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I'm writing about people living in space who can mine asteroids and I'm wondering what minerals or other resources, if any, they might need access to larger planets for. Since planets and asteroids ultimately condensed from the same cosmic debris, I assume they would contain the same basic elements: iron, carbon, magnesium, aluminium, gold, nitrogen, etc. are all found in asteroids so you wouldn't generally need to visit a planet to find them.

So that said, what substances not found in asteroids (especially ones with a known industrial use) are synthesized in large quantities by geological or biological processes and might therefore make planets valuable to space-dwellers?

Earth's petroleum and Titan's hydrocarbon lakes come to mind.

(Assume these people have access to technology like antigravity or space elevators or something else that makes it relatively cheap to lift resources out of a gravity well.)

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    $\begingroup$ Would "Biological products" answer the need, without having to go list a bunch like manure, ambergris, and nutmeg ? $\endgroup$
    – Criggie
    Commented Jun 28, 2020 at 7:52
  • $\begingroup$ I don't know if it is still considered true, but back in the stone age when I was in school, we were taught that an element's scarcity and its atomic weight are inversely related. The bigger the nucleus, the scarcer the element. If that is still true, then your asteroid miners would probably trade with Earth for the radioactive elements at the bottom of the periodic table. Those element's probably exist out in the asteroids but might be easier to acquire en-mass from large worlds with breathable atmospheres. $\endgroup$
    – Henry Taylor
    Commented Jun 28, 2020 at 9:01
  • $\begingroup$ @HenryTaylor the problem is on planets those heavy elements tend to sink to the center where it is basically impossible to extract them, while on asteroids that are easy to get to. $\endgroup$
    – John
    Commented Jun 28, 2020 at 13:14
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    $\begingroup$ Not to be discounted - The water cycle on Earth is responsible for collecting minerals into veins where they can be more easily collected en masse. (as well as being involved in making more complex minerals) In an asteroid, desirable minerals are more likely to be uniformly dissolved in the rest of the asteroid. $\endgroup$
    – notovny
    Commented Jun 28, 2020 at 13:38
  • $\begingroup$ @notovny the tv show opal hunters mentions this as the miners are after the rare black opals and the silica collects in rocks as the water streams carry them down to settle in layers. $\endgroup$
    – user69935
    Commented Jun 28, 2020 at 13:52

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Minerals of biological origin.

Chalk, various types of limestone, marble, coal, fossils, petrified wood, amber, guano.

It is surprisingly hard to find examples of non-biological minerals that are unique to planets. Early solar system conditions allow of the formation of minerals we would not think would be able ot form at a casual look at space conditions. Asteroids rich in hydrates, diamonds, and oxides all exist, minerals that we think of requiring planetary conditions.

Source asteroid diamonds

Source asteroid oxides

Source asteroid hydrates.

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    $\begingroup$ I'm not sure that guano is a mineral (maybe after a few millennia of compression it would petrify into something like coprolite), but otherwise good examples. $\endgroup$
    – RonJohn
    Commented Jun 29, 2020 at 0:56
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    $\begingroup$ guano actually gets compressed into a solid plastic like consistency almost like amber as it ages. We used to use a sample of it as a brain teaser for mineralogy students. $\endgroup$
    – John
    Commented Jun 29, 2020 at 12:22
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Hydrogen and Helium

Light elements are abundant in universe, but are hard to remain in small gravity bodies.

However, gas giants Saturn and Jupiter are the obvious sources in Solar System.

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    $\begingroup$ Hydrogen may be on astroids in water ice or other molecules; I feel Helium is the better example. $\endgroup$
    – Mark
    Commented Jun 28, 2020 at 20:35
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    $\begingroup$ Why would these miners need helium? They can fill party balloons with hydrogen. Which is more exciting because there are birthday candles so when you do squeaky breath you can also blow flames! $\endgroup$
    – Willk
    Commented Jun 28, 2020 at 20:49
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    $\begingroup$ @Mark I have no idea about the OP story, but hydrogen as fuel to fusion nuclear reactors spread in thousands of space colonies, more as propelent to move ships of different sizes may make the amounts of hydrogen in small bodies a bit few. Plus, Saturn has gravity similar to Earth and looks like perfect to one Lando Carlissan's Cloud City. Maybe even witha a Theme Park too, Saturn rings are perfect to mimetize that asteroid field of Hoth. $\endgroup$
    – Rodolfo Penteado
    Commented Jun 28, 2020 at 21:40
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    $\begingroup$ @Willk Liquid helium cooling for superconducting magnets. Your local MRI machine, for example. $\endgroup$
    – Loren Pechtel
    Commented Jun 28, 2020 at 22:27
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    $\begingroup$ Helium is used in coolant systems $\endgroup$
    – Thorne
    Commented Jun 28, 2020 at 22:48
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Concentrated anything.

