OTHER | PHOSPHORUS AND SPACETRAVEL

THE LIMITING FACTOR IN ONE'S ABILITY TO GO INTERSTELLAR much probably...

PACKING UP BREAD AND DIRT BLOCKS

We have so far contemplated how rare phosphorus is, how it arrived here, and why it is so important for Life. Now let’s figure out how important that is for space travel.

Let's start with rations, my proxy for vegetables will be lettuce and cabbage. While milk and poultry will serve as protein proxies. So by mass, a kilogram of meat contains about 1% phosphorus while dairy is around 0.08%. While vegetables contain only  0.04%. I’ll be estimating the needs of the crew based on a 2006 study on the Finnish consumption of natural resources.

Around 25% of the consumption will consist of dairy, while 10% is protein, 17% vegetables, and the rest is mostly water, alcohol, and sugar based drinks. Which provides 170g, 850g, and 58g of phosphorus, respectively, with trace amounts in other sources, throughout the year (~3g/day), which is compatible with the daily flux of phosphorus for a healthy human.

So not only you'd have to be able to store 840 kg of rations per person per year, but also produce it during the trip, after all, cabbage and chicken are also eating.

For crops, the requirements for phosphate fertilization may vary, both corn and alfalfa need about 5.9 kg per ton of crop (5 and 2 tons per acre), and crops only use about 20% per year, so a single fertilization may yield up to 5 crops before it is needed again. Monoammonium Phosphate, a common granular fertilizer, is only 13% P by mass.

So if our ship has to sustain 1000 people for a year, it has to provide 160-170 tons of vegetables a year-round, and 84 tons of meat. And we include at least 991 kg of fertilizer consumed per year.

Because tissue printing and meat culture is already a developing technology I'm initially assuming this meat can be produced without the animal necessarily on-board, nowadays this technology can produce 10kg per hour (per unit I assume), once you have a culture of cells and their nutrients. So those 84 tons are of raw material, including 840 kg of phosphorus.

The conclusion here is that, not only you need 2.04 tons of raw phosphorus per 1000 people, but you also need all of the water, air, but also other elements such as carbon, nitrogen, and calcium to circulate properly around this system, and add a slight surplus there because bacteria will grow in these conditions want you or not, you won't like to wake up one day and find that the nutrients are being drained by the soil itself.

The International Space Station has a pressurized volume of 915.6m³ at 1.0 atm, and a crew of 7 people, though about 13 astronauts have been at the ISS at one time in 2009, several sources cite the ISS optimal capacity as something in between 6 and 10 people, so let’s go with 5 people, since we also have to share those resources with our growing food and unwanted bacteria.

Using that pressurized volume and air density at 1 atm, we get 1,163 kg of air per 5 people onboard, or 232.6 tons per 1000 people. While CO² is disposed of into space by the ISS. Chemical and electrical equipment exists to break CO² into carbon and oxygen, the carbon reacts within the system to create amine compounds, in that case, 40-50% of atmospheric carbon gets trapped in those amine reservoirs.

Active air production can take a few routes, either by burning oxygenic fuel, or by electrolysis of water, the latter which averages at 1 liter of water per person per day, while the released hydrogen is reacted in another unit with carbon dioxide, to produce water and methane.

For water, the ISS is able to recycle 90% of its water — with around 1,514 liters per 2-3 months. While each astronaut is limited to 4.4 liters of water a day (drinking, shower, etc), being conservative we can use between 4 liters astronaut-style, or 384 liters for a more Earth-based routine.

For comfort and leeway into near-future advancements, I’m using:

    Drinking, 4 liters a day

    Shower, 3 minutes, 4 liters a minute, 12 liters per shower, 2 showers a day.

    Food water, about 0.5 liters a day.

    Total: ~28.5 liters a day.

The ISS also has the capacity to recycle between 10 and 20 liters of water a day (apparently), and so we need a daily recycling capacity at least 5-6x that of the ISS, which approaches that many times more equipment since we are at 90% efficiency, we are also carrying enough water to operate 90 days before touching recycled water. So 2,565 liters per person on our trip.

