Following our attempt to measure the colour of HD 189733b (see this post), I have taken a look at what determines the colours of Solar-System giants in the visible. I found the result interesting and in some ways unexpected – at least for me, genuine planetologists probably learn this in first grade.
To first order, the aspect of our four giant planets simply reflects how deep the clouds are in the atmosphere.
The four planets have very different aspects in the visible; three different aspects in fact, since Uranus and Neptune are very similar. Jupiter is a psychedelic maze of white, orange and brown, Saturn is yellow to orange, and Uranus and Neptune are blue (add “-ish” to all these colours because the exact tints are subtle and changing).
On Jupiter the white parts tend to be high clouds (of ammonia), the red parts lower clouds, coloured by organic molecules in the droplets. Traces of blue show in the spaces between the clouds where deeper layers are visible through Rayleigh scattering, or near the edges where the slant viewing angle has the same effect.
“Murky” hazes make the colour of Saturn orange, while the spectrum of Uranus and Neptune in the visible is dominated by Rayleigh scattering and methane absorption. Rayleigh produces blue reflection, and methane absorbs some of the red.
What I didn’t realise is that there is an underlying sequence to these spectra, with more unity than appears from the images.
The key is that as we move away from the Sun from Jupiter out to Saturn, Uranus and Neptune, we see the clouds through more and more of the atmosphere. This for two reasons that reinforce each other. First, as the Sun gets more distant and the temperature cooler, the ammonia clouds sink to lower levels in the atmosphere. They are near 1/2 bars in Jupiter, 1 bars in Saturn and 3 bars in Neptune (methane clouds also appear on Uranus and Neptune, but they cover only a small fraction of the planet and appear quite white).
The second effect is slightly trickier. The gravity on Jupiter is about 3 g. Saturn is only slightly smaller, but much lighter, so the gravity is about 1 g. The gravity is also near 1 g in Neptune and Uranus. A lower gravity means a thicker atmosphere. At the same pressure level, there will be three times more stuff overhead on Saturn than Jupiter. Since the position of clouds is mainly fixed by pressure (through the pressure-temperature profile), but absorption by the rest of the atmosphere is mainly proportional to the amount of stuff, this also implies that sunlight will cross more of the atmosphere to reach the clouds in Saturn, Uranus and Neptune than in Jupiter.
Seeing the ammonia clouds through a significant part of the – slightly hazy – atmosphere is what gives Saturn its yellow colour. Hiding the clouds below most of the atmosphere, thus giving Rayleigh scattering and methane absorption enough matter to operate, is what gives Uranus and Neptune their blue colour. This is analogous to the way colour changes in mountain landscapes on Earth, between the foreground, distance and background. Nearby hills can have vivid colours, more distant mountains can be made slightly brownish and indistinct because of the dust in the air, and very distant mountains are blueish because of Rayleigh scattering.
Thus, all Solar System giants might have a similar aspect if they were “naked”, but Saturn has a thin cashmere sweater, and Uranus and Neptune thick parkas. Only Jupiter shows its gory innards.
This opens an intruiguing perspective: a giant planets more massive than Jupiter (and we know that these are relatively common) would have even less haze absorption and Rayleigh scattering in front of the clouds, and might look like an hallucinatory version of Jupiter, with even the water clouds visible in the deeper gaps.
Extended to the topic of Earth-like planets, it reminds us that the clouds of an habitable exoplanet in the visible might vary strongly with the thickness of the atmosphere and the surface gravity.