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I want to completely and utterly destroy a planetary civilization, while leaving technology and infrastructure (on land) more or less intact.
Essentially, some squishy land bound (bipedal, carbon based) aliens earned the ire of the energy eating rock aliens who have decided to forcibly remove them and take their metals.

By working together the rock aliens can harness and direct ~0.00000001% of the star's output (there are only a few thousand of them) and can use their psychic powers to reflect and bend light. They can also work together to accelerate large rocky/metallic masses (like asteroids). They cannot grip or directly move gasses or liquids.

The rock aliens love metals, especially metals with a high melting point and/or high thermal conductivity. They want to remove all the squishy life forms and atmosphere from the planet's surface so they can take away all the metal on the surface without interference. The squishy aliens have many high profile metal buildings on the surface but also have an annoying tenancy of using particle weapons and mass drivers to inconvenience or destroy the rock aliens.

The rock aliens don't want to bombard the planet with asteroids because that would bury all the useful metal (it is also bad for rest of the story), and the atmosphere is inconvenient to them because it transfers heat (they prefer vacuum for because it doesn't absorb energy as well).

How can the psychic rock aliens remove the atmosphere (and optionally the ocean) in a short period of time (less than ~800 years) with the minimum use of energy and least damage to infrastructure on the surface.

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  • $\begingroup$ Not only is the atmosphere inconvenient for the rock aliens (they are suited to vacuum) but it is hazardous to them as it reduces the effectiveness of their energy absorption. Additionally, I would like to say that the atmosphere has to be removed for story related purposes too, which I will not get into in a comment. $\endgroup$ – Amziraro Jun 24 '15 at 7:42
  • $\begingroup$ If the planet do not have a iron core thing becomes easier, in close proximity the stellar winds can blow off its atmosphere and vaporize any water however this will cause major power and communication failure. $\endgroup$ – user6760 Jun 24 '15 at 12:04
  • $\begingroup$ @user6760 The planet has an iron core and a magnetic field. Naturally shedding the atmosphere isn't really an option (but a good point anyway) $\endgroup$ – Amziraro Jun 24 '15 at 12:09
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    $\begingroup$ Why would they, tho? Destroying a civilization like this requires a major hate-on, and if the solar system in question is anything like ours, they have dozens of significantly sized bodies to extract whatever they want from, all of which have near-zero atmosphere from the get-go. Add to that millions, billions or possibly trillions of asteroids, cometary bodies and other small pieces of rock, and the whole issue becomes moot. $\endgroup$ – Williham Totland Jun 24 '15 at 12:46
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    $\begingroup$ I'm guessing the vacuum cleaner from Spaceballs is out? ;) $\endgroup$ – Doug McClean Jun 24 '15 at 19:24
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Redirecting 0.00000001% of the sun's output is more than sufficient to completely darken Earth and moon from direct sunlight, and even all the other forms of direct and indirect radiation landing on the planet.

Doing so would not only freeze the oceans, but also the atmosphere - you wouldn't be dealing with gases anymore, but much lower volume liquids and solids. The molten core would still be providing a lot of residual heat, so maybe you'd be dealing with oceans of oxygen and nitrogen rather than solids, but the end result is that you'd essentially still be working in vacuum.

If it was necessary to really get rid of the stuff, it's easily done once liquid or frozen. You can either make buckets and drag it out of the gravity well, or, once frozen, redirect all the light into one spot of the frozen/liquid muck you don't want and watch it jettison itself off the planet.

Freezing the planet and reducing the atmosphere to liquids and solids will take a very small amount of time - much less than 800 years. Jettisoning it from the planet, if still necessary for whatever reason, would take longer.

But the pesky heat-sinking atmosphere would, essentially, be a non-issue.

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    $\begingroup$ My thoughts exactly: quick, easy, simple. If you absolutely can't tolerate the frozen gases & liquids remaining, then use your magic energy to launch chunks of it into space. Then again, what are rocks but "frozen" liquid silicates? $\endgroup$ – Jim2B Jun 24 '15 at 14:52
  • $\begingroup$ Brilliant solution. of course the water in the now freezing air might collapse structure on the planet because of ice forming. Water has a bigger volume as ice than as liquid. Think total global Ice age but even faster forming glaciers. Not much would survive such a quick ice age. unless you smart enough to model the planet and let enough heat trough at certain points to cool the planet very uniform. Meaning you reflect a part of the light to the night side of the planet but not much ofcourse. $\endgroup$ – Mellester Jul 21 '18 at 16:53
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So they can manipulate light and larger masses, but not gasses/fluids ?
Concentrate sun-light on one of the poles of the planet until it gets hot enough for a significant portion of the atmosphere in that spot to be accelerated to escape velocity. At the (magnetic) pole the planets magnetic fields are weakest given solar wind maximum chance to blow away the escaping gasses.
This would also create a permanent low-pressure zone in the atmosphere which disturbs the weather over the entire planet to make life very unpleasant for the locals. Additionally drop large asteroids in the oceans to add more heat to the planet and make life even harder for the locals (tsunamis, vapor-clouds, etc.).