Still on water usage, vegetables need 40-80% soil humidity to grow, and so we are looking for around 60% water by volume of soil — and that’s another unknown variable, many crops can be cultivated through hydroponics, but some need soil, assuming we can generate artificial gravity, a more hands-on approaching to vegetable growth will be my route here, we had already established that we can get between 2 and 5 tons of crop per acre (~4,047 m²), let’s settle at 4 tons per 4,047m² for these calculations, and I’m also using ~1 meter deep of soil, so the ship literally carries a farming ground with it.

Humus has an average density of 2.65 g/cm³, clays is 1.7 g/cm³, and clay+gravel stands at 2.7 g/cm³. At thicknesses 12.7cm, 63.5cm, and 24cm for the soil layers, we have a total density of 2.06 tons/m³, which means 8,336.8 tons of soil per acre.

For the vegetables (165 tons) we need an average of 50 acres, for a total soil mass of 416.84 thousand tons, and with 60% humidity, that’s 121.4 thousand tons of water…

Because we need to account for active bateria around, literature shows that up to 5-10% of soil organics are microbes, so basically we have to account for extra “crew” onboard, and thus extra phosphorus, air, and water we need to carry accordingly, bacteria are between 1-2% phosphorus, while I’m assuming their body mass mostly water as well, and so I’m using our humus mass as our reference for organic matter, so about 16.83 kg of bacteria per cubic meter. A single-person ship with a 417 ton garden hoards as much bacterial biomass as extra 30-40 people, and these “people” also consume oxygen and emit carbon dioxide. And so two solutions exist for that:

1. Add more resources on board.
2. Use methanogenic bacteria to even out CO² and O² emissions.

Because the 1st option is too complicated, I’ll opt for the 2nd way.

AEROBIC:

C6H12O6 + 6 O2 + 38 ADP + 38 phosphate → 6 CO2 + 44 H2O + 38 ATP

METHANOGENIC:

CO2 + 4 H2 → CH4 + 2 H2O

The aerobic:methanogenic respirations processes need to happen in a 1:6 proportion to balance out, in perfect conditions. In reality, the biogas conversion rate is about 60%, the unprocessed CO2 could be used to produce carbonate water, but we can’t take that many risks of leakage.

Cultivated soils exhale 7-8 tons of CO2 per hectare per year, so 3.04 tons per acre, and by our counts, a single human would need 0.05 acres of land, so 0.42 kg/day.

Exhaling air from a human contains 4.4% CO2, which amounts to about 0.7-1kg of carbon dioxide per day depending on activity level, because the only function of these methanogens is to process respiration waste, and not biomass, we can get away with ignoring the fermenting time it takes for them to process solids.

With leeway, I calculated the methanogen-to-aerobic respiration ratio to get rid of 1.5 kg of CO2 per crew per day, as 9-10x.

I haven't found anywhere relating microbial mass to methane yield, so I'm using a cow's rumen capacity and methane yield as my proxy biodigestor, with 100 liters rumen populated by methanogens and other bacteria it can produce up to 500 liters (357g) of methane per day, so for 0.5kg of CO2 → 0.182 kg of CH4, we need at most 50-150 L biodigestor per person and their cubes of dirt, which I'm assuming has a sludge consistency 1.3 g/cm³, that's extra 65kg of mass per person, minimum. Or 200 kg per person if you're neutralizing atmospheric carbon through this process.

So here is the thing, a single person in our expedition here needs for a year: be able to produce and farm 840 kg of food.

0.05 acres of land to farm vegetables, 1m thick, 416.8 tons of soil.

3.41 tons of bacteria within the soil, plus ~70 kg of methanogens.

1 kg of fertilizer.

2.66 tons of human usage water.

121.4 tons of water for plants.

62.58 tons of water dedicated to microbes.

Total phosphorus onboard 53kg.

TOTAL MASS (so far) 707.7 tons, or 1.6x International Space Station.

In that regard, I now understand why the ISS crew don't farm for sustenance, instead having small crops for greens and research, but they can have new rations and water be brought to them every 3 months, our interstellar expedition can't.

ARCHITECTURE AND DRY MASS

I'll be using both ISS and MIR as my references/proxies for equipment and infrastructure. I see two ways to approach this estimate, monolithic and modular constructions, monolithic stations are ones composed of a single vehicle or part.

