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I've read the other answers relating to this area (and discovered to my disappointment that my great ideas have been done before... hey ho). But I'd like to get the science right.

I have a nation of humans on a planet which they have arrived at through colonisation and a little light terraforming, but which basically is habitable for them. I want usable metal to be rare, so that the metals they have brought with them are basically all they have.

I understand that stars with planets generally need to have higher metallicity, but I'm talking about usable metal deposits. The humans have lost most of the technology which brought them there and are at a broadly mediaeval-ish level of development (hindered by lack of metals but boosted by having been taught skills they might not have discovered by themselves.)

Could an earth-like planet form without accessible traces of iron, gold, tin, copper, lead etc? Might some of these be more frequent than others?

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    $\begingroup$ How Earth-like does it need to be? Note that our Earth has a magnetic field created in part by its iron core. Without that, we'd be more vulnerable to radiation and possibly loss of atmosphere (although Venus seems to have plenty). $\endgroup$ – Brythan Oct 2 '17 at 20:59
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    $\begingroup$ An iron core wouldn't trouble me, unless that necessarily means iron in the crust. $\endgroup$ – TheSpidermonkey Oct 2 '17 at 21:09
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    $\begingroup$ core composition has no relation to crust composition per say. Our moon has little in the way of heavy metals because it lost them when colliding with our planet early on. $\endgroup$ – anon Oct 2 '17 at 21:22
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    $\begingroup$ To clarify: Earth does not have iron, tin and lead as such, at least not in places accessible to human technology; and while metallic copper does exist it is much rarer than gold. (Small pieces of metallic iron fall from the sky occasionally, but this is a negligible amount.) Metals are smelted from ore. Now, the word "ore" means "dirt from which it makes business sense to smelt metals"; with the correct economic incentives all dirt is ore. $\endgroup$ – AlexP Oct 2 '17 at 23:22
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    $\begingroup$ Please note that in astronomy the term "metal" means "any element except hydrogen or helium" so that, for example, oxygen or carbon would be classified as "metals" to an astronomer. The term "metallicity", which you reference in your question, refers to this definition of metal. So in principle it would be possible to have a star with high metallicity but no "iron, gold, tin, copper, lead etc" at all. (Though in practice there's no mechanism through which such a star could form.) $\endgroup$ – Nathaniel Oct 3 '17 at 5:00
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Could an earth-like planet form without accessible traces of iron

Without iron, you'd need a completely different biochemistry, and would in practice condemn the newcomers to a slow death. Iron is essential for hemoglobin synthesis and humans need to acquire it from food: from vegetables (most green leaves) or already concentrated in muscle tissue (by animals that still need to graze on iron-yielding vegetables). Similar considerations hold for copper, zinc, manganese, selenium and other trace metals.

Now the problem is that when you do have enough iron in the biosphere, iron oxyhydroxides begin to precipitate - all it takes is the appropriate environment (not at all uncommon) and voila, you get bog iron and can begin smelting. You would need some mechanism to prevent this; possibly some bacteria that sequester iron in metallorganic compounds that can not be refined without a lot of metal-based technology. Then most iron would find its way there, and you would need to harvest the bacteria to survive; at the same time, you could do with way less iron in the environment, and for both reasons there would not be iron(III) available to precipitate freely - and what little did, would be again eaten back by the bacteria.

Nutrients trade would be quite the enterprise on your planet - without careful husbandry of bacterial beds, people would suffer from all sorts of malnutrition syndromes.

(Now that I come to think of it, the bacteria might have been gengineered by the original colonists just to allow the planet to be settled, by concentrating/processing the required nutrients).

Let's do this

We start with a mostly silicate-carbon, metal-poor planet orbiting around a Population II (or the hypothetical Population III) star. It is uninhabitable due to unavailability of all except the lightest metals, but it sits smack in the middle of the Goldilocks zone of a suitable star, so terraforming is economically sound.

This is designed to become an agricultural/pastoral world. It exports, if anything, elaborated CHON. Technology will be well-nigh impossible (and this might actually be a desirable trait: whoever said, e.g., that the colonists were volunteers?)

