An iron rain to starve a planet

I have a generic Earth like planet, same atmospheric composition, and the same oceanic composition. The crust of this planet is predominantly silicon based but plenty of iron, nickel and copper. Life started on this planet too and currently covers the entire surface, both above and below the waves.

My objective is to starve the planet of oxygen by removing 50% or more of the free atmospheric oxygen in less than 10 years. The preferred method for doing this is to drop geologically significant quantities of non-oxidized iron onto this planet for a few years in the form of iron dust. Where I get the iron from and how I deliver it to the planet's upper atmosphere is outside the scope of this question.

Can I realistically and dramatically drop the amount of oxygen by dumping powdered iron into the atmosphere and ocean? I've read that iron does a fantastic job of binding to oxygen so that's my preferred approach.

6 Answers
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Earth's atmosphere is $5times10^18$ kg. Nitrogen and Oxygen are the primary components and are approximately equal in mass. 20% of the atmosphere is oxygen by molecule, therefore about 20% of the mass of the atmosphere is oxygen, so there is $1times10^18$ kg of Oxygen that needs to be removed.

There are many oxides of iron, and these oxides form from two different oxidation states of iron atom, iron (II) and iron (III). Let's assume a 2:3 ratio of Fe:O in the iron oxides that we would form, based on the multiple oxidation states of iron that we would dump in the atmosphere. The mass of three oxygen atoms is about 48 g per mol; while two irons is about 111 g per mol. So to react with $1times10^18$ kg of Oxygen, we need at least $2times10^18$.

Dropping this mass of iron onto a planet will convert the gravitational potential energy that this mass had relative to the planet into kinetic energy, released on the planet's surface as it impacts. Assuming the iron comes from outside the planet's orbit, the gravitational potential energy is equal to the energy required to escape the planet's gravity. In kinetic energy terms, we would plug the mass along with the planet's escape velocity into
$$KE = frac12mv^2.$$ The escape velocity of Earth is 11.2 km/s; the total energy imparted to the Earth by the falling iron would be over $1times10^26$ J. Even if the iron came from an orbiting moon, the imparted energy would be at least 95% of this total (a moon that big can only be so close).

Going to the best wikipedia page in the world, we see that this is a problem. This is equivalent to about 20 years of solar energy striking the surface of the Earth, or 200 Chicxulub impacts. If you were going to do this over a millennia or two, that would be one thing, but if you want to do it in ten years, that is another.

How the atmosphere would be stripped

Adding enough iron to a planet to remove all the oxygen from the atmosphere in a 10 year period will be equivalent to hitting it with a dino-killing meteor every three weeks over that period. Since this kinetic energy is dissipated first in the uppermost parts of the atmosphere, the energy will be dissipated as heat in the upper parts of the atmosphere.

The total KE energy of the added iron would not be enough, by itself, to remove the entire atmosphere. But since the iron is added in dust form, it would be a safe assumption that all of its energy would be dissipated in the atmosphere, and most of that in the upper atmosphere.

The total energy addition, divided by 10 years and then by the surface area of the planet is

$$ frac1times10^26 text J3.2times10^8text scdot5.1times10^14text m^2 = 620 fractextWtextm^2.$$

At the top of the stratosphere, particle temperature is in the 270 K range, with a root mean square velocity around 500 m/s, meaning oxygen and nitrogen particles still need ~10.5 km/s of delta-v to escape.

But gaseous molecules are not all travelling the same speed. These particles' velocities are distributed according to the Maxwell distribution, which is itself a $chi^2$ distribution with three degrees of freedom. From the chart in the last link, we can see that the $chi^2$ value for $p=0.1$ with $k=3$ is 6.25 ($k$ is the degrees of freedom). This means that 10% of particles will have a 'value' of 6.25 in a Maxwell distributed set of particles. The mean of the distribution is $k$, which is three, and this is equivalent to the root mean square (rms) velocity of the particles. Thus, if the rms velocity of a group of particles is 3/6.25 = 0.48 times the escape velocity, then 10% of the particles will still be above escape velocity. This is a more likely explanation of how atmospheric escape would work.

In this case, the delta-v required to get 10% of the particles to escape is only 4.9 km/s, so the added iron is bringing 2 mols of atmosphere to this temperate, every second, over every square meter of Earth for 10 years.

Here is where simple math breaks down. The impact of heating in the last second will affect heating in the next second, there is some amount of heat loss through mixing, and other heat loss through radiation back into space. Furthermore, some particles will escape with very high velocities, carrying off a large amount of energy with them. But at a very simple level, the 2 mols per second per square meter is equivalent to $1times10^15$ mols of atmosphere heated until 10% is at escape velocity every second. If there were no mixing or heat loss, this will bring the entire atmosphere to 10% escape in 45 hours.

How much will escape I really can't estimate with accuracy, so I will back off my previous claim the the entire atmosphere will be stripped. A significant portion will be stripped, but there is not enough kinetic energy in the falling iron for it all to be stripped.

Does powdered iron oxidize, thus reducing the oxygen content of the air? Absolutely. But can you use that as a doomsday weapon? Maybe not.

According to the article you linked, 1g of elemental iron can remove 99.99% of the oxygen from 300 cubic cm of air. But how much is that really? 300 cubic cm of dry, room temperature air at sea level masses about a third of a gram, and is about 20% oxygen. So each gram of iron is only removing about 1/15th of a gram of oxygen from the air. Since elemental iron is so much denser than elemental oxygen, and the spaces where we use oxygen absorbers are fairly small, this isn't a problem.