Examples of this design are Salyut 7 and Skylab:

While modular constructions are well        modular, like ISS and MIR:

While modular designs improve a lot on the amount of continuous structure and division of space, I’m actually unsure if the modular architecture can withstand thrust or significant acceleration rates required for space travel due inertia of modules outside of the main axis.

I also assume, given the crew size of both stations, that the total space per crewmember is between 100-200 m³, and from that volume will be calculated the air mass. Because there are a variety of pressurized modules in each station, with different functions and equipment, the exact mass of components vary as well, but within that mass power law, solving for the ISS and MIR pressurized volumes gives their total mass as expected.

A single habitat with 90m³ would weigh on the order of 22 tons ±5 t, while science modules with 30m³ would weigh 5.4 tons ±1.5 t.

Unless we develop FTL drives for such small ships, we won’t need to worry about generating gravity for a trip of a few months. But permanent stay will require some sort of system for gravity, a centrifuge or tether is the logical solution.

Centrifuges are more self-contained than tethers, we can put several habitats along the circumference of the centrifuge and have the crew climb up to the main axl of the ship to move to other parts.

There are a few sites that can calculate the centrifuge size for you, I’m being conservative with materials and putting up 0.5g of acceleration on it, for a radius of 112 meters rotating 2x per minute. Luckily for them, the 112 meter climb won't be a burden, because the gravity will decline the closer one gets to the main axl.

With a module length of 13m and diameter 4.2m (space station sized), with 90m³ of pressurized volume. About 54 of the modules fit tightly end to end on the centrifuge wheel, totaling 4,860 m³, and the rim weighting over 4,000 tons. Because here we have way more space than a few space stations combined, I'm considering it safe to assume we could move part or the operation to the centrifuge.

For an active 5 person crew, we have 0.25 acres of land that goes on another centrifuge with 156 habitat-sized modules side to side, and 69 modules in double line and triaxial symmetry (for balance), used for farming. The whole structure weighs 5,000 tons while dry, plus 2,084 tons of soil and 607 tons of crop water, 7,691 tons total. The farm ring needs lighting as well, let's position a 5kg 280 W full spectrum lamp on top of the plants, one per square meter, extra 5.06 tons.

The whole crew will be using at least 750-1000 m³ combined. Leaving 3,860m³ free for more equipment, recreational areas, more resources, and tool storage. Like proper bedrooms, dining areas, etc. Assuming a few of these modules lead or are designed as aircraft to be used for decent once on their destination, while others serve as genetic banks. Because both centrifuges spin at the same rate, that is, have the same gravity, we can couple them close together.

We need a place to store 315 tons of water destined to nurture microbes and the crew onboard. Large spherical tanks along the main axl can do the job, less pumping work on the way down into the centrifuge areas. 76 spheres with 10m radius can do the job, plus 2-4 extra (just in case), still using our volume-mass relationship we get 3,020 dry tons for the tanks. The best way to arrange them is along the main axl, with about 2m in between them, and arranged in four opposing lines of 20, the length of the water tank section is about 440 meters.

The air mass calculated from the total pressurized volume (~25,110 m³) here is around 31.89 tons of air. Plus our expedition needs 6kg of raw fertilizer per 5 years for the trip.

The sum of all our calculations here yield approximately 15,063 tons, not counting thrusters, fuel, and the whole rig that holds this together. While input of the raw volume gives 33,572 tons. Which I find interesting because the sum of the ISS 915.6 m³ in 22 ton modules give 223,8 tons, about half of the total mass in structure and unpressurized life support and scientific modules. So between 30 and 60 thousand tons is the mass of a 5 crew spaceship.

PROPULSION PROBLEM

We need rockets to make this thing go interstellar, or to go anywhere for that matter. I'll be using the Falcon Heavy first stage as my proxy. At 8.2 MN thrust in a vacuum, exhaust of 3050 m/s, provides 3 km/s of delta-v.

Since we need to escape the solar system, our final cruise delta-v should be above 16 km/s, and so I'm calculated fuel initial mass for 20 km/s delta-v… 20 rockets, each pushing this ship contributing for 1 km/s of delta-v, 69,400 tons of fuel per rocket. And double that amount because we gotta decelerate on our destination.

