As a world builder I like to make solar systems and sometimes when I see an interesting idea I can’t help but wonder could that actually work that way? For an interesting example here is a clip from the movie Pandorum. https://www.youtube.com/watch?v=0xZm-hqIkQA

Now if you notice at the beginning the probe apparently passes by an icy moon similar to Europa, Enceladus, or Triton with those geysers. However, this icy moon Is orbiting an Earth-like habitable planet, that the end of the movie showed also had tropical beaches. Odd, but probably just another plot hole to add to the pile for that particular movie.

But then it happened again.

Now if by some cruel twist of fate you have not yet heard of the Sojourn go check it out, it is amazing with detailed world-building spectacular characters and a story that leaves you wanting more. First episode up on YouTube (https://www.youtube.com/watch?v=xlauJ06yqwc). But the relevance to this question regards the layout of one of the more important solar systems in this universe, shown here. welcome to Centrum Solari

So you can’t help but notice the large innermost planet called Hadimus Which is described by their wiki as a hot ice giant. Now ice giants like Uranus and Neptune are characterized by being smaller than their gas giant cousins like Saturn and Jupiter and having a different makeup. While gas giants are essentially hydrogen all the way down, Neptune Uranus have some hydrogen but also have a mantle that is made of compressed slushy water and ammonia. But would it survive the heat?

Now before I dismissed these ideas as a little bit of fantasy, we should acknowledge that the universe can and has produced weirder stuff, such as Gliese 436 b a planet literally covered in flaming ice. (https://www.youtube.com/watch?v=b6g-bYB2Xmg). So the universe can sometimes seem specificity designed to futz with our heads.

Now with all that said the one thing I would like to know is: Would icy moons and giants be possible within an inner solar system?

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    $\begingroup$ Please, don't take click bait titles as "science". A solid at high temperature and pressure is not flaming nor burning, though it makes for a less appealing video title... $\endgroup$
    – L.Dutch
    Aug 19, 2020 at 8:38
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    $\begingroup$ The universe is a weird place as you say. Although I doubt a moon around an earth like world can hold a atmosphere. If it could, it isn't a stretch to think it has climate zones. An ice giant might survive because of the opposite of global warming, reflecting much of the heat away? $\endgroup$
    – Trioxidane
    Aug 19, 2020 at 8:41
  • $\begingroup$ Polar ice caps on Earth: Am I a joke to you? $\endgroup$ Sep 23, 2022 at 1:02

3 Answers 3


Our own Solar System has ice-cored minor planets in the main asteroid belt (Ceres and Vesta, seemingly, likely at least one of the other bigger ones), and Mars, even with a significant atmosphere, is cold enough that ice is the primary state of water there.

Now, throw in a strong greenhouse effect, and it's rather plausible for a planet near or just outside the orbit of Mars to have moons that are mainly ice with protective sheaths of rubble or frozen mud, and still collect and retain enough solar heat to be warm at the surface -- hence potentially have a subtropical or even tropical climate with a quarter or less of the insolation Earth gets.

For a "hot Neptune", you only need a Uranus. The core of Uranus is already hot, yet it seemingly has virtually the same internal structure as Neptune -- hydrogen-rich atmosphere, but with relatively high percentage of water, ammonia, and cold organics, "slush" mantle of water/ammonia pressure ice, and small rocky core. Except that Uranus is still apparently radiating away the internal heat from the impact that tipped it literally on its side. If that level of heat, internally, has taken millions or billions of years to radiate away, a cold planet that migrated inward would take a similar time frame to heat up enough to disrupt the mantle and core structure. The atmosphere would expand with the heat -- which would make it an even better insulator, allowing the incoming solar radiation to be captured and re-radiated without (much of it) being conducted deeper into the planet. Now, ice moons around a "hot Neptune" not so much -- moons can't pull this trick -- but ocean moons with atmospheres aren't at all out of the question, depending just how hot the planet actually is. If too hot to hold water on the moons, then they'll be mostly rocky, of course, and much smaller than the large moons of Jupiter, Saturn, Uranus, and Neptune with their deep ice (and water) mantles and crusts.


Snowball Earth was an inner Solar System ice-planet. It just didn't stay that way. And judging by Earth's most Mars-like locations, Mars might once have been an ice planet, too.

The problem is that neither Earth nor Mars was ever a Europa-like moon, and if Earth ever was massive enough to be considered a mini Neptune (it wasn't), it was too hot to hold onto that extra mass.

The key example is Earth, though, and Earth's ice lasts while Earth's Moon's does not due to Earth's atmosphere. Without the atmosphere, the water would all escape into space whenever it isn't frozen, and would eventually be lost (see also: Mars). While Titan demonstrates that an ice moon can have a thick atmosphere, my understanding is that this works only because of how far it is from the Sun, and the precise details of its orbit around Saturn. It seems highly unlikely that one could cover Earth's Moon in ice and a layer of nitrogen, without the two very quickly burning away (some of the ice would last in deep craters, as it does today, but most would not).

So to have an inner Solar System ice moon, it probably needs to be more massive than the ice moons with which we are familiar. If the star's luminosity is a little lower, that wouldn't hurt. And even then, it's likely that the moon would go through ice and ocean phases, due to its geology / orbital situation / solar activity / etc.

As for the ice giant, we have found similar-mass and similar-composition exoplanets in their star's habitable zones. The "ice giant" term is a bit confusing, because it's not clear how much of the ices in Uranus and Neptune are literally frozen solid. "Ice" is a weird term once you get away from "solid H2O crystals". And as has been pointed out already, if Uranus retains heat from the impact that knocked the system on its side, then that demonstrates a certain amount of thermal insulation from its outer layers. Really, though, the "ice giant" term is sufficiently new and fuzzy that it seems like calling Hadimus a "hot ice giant" would not hinge necessarily on whether or not liquid ammonia can exist in its mantle. But Uranus suggests that it's not impossible.

So to summarize: the moon would need to be massive enough to hold on to an atmosphere, and the giant would probably be fine but suspiciously convenient characteristics like tilt and impact history (or lack thereof) would help. Maybe the moon is a captured mini-Mars, which benefits from its planet's magnetic field to help sustain its atmosphere, and therefore its ice?


Sure, it can happen. Just depends on a lot of factors. If we take a look at our Solar System, Mercury has an ice side and it is the closest to the sun. And it is unlikely that a planet which is very close to its parent star will have any sort of rotation, or won't be tidally locked. Meaning "most" inner planets will be half-ice.

Things get different when the planet pulls a Venus and rotates anyways. In which case, yeah, that's a big no. The solar winds would erase any form of atmosphere, which lowers the pressure, which decreases the melting point of ice, meaning all your ice and water will be gone rather quick.

There won't be an Inner Europa, so an-ice covered moon, because it is way too hot for ice.

And if your planet has a very thick atmosphere, such as Venus, you won't get any ice either as it will be too hot.

Some weird exceptions will exist, though, where gravity, pressure, rotation and so on just happen to match the perfect way to create an "ice" planet. Although it would still be hot as hell. Also, such a planet must be big due to the gravity needed.

  • $\begingroup$ Mercury doesn't have an "ice side," It's not 1:1 tidally-locked to the sun, the spin-orbit resonance on Mercury is 3:2, with its 55-earth-day sidereal rotation period and 88-earth-day sidereal orbital period resulting in a 176-earth-day-long solar day for points on its surface. $\endgroup$
    – notovny
    Sep 17, 2020 at 10:54

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