin so early, then the LHB would have clearly provided essential help in its development.
In the later, as in my humble opinion, more credible case, it is estimated that the Earth’s resultant magma oceans took around 150 to 200 million years to cool down, which puts the moment that Life became possible somewhere around 4.30 and 4.25 billion years ago, 200 to 300 million years before the LHB, given that the earliest confirmed fossils of bacterial activity date from the LHB period (4 Gya), Life on Earth developed pretty quickly even in those “poor” conditions.
WILD SPECULATION
Approaching it another way: even if the chance of spawning life per “test tube”, say like a 1 liter reservoir, is about 1 in 10¹² per second, that’s one successful reaction every 32 thousand years, and given an ocean with 5.5×10¹⁸ liters, it would still give us 5.5 million events in the very first second, and over a trillion events after 48 hours, even if only 1% of that ocean is truly reactive, the same trillion events happen within 2 months. Now, we don’t really know how many successful events are needed to really spark the biotic state, and so within our time window of 250 million years, it could be as small as 1 in 4.4×10³² as to need a single but flawless success.
IN THE NEXT POST, WE WILL DISCUSS A MINIMALISTIC MODELING OF A SPACESHIP
