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I have two earth clones, in essence, separated by *16550 miles (26350 kilometers). They are, of course, tidally locked, and orbit each other once every 24 hours. These planets orbit a sun identical to ours in the same time as earth, except their orbit around the sun is perfectly circular. Now, I want to add a little bit more to the system. A moon with half the mass of ours.

But there is a catch. This moon is made of ~90% ice. Because I don't mind some materials in there that aren't water, but they must be spread throughout the moon, and not gathered together as the moon's core.

The moon is in the position shown in the diagram below, and it has an orbital period of 72 hours. Assume the ellipses are actually circles.

enter image description here

*16550 miles = a

"Note: I take credit for this graphic." - HDE 226868

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Xandar The Zenon
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    Are you asking about whether the composition is possible or the location? – Jim2B Apr 18 '16 at 05:55
  • @Jim2B Both. Because while it may or may not be possible around Pluto, an earth like planet closer to the sun will be warmer and the tidal forces may rip the moon to shreds. – Xandar The Zenon Apr 18 '16 at 13:07
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    Note: I take credit for this graphic. – HDE 226868 Apr 18 '16 at 22:19
  • @HDE226868 I knew I was forgetting something. I edited e question. – Xandar The Zenon Apr 19 '16 at 02:37
  • Duplicate: http://worldbuilding.stackexchange.com/questions/4969/could-a-planet-made-completely-of-water-exist/4974 – Loren Pechtel Apr 19 '16 at 02:37
  • @LorenPechtel Well, this is significantly smaller, and it is also talking about orbiting other objects at much closer. So while they are definitely related, I don't think it is a duplicate. – Xandar The Zenon Apr 19 '16 at 02:39
  • @XandarTheZenon The answers which show why a pure-water planet is impossible also apply to moons. The fact that moons generally add their own problems of retaining the water doesn't change the basic answer. – Loren Pechtel Apr 19 '16 at 02:43
  • Does the moon have to be old (at least a few billion years), or could it have been created by planetary engineers fairly recently (or possibly captured recently, having previously been on a very eccentric orbit that caused it to spend most of its past history at a much greater distance from the Sun as an icy body), say within the last few million years? Even if it will evaporate as jknappen said, this might at least take a few million years to happen. – Hypnosifl Apr 19 '16 at 21:51
  • @Hypnosifl The formation isn't even relevant to the question, but yes, it could have been created recently by a technologically advanced race. – Xandar The Zenon Apr 20 '16 at 04:17
  • Formations of planets like this generally occurs in the outer bands, then the body 'migrates' to the inner planets. We've discovered a few water planets now, no reason this moon can't form naturally. Do you need the setup stable over time? Or can this be a temporary (still multi million year) setup that will eventually collapse? – Twelfth May 05 '17 at 21:33
  • Note that the orbital arrangement shown is likely to be unstable, circumbinary setups have a "critical radius" below which a third, lighter body orbiting the center of mass of the larger bodies is likely to be flung off or pulled in. https://what-if.xkcd.com/150/ mentions this with respect to a planet with two stars. With a moon around a binary planet, it's probably worse, since the sun adds another force to make the whole setup even less stable – zstewart May 05 '17 at 22:23

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Europa has a water/ocean layer estimated to be about 100km deep. and it has an icy crust at least 10km thick.

While Hydrogen and Oxygen are the 1st and 3rd most common element in the universe, making a water moon theoretically possible (because there are a lot of elements to make plenty of water). I suspect it would take some strange occurrence to have a water moon without at least a small iron core.

The core would be the main 'gravity' source to 'attract' the water, like a snow flake forming around a dust particle. It might be able to keep collecting more and more 'water', though it would be likely to collect other materials along the way, which would gravitate toward the center forming/enlarging the core. Every asteroid hit would have material that has to go somewhere. So just by the act of forming it should have some kind of none water core. Outside of it being manufactured of course.

The Solar system would need to have an unusually high percent of water vs. other minerals in order to get it to be mostly water, and in that case, both planets would likely be composed of very large amounts of water too, unlikely to have any land that sticks out of the oceans.

The moon would also likely have a fairly thick crust of ice significantly reducing the rate at which it is evaporating by solar wind into space.

bowlturner
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Your moon will not be stable and evaporate (due to Atmospheric escape) into the space.

Orbital mechanics should be OK (at least for some time; the Three-body problem is notorious for its chaotic nature and the non-existence of analytic solutions).