Asteroids have everything mixed. Eventually, heavier minerals are deeper inside, if the asteroid has long enough molten past. They lack atmosphere and hydrosphere that can selectively dissolve, transport and precipitate minerals.

Volatiles.

Water, amonia, gases of any kind. Evaporated long ago. A tiny amounts may be found trapped in the crust. Comets are better targets for these.

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Concentrated uranium & thorium ores.

Obviously, rocky asteroids contain uranium and thorium as well, but only in very low concentrations. One might think that the density of such substances would result in them being trapped in planetary cores--and to some extent, they are--but the chemistry of uranium and thorium results in their compounds being preferentially concentrated in rocky crusts, which makes mining differentiated bodies like planets (or at least large spherical asteroids / minor planets) for them much easier than sifting through undifferentiated asteroidal material.

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    $\begingroup$ Specifically, any elements listed as lithophilic in the Goldschmidt classification tend to be enriched in crust of differentiated planets. This includes uranium and thorium, and indeed all naturally occurring lanthanides and actinides. $\endgroup$
    – Ilmari Karonen
    Commented Jun 30, 2020 at 8:19
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The most obvious ones are those made by living beings, but there are also those created in the presence of liquid water (like the ones they identified on Mars) or atmosphere (whether because of free oxygen, or because of the other gases in a reducing atmosphere, like that of every other planet in the solar system, or Earth's before blue-green algae).

There's also possibility that gravity changes the formation of the minerals. Usually this would be in the direction of making them more irregular, but it might also make them more dense, like diamonds.

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Ores. Most of the things we mine have been concentrated by hydrothermal or long-ago biological processes, which is the only reason they can be mined economically.

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Anything that needs high heat and pressure to form. Diamonds are the standard example but others exist.

Or even granite. It better be pretty cheap to lift, but maybe granite tile is all the rage amongst the wealthy.

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Every mineral that needs liquid water to form, i.e. minerals formed in hydro-thermal processes. Since small cosmic bodies such as asteroids lack liquid water and hot cores, hydrothermal processes are very unlikely to occur.

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I'd believe that you shouldn't be able to find any mineral or rock that must have some kind of differentiation in their genesis, since most asteroids don't have the temperature or the time or the mechanisms to do such processes. Their composition is pretty much always iron-niquel. The rocks I'm talking about are things like granites, andesites, or acid rocks.

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Basically any material that was build up by our oceans. So again lime(stone), any form of salt deposits (being regular NaCl "cooking salt" AND other salts used for solid fertilizers)

Then stuff that formed under heavy compression deep below earths crust - like diamonds and other half / full jewels - smaller asteroids simply lack the pressure and conditions to allow them to build up

But also magmatic material up to granite and similar like diorite

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Simple sedimentary or metamorphic rock. Seriously.

Asteroids don't have the flowing water needed to form sediments, nor do they have the tectonic activity needed to compress and heat rocks to make metamorphic ones. If you want some nice sandstone for your fancy space hotel, you'll need to import it from Earth or Mars. It might be possible to manufacture it, but I expect that would need specialised machinery.

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Plutonium and product only obtainable thru fission

If you're talking about elements(as opposed to molecules), then Plutonium can naturally occurs on planets (see the Oklo Natural nuclear fission reactor) but not in asteroid.

Note that even inside the Oklo mine, you can not currently find Plutonium (it decayed long ago), but your civilisation can certainly find such a natural reactor still active somewhere and providing large quantities of Plutonium or ruthenium 100.

With similar phénomenon, you can also get Tritium, usefull for nuclear fusion

Of course, our human civilisation can obtain those element using artificial controled fission. It's up to you to imagine why this would be unpractical in your universe.

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Agricultural products

Sure, you can grow small amounts of food in orbital growhouses and hydroponics. But planets provide you with plenty of space for large-scale farming, forestry and animal husbandry. When launch costs aren't too expensive in your world, then planetary agriculture might be far more economical than orbital agriculture.

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Bitcoin

Because you can fit more server farms on a planet than on an asteroid. Also the Deep Space Network has very high latency, you get better internet on Earth's surface.

Drugs

You can't grow weed and magic mushrooms on space rocks.

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    $\begingroup$ Also, planets with atmosphere and liquid water are better for cooling your mining farm. Due convection. $\endgroup$
    – user28434
    Commented Jun 30, 2020 at 9:18
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    $\begingroup$ Nope! In the end what limit the cooling of a body is its surface area. And for a given quantity of rock, you'll get much more surface by splitting it in small, irregular shapped asteroid rather than 1 big spherical planet $\endgroup$
    – Madlozoz
    Commented Jul 2, 2020 at 9:45

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