Worldbuilding & Climate: How does the sun affect a simple sphere with atmosphere?

Question

This Query is part of the Worldbuilding Resources Article.

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I'm working to build a procedure that will help me understand the climate of the worlds I create. I'm beginning with simple and building to complex because, frankly, lacking anything that could be construed as a PhD in climatology in even the weakest light with Def Leppard playing at distracting levels in the yard next door, I need to begin at the beginning.

So let me pause and tell you a joke. The Mob was tired of fending off the cops when they fixed the races, so they "invited" a physicist to build them a simulator that would predict the winning horse every time. After months of "motivated" labor, the physicist finally produced his masterpiece. A delighted mob bet big on "Haley's Shadow," with odds 9-to-1 to win — and lost everything. When the Mob suggested that a bit more than a pound of the physicist's flesh would be required, the very perplexed man said, "I don't understand! It worked fine with my spherical horse!"

So, let's start with a spherical horse.

What are the specific weather patterns that would develop and (I presume) stabilize over time under the following conditions?

• Given a star similar to Sol, which is a G2V class star with a solar luminosity (L☉) of 1.0.

• Given the sphere (I hesitate to use the word "planet" at this point) is always within the star's habitable zone.

• Given a sphere of mass and volume similar to Earth.

• Given an atmosphere with Earthlike composition and density.

• Finally (and this is the important part), the sphere DOES NOT rotate, DOES NOT orbit, has a perfectly smooth surface, and the surface DOES NOT contribute to climatological effects. (I believe there's enough fiction in this single bullet to justify asking the question here... but y'all can tell me otherwise.)

I'm looking for a first-step explanation. Simple, simple, simple, simple, simple. With one exception...

It would be cool if the answer could accomodate variations in solar luminosity and the sphere's (OK, the planet's) volume. Or, if it's more appropriate, an explanation as to why solar luminosity and planetary volume don't matter.

I can actually imagine an argument like, "as luminosity increases, the habitability zone is pushed out, ditto with planetary volume, thus the general effect is always the same... at least if you want human-like life....

Which, of course, I do.

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EDIT:

1. When I say the surface of the sphere does not contribute to climate effects, I mean that I want to deal with water, soil, elevation, etc., in a later question. Please assume this question is about the atmosphere and only the atmosphere. It's a gas dynamics question around a shape that provides gravity for the sake of the atmosphere and nothing else.

2. Yes, this question will lead to a good understanding of how climate works on a tidally-locked planet. But that's an issue for later.

3. Yes, assuming no orbit, no rotation, no surface effects is absurd. By the same token, all freshman physics classes are absurd becasue they all start with spherical horses. I did that on purpose, folks. It's impractical to hand a first-year physics student a graduate-level textbook in an effort to just jump to the solution. (If you don't believe this, it's been a while since you were a freshman....)

Answer 1

Interestingly enough, in all but two points, it sounds like you're describing Mercury.

Mercury is not in the habitable zone (and of course it's smaller than Earth), but being more or less tidally locked it's the same for all intents and purposes as a planet that is stationary in relation to its sun.

Justin's answer would at first blush be vindicated by Mercury insofar as Mercury has had any atmosphere it may have had baked off. That said, let's put it in the habitable zone, but up the very back of the bus, so to speak. In other words, let's make it as cold as possible while still allowing liquid water. I'll leave this for the next question; all I'm doing now is using water as a gauge of habitability, not as a thermal mass.

It's been a LONG time since I did fluid dynamics in any fashion, but my understanding is that a single heat source (and direction) will provide some energy, and that energy will create some turbulence. It has to. While Earth's rotation and the Coriolis effect creates turbulence in a known manner, I'm deeply suspicious that the greenhouse gasses allowed by your atmosphere would trap much of the heat close to the sphere, meaning that you end up with high pressure systems on the day side, and low pressure systems on the night side (heat adds energy, causing the gas to want to expand). The warmer heat near the surface of the sphere (which is retaining some of that heat and helping increase the warming of the atmosphere in an imbalanced way, biasing that close to the sphere itself) rises, creating a convection current because as the pressure reduces at altitude (Boyle's Law) the temperature will drop (slightly) and you have a current that will at least circulate on the day side, but because of the pressure differential will likely start to cascade through to the night side in a very different manner to that we would normally see with the introduction of kinetic energy (rotation).

While I'll leave water for the later question, it does give rise to the question of what your sphere is made of. Assuming it's as good a thermal mass as water, then you can expect your sphere (at least on one side) to store a lot of energy, creating a differential in thermal input that favours the atmosphere close to the surface. This capability to act as a thermal mass is perhaps key to whether or not your atmosphere will survive.

Why further out (coldest possible habitable zone)? Because of Neptune. If you look at Neptune, you see massive winds across the face of the planet that seem to almost ignore the Coriolis effect. Why? Lack of energy basically.

Once a wind gets going on Neptune, there's not enough kinetic or thermal energy to stop it, so it just keeps on going.

The point being; I'm pretty sure that you can maintain an atmosphere on a tidally locked 'sphere' provided you don't introduce all the thermal energy all at once, and you allow for the sphere to retain heat energy as a thermal mass of some kind.

Answer 2

I would recommend reading Life on a Tidally Locked Planet to glean a few ideas about tidally-locked planets, though it is not focused on weather alone.

This has actually been studied, though papers are not publicly available, but reading the abstracts of The Inner Edge of the Habitable Zone for Synchronously Rotating Planets around Low-mass Stars Using General Circulation Models, there appears to be a threshold of a 10 earth days orbital cycle. For orbits less than about 240 hours, an upper atmosphere jet stream, drives a reasonably effective global atmospheric circulation that moderates the extreme temperatures that would otherwise occur, making the planet potentially habitable.

Tidal-locking would be more common in planets that orbit their star closely, and and 240 hour orbit around a red-dwarf could easily be in the habitable zone.

A G2 star would be far too hot for a 240 hour orbiting planet, and a habital zone orbit around a G2 star would be far too slow to generate the necessary jet-stream to drive global circulation.

Answer 3

The answer, I am afraid, is elementary.

Dark on one side, light on the other, hot with increasing heat on the bright side, cold with increasing cold on the dark side, with no possibility of tomorrow. Or today, either. Since there ARE no days, or nights for that matter. Summer will never come, nor will winter. Not even fall or spring.

And absolutely no chance of precipitation. Rain OR snow. EVER. And no change in cloudiness. Same today, and today, and today. Forever.

The atmosphere, I am afraid, would just burn off on the hot side, and be frozen on the dark side. And it would have zero humidity. Period. No surface source of moisture.

But of course you can always play around with the composition of the atmosphere, even though there is no vehicle for adjusting the content of the atmosphere (no evaporation, condensation, volcanoes, surface features, or wind patterns, assuming the planet surface is perfectly homogeneous, and every part heats up or cools down equally).

EDIT

But if you want more DETAIL (as opposed to thought) the dark side would approximate absolute zero as there is NO source of heat. The bright side would approach extremely high temperatures, as there is no cooling effect. I am afraid the atmosphere on the dark side would be so low in temperature that it would freeze (no matter what the composition) and frozen stuff does not typically move 'in the wind'. I would expect the atmosphere on the hot side would be so hot as to burn it off directly into space, or if it went to the dark side, it would freeze. I can't imagine any scenario where there would be a retained atmosphere, except maybe convection currents on the hot side with whatever atmosphere was left.

Oh, and the horse would always win the race. Or loose it. Because there would only be one horse. Coming in first or last would be the same thing,