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I know for example there is an interesting gap in the amount of lithium, beryllium and boron compared to Helium and Carbon, but I don't know how useful they are. Here is a graph showing relative abundance of each element.

abundance of elements https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements#/media/File:Elements_abundance-bars.svg

I am trying to figure out, as humans start to spread over the galaxy, what atomic elements would still be rare enough that they would be actually trade, instead of being part of some kind of post-scarcity economy. (example: you can find hydrogen in any star, tons of it... so obviously you don't need to trade it, you can find it in whatever place you are).

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    $\begingroup$ Although hydrogen is indeed in stars, getting it out again is a non-trivial challenge. Even a gas giant is probably more effort than it's worth unless you need tremendous quantities of the stuff. $\endgroup$
    – Cadence
    Aug 12, 2020 at 6:04
  • $\begingroup$ @Cadence just scoop it out of a nebula. If you can do interstellar travels as an everyday thing, you can scoop nebulae. $\endgroup$ Aug 12, 2020 at 14:55
  • $\begingroup$ @Renan even if you can, how much material would say a ship with 1km of scoop get if it did one pass through the average nebula? Assuming there is such a thing as "average nebula". $\endgroup$
    – Demigan
    Aug 12, 2020 at 17:11
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    $\begingroup$ @Demigan they are also very large. The Eagle nebula is 70 light years across. With a square kilometer scoop, you'd get 1.66 × 10e52 particles. If every particle is a hydrogen pair, that's about 2.5 × 10e29 moles of H2, or about 5 × 10e29 grams. That's about 100 Earths. Two more scoops and you have a jovian mass. A thousand ships in parallel will get you a sun. The Tarantula nebula is 25 time larger, so you'd need just 40 scoops to form a star. Or you could make larger scoops and do it with less passes. $\endgroup$ Aug 12, 2020 at 18:45
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    $\begingroup$ @Renan How much of that mass would you need to convert to energy/exhaust to collect and transport the rest? Stars are a much more convenient source of material, as gravity has already collected it into a single location for you. $\endgroup$ Aug 13, 2020 at 16:12

6 Answers 6

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People need boron, because plants need boron.

https://en.wikipedia.org/wiki/Boron

Boron is a chemical element with the symbol B and atomic number 5. Produced entirely by cosmic ray spallation and supernovae and not by stellar nucleosynthesis, it is a low-abundance element in the Solar System and in the Earth's crust.[11] It constitutes about 0.001 percent by weight of Earth’s crust.[12] Boron is concentrated on Earth by the water-solubility of its more common naturally occurring compounds, the borate minerals.

This is why boron is rare. Even the boron that is out there in the universe would be dispersed without a planetary hydrologic cycle to concentrate it.

Humans need plants to eat, and plants need boron.

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/boron-on-plants.htm

Artificial hydroponics operations require plant micronutrients and boron is one that might be difficult to find locally in any concentration that made it efficient to collect. Shipping boron pellets would be reasonable.

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    $\begingroup$ Possibly relevant. $\endgroup$ Aug 12, 2020 at 19:08
  • $\begingroup$ great. now another thing to worry about, Peak Boron. $\endgroup$
    – Michael
    Aug 14, 2020 at 2:09
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    $\begingroup$ @Michael - maybe it would be better if you made that your rap name? $\endgroup$
    – Willk
    Aug 14, 2020 at 2:57
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    $\begingroup$ @Willk Yes! Now I have something to replace "Morons from the Planet Boron" I came up with when I was a kid. $\endgroup$
    – Michael
    Aug 14, 2020 at 4:30
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Lithium, like boron (covered in another answer), is relatively rare because few processes have produced it since the Big Bang created a tiny percentage -- but it's useful both as a chemical and as a component of one of the "easiest" pathways to nuclear fusion -- including aneutronic fusion of lithium and deuterium, which is likely to be very important over a long time frame.

The lightest and most reactive of the alkali metals, we use lithium compounds for soaps that make grease, in drugs, as an alloying agent -- the Space Shuttle had lithium-aluminum alloy for the external tank for the last half of its service life -- and, in its chlorate and perchlorate forms, as an oxidizer. There surely are many other industrial uses for lithium, but one of the most important at present is in high energy density, long life rechargeable batteries crucial to electric transportation.