What little dense elements the planet had (iron and nickel, essentially, together with any heavy metals from the Population III dust) has sunk towards the core, and is not practically reachable, but there's a nice asteroid belt not too far out. Ice comets are launched at the planet, while asteroids - they, too, metal-poor - are ground down to pebbles and dust, and the latter electromagnetically separated - in space, you can build the equivalent of an enormous mass spectrometer; particles with relatively high content of desirable elements are condensed again (sinterized?) and launched towards the planet. This goes on for a long time (self-replicating Von Neumann robots might come handy - of course they'd have to balance the need for metals and heavy elements to build themselves).

All the projectiles burn in the atmosphere, and water vapour and metallic ashes start floating down (metallogenic hits would be targeted towards the center of the continents to avoid losing metals to the seas). After many years, the surface is covered with a thin layer of metallic oxides, that rains force to seep downwards.

At that point the planet is "seeded" with algae, cyanobacteria and very basic (and sturdy) life-forms, that begin the transformation of the soil and the oxygenation of the atmosphere.

Other years pass by, and plants are seeded on the planet. These are much more aggressive and efficient in recycling the topsoil.

Water and mineral meteors will still be sent into the atmosphere: carbon dioxide, water and ammonia from the Oort cloud equivalent of this world will supply all the CHON we might ever want.

We start needing to sequestrate hydrogen; one (risky in the long-term) possibility is to stabilize it to methane (carbon is abundant thanks to carbonaceous chondrite meteors) and store in in undersea methane clathrate beds. Natural oil is another possibility, using biological Fischer-Tropsch processes to form oil reservoirs.

Several organisms are seeded, tailored to ensure a uniform spread in topsoil elements. Similar to worms, they would break down possibly dangerous concentrations of elements while aerating the soil (and there goes our bog iron).

Finally, there is oxygen enough to sustain Earth fauna, which is seeded from frozen ova by artificial robot wombs.

The whole "planet factory" could have been sent on a straight, high-acceleration trajectory towards the target planet, while a colonization ship follows. Such a scheme is presented in Robert J. Sawyer's Golden Fleece: the colonization ship spends the first four years, subjective time, going round and round the Solar System, accelerating to the speed of light. Then it sets off for a forty light-years travel to Colchis. There it will decelerate for another four years subjective time. The fifty years of the interstellar leg of the journey only take a few subjective days due to relativistic time-dilation, and the travelers will have experienced only a eight-year voyage, which is doable without suspended animation or complex generation ships.

Except that the whole scheme is a hoax. Colchis, at the beginning, is a uninhabitable planet whose probe images have been faked. The ship computer alters the flight plan so that the ship spends some additional subjective weeks, corresponding to thirty thousand years, whizzing around the Solar System at a much higher speed than officially planned. In this time, AI probes will land on Colchis and terraform it to match the fake images.

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  • $\begingroup$ I would give this answer multiple up votes if I could. $\endgroup$ – DPT Oct 2 '17 at 23:10
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    $\begingroup$ Many thanks for this really helpful answer. I'm now seeing the problem as not "can the planet have no iron?" but "how can the planet support terrestrial life (inc trees, grasses, animals) without usable iron?" The bacteria idea is ingenious but would create plot-related problems. Could it be as simple as they've lost the knowledge of smelting? The colony were transported in embryonic form and raised for a few generations by robots, which taught them essential survival skills until their stored energy ran out. It would be an elegant fit if the "mining and smelting robot" had malfunctioned. $\endgroup$ – TheSpidermonkey Oct 3 '17 at 7:32
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    $\begingroup$ @TheSpidermonkey - As with most "lost knowledge" scenarios, it's hard to pull just one block out of the wall. While I personally cannot smelt iron, knowledge that extraction of iron is possible would be sufficient to start working it out from base principles again. And unless there's a plot-specific reason (a la David Weber's Safehold series) to not include major technological teachings in your colony's upbringing, it's terminally foolish to cripple your own people that way. $\endgroup$ – jdunlop Oct 3 '17 at 16:39
  • $\begingroup$ @TheSpidermonkey, apart from not having the knowledge, one thing that would prevent them from smelting would be energy. Without easily accessible fossil fuel or biomass that can spared, they would not be able to make any metals no matter how much ore they had. $\endgroup$ – Kratz Oct 3 '17 at 21:08
  • $\begingroup$ @jdunlop - depends on the block, I think... going with bacteria that sequester iron, there may be iron deposits that look different from known ones (rusty red vs a bright yellow iron-rich bacteria slick, on earth). If the bacteria on this planet makes some (major) deposits, say, bright blue (not found on earth) it might not be recognized as iron ore. Loosing a second bit, like the machine that tests for minerals or how to find ores without it, is then the end. Prior knowledge might leave them looking for what isn't there (or not there in useful amounts) instead of finding what is there. $\endgroup$ – Megha Oct 4 '17 at 2:20
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There are four mechanisms I can think of that you can go to for this; Impoverished Crust, Quiet Land Hungry Sea, Mineralisation Skew, Excessive Traces, so here's the short notes for each mechanism:

