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This Query is part of the Worldbuilding Resources Article.

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.

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....)

elemtilas
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JBH
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3 Answers3

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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.

Tim B II
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  • The question specifically mentions to ignore any thermal mass effects of the planetary surface. The surface has no effect on the 'weather'. Thus, it must be assumed that it has zero heat flux. I know, impossible, right? But that is what the question asks. Eliminate all variables. As soon as you allow heat transfer through the core, the dynamics change, and it is more than just the atmosphere that is moderating weather. If you assume perfect heat transfer through the planet, the back side is the same temperature as the bright side, irregardless of the atmosphere. – Justin Thyme Feb 01 '18 at 17:38
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    A 'day' on Neptune is 16 hours, so it spins much faster than earth. The planet surface is petty much equally warmed by the sun. Heat is equally distributed around the planet. Mercury is the best analogy to the OP requirements, and was basically my model for what would happen, although the temperature differential between the hot side an the cold side is much greater. But it is not a perfect analogy, because there would be some thermal transfer within the body of the planet due to convection. – Justin Thyme Feb 01 '18 at 17:47
  • Hi Justin. Yep, I missed the thermal mass part because it was hidden in 'planet doesn't cause any effects' which I assumed (given the surrounding passage) meant in a kinetic context, not thermal. While I agree with you about Mercury (an Neptune), eventually heat has to dissipate into space, meaning that if you keep energy input closer to the equilibrium of heat dissipation, thermal currents would occur through pressure differential alone I suspect. I still think your atmosphere might survive without planetary thermal mass IF you introduce the heat slowly enough, or am I wrong? – Tim B II Feb 01 '18 at 19:28
  • Both you and JBH might be interested in the following article How to Get an Atmosphere which gives a pretty good description of how atmospheres are created in the first place. The factors that ensure an atmosphere are also the factors that will affect climate. If you have one, you have the other. You can't HAVE an atmosphere without thermal mass. Specifically, an internal source of heat. – Justin Thyme Feb 01 '18 at 22:33
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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.

Gary Walker
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  • Should that be 'LESS than 240 hours' or 'MORE than 240 hours'? The term 'threshold' implies that it has to be MORE. From the cited article abstract ' For an Earth-sized planet, the dynamical regime of the substellar clouds begins to transition as the rotation rate approaches ∼10 days. These faster rotation rates produce stronger zonal winds that encircle the planet and smear the substellar clouds around it,' That is, it takes FASTER rotation. – Justin Thyme Feb 01 '18 at 17:25
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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,

