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I can think of at least 2 movies that had aliens invading Earth for water.

Is this a feasible motivation? I was thinking that

  1. Water is H20, i.e. dihydrogen monoxide
  2. The vast majority of the universe is made of hydrogen
  3. Oxygen is also pretty common as it's the 3rd most common element in the unverise
  4. We can combine the raw elements into water, so certainly an alien species so advanced as to be capable of interstellar travel to Earth could also easily combine these 2 abundantly available elements into water

But OTOH I'm not an expert and large teams of people working on big budget Hollywood films seem to think it's a reasonable motivation.

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    $\begingroup$ As explained below by Thucydides, the answer is emphatically "no". There's way too much free-floating water out in space to justify invading any planet for it, much less one occupied by a species of intelligent, pissed off bipeds with nukes. $\endgroup$ – Z.Schroeder Oct 11 '16 at 2:33
  • $\begingroup$ I'm curious, what movies? $\endgroup$ – Phoenix Oct 11 '16 at 2:34
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    $\begingroup$ Needing water is just our cover story. We are really here to harvest slaves ...oops, I shouldn't have said that. $\endgroup$ – Henry Taylor Oct 11 '16 at 3:09
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    $\begingroup$ Earth water just has an unbeatable flavour, far superior to any back-alley ice-giant, making it a popular drink at affluent alien cocktail parties. $\endgroup$ – Harry David Oct 11 '16 at 8:02
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    $\begingroup$ The aliens would only take our water if the goal was not to get water, but to deprive us of it. In other words, the visit is for ideological reasons, and not economical or technological ones. $\endgroup$ – EvilSnack Oct 16 '16 at 22:07
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Any alien species which can travel through space will be passing far more water on the way to Earth than actually exists on the planet. The Oort cloud is a mass of comets estimated to be trillions in number and massing at least 5× Earth masses.

Coming closer they then pass the Hills cloud, which may be 5× greater in mass than the Oort cloud, the "Scattered disk" and the Kuiper belt.

enter image description here

Oort Cloud

So they pass through up to a light year worth of water frozen into cometary cores before they even reach the outer edge of the Solar System. Once they get into the Solar System, they pass the various gas giant planets, where the moons are largely masses of ice and liquid water. Europa alone has 3× the water of all the oceans of Earth.

Finally, if they do actually come to Earth to get water, they will be dragging the water out of a deep gravity well, adding a large extra energy cost to every gram of water they take. Frankly, this is going about things the hard way, and smart aliens who were after water could be mining the Oort cloud right now at a negligible energy cost and we wouldn't even know it.

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  • $\begingroup$ But comets, or at least the one examined by the Rosetta mission, contain a lot of other stuff mixed in with the water/ice. Maybe the aliens want relatively pure water (from e.g. the Great Lakes) or sea water, without the hassle of separating out all the other gunk. $\endgroup$ – jamesqf Oct 11 '16 at 4:50
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    $\begingroup$ @jamesqf whatever purity increase might be gained by getting "fresh" water from Earth, is much, much more hassle than purifying the water available in space. The energy cost of escaping the gravity well is far more significant than anything else involved. $\endgroup$ – Nij Oct 11 '16 at 10:49
  • $\begingroup$ @Nij: You don't know what technology the aliens have available. Perhaps they have abundant energy and a handy mass transporter, but would find it inconvenient to design & build a comet-purification system. $\endgroup$ – jamesqf Oct 12 '16 at 5:37
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    $\begingroup$ @jamesqf if they have abundant energy, that and a big kettle are enough to purify water, it's practically Bronze Age-level technology. $\endgroup$ – Nij Oct 12 '16 at 6:06
  • $\begingroup$ It also bears pointing out that the Sun's gravity well is deeper at the radius of Earth's orbit than Earth's gravity well is at the Earth's surface. $\endgroup$ – EvilSnack Oct 16 '16 at 22:03
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Not remotely. The rings of Saturn contain 26 million times more water (ice) than has ever flowed on Earth.

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  • $\begingroup$ Oh yea, that's a good point! +1 $\endgroup$ – Hack-R Oct 11 '16 at 2:46
  • $\begingroup$ Can you give some numbers? 26 million times more water than found on Earth just doesn’t make sense. $\endgroup$ – JDługosz Oct 11 '16 at 21:32
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    $\begingroup$ Data from Wikipedia: the rings are about 99.9% water, mass from 10^19 to 10^20. Earth's hydrosphere is 99.7% water, mass about 1.3×10^18. "28 million times more water" is exaggerated by at least five orders of magnitude. $\endgroup$ – Nij Oct 12 '16 at 6:14
  • $\begingroup$ youtube.com/watch?v=mTklQj7sXr8 about 50 seconds in. $\endgroup$ – rek Oct 12 '16 at 13:52

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