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I am currently working on one of the planets in my system, called Opea. Opea is a roughly earthlike planet in the furthest edges of its star's habitable zone (receiving half the sunlight as Earth), with a thin atmosphere (0.16 bar). I am trying to build it so that it is warm enough for the equatorial band to be liquid, with the rest being frozen ice caps. However, I am trying to make it so that the land-based glaciers are very thin or preferably non-existent. This is for a variety of reasons, most notably to make terraforming easier (as there won't be any major sea level changes).

I have considered that since it will be below freezing everywhere on the planet (with the equatorial band only being liquid because of its ocean's high salinity), but I am unsure of the science of it. Since evidence suggests there can be snow at any temperature, that may mean that over billions of years there will be buildup, leading me to believe there would have to be some way for it to be warm enough for any snow buildup to melt, but not warm enough to heat up the whole planet. Is there even a way to do this?

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    $\begingroup$ Science requires trade-offs. How habitable do you need it to be? A thin crust with active volcanism could do it, but that means a lot of ash in the atmosphere. A high ammonia percentage would lower the freezing point of water, but even if not in the atmosphere, it would make the water hard to drink without filtration and impossible to farm in. Settling for thin polar caps means low overall water content or very low evaporation - bad for farming. Do you see what I mean by trade-off? Can you edit your post with your expectations for how habitable you hope the planet will be? $\endgroup$
    – JBH
    Commented May 30 at 1:08
  • $\begingroup$ Are you willing to accept things like high axial tilt or an eccentric orbit that regularly takes the planet closer to its sun? $\endgroup$
    – Monty Wild ♦
    Commented May 30 at 3:27
  • $\begingroup$ Are sea level changes really so problematic? They seem much less disruptive to a story and immersion than any mechanism for keeping the land clear of glaciers. Whatever you come up with is going to seem contrived, unbelievably specific to this particular situation. You're warming a mostly-frozen planet and increasing its atmosphere several times over at minimum, some sea level changes seem like something that is both entirely expected and entirely within your ability to handle, and more of an interesting background detail than something to write out of the story. $\endgroup$
    – Christopher James Huff
    Commented May 31 at 15:14

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If the world is cold enough that there can be snow and ice build up, and thus glaciation, at all, or almost all, latitudes, which is possible, it's happened before on Earth and which you seem to have about right. Then over geological time ice will spread more and more uniformly across the globe as summer evaporation/sublimation of ice deposited at higher elevations allows that moisture a chance to be precipitated at lower altitudes. On long enough timescales this process will globally smooth the ice depth based on annual average irradiation intensity. The poles will still have thicker deposits but there won't be much in the way of "ice topography", no notable ice caps as separate entities or major prominences above the general landscape. Glaciers are also highly erosive so while there will still be some emergent peaks most of the land, if any, under the ice is going to be ground and filled flat.

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Astronomically recent calamity

This might not hold up to serious scientific scrutiny, but I don't see [Scince-Based], so here's an idea:

Your planet had a moon for billions of years. Sometime "just" millions of years ago, tidal drag finally caused the moon's orbit to decay past the Roche limit and ripped it apart. Countless impacts left the planet's surface molten for millennia, but eventually it cooled down again. There just hasn't been enough time for thick glaciers to form, yet. In fact, the planet is still pretty warm compared to its equilibrium temperature - but that's millions of years away, and the humans plan to have added a dense greenhouse gas atmosphere within a few thousand years, so it doesn't matter.

This also gets you some planetary rings, which are cool. Ordinarily bad for life, but if you have the kind of energy sources you need to get serious about interplanetary travel and terraforming, you can probably laser any big pieces out of the sky as their orbits decay.

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