Most likely the locals would be dead, or in no condition to operate their weaponry, because of collapse of their society, in just a century or so. With the locals no longer a threat you can start using more direct methods on the surface.
Here I'm thinking on building, on the ground, some sort of chemical installation(s) that bind the atmospheric gasses in solid materials, thereby taken them out of the atmosphere.

Obviously you don't want to heat the planet too much. Melting the metals wouldn't be too bad, they are still easily accessible. But evaporating them would be problematic as they would disappear together with he atmosphere...

If the aliens are powerful enough to stop the planet rotating all things are easier. A non-rotating planet can't sustain a significant magnetic field protecting it's atmosphere. The solar wind would strip the planet from its atmosphere very quickly. (Again: accelerate by heating the gasses.)

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If you want to strip the atmosphere, just do so. Not quite as flippant an answer as it may seem. If you look at the planet from the direction of the sun at a reasonable distance, the atmosphere will have detectable thickness, it will be possible to focus light into a laser beam in such a way that it hits the atmosphere but not the surface. If you use a wavelength that is efficiently absorbed by the atmosphere this will impart energy to the atmospheric molecules and increase the rate of escape. Essentially you can boil off the atmosphere from top down although the light will also have some momentum which will also be imparted to the molecules and help in the escape.

Some of the heat will leak to the planet as thermal radiation, but if you pick an uninhabited area such as polar areas or oceans the effect should be minor. And you want to kill anyone anyway so who cares.

Once the atmosphere has been reduced to irrelevancy, you can't get entirely rid of with this method, the oceans will be gone to. All the water will evaporate due to low pressure. Apart from the water that will freeze and then sublimate. The difference is not really important for your purposes. Freezing will happen since the thinner atmosphere will lose its ability to retain heat.

Second option is to redirect solar radiation in such a way that it does not hit the planet at all. This will cause the atmospheric gasses and all water to freeze. No atmosphere, no oceans, everyone will die, and structures will not be broken by this. Although they will buried by frozen atmosphere and the weight might have collapsed them. But you can simply unfreeze and then make the planet easy to digest at your leisure. There will be nobody to bother you, even nuclear shelters assumed atmosphere still exists outside.

Third option is to simply heat the surface until everybody dies. While this would be against the parameters of the question, I am not convinced tasty metal structures would become unedible simply because they have some melt damage.

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How about first reflecting a massive amount of light from the sun onto the planet to heat up the atmosphere, this could boil the ocean and make it impossible for a large civilization to live on.

With regards to stripping the atmosphere it may be possible to change the orbit of the planet for a close fly by of the sun, solar winds may be able to slowly strip away the atmosphere although it would be much more effective if you're able to alter the magnetic field of the planet.

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Arrange for a sequence of very heavy asteroids to fly by on near-miss patterns, each one will fly through the atmosphere and skim just over the surface of the planet. Each one will blast out a substantial chunk of the atmosphere so if you set up a sequence of them it will act almost like a bucket chain, with the cumulative effect gradually stripping the atmosphere off the planet.

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The Direct Approach

A typical planet does indeed have a lot of metal on its surface, but there's a place where it's likely to have far more metal, that's far denser and readily extracted: Its core.

Instead of stripping the atmosphere, the rock aliens could focus the energy they can harness from the star and just pump it as directly as possible into the material the planet is made from.

If they transform it into long-wave radio signals that can be captured using the core (or the magnetic field) of the planet as an antenna, they can then pour the energy into making the planet shake and heat up; the principle very much the same as an induction stove top.

With a Sol-like star and an Earth-like planet, the aliens would be able to harness about 40 petawatts. If all of the harnessed energy can be modulated to cause induction in the core of the planet, that would rapidly make life suck on that planet.

After hours or minutes, tectonic activity would have increased noticeably, with some zones rapidly being plagued by near-constant magnitude 6 earthquakes and massive volcanic eruptions.

After a couple of weeks or days, if any supervolcanoes were already on the brink, they would now be erupting, with verneshots a distinct possibility.

After a couple of months or weeks, any other potential supervolcanoes would be going off, and verneshots and other similarly dramatic phenomena would be commonplace. All life would be long dead. The planet would be seeing massive outgassing, leaving the surface a thinly atmosphered mess of molten rock.

After about a year, the planet would be reduced to rubble, and further attempts at induction would be fruitless, as the bits of rubble formerly making up the planet would be too incoherent, but at this point, they would have easy access to all the iron, nickel, tungsten and other materials they could possibly want: An entire planetary core worth.

This doesn't preserve the surface, of course, but why bend over a dollar to pick up a dime? The core is where the real action is.

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Reactive metals will strip the ozone, the oxygen and then the nitrogen from the atmosphere and the finely powdered metal will then precipitate out and fall to earth creating a thin blanket of oxide, nitrite, and hydroxide sand that can be harvested for processing and reuse. Widespread Ozone destruction will remove the irritating carboforms too. The ocean is a hard one, there's so much of it and water is reasonably stable, a lot of it will boil off when the atmospheric pressure drops enough and if you use Series I Metals, Lithium, Sodium etc... they'll react to strip the oxygen out of water too and the hydrogen will just boil off into space.

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