If you were standing on the proplanetary side of a habitable moon of a gas giant, and the moon had a thick enough atmosphere to make the sky blue, how would the gas giant look during the day? Would it look faint? Would you see the entire planet or just whatever side is more illuminated by the sun (I'm imagining the sun peaking out from behind the planet and illuminating one side, while the other fades to blue)? How would the planet look during the nighttime, when the proplanetary face of the moon is facing away from the sun and the planet is illuminated? Would it be bright and more saturated? And does this all depend on the density/thickness of the moon's atmosphere?
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4$\begingroup$ I don't think there could be a definite answer to this question. It is too generic. How a gas giant would appear as seen by its satellite depend on the size of the planet, it's albedo, the brightness of the star, the distance of the star, the distance of the satellite, the thickness and composition of the atmosphere... $\endgroup$– PrallaxCommented Jul 29, 2022 at 6:19
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2$\begingroup$ Maybe you could help by editing the question and fixing some of these parameters. You could say, for instance, how does Saturn look like when seen from Titan. Or, how would Jupiter look like if seen from a satellite orbiting at X distance and with the same atmosphere as Earth $\endgroup$– PrallaxCommented Jul 29, 2022 at 6:23
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$\begingroup$ @Uhoh the question specifies a habitable moon. Which habitable moon in our solar system should Elhammo choose as his example as you suggest? $\endgroup$– M. A. GoldingCommented Jul 30, 2022 at 17:17
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1$\begingroup$ M. A. Golding - Exactly. I can just look up a photo of Jupiter from Europa if I want, but I'm interested in how a gas giant would appear through an atmosphere (at different times during its orbit). $\endgroup$– ElhammoCommented Jul 30, 2022 at 18:41
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$\begingroup$ You can actually see Jupiter in the daytime (from Earth), it's just too small to see any detail. But a good approximation would be what it looks like through a scope in the daytime: google.com/search?q=jupiter+in+daytime $\endgroup$– Greg MillerCommented Jul 31, 2022 at 20:47
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Partial answer: the planet will appear roughly three times as bright as the Moon does in our daytime sky! (if the sky is like ours) and the part of the disk not sunlit will look "invisible" or "missing".
How big it will appear, or the fraction illuminated vs time will depend on details like the ratio of the moon's orbital distance to the planet's diameter. I'll leave that for other answers to address, but it's going to vary a bit like the phases of Earth's moon as seen from Earth.
This is an interesting question and it can actually be answered fairly independently of which moon, which orbit or which gas giant!
What we really need to nail down is which atmosphere!?
Since both the moon's atmosphere and the planet are illuminated by the same Sun at essentially the same distance, that drops out of the problem. We just need to look at the ratio of the sky brightness to the planet's apparent surface brightness.
Luckily our Moon presents us with an excellent example.
- The same way that a wall does not get brighter or dimmer as we walk towards or away from it, the average surface brightness of a resolved, extended object like the disk of a planet or moon does not really increase or decrease if we are closer/further from it. The total light varies as 1/r2 because the apparent size varies like that.
- The albedo of the Moon is pretty low actually, roughly of order 10%. It's actually slightly orange/brown because it's less reflective in blue and more reflective in red. For more on that see this answer to Why is this moon red?
- The albedo of gas giants in our solar system is 3 to 4 times higher than the Earth's moon!
- At the same time, the light in Earth's Diffuse sky radiation (i.e. blue sky) is quite blue due to Rayleigh scattering so while it might interfere with the blue hues of the planet's disk, the rest of the visible spectrum will shine brightly through. If the planet somehow had a "great blue spot" it might be nearly invisible behind a bright blue sky. For more on that see Why does Unity look transparent? and answers there.
So as long as the Moon's atmosphere has a blue sky that isn't overwhelmingly brighter than Earth's,
The gas giant would have a surface brightness ~3-4 times as bright as the Moon in our sky! (if the sky is like ours)
And there's the rub. A moon will likely have a lower surface gravity $g$ than Earth, which means that the scale height of the atmosphere will be proportionally larger: $$H = \frac{kT}{mg}$$ where $m$ is the average mass of a molecule of atmosphere.
So if the moon has a surface gravity of 0.1 and everything else is the same (it won't be) then the scale height will be more like 80 kilometers compared to Earth's ~8 km. With an atmosphere 10x thicker the sky will be much much brighter.
So let's hope the two effects cancel; albedo 3-4x higher and Rayleigh scattering 3-4x stronger making the planet's surface brightness relative to the "blue sky" in front of it about the same ratio as the Earth's moon behind our daytime sky.
| Body | Bond Albedo |
|---|---|
| Moon | 0.11 |
| Jupiter | 0.343 |
| Saturn | 0.342 |
| Uranus | 0.300 |
| Neptune | 0.290 |
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