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I want the setting in my story to be a moon. This moon has an ocean, and it's important to the plot that the surface (7-10 feet) freezes during winter and melts during spring. What would all of the conditions involved be?

Its planet is a gas giant about the size of Neptune. The moon is about the size of Saturn's Titan. The salt content is the same as Earth's oceans. There is also very little land.

The Square-Cube Law
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  • You are asking two different questions here: How hot would summer get is totally unrelated to what would it take to freeze/thaw the ocean. – L.Dutch Aug 24 '20 at 04:29
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    Suprisingly, there are quite a few bodies of water on Earth which freeze in winter and thaw in summer. I have heard that there is a small country (named Russia? something like that) which has for centuries been confronted with the problem that most of their small insignificant ports (St. Petersburg? Vladivostok? something like that) freeze in winter. It has apparently been a great force in shaping the foreign policy of that feeble and mostly unkown country in search of a mythical warm water port. – AlexP Aug 24 '20 at 04:37
  • We need a few details here: the mass, composition and size of the moon, the composition and mass of its oceans and atmosphere, and the mass of its planet and the moon's orbit, as the planet may cause considerable tidal heating. – The Square-Cube Law Aug 24 '20 at 04:44
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    Downvote for lack of basic research and/or lack of detail in the question. For example: if the ocean is just plain water, how cold would it need to get to freeze? Any temperature below the freezing point of water of course. How warm would it take to melt? Above the melting point of water of course, which incidentally, is the same as the freezing point. This info can be easily looked up, even if you wanted an ocean of some other material such mercury, sodium, nitrogen, oxygen, whatever. So either this question is poorly researched, or the OP isn't verbalizing what they actually want to know. – DKNguyen Aug 24 '20 at 05:10
  • (1) You do understand that there is no way for the entire moon to have the same climate, right? (2) Let's take a simple example. The northen part of the Gulf of Bothnia freezes every winter and thaws every summer. Tornio is a Finnish town on its shore. Look up its climate. What more do you need? – AlexP Aug 24 '20 at 21:06
  • A moon the size of Titan that experiences seawater-melting teperatures, is probably going to lose that water though evaporation and atmospheric escape over astronomical timescales. https://en.wikipedia.org/wiki/Atmospheric_escape – notovny Aug 25 '20 at 12:32
  • @AlexP actually, if the answer dealt only with the equator, the poles take care of themselves, don't they? (Although that reference to Tornio was fabulous. Great way to help new worldbuilders realize that reflecting real life is 90% of the battle). – JBH Aug 25 '20 at 22:07
  • In relation to @notovny's comment, this Worldbuilding question already addresses the issue of smallest planetary mass that can retain an atmosphere. For the sake of realism, that's as small as your moon can get and still have an atmosphere. Conveniently, it's actually possible for a moon the size of Luna to hold an atmosphere - under the right conditions. – JBH Aug 25 '20 at 22:10

2 Answers2

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28.4 degrees, but...

Ocean water freezes just like freshwater, but at lower temperatures. Fresh water freezes at 32 degrees Fahrenheit but seawater freezes at about 28.4 degrees Fahrenheit, because of the salt in it. When seawater freezes, however, the ice contains very little salt because only the water part freezes. It can be melted down to use as drinking water.NOAA

There are roads they build across frozen ocean every year in Alaska. They spray fresh water on top of the frozen salt water, because the fresh water freezes harder. Famously, Lake Baikal had a rail line built across the lake each winter until a bypass was constructed. Depth of ice is critical for what you want to do with your ocean, and the colder it gets, the thicker the ice. Also, Atmospheric pressure can greatly affect temperatures of freezing and I don't know what the air pressure is on your moon. Currents can also affect this, as warmer or colder water than air will freeze differently. Freezing is often at the shore first, and you often get a hilly mound of ice at the coastline. Wind can affect freezing in unpredictable ways; ask a meteorologist if it's important. Ice usually freezes at the surface first, and ice floats, so you get a layer of slush which gradually solidifies. It will all freeze into t he equivalent of the polar ice cap given low enough temps and time. Here's another link to NOAA about ice formation and depth NOAA #2

I'm not sure how variable gravity and solar radiation might affect your freezing. There are a lot of variables to consider to tweek freezing point, so just make it really cold unless subtle differences are important to the plot. At that point, you'll need to do some more extensive research.

DWKraus
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  • You are inferring STP conditions, but nowhere the OP stated that pressure is 1 bar. – L.Dutch Aug 24 '20 at 12:35
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    @ L.Dutch - Reinstate Monica♦ In all that, I did specify that atmospheric pressure can affect freezing. Mostly just trying to communicate a few of the variables to ice formation. I think they just need to make conditions something like -20 degrees F and let things really freeze like it does here in Minnesota. That eliminates a lot of variables. – DWKraus Aug 24 '20 at 13:20
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It depends on the atmospheric pressure.

Even on Earth, freezing point and boiling point vary depending on altitude.

More information is needed.

Seasons are weird on moons

Moons are almost always tidally locked to their planets, so their days and months are the same thing.

Moons are also almost always approximately on the same plane as the planetary equator, so they share seasons with their planets. The main problem here is that the slow day will likely overwhelm the effects of the season.

(This formerly said that moons are in the same axis as the planetary orbit, which is incorrect and had some insights about sun distance not mattering which are no longer relevant)

Let's suppose you had an exotic moon that revolved around its planet roughly once per 24 hours. This would imply that the moon is very close to its planet and would suffer pretty severe deformation, and quite possibly would fall within the Roche Limit and disintegrate (i.e. your setting is impossible in this case). Days would have to be pretty short for seasons to matter more than day/night cycles on your moon, so you can't make days too much longer than 24 hours. You will need to consult with an astrophysicist to determine remotely plausible conditions.

Beefster
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  • The inner moons of the Solar Sytem's planets most often orbit very near the plane of the planet's equator, rather than the plane of the planet's solar orbit .In the situation where there's a significant axial tilt for the planet, moons that orbit in the plane of the planet's orbit are actually fairly rare. – notovny Aug 28 '20 at 04:19