# What could cause a difference in coloration between the hemispheres of a gas giant?

Would it be possible for a gas giant to have differently colored hemispheres?

E.g. the northern hemisphere appearing blue when looked at, while the southern hemisphere would appear brown? (or any other combination of coloration such as red & green, etc.)

What processes would cause such a world to be formed?

• this might help you out: en.wikipedia.org/wiki/Sudarsky%27s_gas_giant_classification the colour is affected by temperature so I don't think so – Styxal Apr 12 '17 at 8:17
• look at the bands of jupiter: each band goes in an opposite direction to the surrounding bands, suppose you have the northern hemisphere spinning clockwise and the southern hemisphere spinning anticlockwise then you may get different gasses in them therefore different colours – Alex Robinson Apr 12 '17 at 8:19
• @Cursed1701, how can the norhern half of a planet spin in different direction with respect to the southern half? – L.Dutch Apr 12 '17 at 8:30
• @L.Dutch a writers best friend - Handwavium – Alex Robinson Apr 12 '17 at 8:30
• @Gstestso I've had a go at trying to make your question a little more structured/clearer (in regard to both: title & body) - please feel free to roll back to your previous version should I have misunderstood your intent :) – dot_Sp0T Apr 12 '17 at 9:49

The planet should be visible in one color from a long distance according to @Styxal's answer.

On the other hand, when you are close enough to see clouds in its atmosphere then you have some other options:

• Jupiter has the Great Red Spot and we don't know exactly what is it.
• [Sci-fi] Imagine a gas which reacts with light from the star (chemically or like a laser) and this reaction causes it to change color for a short amount of time. An observer will see an area with a different color and a gradient around this area due to winds.
• [Alien] A very, very big factory which pollutes the stratosphere. It would be a spot of different color with a gradient around it.
• [Alien] Messaging system. Aliens change the color and the observer from another star could see it and make a decision - send supplies or an army
• you've made me think here, and something like light pollution could maybe change the colour of one side – Styxal Apr 12 '17 at 9:45

Some others have pointed out Sudarsky’s colors, but ignored that this is not the whole story:

The discrepancy [that neither Jupiter nor Saturn actually look like Sudarsky’s type I color] can be partially accounted for by taking into account non-equilibrium condensates such as tholin or phosphorus, which are responsible for the coloured clouds in the Jovian atmosphere, and are not modelled in the calculations.

So, a “non-equilibrium process” produces a strong color, and is confined to one hemisphere either because the colors vanish before it crosses the equator, and the process is associated with the location.

Consider that spots like the Great Red Spot can move around but can't cross tbe equator. Suppose that's true for air mass in general — the convection cells are separate for northern and southern. So, if something like a Big Red Spot was spewing tholins into the air, it would tint the entire hemisphere. But the mixing of air across the equator is much smaller in magnitude, and the color has a limited lifespan (either it disappears as molecules break down, or it returns to its proper deeper layer). So, it doesn’t color the whole planet.

Note that Saturn has an unusual feature at one pole. Something that exists at one pole only would be perfect for producing the color: either dreging it up from a deep layer, or producing molecules through some energetic process.

Another possibility for the latter would be the presence of a star bright in UV located in the north (or south) sky. This causes novel molecules to be produced where the star is visible. The slow production is balanced with the slow disintegration and slower cross-hemisphere mixing of the material.

A more exotic solution would be related to hyperfine splitting or shifting. Some chemical process happens slightly differently depending on whether it’s in a north or south magnetic field; this slight change gets magnified through shifting equilibrium etc.

Well, that seems difficult since the magnetic field alone won’t break the symmetry. But you don’t need anything, if the breaking is spontaneous. You have groups of interacting molecules that can settle into different possible equilibrium states. It happens to be in different states in north and south hemispheres, thanks to low mixing between them. It might be seasonal, with different temperature making the difference; so the summer and winter hemispheres will have characteristic colors.

If you have a planet which is tidally locked with its star (a configuration which would be slow to achieve especially if there are other planets, but not impossible), you can get massive temperature disparities. The problem here is that convection streams in the atmosphere would distribute the heat, minimising the temperature difference.

A $~200K$ temperature difference isn't extreme however. In this case the dark side would allow the chemical interactions to form ammonia clouds, at the rim in the twilight you would see water vapour clouds.

Gas giants are known for their long lasting storm patterns. It is possible that the kinds of storms of the light and dark side would disintegrate at the boundaries due to chemical changes, at which point there would be little convection between the two sides, and the sides would retain their relative temperatures.

This is all highly speculative, I have no concrete hypothesis or evidence to back this up, but I can't disprove the principle.

Your example specified north/south hemisphere, but perhaps you're open to east/west. This answer is weird and may need some refinement, I admit.