You could easily make a case that lithium and boron are the two most crucially rare elements in the periodic table; we'll need both for many purposes for at least as long as humans continue to inhabit bodies like the ones we wear now.

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Hasn't been mentioned yet, but beryllium has a rather low abundance, and is a remarkably useful metal due to a number of unusual properties it exhibits, namely:

  • Despite being an alkali-earth metal, beryllium is actually remarkably non-reactive. It doesn't even form oxides in regular atmospheric conditions unless you heat it to high temperatures.
  • It has an unusually good structural properties for such a light element, exhibiting high flexural rigidity and thermal stability as well as a bulk modulus only a bit lower than that of steel, a Young's modulus and shear modulus higher than those of steel, a very low Poisson ratio, and a reasonably high hardness by most measures of hardness. These properties combined with it's very low density make it extremely popular for making mirrors that are either very big (such as the one on the James Webb space telescope) or are very small but must be moved very quickly (for example, those in the optical fire-control systems on the German Leopard 2 main-battle tanks).
  • Due to it's very low atomic weight, it's relatively transparent to ionizing radiation. This, combined with the above mentioned structural properties, makes it very widely used for applications which require blocking out visible and ultraviolet light but allowing ionizing radiation through.
  • It has a very high thermal conductivity, making it useful for thermal management applications. Beryllium oxide retains this high thermal conductivity while also being a good electrical insulator, making it useful for cases where both properties are required (though it's not much cheaper than other options).
  • Relatively small amounts of beryllium have a big impact on the bulk structural properties of other metals it's alloyed with. Beryllium copper is a particularly good example of this, where between 0.5% and 3% beryllium content in otherwise mostly pure copper produces a remarkably durable, non-sparking and non-magnetic material that's excellent for use in tools used in hazardous environments.
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    $\begingroup$ It's also really toxic in dust form. In amounts small enough to be invisible to the naked eye. $\endgroup$
    – DKNguyen
    Aug 13, 2020 at 3:29
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Considering a spiral or barred spiral galaxy, you will find all elements from the periodic table (except the artificial ones). The interesting thing is how they are distributed. I found this paper:

Conclusions that span the galaxy types treated here are as follows. All galaxies, on average, have heavy-element abundances (metallicities) that systematically decrease outward from their galactic centers while their global metallicities increase with galaxy mass. Abundance gradients are steepest in normal spirals and are seen to be progressively flatter going in order from barred spirals to lenticulars to ellipticals. The distribution of abundances N(Z) versus Z is strongly peaked compared with simple closed-box model predictions of chemical enrichment in all galaxy types. That is, a "G dwarf problem," commonly known in the solar cylinder, exists for all large galaxies.

For spiral galaxies, local metallicity appears to be correlated with total (disk+bulge) surface density. Examination of N/O versus O/H in spiral disks indicates that production of N is dominated by primary processes at low metallicity and secondary processes at high metallicity. Carbon production increases with increasing metallicity. Abundance ratios Ne/O, S/O, and Ar/O appear to be universally constant and independent of metallicity, which argues either that the initial mass function (IMF) is universally constant or that these ratios are not sensitive to IMF variations. In the Milky Way, there is a rough age-metallicity trend with much scatter, in the sense that older stars are more metal poor.

In laysman terms: you will find everything you need more easily as you move towards the core. Just the same, as you move away from the core elements heavier than helium get increasingly rare.

For human survivability, the rim of the galaxy is safer. The hub is where you find the most black holes and where you get the most supernovae. But those places are also poor in... well, everything. So you would be bringing metals from Earth and other planets to the rim. Notice that I'm talking about metals in the sense of astrophysics - much to the dismay of chemists, astrophysicists will call anything heavier than helium.


By the way: the gradient is very noticeable close to the midplane of the galaxy, not so much when away from it. I got the image below from an article on mapping the outer Milky Way:

Metal mapping of the Milky Way
Source

So you may have to export metals to the "northmost" and "southmost" reaches of the galaxy even when close to the hub.

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    $\begingroup$ So you would be bringing metals from Earth and other planets to the hub. Do you mean that you would bring metals from Earth to the rim? Or from the hub to Earth? $\endgroup$
    – cowlinator
    Aug 12, 2020 at 19:32
  • $\begingroup$ @cowlinator from Earth to the rim. Thanks for the heads up, I fixed it :) $\endgroup$ Aug 12, 2020 at 19:41
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A lot of people have made mention of light elements. However, these won't be an issue to an advanced civilization.

