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Alien AI DoomFleet has a mission: Destroy threats to their creator. They do this by detecting the signature of FTL drives, going to the system, and blowing everything up, up to and including the homeworld.

For thematic purposes, I want the worlds to be stripped of their atmosphere. I'm not sure where to even start with this. "Simply" bombarding with explosives, even anti-matter explosives, seems like it would take far too long to be worthwhile.

So my question is: How do you go about removing the atmosphere from a planet? I'm not particularly interested in what would happen afterward, as that is pretty well discussed in this question/answer - I'm looking specifically for mechanisms to remove the atmosphere. Preferably fairly quickly.

Notes: Time, while not extremely restricted, is not unlimited. We can't necessarily park something in orbit and wait several years. There's other planets to be exterminated, as the DoomFleet's FTL is relatively slow. Additionally, we want to strip the atmosphere and be done with it. We're eliminating threats, not establishing beachheads, so we don't want to worry about maintenance and the like.

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One possibility is to heat the surface. This is "comparatively" easy since the bad guys could for example deploy an enormous solar sail (the term is apparently mega-statite) and use it to cook off the planet. It would double up as an extermination weapon, rendering the surface uninhabitable.

Exciting and focusing some kind of coronal mass ejection to the extent of stripping the atmosphere away is also efficient, provided these Berserkers - or Amadiros? - have the technology to pull it off. It would probably be difficult to strip off all of the atmosphere in a single go, but nobody says that once overwhelmed the system's defenses - they should do that in any case - they can't install themselves near the sun and start taking pot shots at the planet. This is the strategy preferred by the Ekhat of The Course of Empire.

Another relatively low tech possibility, with the advantage of being very hard to defend against and the disadvantage of only working against oxygen-nitrogen atmospheres (and requiring terrifying resources), could be to bombard the planet with magnesium pellets. The problem is of course to procure the magnesium in the first place: that much metal (the mass of Earth atmosphere is 5.1480×1018 kg) would be equivalent to almost a tenth of the maximum estimated mass for the Chicxulub impactor.

The DoomFleet would need to mine magnesium from somewhere and bring it along from system to system. Then the reservoirs could be placed in space in such positions and with carefully adjusted speeds to hit on the victim planet from all directions, at the same time.

The first pellets would burn violently in oxygen, forming magnesium oxide powder and raising the atmosphere's temperature enough to allow further reaction of magnesium with nitrogen (actually this wouldn't even be needed - the reentry heat would suffice to that). The resulting magnesium nitride is solid, but it would quickly react with water vapour from the now boiling surface of the oceans to form magnesium hydroxyde and ammonia, which would mostly dissolve in the remaining water while the dissolved oxygen would get outgassed (completely depleting the oceanic oxygen would take a long time). A large part of the available oxygen would be consumed by the fires engulfing the planet, though - forests, cities, all kinds of organic matter.

After a comparatively short time, the atmosphere would be replaced by a boiling fog of low-pressure water vapour and ammonia, and the planet would see its albedo significantly increased. I expect this to lead to a rapid cooling of the atmosphere (and the surface). After that, without the greenhouse effect from the atmosphere, the surface cools down to about -18 °C and we are left with a very thin, sterile atmosphere - carbon dioxide, ammonia, nitrogen, water vapour, argon - on a frozen surface. The remaining organic matter would capture what little oxygen still seeped from the oceans.

It's unlikely that any life form could survive the firestorms, the nuclear winter, the change in the environment and the subsequent onslaught of UV radiation after the disappearance of an ozone layer.

However, a more efficient way of destroying the planet would be to let the atmosphere alone (directly) and locate instead a suitable asteroid (or more than one) to deorbit and smash into the target homeworld. It would take the same energy as to move a comparable mass of magnesium metal. This is the preferred tactics of the Achuultani of Empire from the Ashes.

Now that I've thought on this a little more, so large a quantity of almost anything arriving at several kilometers per second would either burn in the atmosphere or contribute to its heating by friction, and make the planet uninhabitable no matter what.

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    $\begingroup$ That's a horrible abuse on chemistry! Remind me to never let you near a chemistry set (especially a planetary scale one)! $\endgroup$ – Cort Ammon - Reinstate Monica Mar 19 '17 at 20:43
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    $\begingroup$ I do prefer asteroid impactors :-). But seriously, my chemistry lessons are years in the past, so I might have easily overlooked something. Actually, I certainly did; I might easily have overlooked something crucial. Do not quote me for your atmosphere-stripping needs. $\endgroup$ – LSerni Mar 19 '17 at 21:12
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    $\begingroup$ Can't say I agree with the chemistry, but the solar flare idea is great. I'd revise it to a Coronal Mass Ejection though. $\endgroup$ – Joe Kissling Mar 19 '17 at 21:47
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    $\begingroup$ That's an insane amount of magnesium. Not really viable unless you happen to have the capability to mine several dozen planets worth of it. Even then, if FTL travel in this universe is even slightly expensive, it would be uneconomical to move that much stuff. $\endgroup$ – Gryphon - Reinstate Monica Mar 20 '17 at 2:33
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    $\begingroup$ Planetary Impactor levels of magnesium makes me giggle far too much. The CME seems like a great idea - Would it be possible to simulate one without sundiving, or would that be even more energy/technology intensive? $\endgroup$ – Andon Mar 20 '17 at 3:39
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If your fleet has a very powerful anti-gravity device, simply suspend the planet's gravity for long enough for much of the atmosphere to exit the planet's gravity. If your fleet doesn't have very long, you could drop a device to do this onto the planet and leave. If you don't want somebody finding it, give it a self-destruct after the atmosphere is gone.

