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I was thinking about aliens and space colonization and what the probability is of uranium ores being found on other planets.

Would they be common? rare? medium? who knows? the cosmic abundance of uranium-238 is 1 to 1 trillion particles.

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    $\begingroup$ Do you mean Earthlike planets, with oceans, plate tectonics, and so on? Then almost certainly they would be found in similar abundance to Earth. Ores are generally formed by geothermal processes involving water, which dissolve & concentrate particular elements & compounds. $\endgroup$
    – jamesqf
    Commented Nov 26, 2021 at 17:14
  • $\begingroup$ maybe, but there's more uranium in the crust than the core. i think that uranium most likely came from meteors. $\endgroup$
    – user91668
    Commented Nov 26, 2021 at 18:36
  • $\begingroup$ "more uranium in the crust than the core", evidence? Doesn't matter where it originally came from, and what's the difference between meteors and planetesimals that accrete to form the planet? The point is that there's going to be some in the crust, and those geothermal processes will concentrate some of it. $\endgroup$
    – jamesqf
    Commented Nov 27, 2021 at 2:54

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the cosmic abundance of uranium-238 is 1 to 1 trillion particles.

It seems like very rare, but let's roll some numbers.

The mass of Earth is $5.9 \times 10^{24}$ kg. The molar mass of iron is 55, meaning that a mole of iron has a mass of 55 grams.

That means that, roughly speaking, Earth is made of $10^{26}$ moles of iron, if it was made of entirely iron. We can do that in a fist approximation, since the atomic weight ranges between 1 and 100, so doesn't really affect the order of magnitude of involved moles on this scale.

1 trillion is $10^{15}$, so this calls for about $10^{11}$ moles of uranium 238 on Earth, or $238 \times 10^{8}$ kg. That's quite a bunch, isn't it?

Long story short, once you star playing with big numbers, even very rare events, like the presence of uranium, become sensible. Of course, as the numbers show, it will be rare, but don't confuse rare with not present.

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  • $\begingroup$ I would think more important than present or not is whether it is accessible. You could have a great supply of U238 somewhere but if it's buried too deep (or worse, on an asteroid somewhere) it might as well not exist. $\endgroup$ Commented Nov 26, 2021 at 7:58
  • $\begingroup$ Except the case of "Generation ships" (in one form or another), space fairing is likely to require levels of energy large enough to blow out a small planet. This is not to say that one will start demolishing the planets at the destination after fissile minerals - that is to say that you'd better solve your energy problem before you leave towards your destination (I'm endlessly amused when being shown a SciFi movie with the characters sorting out some rather thin power cables falling on his head in case of a damage - take a look at the jumpstart cables and think carry only 1.2-2kW). $\endgroup$ Commented Nov 26, 2021 at 8:15
  • $\begingroup$ "That's quite a bunch, isn't it?" - nop, lol, u talking about 23 million tons, extracted from a whole planet, a whole planet Karl. A planet is a bunch, but 23 million tons isn't, lol. U asked we answer(TM) $\endgroup$
    – MolbOrg
    Commented Nov 26, 2021 at 10:46
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    $\begingroup$ @MolbOrg - 23 million tonnes is 8 million tonnes more than the theoretical extractable reserves of uranium on Earth, so yes, it really is quite a bunch. $\endgroup$
    – jdunlop
    Commented Nov 26, 2021 at 17:32
  • $\begingroup$ "the cosmic abundance of uranium-238 is 1 to 1 trillion particles", Yes, but 98-99% of the matter in the universe is either hydrogen or helium, not much of which is going to stick around on a terrestrial planet. $\endgroup$
    – jamesqf
    Commented Nov 27, 2021 at 2:58
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The most important question is "does the planet in question have abundant oxidizers?"

Uranium Oxide

Heavy metals, left to their own devices, tend to sink towards a planet's core. But uranium oxides, fluorides, and chlorides are all much less dense. So as long as, during the early stages of the planet's formation, these compounds form (or arrive in that form), you can expect uranium to be abundant in the crust to the same degree that it is in Earth's.

(This is less true ten billion years ago, or so, before there were enough supernovae, but I'm assuming a relatively contemporary planet.)

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Supernovae and planet age

The abundancy of Uranium and other heavy elements depends on the proximity of supernovae at the time your planet was formed from the proto-planetary disk. It also depends on the age and development stage of your planet. My answer is mainly a link,

https://world-nuclear.org/information-library/nuclear-fuel-cycle/uranium-resources/the-cosmic-origins-of-uranium.aspx

Uranium is a heavy substance tending to occupy the mantle and core. But there are reasons uranium will end up in a planet's crust, resulting in ore that can be harvested by your colonists. To get Uranium into the crust of your planet is a matter of time: geological age. Uranium tends to move into the crust because it prefers to combine with minerals in the crust rather than iron, according to the article: "the characteristic of uranium which makes it combine more readily with minerals in crustal rocks rather than iron-rich ones"

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