But that means you'll need to drop three times the mass of Earth's atmosphere in iron filings in order to totally deoxygenate the planet. Since you specified 50% oxygen removed, you need 1.5x the mass. Earth's atmosphere masses 5.15 * 10^18 kilograms.

I daresay you can find far more efficient ways to kill a biosphere with eight billion billion kilograms (short scale) of iron. Like say dropping it from orbit in a big mass, or a lot of little masses. For scale, we're talking about around six million years of current industrial iron production levels.

I do not have an answer, but I do have a few considerations which would effect the feasibility. They are called, collectively, the Law of Unintended Consequences.

Rapid oxidation, of course, is called 'fire'.

The rusting (oxidation) of iron is exothermic.

I remember in high school burning steel wool in pure oxygen. Just a small quantity of steel wool produced a tremendous amount of heat. I also remember iron fillings spontaneously igniting in pure oxygen. I would suggest that the amount of iron fillings to remove the amount of oxygen that has to be removed through rapid oxidization, as calculated by other posters, in just ten years, would result in one colossal exothermic reaction that would literally set fire to the entire atmosphere.

The danger of self ignition in scrap cargoes is well known, especially
when there is contamination with cutting oil, cast iron borings and
organic flammable materials.

from Spontaneous combustion

Another factor would be any magnetic fields around the planet. Everyone, of course, is familiar with the demonstrations of sprinkling iron filings in a magnetic field. I have this vision of huge bands of iron filings forming in the lines of magnetic flux around the planet, suspended in the air.

A third factor, is lightning. Iron conducts, and establishing a huge conductive path around the planet would play havoc during any lightning storm. The iron fillings would be immediately vaporized. The result would be molten iron vapor, not iron filings. Could there even BE lightning? Or would there be continuous lightning? A corona?

Fourth, what WOULD be the effect of that much particulate iron in the atmosphere on the inhabitants? Blood carries oxygen by way of the iron contained in it. Would the body be able to 'breathe' and survive on the iron particles, instead of free oxygen which then gets bound to iron in the blood? Living mammals on earth have an excellent ability to extract oxygen from iron. Would this technique be counter-productive? Instead of oxygen starvation, would the mammals end up oxygen enriched?

Fifth, what happens to the oxidized iron fillings in the atmosphere? There is no real reason why they would all fall to the ground. They would be just like any other dust particles in a dust storm. Maybe rain would cleanse the atmosphere of them. But in the meantime? Would the iron dust block out the sun, cooling the planet, or would the iron dust absorb heat from the sun, warning it even more?

Last on my list, what would be the consequences of this much iron oxide on other materials? Iron oxide and aluminum, for instance. The aluminum 'robs' the iron oxide of its oxygen, forming aluminum oxide. The iron is recovered.

The Law of Unintended Consequences is an immutable law, that tends to but a kibosh into the best laid plans of humans.

Yes, and this is exactly what happened in Earth's past (although in reverse, kind of).

This is called the Great Oxygenation Event, and one of the most striking evidence of that in the geological record are the Banded Iron Formations. Here's one:

(From https://www.flickr.com/photos/jsjgeology/18602978984/)

What happened was the beginning of photosynthesis, cyanobacteria producing free oxygen to the atmosphere. But, this oxygenated atmosphere was not in equilibrium with the oceans that had soluble ferrous iron (Fe²⁺). The oxygen then bonded with the ferrous iron to produce insoluble ferric iron (Fe³⁺) causing the deposition of the banded iron sediments.

Eventually, the ocean ran out of ferrous iron, allowing the atmosphere to accumulate large amounts of free oxygen.

In your case, instead of injecting oxygen, you can inject ferrous iron to the oceans or atmospheres. This is easier scientifically because you don't need a source of metallic iron (Fe⁰) and ferrous iron (Fe²⁺) can be easily sourced from volcanoes or seafloor hydrothermal vents, etc.

Note that in earth's history this occurred over millions of years. If you want this to happen in 10 years you need a lot of iron and it has to be extremely reactive. But we all know (unfortunately) that it's possible to change the atmosphere's chemistry very fast.

You're going to have a bit of a problem here. There is approximately 10^21g of oxygen in Earth's atmosphere. Rust is Fe2O3. One mole of it contains 48g of oxygen and 112g of iron. Thus to get rid of all the atmospheric oxygen you'll need 2.33x10^21g of iron, half that (1.16x10^21g) to reduce it to 10%. To accomplish your objective you'll need a ball of iron 650km across!

As for how to accomplish it--make it into small pieces and drop it on the Earth from a fair distance out. It will hit fast enough to burn up on entry--it's going to rain down as rust, not as elemental iron.

The only question is whether you can accomplish this within your 10 year timeframe without burning the planet in the process.

It is plausible. Iron does bind oxygen. The banded iron formation are huge Precambrian rust deposits, as iron bound up oxygen in the newly oxygenating Earth. If iron could bind oxygen then it could now as well.

This scheme would work better the less ocean you have. In the ocean your scheme could backfire, at least initially. Iron is scarce in the open ocean and it is a limiting nutrient for open ocean photosynthesizes. Iron fertilization has been shown to boost photosynthesis in the open ocean and has been proposed as a method to sequester CO2 and remedy global warming. Your iron drop might increase oxygen.

The other thing about dropping iron over water is that it might sink to the bottom and be buried, never getting a chance to react with oxygen.

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