This is just out of question unless we can fancy ourselves some really good non-chemical fuel thrust source, like those ion or nuclear fission plasma thrusters, at 5-10% the speed of light, it is feasible to reach nearby star systems in a few decades to centuries.

MISSION LAYOUT PROBLEM

OK, so there is no way we can arrive at the destination with the same crew we've put in, basically.

We can't put out a sleeper mission, and what's the point of uncrewed missions in this post? We want humans. And so we don't need humans for like 99% of the time for this to work. Granted that until we solve the thrust source problem, we will also get to the artificial uterus technology, and also good enough AI that can raise our little crewmembers into colonists once they arrive. And so, depending on how complete our mission layout is (raise them on orbit or ground), we can basically discard all our work until here about human sustenance in space…

So one way we can solve the ship mass problem is to simply diss the entirety of the human sustenance modules and keep only the essential, in a way, Avatar’s Interstellar Vehicle Venture Star does this job well, with its payload necessitating little space aboard, since the crew is asleep for most of the trip awakening prior to the descent procedure. Except what is more realistically likely to happen within the next century is to solve the artificial intelligence problem, and the ethics of raising humans in space without parents or choice of career.

REMINDER

Your destination MUST HAVE enough phosphorus in acessible form to sustain a growing population, else you're stuck.

-M.O. Valent, 05/06/2022

OTHER | PHOSPHORUS AND LIFE

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:

1. Verify if it is in the habitable zone. 

2. Verify mass and size, be certain it is dense enough to have a solid surface.

3. Verify star and planet for a decent metallicity.

4. 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

- M.O. Valent, 30/05/2022

Water-Carbon based Life | Why is it the way to go?

THE 6TH BEST ATOMIC FLAVOR - CARBON

Ah... Carbon, the Periodic Table's main subject for most high-school students together with Oxygen and Hydrogen.

Carbon is an interesting sort of element, and I think the vast majority of you are used to its amazing properties. Carbon is an extremely stable element, it is able to form different lattices, which vary from graphites to diamonds, when in the presence of other elements it can make up to 4 electronic bonds with them. For the industry apart from these characteristics, carbon is extremely important in base reactions to refine metals, build polymers, and drastically increase the heat resistance of certain components, among other uses.

In general, carbon plays an important role in our society, and as it is of common knowledge at this point, our ecosystem and life as we know it.

There is one question that utterly bothers me about carbon, and has kind of always been in sci-fi writer's heads for quite a while, "is carbon-based life is the only way to go?".

That often if not ALWAYS come with the extra of questioning the place of hydronium oxide as well, a stable and highly corrosive substance - also known as Water - as a solvent for life.

A SUBSTITUTE FOR CARBON?

Wikipedia cites at least 17 hypothetical alternatives to the Water-Carbon based life (among other affairs) we have on Earth. Talking about building blocks, ie, substitutes for Carbon, we often stumble upon Boron and Silicon, some outliers go as far as to consider Arsenic and Sulphur too.

BORON

Boron and Borane are interesting interactions, but their abundances nonetheless rule them out as a likely possibility at all, Boron is even rarer than Fluorine, akin to Vanadium.

Take for example diborane (B₂H₆), it is only liquid between -164,8°C and -92,5°C, a 72,3 K window in the energy spectrum and a very low energy window. Needless to say, diborane cannot exist in an environment with oxygen or water too, as it reacts violently to form boron trioxide (a powder) and boric acid (also a powder).

SILICON

Silicon is notable for making 4 bonds like Carbon does - but unlike carbon, organosilicon reactions have to take place at low temperatures because the optimal medium for these reactions are alkenes, alkynes and ketones. If the medium is acetylene, the temperature of the world has to be stable at -80°C and at 1,27 atm. For ethylene, the temperature must be between -170°C and -103°C. And for Acetone, it is between -95°C and 56°C. 

The major problem here is that the most energetic molecules that can be derived here need a lot of energy to be broken and release their energy in exchange, see Ketoses for example, way above the boiling point of many of the proposed solvents.

