Everything else being equal, how would reduced planetary mass and gravity affect the behavior of the atmosphere and the climate?
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$\begingroup$ Can you make your alternate Earth denser, so that the surface gravity is the same as on our Earth? $\endgroup$– void_ptrCommented May 23, 2022 at 22:43
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$\begingroup$ No, the point is I want a planet with the lowest realistic gravity possible. But I don't know how to envision the climate on this planet :/ $\endgroup$– ElhammoCommented May 23, 2022 at 22:45
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1$\begingroup$ @N E Techno Tech Blog - I've already done the research to figure out how realistically low a planet's gravity can be while still being strong enough to maintain a long-lived atmosphere (this involves the mass and density necessary for strong internal dynamo and magnetosphere, as well as an escape velocity high enough to hold on to water vapor), so that's not an issue. I guess I'm just wondering how a less dense but more expansive atmosphere would behave. And would a smaller planet heat more evenly or less evenly? $\endgroup$– ElhammoCommented May 23, 2022 at 23:11
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$\begingroup$ @Elhammo Yeah, I just checked out how much less dense Mars actually was and was surprised by the result. Earth is just very dense. So I realized that my comment didn't make much sense after I posted it. $\endgroup$– N E Techno Tech BlogCommented May 23, 2022 at 23:32
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$\begingroup$ Hello Elhammo, welcome to worldbuilding. I'm trying to go out of my way to not close new user questions today... If you read help center and help center you'll learn that this question is far too broad. Climate is complex when a lot of variables are known - and you've given us practically none (what kind of star? how far from the star? What makes the planet less massive? (That affects the magnetosphere....) Are there moons? Is there water? How much? (less massive comment, can change the surface area...)) This Q is not specific. Qs are required to be specific. Please read those pages. $\endgroup$– JBHCommented May 24, 2022 at 3:45
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2 Answers
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If Earth had the same diameter and it's mass was reduced by 25 percent, its gravity would also be reduced by 25 percent, but it escape velocity would be 86.6 percent what it is now.
To put that into perspective, Earth's current escape velocity is 11.184 km/s and 86.6 percent of this is 9.686 km/s. The escape velocity of Mars is 5.025 km/s. This means its atmosphere would be less dense than what it is now, but denser than that of Mars.
From this diagram,
The atmosphere would contain fewer volatile gases and quite possibly less water vapor. Retaining nitrogen and oxygen would not be an issue. With less water vapor there would be less rain.
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A colder world
I am unclear about what you mean by the "atmosphere scaled down proportionally"? Is it just the surface pressure that is 25% less than on Earth? Or is the atmosphere's total mass 25% less than that of the Earth's, just as the mass of your planet is scaled down?
If the total mass of the atmosphere is the same as Earth's atmosphere, the surface pressure would be 25% less, since the (gravitational) weight of the atmosphere would be 25% less, similar to the air at altitudes of ca. 2.5 km. At this altitude on Earth, the air is roughly 15 °C colder than at sea level, and so I imagine will the surface of your planet (unless I am missing something; I am not an atmospheric scientist). The average surface temperature of Earth is 14 °C; such a reduction would bring the average surface temperature to just below freezing (since pressure doesn't change freezing temperatures much). Much more of the ocean will be permanently frozen, and winds (which are driven by heat) will be less strong overall.
If the total mass of the atmosphere is 25% less than Earth's, the surface pressure will be 44% less than on Earth - roughly the same as on a 5 km mountain top on Earth. By the same argument as above, that will make surface temperatures roughly 32 °C colder than on Earth, or down to -18°C. Even Nuuk, Greenland, is warmer than this in winter. This will make for a much colder planet, with only equatorial regions being habitable.
answered May 24, 2022 at 13:09
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$\begingroup$ Thanks for the reply! I meant 25% less mass in the atmosphere as well. But maybe I'll keep the mass the same. In that case, I don't think it would be as cold as earth altitudes of 2.5 km, bc the air column at sea level on this planet would be a lot "thicker" (extending higher up) right? And therefore trapping more heat? Especially with the same mass of atmosphere on a smaller planet, the air column over any point should contain more air. Gravity would just be putting less downward force on it. So I'm not sure how that would affect temperature, but I don't think it would be that much colder. $\endgroup$– ElhammoCommented May 24, 2022 at 16:39