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I'm designing a hypothetical lower-gravity planet with 0.47M, 0.79r and 0.76g, with a similar density to Earth. I've already determined that this mass, radius, and density will allow my planet to sustain a long-lived internal dynamo and strong magnetic field, as well as have an escape velocity that will permit the stability of water vapor in the atmosphere. I want to give the planet an Earth-like atmosphere (in terms of composition), but the fact that the gravity is lower might mean that the atmosphere would be less dense and expand farther out. I'm worried this might make the planet too cold, and I don't want a snowball planet scenario.

My question is - if I want the planet to be at least as warm as Earth was during the last ice age, what factors should I tweak to make the planet realistically warm enough? (I'm thinking maybe increasing the total mass of the atmosphere, ocean size/depth, rotation rate, etc). I have some general ideas but no confidence that they make sense. Does anybody with a background in astronomy know?

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Generally, you want to keep your infrared optical depth high in the lower atmosphere, you do this with greenhouse gases, like $\rm H_2O$, $\rm CO_2$, $\rm CH_4$, or many other asymmetric molecules.

Once you have the optical depth, you can compute the surface temperature from https://ui.adsabs.harvard.edu/abs/2010A%26A...520A..27G/abstract , Eqn. 27.

In your lower gravity you can still keep the atmosphere from escaping by invoking a more severe cold trap, such as by efficient infrared cooling where the solar bolometric radiation is absorbed, again $\rm CO_2$ can do that job.

Those are rough guidelines however, and none of this makes a self-consistent realistic atmosphere. For that you'd actually have to run models. Or the crowd over at world-building might help you in their free time, but don't expect rigorous answers there.

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    $\begingroup$ Thanks! So, keeping the infrared optical depth high means not allowing as much infrared to pass through the atmosphere? That's just the greenhouse effect, right? And how does a more severe cold trap form? Is this something that could happen on a smaller planet with a substantial atmosphere? Or is it more likely to happen on such a planet - maybe a feature of the expanded atmosphere? $\endgroup$
    – Elhammo
    Commented May 24, 2022 at 19:19
  • $\begingroup$ Ok I can't find the answer to this cold trap question so I've come back here lol. If I placed the planet a little further from the sun but had a strong greenhouse effect going on to warm the planet, would the planet being further from the sun ensure a stronger cold trap, preventing the escape of water vapor? Thanks! $\endgroup$
    – Elhammo
    Commented May 25, 2022 at 0:44
  • $\begingroup$ @Elhammo: Yes, that's the greenhouse effect. Increasing the ghg while keeping the gas type constant, would mean to have more atmospheric mass, in this case your cold trap just moves upwards with the rest of the temperature-profile. But look at Venus: No cold trap needed, if the entire atmospheric column is efficient at cooling anyway. $\endgroup$
    – AtmosphericPrisonEscape
    Commented May 25, 2022 at 11:59
  • $\begingroup$ @Elhammo Atmospheric pressure barely plays a role in the evaporation rate of water from an ocean, lake, or pond. The driving factors are the temperature at the surface of the body of water, the temperature of the air just above the body of water, the relative humidity of the air just above the body of water, and the wind velocity. $\endgroup$
    – David Hammen
    Commented May 26, 2022 at 15:59
  • $\begingroup$ @David Hammen - Lower atmospheric pressure at higher altitudes lowers the boiling point, though. So wouldn't that also increase evaporation? And what about lower gravity? I assume that would increase evaporation because the water molecules would be held less tightly by the planet. Getting lightly off topic, do you think it would rain more on a lower gravity planet? $\endgroup$
    – Elhammo
    Commented May 26, 2022 at 17:55

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