Si-C bonds are weaker than C-C and C-O bonds, but Si-O bonds are stronger than the others, hence why we find silicon mostly in rocks bound to a given amount of oxygen - and even if somehow there is any free silicon around the planet, we can discard its viability as building block as it is substantially less electronegative than hydrogen. Which helps maintaining molecules together but not so close, in which case Silicon becomes the positive end of the molecule. It is so "useless" for base life chemistry because it is very damn stable, and silicon chemistry requires lots of energy and sometimes even Platinum catalysts.

ARSENIC AND SULFUR

These two elements oxidize each other, and react with water and oxygen as well, both are unstable chemicals and their basic compounds are like time-bombs that run faster the hotter it gets (really fast at +100°C), from which Arsenic is the most interesting. Arsine is stable at narrow range of temperatures (-111°C to -62°C). Arsenic compounds tend to also form metallic complexes and crystals which are by default absurdly stable and nonreactive given the planetary conditions. Arsenic is also a billion times rarer than Carbon.

A SUBSTITUTE FOR WATER?

Oxidane, or chemically H₂O, is a very peculiar molecule in several ways, in general that is linked to its electronegativity differential.

Oxygen is one of the most electronegative elements in the periodic table, that is, it has a strong tendency to attract electrons to itself when bonding, that's what gives oxygen its oxidative properties. In turn, hydrogen cannot hold its single electron near the oxygen, so the electron pair is mostly present around the oxygen while the nude protons cluster on one side of the molecule, giving the molecule an overall positive and negative ends. This is an important quirk of water, because it allow water to stick to itself through hydrogen bonds. So in water, every water molecule is connected to at least other four molecules, and so forth.

From this, water is particularly resistant to atomic motion - that is - resistance to heating up. Water has an absurdly high heat capacity (4,2 J/g°C), and it helps it to remain liquid in wide array of pressures and temperatures.

But let's look at other elements near oxygen, if those characteristics are from polar substances, sure of water's neighbors might be just as good.

If you add Hydrogens to Nitrogen, Oxygen and Fluorine - you get Ammonia (NH₃), Oxidane (H₂O), and Fluorane (HF).

Based on sheer electronegativity and bonding strength, fluorane should be stronger than other substances. Except it is not, because molecule geometry also matters, for the maximum strength of bonds, fluorane would have bond linearly with other molecules, which it doesn't because fluorine is so much electronegative that it keeps from bonding with more than 2 molecules at a time - instead bonding in zig-zag patterns, like so:

Likewise, when we look at Ammonia, its molecule can only bond with 3 others...

So actually, the boiling point temperature goes like: H₂O > NH₃> HF because of the number of bonds they are able to make.

The polar nature of water also makes it an optimal solvent for the majority of molecules, because the water molecules can nudge themselves into the weaker bonds of substances and separate their components through hydrolysis, take for example how paper dissolves in a water because the water molecules break the cellulose bonds separating the glucose monomers.

Oxygen is also way more abundant in the universe than any of the other two elements, oxygen is tens of times more abundant than nitrogen, while nitrogen is hundreds of times more abundant than fluorine.

WHAT ARE POSSIBLE EXOTIC WORLDS?

Given what we've seen here so far, there is reason to believe that Water-Carbon based life is, if not dominant by a vast margin (say like, "90%"), the only type of biochemistry possible in the Universe as we know it.

Besides the small-scale chemical properties of oxidane, it also serves a huge temperature buffer substance, narrowing down the temperature variation in the atmosphere of the planet because of its high specific heat capacity, the other fluid that is just as good in this is Ethanol (and it is just half as good). Any planets sporting other volatiles would have wild temperature variations between day and night or throughout the year unless they were extremely distant from their stars with circular orbits.

Silicon is still narrowly up to debate because how it still holds potential for the genesis of proto-organic molecules.

The only other water-analog molecule I cannot find strong evidence against for possibility biochemistry is hydrogen sulfide (H₂S) - all other hydrides can be easily destroyed by the primary atmosphere of a young planet, such as how selane can be created by aluminum compounds and water in the early crust, but dissociated into fine selenium ash and water when reacting sulfur dioxide from volcanic activity.