Justin Thyme
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  • So, I can't assume a high pressure zone sunward, a low pressure zone spaceward, and turbulence around the edges? I congratulate you for pointing out that, without all the possible variables, you can't get all the possible effects. But I'm downvoting your answer because you haven't put any thought into this at all. – JBH Feb 01 '18 at 01:28
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    There are a lot of very strong statements here, and nothing really to back them up. – HDE 226868 Feb 01 '18 at 01:30
  • Since you have not built in ANYTHING that will modify the climate, and absolutely nothing that moderates it, I can not see any factor that would CHANGE it. No factors that would change the atmosphere. Nothing that would change the elemental composition of the atmosphere. No spin, no Coriolis effect. No moon, no tidal drag of the atmosphere. The only change would be due to unpredictable meteor impacts. I thought about the potential air currents from air heating and rising at the dead-center bright side, and cooling and falling at the dead-center dark side. – Justin Thyme Feb 01 '18 at 01:47
  • ctd But these would be unchanging and static. I am afraid without any reference to the composition of the atmosphere, or its thickness, calculations regarding heat flow are virtually impossible. At optimum (for homeostasis) I assumed most heat that arrived on the bright side would be conveyed to the dark side through these currents, and then radiated into space. No radiation sources, no magnetic field, no ionization sources, there does not have to be much thought put into it. It can not be anything BUT a stabilized weather pattern as described. The weather is that there IS no weather change. – Justin Thyme Feb 01 '18 at 01:53
  • ctd The worst case scenario, as I stated, would be that the high pressure on the bright side would carry all of the atmosphere into the stratosphere, where as I said it would burn off into space. That would draw the atmosphere from the dark side, until the dark side became cold enough to freeze the atmosphere. You have not allowed for ANY water source. I can't imagine the atmosphere being much other than hydrogen, maybe helium, unless it was deposited by some asteroid, in which case the composition would be static. Unless more asteroids replenished it, but that is unknowable. – Justin Thyme Feb 01 '18 at 02:01
  • ctd When you eliminate all the variables, there isn't much left. As I said, there is only one spherical horse in the race, and that horse is not hard to describe. – Justin Thyme Feb 01 '18 at 02:02
  • As stated in the qustion: atmosphere is Earth equivalent. There's obviously a thermal gradient between the sunward and spaceward sides of the sphere. There is gravity that is acting tangentally to all atmospheric effects. The solar effect at the equator is greater than the solar effect at ther terminus. The air is not standing still with nothing to do, gas dynamics are still in play. I suspect there's a lot going on. Spherical horse, Justin. You did it as a freshman in college. Assume the atmosphere doesn't boil into space, what would it do? – JBH Feb 01 '18 at 06:54
  • And to make a point, the entire premise of this exercise is to avoid modeling an infintely complex system with itself. As an engineer I had to begin designs with boxes that said "magic happens here" all the time. It doesn't invalidate the initial design, it allows the system to be scoped and interactions to be identified before delving into the details of compartmentalized operations. If you don't do this in your life now, you surely did so as a student. – JBH Feb 01 '18 at 06:57
  • You have OVER-simplified. The starting point should NOT be an atmosphere, it should be some form of more efficient heat transfer. The dark side can NOT stay at near absolute zero, and the bright side unbelievably hot, without getting what I have described. The earth's atmosphere can not just be assumed, it is a product of ALL of earth's climactic variables. Oxygen levels, CO2 levels, the ozone layer, the humidity level, particulate level, all have been a RESULT of the heat exchange due to the earth's rotation. – Justin Thyme Feb 01 '18 at 18:02
  • I am well aware of 'black box' design. Reverse design. You start with what you want, and go backwards to keep adding what you need to get there. But in this case, you are adding the wrong thing to your black box as your first variable. You start with the most significant variable, and go to the least significant. It's like designing a hydro-electric project, and starting with the size of the generators, assuming that the water flow is adequate, then looking for a site to match. The SITE is the crucial variable that drives all other design. – Justin Thyme Feb 01 '18 at 18:18
  • In this case, planetary rotation is one of the MOST significant variables, along with water on the surface so that it can GET into the atmosphere. Atmospheric effects come after. – Justin Thyme Feb 01 '18 at 18:18
  • @HDE 226868 Yes I suppose it COULD be dark on the side towards the light, and bright on the dark side. It could be cold on the heated side, and warm on the side away from the heat source. It might have a day without rotating, and it might even have seasons without any orbital change. It might be unlike any planet in our system, and start with an atmosphere other than helium or hydrogen, or this atmosphere might not be burned off the way it was on every planet in our system. – Justin Thyme Feb 04 '18 at 00:10
  • ctd It might have an atmosphere with out any internal heat source from the planet, unlike ANY other planet or body with an atmosphere. And maybe it might have water in the atmosphere, even though there is none on the ground. No ice, either. No hydrologic All of that is pure speculation on my part. The planet Mercury, after all, is just a figment of our cosmological imagination. But dang it, I absolutely insist there is only one spherical horse. – Justin Thyme Feb 04 '18 at 00:15
  • -1 > Comments after the fact do seem to slightly address some of the OPs original question, but the answer, as posted, doesn't do what the comments of the answerer claim that they do. For example, one comment says "...as I stated, would be that the high pressure on the bright side...", but this isn't actually stated in the answer, only in the comments. The question was about what the weather pattern would "be", not about how it would "change" but the answer only mentions changes, and doesn't actually describe the existing weather, as the asker originally asked. – Harthag Nov 12 '18 at 17:17