  1. Impoverished Crust, the world as a whole is extremely rich in metals but they're all deep in the Core, the Mantle and Crust are almost purely composed of light metal Silicates, you get a geologically active world, possibly more so than Earth even. There's enough traces of heavy elements, including metals to support a, slightly spare, biosphere but Metallurgy is out.

  2. Quiet Land Hungry Sea, on Earth with the exception of Limonite and similar Hydroxide Ores, most ore bodies form in the Oceans due to either Oxidation of chemically dissolved elements in seawater or anaerobic processes that form Sulfurous compounds in the seafloor oozes. The metals in these ores have come from the land, they're eroded from primary igneous minerals and washed out to sea chemically dissolved in river water. These ores are then Uplifted by geological processes to the point where they're accessible. On a world without major tectonic activity, like Larry Niven's Destiny, elements are still washed into the sea and mineralise, but there are few places where the tectonic mechanisms to return those elements to land are active.

  3. Mineralisation Skew, without exception, that I can think of, commercial Ores on Earth are limited to the following forms, Oxides, Sulfur compounds, Hydroxides, or Carbonates, these all have one thing in common; they're relatively easy to decompose through heat and/or reduce with Carbon leaving elemental metal behind. In a world where Silicates dominate metallic geochemistry thermal smelting does not work because Silicate minerals are basically fireproof. This will also complicate biological uptake of certain elements as primary minerals in the Regolith will be much harder to break down.

  4. Excessive Traces, ultimately what elements, including metals, are accessible in a particular planet's crust isn't really about what's there but what form it is in and what extraction techniques you are able to employ. Iron forms several "pure" compounds that can be used as ores, Pyrite, Hematite, and Magnetite, but it also forms a host of "impure" ones including, Ilmenite a Titanium-Iron oxide and Chalcopyrite and Bornite which are actually used commercially as Copper ores. Ilmenite has a far higher smelting temperature and requires specialised equipment to extract the Iron because of the Titanium "contamination", similarly Chalcopyrite and Bornite make poor ores of either Iron or Copper in basic thermal smelters due to contamination with the other metal, trace Iron in Copper, or Copper in Iron, has the effect of making the desired metal brittle and unworkable. In this way relatively high levels of rare elements like Titanium and Tungsten would "pollute" ores of metals that can be extracted using simple, "low-tech" methods.

Please Note in all cases Aluminium will be present in vast quantities but not accessible with Medieval technologies. Gold may also be available in quantity under any of the proposed scenarios as it doesn't participate in rockforming mineralisation so it is relatively mobile and it sticks together when atoms come into contact with each other, in short Gold concentrates wherever it occurs.

If you want/need elaborations let me know.

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Could an earth-like planet form without accessible traces of iron, gold, tin, copper, lead etc?

Possibly. However, it would be hard to provide an environment suited for living organisms, as many biochemical processes require the presence of trace minerals, e.g. iron is needed for hemoglobin. If you wanted to make that environment habitable, you'd need to introduce a way of replenishing access to said trace minerals.

Could an earth-like planet form without human access to "industrially" usable amounts of iron, gold, tin, copper, lead etc?

