I am seeking information and ideas from all you clever people.

What I am trying to imagine is a chemical reaction that would change Earths atmosphere rapidly into something else which would be more suitable for an alien race. This would no doubt make the Earth's atmosphere unbreathable for humans thus creating a ready made plan for Aliens to populate.

Ideally this would be a weapon that would start some sort of a chemical chain reaction in Earths atmosphere. It might only be a small change. Small enough to kill off the humans and thus making it breathable for the aliens.

*Update: Am amazed and delighted at the responses. Such good ideas and suggestions. Thanks everyone who has taken the time to comment and answer. *

  • $\begingroup$ What is the chemistry of these aliens based on and what is dangerous for them? We cannot answer without this information. $\endgroup$
    – L.Dutch
    Jun 20, 2019 at 6:49
  • $\begingroup$ What counts as "rapid"? How about nudging human civilisation towards rapid industrialisation coupled with an economy fulled by short-term growth (and short-term decision-making) which gradually increases the amount of CO2 in the atmosphere until most of them die and the aliens can easily invade. $\endgroup$
    – thosphor
    Jun 20, 2019 at 14:31
  • $\begingroup$ As there is no indication of what atmosphere alien needs. I guess the solution is altering the atmosphere enought. Well simply go full speed in the atmosphere, the expending plasma will do the rest. If you need a more old atmosphere, like red lava ball, use your weapon to break the crust. You need to add some gaz? Launch an asteroid made of that element, impact will take care of the vaporisation for you. $\endgroup$ Jun 20, 2019 at 15:11
  • $\begingroup$ A few responses...I'm not sure what the chemistry of the aliens is based on. Whatever suits this narrative (weapon, destruction etc). I think i'm looking for something a little more 'rapid' than nudging civilisation towards its own destruction. Isn't this what we're doing ourelves anyway? Breaking the crust might be an idea... $\endgroup$ Jun 21, 2019 at 23:27

4 Answers 4


The raw problem here is simply how big the atmosphere is. Earth's atmosphere masses on the scale of 5 billion billion kilograms. Adding something to it on a planetary scale is much harder than it sounds.

A chemical "chain reaction" would involve transforming the elements already present in the atmosphere into a new configuration. The problem is that the elements in the atmosphere quite like the configuration they're in. Diatomic nitrogen (N2) is generally considered stable and inert, as is argon. Oxygen is highly reactive, but it needs something to react with, and the relatively high levels of oxygen in the atmosphere mean anything susceptible to it at normal temperatures will have been oxidized long before the aliens get involved.

At higher temperatures, oxygen will react to itself (creating ozone) or with nitrogen gas (creating nitric oxide and a variety of related compounds). Both of these are nasty, and quite toxic to humans, although they tend to be short-lived and so presumably wouldn't factor into any kind of permanent alien atmosphere. They also require a lot of energy input: nitrogen combustion occurs in the neighborhood of 2000 °C, and ozone is usually generated by corona discharge. Not something you want unless your alien terraforming (xenoforming?) device is supposed to double as a death ray.

One option would be to introduce a substantial volume of new material into the Earth's atmosphere. The problem, again, is mass. A largish meteor like the Chicxulub impactor might mass somewhere in the vicinity of 10% as much as the atmosphere. But then, that impactor is also thought to have caused the K-T extinction, one of the largest mass extinctions in Earth's history. Spreading out the impacts will reduce the effects on the Earth (although it still won't be much fun), but it will take longer.

The last, probably most practical approach is in situ resource utilization - basically, planting factories on Earth that use materials already present here to change our atmosphere. Of course, that's a long-term solution and rather noticeable for the poor saps already on the planet.

For further reading, you might be interested in ideas for terraforming Mars, essentially the same problem from the alien's point of view.


Ultimately there isn't a single small device or chemical that could set off a massive chain reaction on Earth. if there was chances are it would have already happen. The Earth's been hear a few years and any number of things have happened to it over that time, from asteroids to distant stars supernova bathing it massive amounts of gamma radiation to contracting a nasty skin irritation call humans.

