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Let's start with a little backstory--our planet underwent an impact-coalescence cycle only once, 4.5 billion years ago, when a Mars-sized object named Theia destroyed the infant Earth in a glancing blow. Since then, Earth has been orbited by a ball of rock 2,159 miles wide from a distance of 238,900 miles. It also has the following:

  • Four parts per billion of gold
  • 1/30 as much palladium as gold
  • 1.5 parts per million of tungsten
  • Germanium rated as the 32nd most common element
  • Phosphorus rated as the 11th most common element in the crust

In this alternate Earth, the planet is orbited by a moon 3,274 miles wide, with a rocky crust but a mantle and core of pure iron, from a distance of about 665,000 miles. It also has a higher concentration of the listed elements--so much higher that gold is now 75 parts per billion, as common as silver. Would this sort of specific detail be possible if Earth were hit once by a Theia-like object?

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  • $\begingroup$ "Would this sort of specific detail be possible if Earth were hit once by a Theia-like object?" I'm confused by your question, since we already know what what happened when Earth was hit by Theia. (Maybe you're being to clever with your backstory. Keep it simple, so we don't get confused.) $\endgroup$ – RonJohn Sep 9 '18 at 4:29
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    $\begingroup$ By "Richer" I assume you mean simply more abundant in precious metals and are ignoreing any inflationary affects? $\endgroup$ – Ummdustry Sep 9 '18 at 7:29
  • $\begingroup$ I am not sure what this question is actually asking. Is it about whether this impact scenario is realistic? Or does it ask about the economic consequences of having a mineral-rich moon? $\endgroup$ – Philipp Sep 9 '18 at 12:20
  • $\begingroup$ @Philipp "Would this sort of specific detail be possible if Earth were hit once by a Theia-like object?" If you read the entire question you'd see that the question is stated very clearly. $\endgroup$ – The Great Duck Sep 9 '18 at 15:27
  • $\begingroup$ You say "It also has the following." To be 100% clear, what is "it?" The Earth or the moon? Please edit your question with the clarification. $\endgroup$ – JBH Sep 9 '18 at 16:06
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First,

with a rocky crust but a mantle and core of pure iron

If both the mantle and core are pure iron, what differentiates them to mantle and core?


Now - to your question:

Would this sort of specific detail be possible if Earth were hit once by a Theia-like object?

No. Earth getting hit once by a Theia-like object would cause just about the entire planet to melt and mix. This is not what you want. This mixing will cause the now enriched-mantle top layer to come into contact and mix with the liquid iron core, before separating again. This mixing will cause the liquid iron to sequester all the precious metals (gold etc) you delivered by the impact. My previous answer to your question explains this process in detail:

If Earth's Core had ALL Of the Heavy Metals

How can this work?

The most likely way to make this work is by supercharging the "late veener". You want to avoid melting the entire rocky mantle. For example something like this:

enter image description here

This essentially causes the solid rocky mantle to be an insulator or a barrier that impedes mixing between the newly-enriched now-liquid mantle layer, and the iron core (whether solid, liquid, or both; doesn't matter now).

I'm not an impact physicist, but my gut feeling suggests this will require either a small Theia-like object so the total amount of energy delivered does not melt the entire planet, or several smaller objects over a longer period of time, possibly million of years. This will allow the precious metals to mix in the mantle, without being sequestered in the core.

And whatever you do - have plenty of oxygen around!

Oxygen will cause the core to be smaller, and it also help keep the precious metals in the rocky mantle.

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  • $\begingroup$ mantle generally by definition means liquid $\endgroup$ – The Great Duck Sep 9 '18 at 15:30
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    $\begingroup$ @TheGreatDuck no, this is one of the greatest misconceptions about earth’s mantle. The mantle is mostly solid. It has very small partially molten zones, but they are volumetrically negligible. $\endgroup$ – Gimelist Sep 9 '18 at 17:20
  • $\begingroup$ Fair enough but the point remains that the asker is likely operating under that misconception. $\endgroup$ – The Great Duck Sep 9 '18 at 17:48
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I don't think a planet sized object can do it; both Earth and the object don't just melt, too much of it actually vaporizes (Not in the Star Trek sense, but the scientific sense; being heated beyond the liquid state (melting), beyond boiling, to the gaseous state: vapor; like steam for water).

The amount of convection and random movement in this hot state will make it impossible to separate the components of the striking object and the parts of the Earth's crust and mantle that vaporized. Some of it settles back on Earth, some orbits the Earth.

I do think it is possible, if your mass and distance and orbital periods are all consistent, for the moon to be of a different mass and in a different orbit around the Earth.

Your better bet, to increase the heavy metals on Earth, would be a period of heavy bombardment by smaller meteoric impacts that do NOT penetrate too deeply. The Chicxulub meteor that wiped out most dinosaurs 65M years ago left a noticeable planet-wide layer of iridium behind, but the impact crater itself was only 93 miles wide and 12 miles deep.

Now supernova are required to produce heavy metals like gold and palladium, and when they do, they produce them in massive quantities and spray them into the universe. It is plausible science that some stray rocks made entirely of such metals from a nearby supernova could impact a planet like earth, vaporize like the Chicxulub meteor, and spread those metals uniformly on the Earth. You could put the timeline anywhere, even billions of years in the past.

In the link, you can read about the Multiple Impact Theory; many other impact craters occurred around the world at nearly the same time as Chicxulub. So another possible scenario is that a heavy metal asteroid was gravitationally captured by the sun and was in its own unusual orbit as a result. Since it wasn't in sync or in the plane with the rest of the debris around the sun, eventually a collision with some other large rock broke it up and the pieces ended up on a collision course with Earth; but (like the Shoemaker Levy asteroid) broken up into a 'string of pearls' of various sizes, none of them particularly catastrophic, that basically sprayed the Earth with heavy metals over the course of a few years. That would lead to a less uniform distribution of heavy metals; you'd have regions tens or hundreds of miles with high concentrations, while other regions had lower concentrations. Of course then you also have continental drift, earthquakes and other forms of mixing so the regions would not be just circles on the map; and water and rivers could move and even further concentrate the heavy metals far from their deposit areas. That is why we pan for gold in streams and rivers.

I'd suggest some kind of bombardment like this. This would better explain a differential between the two bodies. The bombardment could occur on the planet (like Chicxulub) or on the moon: From its cratered appearance it has clearly taken some major hits for us many times in the past.

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The answer to this question is alluded to in the answer by Gimelist. The key is that a very large impact delivers enough energy to cause a core-forming event during which the vast majority of the iron in the projectile (and any iron in the planet's mantle) sink into the core. During this process most iron-loving "siderophile" elements follow the iron and are thus removed from the mantle and crust. From the ones that you mention, Gold, Palladium and Tungsten are siderophile (see here).

All of the mineable gold on Earth was delivered in the "late veneer". The late veneer refers to material delivered to Earth after the last core-forming event in the form of impacts from objects small enough not to trigger a core-formation event (think, smaller than the Moon or so; Theia was likely ten times more massive). Earth's veneer was very small, comprising only about 0.5% of Earth's mass. It's easy to imagine a much higher proportion of an Earth-like planet's mass coming as a late veneer -- simulations indeed find that this can happen and that the amount of veneer is inversely correlated with the timing of the moon-forming impact (earlier Moon formation implies more veneer; see this paper of which I am a coauthor).

So, if the Moon formed earlier the Earth would have more gold (and other siderophile elements)!

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