Suppose you have a moon in geosynchronous orbit so it is always over the same half of the planet. That moon would reflect light onto the one hemisphere. That light would shift the reflected color of the planet. If the gas of the planet is particularly reactive to, for example, UV light, the extra light from the moon might make a significant difference in the apparent color.

So what makes the moon glow? Well, a phosphorous moon in an oxygen atmosphere might do the trick. Our Earth moon is heavy with phosphorous. It lacks an atmosphere. But if it had one, you might get continuous discharge from the moon glow. https://www.thenakedscientists.com/get-naked/experiments/phosphorus-moon

I'm stretching a lot of science here to create plausible scenario. Some of the other commenters may be able to poke holes and refine it.

• Wouldn't the burning rocks be a temporary phenomenon and then get used up? – JDługosz Apr 12 '17 at 13:44
• @JDługosz Sure, but it could potentially burn for a long time... processes on the scale of moons can be very long... for example, look how long a moon has been raining on Saturn. – SRM Apr 12 '17 at 21:13

The simplest explanation would be seasons. The gas giant has a tilted rotation axis, so one pole is in sunlight all day long, the other in darkness, for six "months" (which could last several years).

This affects wind patterns and surface temperatures, and results in different compounds floating to the visible surface.

This behaviour can become even more extreme if life enters the equation (you could have photosynthetic "algae"-equivalents flourishing during summer and hibernating throughout the winter).

Styxal's answer is but one part of the story. The colouration is however not affected only by temperature but also by chemical composition.

This means that if through some means you could have your planet have a markedly different chemical balance in the northern as compared to the southern hemisphere (possibly combined with different temperatures) you might end up with a planet that appears to have different colours between the hemispheres.

However, this is harder than you might think. How are you going to prevent diffusion of parts of the atmosphere, eventually homogenising the entire ball into one more or less consistent chemical cocktail?

You'd need some very strange wind patterns near the equator, and going down very deep into the atmosphere to prevent that happening, patterns I'm far from convinced could ever develop long term in nature even on a far smaller scale than that.

For a gaseous giant planet to be blue it would need an equilibrium temperature of between 350K(170 °F, 80 °C) and 800K(980 °F, 530 °C). To be red/brown the equilibrium temperature would have to be less than around 150K(−190 °F, −120 °C). This is because of the density of gases and how they refract light I guess.

With some hand-waving you could somehow make one side of your planet an ice giant planet such as Neptune, which has a different make-up of only 20% hydrogen and helium compared to gas giants which are (in the case of our very own Jupiter and Saturn) more than 90% hydrogen and helium. But ice giants are formed differently and you would expect the atmosphere to all just mix together so I don't know how that would work. Maybe with some of @Cursed1701's handwavium :-)

Sudarsky's gas giant classification (Wikipedia)

Ice Giant planets (Wikipedia)

I would also like to add that you could possibly use light pollution as an excuse if you intend to have your planet be inhabited.

• Neptune is clearly blue, and it's only 70 K. Why do you think that blue pigments can only exist at a certain temperature range? – JDługosz Apr 12 '17 at 12:57
• Neptune isn't a gas giant it's an ice giant @JDługosz – Styxal Apr 12 '17 at 13:02
• Can you cite a mechanism and explain in your answer? Why would certain pigments only exist in a specific temperature range, and what do you know about the composition of the OP’s planet? – JDługosz Apr 12 '17 at 13:09
• www8.nationalacademies.org/ssbsurvey/… Ice giants are only 20% hydrogen and helium compared to gas giants which are (in the case of Jupiter and Saturn) more than 90% hydrogen and helium. They're known as ice giants because their constituent compounds were ices when they were primarily incorporated into the planets during their formation, either directly in the form of ices or trapped in water ice. I suppose it could be possible to have one half of the planet be made differently though with some handwaving, like Cursed1701 said in the comments. – Styxal Apr 12 '17 at 13:26
• «, I don't study this or anything, I just spent a bunch of time on google before answering» that's normal for these kinds of subjects as we have around here! – JDługosz Apr 12 '17 at 13:43

## Axial Tilt

It wouldn't be a year-round solution, but if the planet's axial tilt was very far over (like Uranus's), then during each hemisphere's summer, it would receive almost the total amount of incoming sunlight. That could be enough difference to cause a visible change between the two hemispheres. The effect would lessen and vanish during the spring and fall, though, as it swapped hemispheres.

Giant Impact

What if the gas giant suffered an impact that discolored the cloud tops? Look at what happened with Comet Shoemaker–Levy 9 broke up and struck Jupiter over a several day period.

If your gas giant suffered a concentration of impacts on a much larger scale in one hemisphere you could get the effect you are looking for. The winds of the planet would likely keep the soot kicked up from the impacts segregated into one hemisphere. A bombardment severe enough could leave marks that are visible fro years.