Chain reactions usually form when a huge amount of energy is released by the initial reaction and that causes it to continue but it has diminishing returns. if there was a way to have a continual reaction go without the addition of new energy then it would break a few fundamental thermodynamical laws...

Pretty much everything i can think of that could deliver enough energy to have an effect on the atmosphere would have such a massive effect on the locality that it initially happens that its more of a conventional weapon then a chain reaction.


The planet is pretty happy the way it currently is, the Atmosphere has been almost exactly the same for a few million years (until humans began industry at least), same with the rest of the planet any effect that humans or aliens have disappears pretty quickly in the grand scheme of things and the planet returns to how it wants to be

The Chernobyl Accident had worldwide effects raising background radiation across almost all of Europe and touching most continents, but its chemical and biological global effect was minimal, and it didn't last, and even the local area now 20 years on is recovering. its economic and social effect was huge and lasts to this day, but that's it really apart from a small localised area


They could certainly release a bioengineered organism in the water that could ingest the water and anything it can find, breed like bunnies and then expel some chemical required in the alien atmosphere, but it would definitely take a while to work and even then it would not be a sure thing...


If your Aliens are oxygen breathers, but in a different range of humans tolerances.

Humans die at oxygen levels < 14%-16% from anoxia. Really fit people can tolerate lower levels, sick people need higher levels.

So if you aliens were fine down to 10% then they could dust the planet with powdered iron, particularly our cities. It would react quickly and form rust, binding nascent oxygen in the atmosphere. And, powdered aluminum, tin, and many other metals would work. There are also oxygen absorbing zeolytes that would bind oxygen until released by heating it or other chemical means.

I think it would take a lot of material like Iron, they could concentrate it on cities. The center of the dusting would have the lowest oxygen concentration, and it would rise toward the edges of the dusting.

And, another idea is are giant machines that liquify oxygen and store it. Takes huge amount of power, but areas around machine oxygen concentration goes to near zero and rises the further you get away from machines.

Another option is a catalytic reaction with Nitrogen, Carbon Dioxide, and Water Vapor, producing Hydrogen Cyanide, Nitric Oxide, and Diatomic Oxygen in equal proportions. The reaction would need to be fast acting since the first two are very reactive molecules. Areas where the catalyst where dropped would be dead zones.

  • 2
    $\begingroup$ The iron filing plan was examined here. It turns out, it's a lot of iron - enough that you could pack it into a ball and just Armageddon us to death instead (assuming you have a plan to stop Bruce Willis). $\endgroup$
    – Cadence
    Jun 20, 2019 at 7:10
  • $\begingroup$ @Cadence: Why is that a worse plan for the aliens? They get terraforming and pest extermination in one nice neat package... $\endgroup$
    – Joe Bloggs
    Jun 20, 2019 at 9:04
  • 1
    $\begingroup$ @JoeBloggs That's certainly true, but the phrasing of the question led me to believe they didn't want a traditional doomsday scenario. $\endgroup$
    – Cadence
    Jun 20, 2019 at 9:10
  • $\begingroup$ Great answer. Thanks. The Iron idea might work and the catalytic reaction. Will consider both and see where I get to. Like you said, just need to change the oxygen slightly (ie down to 10%) just to kill us humans off. $\endgroup$ Jun 21, 2019 at 23:37

Alien Dark Matter

OK, this one is going to be more on the edge of science fiction than real science.

Let's suppose that your aliens need an 'element X' in the environment to breathe, which is toxin for earth's life and can be easily made using their high-end tech, earth's oxygen, and sunlight.

This High-end tech is nothing but a "tiny alien dark matter particle" (non-natural and unstable), that when collides with an oxygen particle produces element X and 2 such particles.

O2 + dark particle -> (Element X) + (2 * dark particle)

Given that this process only requires energy from sunlight to happen, all your aliens need to do is release a single such particle into our atmosphere and simply wait for the power of 2 to take over the planet.

Note: The Element X, could be any known or unknown element.

  • $\begingroup$ Sounds great. I like the 'element x' idea. Like Blade Wraith said above, if it was easy it would already have happened. I may have to get creative with the chemistry. Thanks! $\endgroup$ Jun 21, 2019 at 23:40

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