So we have team of carbon as a building block, and either oxidane, ethanol, and sulfane as solvents (heat capacity 4,2 > 2,4 > 1,0 J/g°C), each with decreasing boiling points and decreasing commonality.

OTHER TYPES OF EXOTIC BIOCHEMISTRY...

Within our established limitations here, we can have:

ALTERNATE CHIRALITY

Chirality is an asymmetric property of certain molecules, that is, if your superimpose the reflections of this certain molecule on top of another, they won't match.

In the example above, molecules 1 and 2 have the same structure and chemical formula - but they are mirror images of the other, molecule 1 has the red radical on the Right side while molecule 2 has the red radical on the Left side.

It might not be that big of a deal at first, after all, it is chemically identical, right? Well, it happens that molecular machinery in life will not be able to fit the lock&key mechanism of their chemical reactions half of the time, and so it adapts to only accept and use one chirality of a molecule at a time, and sometimes molecules have drastically different effects if they are flipped on their chirality to one organism - and so the ecosystem of Earth has adapted to use only one chirality of glucose, only one chirality of ATP, and only one chirality of vitamin B, and so on so forth.

Because of this, had luck or the conditions of our planet been any different, it is possible that life worked its way around other sets of chiralities, and so the same may be valid for other planets. Imagine Earth food being poisonous to aliens because it reacts different within their organism, and vice-versa - there is N possibilities within this field.

ALTERNATE DNA STRUCTURE

Under general circumstances, a DNA or RNA world is a given to form with 4 base pairs, ATGC - but these are not the only possible base pairs for a DNA-like structure, under very specific conditions (a lab as far as we know nowadays), the base pairs PZBS or d5SICS-dNaM may be included, partially mixed, or be the only base pairs akin to ATGC is in our DNA, and so depending on the nature of life in a certain planet, DNA may be partially compatible or not compatible at all.

If this really holds true, assuming 4 bases is the minimum optimal number of bases you need, there are at least between 4 and 15 other possible DNA alphabets than our own, opening a world of exotic amino-acid synthesis and gene expression.

"CARBON CHAUVINISM"

Carbon chauvinism is a relatively recent term to designate the quality of those who assume that extraterrestrial life must be similar to life on Earth. In particular, the term applies to those who assume that the molecules responsible for the chemical processes of life must be based on carbon as the main structural element.

This attitude suggests that humans, as carbon-based life forms that have never found life beyond their planet, may have difficulty conceiving the existence of alternative biochemicals. The term was first used in 1973, when Carl Sagan described this and other human chauvinisms that limited his imagination about possible distinct forms of extraterrestrial life in his book The Cosmic Connection.

However there is sufficient mathematical and experimental proof of carbon's superior thermodynamic and chemical properties. Such ideas are not akin to Eugenics or the belief in Miasma, but more like the theory of relativity, which holds its predictions, and has taken several decades for the nearly full understanding and observation of described phenomenon, such as space curvature and black holes. It is such a fundamental property of chemistry one would need to completely re-write physics in order to make non-carbon based life possible at all.

It is not a matter of preference, but a matter of understanding that the surrounding chemistry of life, such as energetic molecules like ATP and structural cell membranes can only exist within the boundaries of water-carbon chemistry. One cannot completely rule-out the possibility until we have explored a substantial number of planets, but recognize other hypothetical biochemistries as significant possibilities is outright foolish and a pseudo-skeptical posture - alike people that "doubt" the existence of outer space like planets and other suns because the sky is so "fundamentally" different from the earth that our physics should just break. Or doubting the existence of atoms, despite chemical and physical phenomena being accurately described by atomic theory.

Now, I'm not willing to touch on the subject of aplanetary life, like microscopic string-particle things inside stars or in deep space, because it is not scientifically falsifiable, and thus one cannot put themselves through the problem of discussing it.

But I can see how one sees faces carbon problem like how we see today the ancient Greeks discussing the possibility of atoms in the first place. The problem with the idea that we may only be on the tip of the iceberg is that nowadays, we have methods and a vast disparity of technological superiority. We do understand the fundamentals of physics, and arguing against those fundamentals is like arguing against trigonometry - one just can't...

- M.O. Valent, 14/11/2021