Sir Cornflakes
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    The "Atmospheric escape" link mentions the simplest form of escape is 'Jeans escape' where the temperature of the body means that some molecules randomly achieve escape velocity in collisions. But googling for some info on this, I turned up this article which in Fig. 3 on p. 4 mentions the rule of thumb that Jeans' escape is only "considered important" if the "thermal velocity ... at the exobase" exceeds about 1/6 of escape velocity, and shows the temp vs. 1/6 escape velocity lines for some gases. – Hypnosifl Apr 18 '16 at 20:27
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    (cont) And there's a graph on that page that shows the thermal velocity at exobase for different gases plotted as a function of surface temp. and 1/6 escape velocity. For a body with the density of water and the mass of about half the moon, 1/6 the escape velocity at the surface would be about 0.26 km/s, that would seem to be well under the line of water at Earth or moon temps (meaning the thermal velocity at exobase is significantly higher than 1/6 escape velocity), confirming what you said about the atmosphere escaping quickly. Only if the temp were around 50 K or less would it be stable. – Hypnosifl Apr 18 '16 at 20:48
  • Um, why couldn't the moon hold on to an atmosphere? The question never stated that it didn't have one. – HDE 226868 Apr 18 '16 at 22:23
  • @HDE226868 - The question didn't state it, the point is that physical facts the questioner probably wasn't aware of (like the fact that an atmosphere will be lost to space in a few million years if the average velocity of an atmospheric molecule at the atmosphere's temperature is too close to the body's escape velocity) imply that the moon won't be able to hold on to the atmosphere. And as the atmosphere is lost, the outer surface of liquid water will begin to boil off once the atmospheric pressure gets too low. – Hypnosifl Apr 18 '16 at 23:18
  • @Hypnosifl I didn't think it would be able to attract an atmosphere anyway, I was just wondering if the moon would boil away, become a frozen moon, or something similar. – Xandar The Zenon Apr 20 '16 at 04:19
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    @XanderTheZenon - Even if it didn't start with an atmosphere when planetary engineers created it as an iceball far from the Sun, it would create an atmosphere of water vapor around itself once moved into orbit around the Earths, since the water on its surface would start to evaporate. Although the moon as a whole won't be in equilibrium, you should get a sort of transitory equilibrium between the liquid surface and the atmosphere, where the rate the atmosphere loses water molecules to space equals the rate it's replenished by the liquid surface conintinually evaporating. – Hypnosifl Apr 20 '16 at 05:44
  • If this were true, how can Europa have an ocean? Maybe these processes occur at such a slow rate that such a moon could be semi-stable for a few 100k years ? That would probably enough for the story ? – Falco Nov 26 '18 at 12:47
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    @Falco Europa is much colder due to its distance to the sun. The so-called ocean is covered by a thick layer of ice hindering its evaporation. – Sir Cornflakes Nov 26 '18 at 13:15
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A moon which is 100% water is possible in theory, but it could only occur in reality if it was an engineered artifact rather than a natural moon.

For any planetary body to form, there must be some sort of gravitational "nucleus" for other particles to be attracted to. Your planetary engineers could start with a giant "ice cube" of purified water (all that matters is the mass of the object you are starting with, not the composition), and it will soon be attracting everything nearby, with the attraction growing exponentially as the mass increases.

Of course, this means that everything including rock, dust bits of carbon etc. will also be swept into the mix, so your engineers will have to put the ice cube nucleus in a cloud of 100% pure ice crystals. The mass of the in falling ice will increase the pressure and temperature of the growing body, and eventually melt the core ice cube. At this point, any matter trapped in the ice cloud will start falling towards the core, so you will get a small core of other elements unless the ice cloud is maintained at a very high level of purity.

As the water clump gets deeper, the core will eventually resolidify as the water assumes one of the multitude of forms of ice. This pressure ice is different in properties than the ice we normally see in our drinks or floating on the ocean (for an introduction read: https://en.wikipedia.org/wiki/Ice), but a lot of this depends on the exact conditions being encountered, small moons like Europa or the Moon may not have enough pressure at the core to develop an "ice" core or mantle. A planetary body with a standing column of water 1000km deep will almost certainly have a core and mantle made up of various forms of ice.

Since we formed this body in deep space where an ice cloud could be generated and remain frozen, there is no reason to suppose that the surface won't (edited from "will". Autocorrect sucks!) quickly crust over with a layer of water ice (ice 1) after the initial heat of formation dissipates. This will also vary depending on the size of the moon, external heat sources and so on. Since there are no internal sources of heat, the moon will eventually freeze solid, and you will have what looks like a white cue ball floating serenely in space.

Gryphon
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Thucydides
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  • Did you mean to say "there is reason to suppose the surface will crust over with a layer of water ice", or are you saying the surface would remain uncrusted even "after the initial heat of formation dissipates", and assuming it remains at the same distance from the Sun? Also, if the planetary engineers bring it closer to the Sun into the habitable zone (where I assume the two Earth clones in the question are supposed to be), would you agree with jknappen that a mass of 1/2 the Earth's moon would be too small, and the body would evaporate fairly rapidly (say, a few million years at most)? – Hypnosifl Apr 18 '16 at 18:44
  • I think you misunderstood me, when I asked about the possibility I did not refer to the formation, I was wondering if it would eventually boil away due to its location. Besides, I understand how ridiculous it would be to have 100% ice even if it was built. Hence 90%. Because distilled water is hard to come by. – Xandar The Zenon Apr 18 '16 at 19:56
  • Yes, I meant it will crust over, but autocorrect has other ideas. As for will it boil away, since we are inside the "snow belt" according to your description, then the answer will have to be "yes". If it was engineered outside the snow belt and brought into its current orbit, it will also boil away unless steps are taken i.e. a sunshade. – Thucydides Apr 18 '16 at 22:31
  • Autocorrect knows about physics :-) Of course, it will boil away, because the sun-lit parts of the moon surface will be hot enough for the ice to evaporate. By the way, it will be a great cosmic spectacle for any observers on the twin planets: The tail of vapour will illuminate the nightly sky as a giant coment. – Sir Cornflakes Feb 20 '17 at 17:17