Why do Jupiter’s moons have so much water by mass? Did all the bodies in the solar system start out with this much water and the planets closer to the Sun simply lost it to space?

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    $\begingroup$ could you please add more context and research into your question by giving some numbers and comparing those to other bodies, for example Earth ? Thanks ! $\endgroup$
    – mysterium
    Aug 15, 2022 at 18:15
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    $\begingroup$ Given that water is the most common chemical compound in the universe, you might rather cogitate on why some bodies don't have a lot of water. $\endgroup$
    – John Doty
    Aug 17, 2022 at 17:44
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    $\begingroup$ It isn't just Jupiter's moons. Nearly all of the non-giant bodies in the outer solar system have substantial amounts of water ice. The giants also have plenty of water in them, under lots of molecular hydrogen and other light stuff that smaller bodies don't have the gravity to retain. $\endgroup$
    – Foo Bar
    Aug 18, 2022 at 11:49

1 Answer 1


tl;dr: They have more water because they captured it as ice, and it’s easier to hold onto ice than water vapor. Planets (and by extension, moons) beyond the frost line were formed with ice as a part of their cores, so they in a sense started off with more water than terrestrial bodies closer to the sun.

A surface level explanation boils down to one thing: the frost line.

There is a line (roughly twice the Sun-Mars distance at ~3 AU) beyond which water can exist as ice for an indefinite amount of time exposed to the Sun (that of course is not to say ice can’t exist inside this region, but rather that it needs accommodations like being obscured from the Sun for large chunks of time).

Because water can exist as a solid beyond this point, it can (mixed with rocks and metals) contribute to the formation of planetesimals which leads to planets, leading to much larger planets (gas giants).

Inside the line, water is vaporized and only kept by planets with masses large enough to hold onto it.

As you can see, by this reasoning gas giants (and by extension meteors, moons, dwarf planets etc) beyond the frost line are just naturally going to have more H2O because it’s a lot easier for a planet to hold onto ice than water vapor, seeing how the latter can escape if the Boltzmann distribution of its speeds has a big enough tail beyond the escape velocity of the body.

Once ice is captured, methods like geothermal heating can melt ice closer to the core of the body, leading to a situation like Europa where there is believed to be liquid water under its surface.

While I’ve probably glossed over a lot of important details, this is a (hopefully) digestible explanation.

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    $\begingroup$ Why doesn't the ice sublimate though? I have ice cubes in my freezer - unexposed to sun, and over long time periods they get smaller and ice deposits all over the drawer. I'm sure it would take much longer for water to sublimate from a moon, but then again planets were around for much longer than my ice cubes. $\endgroup$ Aug 17, 2022 at 10:57
  • $\begingroup$ Wouldn't it make more sense to call it a 'frost sphere'? $\endgroup$
    – Jojo
    Aug 18, 2022 at 6:06
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    $\begingroup$ @TomášZato-ReinstateMonica Ice sublimates slower the colder it gets - temperatures on Europa, for example, are around -225 degrees F / -143 C. This is significantly colder than your freezer and would make ice take much, much longer to sublimate. $\endgroup$
    – Skyler
    Aug 18, 2022 at 13:47
  • $\begingroup$ You virtually don't sublimated any ice any more at 130K $\endgroup$ Aug 19, 2022 at 23:46

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