Sure. Multiple possibilities:

  • Even out the concentration of metal (ions) over the whole surface. Organisms only need trace amounts, so they should be fine for the most part. Could be formed by having an large ocean evaporate over billions of years.

  • Have few ore deposits only available in hostile environments unsuitable for prolonged human activity, e.g. near active volcanoes, under deserts or in rocky mountain ranges. Maybe those ore deposit locations are known, but to use them more advanced technologies would be required (including a way to feed all miners etc.). These deposits could additionally be spread out, so if you'd need multiple different metals, you'd have to support multiple mining outposts a large distance apart.

  • Just have a very small amount of land mass above the ocean level. Mining operations get a lot harder if you have to worry about dozens of meters of water above your miners heads.

  • If you can't limit access to metals themselves, limit access to fuels: To smelt metals from ores, you need vast amounts of heat - which in turn requires access to fuels. The easiest accessible fuels available on earth are wood and coal - but those might not be present on the target planet (maybe someone forgot to include trees in the terraforming process? And coal needs a lot of time and organic matter to form, so there might not be any if living organisms were only recently introduced to the planet).

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  • $\begingroup$ Re "These deposits could additionally be spread out": Look at copper and tin (for making bronze, which is far easier than iron for an early civilization, other than the trade problem) for a real-world analogue. $\endgroup$ – o11c Oct 4 '17 at 15:28
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You could model your planet on the "dwarf planet": Ceres.

Ceres is more similar to the terrestrial planets (Mercury, Venus, Earth and Mars) than its asteroid neighbors, but it is much less dense. One of the similarities is a layered interior, but Ceres' layers aren't as clearly defined. Ceres probably has a solid core and a mantle made of water ice. In fact, Ceres could be composed of as much as 25 percent water. If that is correct, Ceres has more water than Earth does. Ceres' crust is rocky and dusty with large salt deposits. The salts on Ceres aren't like table salt (sodium chloride), but instead are made of different minerals like magnesium sulfate.

Ceres low density is because if it has a metallic core (like Mars or Earth) it is much smaller than those of the big league planets. Ceres might be rock (siliceous materials like the earth crust) at the core. As noted in the excerpt there is proportionately more water and mineral salts than on earth. I see in some articles mention of graphite on the surface as well.

You could scale up a Ceres-like body (perhaps formed of an agglomeration of Cereses?) to make a world of the size and gravity you wish for your story. An agglomeration of compositor asteroids would also lead to the possibility of one part of your planet having a very different composition than the rest.

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Sure, there are no guarantees in crust composition of planets.

However, one way to certainly reduce composition is to just say it was mined out by another race millennia's ago depriving it of various useful minerals.

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    $\begingroup$ Thanks for the answer! I'm keen to avoid the "already mined out" solution, for plot-related reasons. But if there can be a habitable earth-like world with a breathable atmosphere that just happens not to have usable amounts of (to us) common metals, then that works great for me. $\endgroup$ – TheSpidermonkey Oct 2 '17 at 20:58
  • $\begingroup$ Like I said "there are no guarantees in crust composition of planets" $\endgroup$ – anon Oct 2 '17 at 20:59
  • $\begingroup$ There's no guarantees of almost anything. What about probabilities? Any citations (or commonly known data)? $\endgroup$ – jpmc26 Oct 2 '17 at 22:23
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    $\begingroup$ We have just recently been able to observe planetoids of other systems. From what we have observed the outcomes can be wild. A lot depends on the matter present at the systems birth as well as behavior of coalesced objects during its genesis. Im sure someone has attempted to try and shoot some probabilities but realistically they cant just yet. You also have to consider what arrives and is available to arrive after the crust solidifies as that is what is most likely to be on the crust in terms of heavy metals. $\endgroup$ – anon Oct 2 '17 at 22:36
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It turns out that the Earth is actually low in gold and silver, compared to the rest of the solar system. The type of magnesium found on Earth is also different (tends to be heavier) than magnesium found in the rest of the solar system.

So we know that a planet can form with less of some metals than it should have and that the resulting planet can be habitable by us (in some circumstances).

The question is then how does that happen... The short answer is, scientists think the gold, silver, and lighter magnesium evaporated1,2.

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