If you don't have anti-gravity, you may have to do this on a gas specific basis. I'm going to assume an earth like atmosphere, so you have mainly nitrogen and oxygen to deal with. Nitrogen could be destroyed using a biologically engineered nitrogen fixing bacteria to do this at an extremely fast speed. You would have to leave this on the planet because even with an extremely effective bacterium this would take a while (years, not millennium though). Next the oxygen. It's really hard to get rid of oxygen. The boiling point is around -180 degrees Celsius, so that's out. The best way I could come up with is to have your bio-engineered bacteria also somehow react the oxygen into a solid/liquid compound that would then become common on the planet. Iron oxide would make your planet look like mars, with red dirt and sand. It would likely have to mine this substance, so it would be an interesting organism. Maybe include it attacking people/developing sentience/something cool as a plot point?

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    $\begingroup$ While the anti-gravity would take far too long for the scope of the story, and I'm pretty sure I won't use bio-plagues, I love the concept of a bio-plague since it's essentially fire and forget. Also terrifying. $\endgroup$ – Andon Mar 20 '17 at 3:35
  • $\begingroup$ Thanks for the upvote (that was you, right)! The bio-plague was rather inspired, if I do say so myself. I love the idea of it outgrowing it's original purpose post-atmosphere and doing something weird. $\endgroup$ – Gryphon - Reinstate Monica Mar 20 '17 at 3:43
  • $\begingroup$ Please tell me why you downvoted. I would like to improve this answer if possible. $\endgroup$ – Gryphon - Reinstate Monica Mar 21 '17 at 18:38
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As long as you are not against killing all life on that planet in the process, by means others than suffocation, there are some reasonably fast options.

One option would be to use relativistic bombardment. Checking http://www.projectrho.com/public_html/rocket/usefultables.php (which is by the way, really great site if you are considering writing s-f) Boom Table, you need around 3.2*10^26J to strip Earth-like planet off atmosphere. Assuming you can get impactor running at 99.99% c, it would take around 100000 ton of asteroids (better having number smaller ones than one big in this case).

I wouldn't rule away antimatter bombardment - you would need around million tons of antimatter, but you don't need to accelerate it to 99.99% c :) And million tons is not that much on cosmic scale - we are talking about 100x100x100m cube of anti-water, or a bit less of anti-rock.

Another option, using locally-sourced material, would be to ask the local star to become more violent. This goes a lot more into handwavium department, but having huuuuge solar flares could do the trick (this was explored for example in https://en.wikipedia.org/wiki/Commonwealth_Saga).

If you could do a magic space warp thingy around the sun, 1 second of directing all its output toward the planet would be also enough.

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The sun can already do this and has done to mars

Our magnetic field keeps solar radiation from stripping away our atmosphere. Without a magnetic field the sun seems pretty effective at it.

So we need to find a way to remove the planet's magnetic field. There are two methods of this:

  • Cool the core: This would result in not only the loss of a magnetic field but also the a massive drop in temperature on the planet making it much more difficult for those in underground bunkers to survive.
  • Slow the planet's rotation: It isn't clear how long it would take for this to remove the magnetic field but the molten core would slow over time and stop producing the magnetic field. Slowing the planet could be achieved by moving the moon (increasing it's angular momentum and so, to conserve it, slow the earth's rotation).

The benefit of this is that you don't need to know the composition of the atmosphere before going there.

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    $\begingroup$ I've seen other questions asking about adjusting planetary rotation, and that's a LOT of power/energy involved! $\endgroup$ – Andon Mar 20 '17 at 3:37
  • $\begingroup$ @Andon I think however you do it a lot of energy will be involved. I assumed your advanced civilisation would have access to large sources of power. $\endgroup$ – Lio Elbammalf Mar 20 '17 at 7:17
  • $\begingroup$ Well, yes. This is true, but modifying rotation involves simply staggering amounts of very precise energy. $\endgroup$ – Andon Mar 20 '17 at 10:11
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    $\begingroup$ My bioplague doesn't require more energy than whatever it takes to make it and shoot it safely to the surface. You could probably just sprinkle a bunch of bacteria down from orbit by shooting them backwards so they slow down and fall. See my question for the details. $\endgroup$ – Gryphon - Reinstate Monica Mar 21 '17 at 10:28
  • $\begingroup$ @Gryphon Then it depends on how realistic we're being. You propose a bacteria which can remove an atmosphere and still survive through the dramatically changing pressures it is creating (since you're requiring that it cannot expel any gasses). Given such a specific action your method is also restricted to only working on certain planets. Of course you can claim your bacteria works but you need a lot of hand-waving to do it. That isn't a problem as such, only that when I said it would require a lot of energy I was discounting hand-waving. $\endgroup$ – Lio Elbammalf Mar 21 '17 at 13:16
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The Alien DoomFleet only has to tow a gigantic parasol or space sunshade into place above a planet blocking its insolation from its primary star. Once no ambient stellar radiation is warming the planet its temperature will fall until the atmosphere freezes out and the planet's surface is covered in a frozen layer of its atmospheric gases.

The DoomFleet sends in armies of robots and evacuation equipment to scrape the frozen atmosphere off the planet's surface and transport to wherever they want to transport it. Perhaps to planets the DoomFleet's creators want to terraform.

Freezing planets in this way is also an excellent way of exterminating the inhabitants of planets. Freeze, scrape and shift and a planet's atmosphere is gone. Not as spectacular as some methods, but guaranteed effective.

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