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I'm envisioning a planet around a sunlike star, with gravity and temperature similar to Mars, but covered with a thin shell of water ice (with CO2 ice at the poles). The atmosphere's pressure and composition are similar to Mars, albeit with more water vapor as would be expected due to water's prevalence on the planet.

Given the low pressure/gravity and abundance of water ice, I'd expect the atmosphere to saturate with ice crystals, similar to how the martian atmosphere is full of dust. On Mars, Mie scattering due to this dust turns the sky yellow-brown. What I want to know is, what do these ice crystals do to my planet's sky? What color is it, and how foggy does it look (if it has moons/rings, could you see them)?

My understanding is that in the low-pressure limit, ice crystals form hollow hexagonal columns of order 10 microns in length. Since they're highly non-spherical, Mie theory isn't easily applicable (not to mention that ice has very different optical properties from dust). I'm tempted to say that the ice simply turns the sky white, like the color of ice crystals in clouds on Earth, but I have no basis to assume this is still true at much lower densities and in vastly different atmospheric conditions.

For bonus points: What does the planet's sky look like near the sun? I would guess there's a halo effect, but how visible is it, given the low atmospheric density? Again, I'm not positive that this effect isn't unique to Earth-like atmospheres.

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  • $\begingroup$ "Given the low pressure/gravity and abundance of water ice, I'd expect the atmosphere to saturate with ice crystals": Why? I for one would not expect it to be saturated with anything. Water ice does not behave all that much like basalt or whatever rock, and I would not expect it to be eroded into dust. (And here on Earth, we do see quite frequently the sky covered with clouds of ice crystals. They appear cool white (high color temperature), and the sun and the moon do indeed often produce 22° halos when shining through them.) $\endgroup$
    – AlexP
    May 17, 2022 at 11:49
  • $\begingroup$ Sorry for the sarcasm, But have you ever seen clouds? Whole lot of those are full of ice crystals $\endgroup$
    – Topcode
    May 17, 2022 at 12:36
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    $\begingroup$ I mentioned the clouds, geez! Nobody actually reads the whole question, do they? $\endgroup$
    – Ventifacts and Yardangs
    May 17, 2022 at 16:22
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    $\begingroup$ I would definitely expect both 22° and 46° halos to be prominent. The only requirements for them are (a) a near-point-source of light behind the atmosphere (sun, moon); (b) hexagonal prism crystals (for 22°) with flat bases (for 46°); and (c) sufficient density of ice crystals along the path. $\endgroup$
    – addaon
    May 17, 2022 at 19:10

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Mother of pearl.

Curiosity rover spies colorful iridescent clouds on Mars

mother of pearl clouds

"If you see a cloud with a shimmery pastel set of colors in it, that's because the cloud particles are all nearly identical in size," said Mark Lemmon, an atmospheric scientist with the Space Science Institute in Boulder, Colorado, in a statement. "That's usually happening just after the clouds have formed and have all grown at the same rate. "I always marvel at the colors that show up: reds and greens and blues and purples," Lemmon said. "It's really cool to see something shining with lots of color on Mars.

I wish the picture was more what Dr Lemmon is describing - a rainbow colored cloud. I here assert that is how your clouds will look because that will be awesome.

Then when people need to settle down they can be plain white for a while.

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    $\begingroup$ Interesting! I was hoping the thin atmosphere would lead to some interesting effects, and I wasn't disappointed. I wonder if having more water in the atmosphere would change these filament-like cloud formations. Either way, though, this helps me paint the scene in my mind a lot better, so you get an upvote from me $\endgroup$
    – Ventifacts and Yardangs
    May 18, 2022 at 22:17

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