<p>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 <a href="https://en.wikipedia.org/wiki/Frost_line_(astrophysics)" rel="nofollow noreferrer">frost line</a> 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.</p>
<p>A surface level explanation boils down to one thing: the frost line.</p>
<p>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).</p>
<p>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).</p>
<p>Inside the line, water is vaporized and only kept by planets with masses large enough to hold onto it.</p>
<p>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.</p>
<p>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.</p>
<p>While I’ve probably glossed over a lot of important details, this is a (hopefully) digestible explanation.</p>

CC BY-SA 4.0
2022-08-17T10:57:54.953Z
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.
Tomáš Zato
CC BY-SA 4.0
2022-08-18T06:06:45.233Z
Wouldn't it make more sense to call it a 'frost sphere'?
Jojo
CC BY-SA 4.0
2022-08-18T13:47:46.493Z
@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.
Skyler
CC BY-SA 4.0
2022-08-19T23:46:21.743Z
You virtually don't sublimated any ice any more at 130K
planetmaker
CC BY-SA 4.0
2022-08-15T19:28:42.833Z
2022-08-19T06:11:14.473Z
2022-08-19T06:11:14.473Z
Justin T
<p>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?</p>

CC BY-SA 4.0
2022-08-15T18:15:35.997Z
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 !
mysterium
CC BY-SA 4.0
2022-08-17T17:44:48.630Z
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.
John Doty
CC BY-SA 4.0
2022-08-18T11:49:06.867Z
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.
Foo Bar
<jupiter><natural-satellites><planetary-science><water><galilean-moons>
Why do Jupiter’s moons have so much water?
Elhammo
# Why do Jupiter’s moons have so much water?

(Asked by: username_0 on 2022-08-15)

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?

**Tags:** jupiter, natural-satellites, planetary-science, water, galilean-moons

> **Comments:**
> **username_1**: 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 !  
> **username_2**: 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.  
> **username_3**: 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.  

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## 1 answers

### Answer by username_4 on 2022-08-15. Score: 73

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](https://en.wikipedia.org/wiki/Frost_line_(astrophysics)) 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.

> **Comments:**
> **username_5**: 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.  
> **username_6**: Wouldn't it make more sense to call it a 'frost sphere'?  
> **username_7**: @username_5-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.  
> **username_8**: You virtually don't sublimated any ice any more at 130K  

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