For instance, if you have a spaceship capable of traveling light distances, then surely your civilization has succeeded in developing fusion reactions. In fact, here is a source where with current technology we have succeeded in using fusion with net positive energy on a very small scale: https://www.iflscience.com/physics/nuclear-fusion-reactions-see-net-gain-energy/

This tech would enable you to generate anything from hydrogen up to Iron with no problem. After Iron is the real question.

To solve that problem we look to nuclear reactors. Even with today's tech, we can generate heavy elements in nuclear reactors. Russian pushing the boundary of Periodic Table The only real limit is cost. It is more expensive to make gold than to mine it. So the real question about the heavy elements is "how expensive is space travel". If it is really expensive, it might be cheaper to just make any element you need locally. Then the cost imbalance between mineral sources would affect local economics but there would be limited trading of raw materials:

A that point, it is perhaps likely that what is traded isn't raw materials, but more likely finished goods. OR perhaps someone familiar with isotopes might know of one that isn't available by fusion/nuclear reactors and would be valuable to have on hand. This could be a potential trade raw material.

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  • $\begingroup$ Why would isotopes not be available by nuclear physics? $\endgroup$ Aug 13, 2020 at 23:31
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    $\begingroup$ @GarrettMotzner: Not easy to produce in bulk by the reactions that practical fusion reactors use, whatever fusion tech works most easily. It seems plausible that this might be the case. Bombarding targets in particle accelerators to make arbitrary isotopes is much slower than fusion in a plasma. Making every element up to iron sounds ambitious, though. $\endgroup$ Aug 14, 2020 at 3:58
  • $\begingroup$ Maybe ambitious, but remember, if we can advance our own tech, we can get net positive energy from it. So if there were a couple of rare materials, then you use all your fusion energy plants and stop the process at the desired element. And remember, they have already solved the light speed issue which seems a lot more ambitious to me than generating materials. But who knows. I don't disagree it could be harder than I imagine. Just an idea. $\endgroup$
    – brian_ds
    Aug 14, 2020 at 13:02
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Heavy elements are rare everywhere due to their unlikely chance of creation in stellar processes/supernovae, especially radioactive ones.

Lanthanides/rare earths like Y, Yb are useful for superconductors.
Post transitions like Tl are usful in fission reactors. Transuranics like U235, Pu239 are useful in reactors and weapons.

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    $\begingroup$ If you're engaged in interstellar materials trade, I don't know how useful fission reactors will be, because you've clearly solved the energy problem. $\endgroup$
    – jdunlop
    Aug 12, 2020 at 19:30
  • $\begingroup$ @jdunlop you could trade some for dilithium crystals with pre-warp civilizations that arm in an arms race phase. By trading with both sides, you can escalate demand. If they are pre-fission, you could throw in schematics and even create the arms race in the first place if one didn't exist. $\endgroup$
    – Bohemian
    Aug 12, 2020 at 22:39
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    $\begingroup$ Sure, but if they're pre-fission, you don't need to haul fissionables from different solar systems, you can mine elsewhere in their solar system and trade in-system, leaving with low-mass, high-value items like cultural artifacts or magic warp crystals. (Also worth noting that in arms races here on earth, uranium shortages were not - after the initial purification hurdles - the thing limiting weapon creation, so aliens delivering a thousand tonnes of the stuff wouldn't make much difference.) $\endgroup$
    – jdunlop
    Aug 12, 2020 at 22:51
  • $\begingroup$ @jdunlop I would not discount fission so quickly. While it would probably not be a primary energy source, radioisotopes are still useful in things like RTGs (for when you need a small, compact, solid state power source that will last practically forever) or for small nuclear warheads. Of course, whether there is sufficient demand for those applications to warrant interstellar trade is another matter. $\endgroup$
    – BBeast
    Aug 13, 2020 at 2:53
  • $\begingroup$ And it is possible that fission reactors may be cheaper than their more advanced alternatives, in which case there would remain a demand for radioisotopes. $\endgroup$
    – BBeast
    Aug 13, 2020 at 3:22

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