What can I see on the flyers' world?

Question

Planetary conditions: ~0.5x Earth gravity (guess: 0.9x Earth density, 0.777x Earth radius, so about 0.42 Earth masses... but that's probably not important). Atmosphere broadly similar to Earth but ~10x surface density and pressure. Because of half gravity, scale height is 2x Earth, ~17km, so column mass of air ~20x Earth. Surface temperature also similar to Earth.

This is the sort of planet where it's easy to fly (hence my title). But what other effects does it have? Specifically, how does it affect what you can see?

Based on rough figures I've seen... I expect the optical depth will be greater than 1. Thus, you can't see stars on a clear night, and the planet's sun will be visible as an ill-defined bright patch as if through thin cloud. It won't be severe enough to prevent seeing to the horizon, though.

I'd like someone who knows the science and math behind this to check these conclusions and see if there are any other obvious effects I'm not thinking of.

Answer 1

According to Wikipedia, increasing atmospheric density would increase the index of refraction for the atmosphere, causing astronomical objects, such as the sun and moon, to appear higher in the sky. This would cause the sun and moon to appear to rise earlier and set later than they actually do.

One other effect is magnification of astronomical objects. Since the gravity of the earth is less, the atmosphere would be larger. Since the atmosphere is also denser, it would act like a lens, magnifying the sun and moon. This would cause the planet to be brighter and hotter than it usually would.

Answer 2

A few effects off the top of my head:

Sky Coloration

A thicker atmosphere favors higher frequencies of the sky at midday and lower frequencies at sunset due to Rayleigh scattering. If your planet orbits a boring old G-type main-sequence star like our sun, its sky will likely be a dim violet, as the dominant frequency should be somewhere in the ultraviolet, and at sunset, it should be dim red or black.

If your planet orbits a red dwarf, on the other hand, the sky could be orange, yellow or even green at midday (still red at sunset) depending on things like the amount of dust in the atmosphere (that causes a separate process called Mie scattering, which is why Mars's sky is yellow). No matter what, there won't be a lot of light reaching the surface (though probably still more than how much reaches the Earth at night), but anything that evolved there will be used to that.

Climate

High pressure at the surface makes it much harder for surface water to evaporate, so your planet will tend to be drier than Earth. That said, climate is incredibly complicated and full of feedback loops, so far instance if you have a large forest biome, the water exhaled by the plants could still result in rain on its own.

Since your planet has a high cloud layer and high atmospheric drag, rain will take a long time to reach the surface, and indeed will likely evaporate and condense multiple times before touching land. This means that rain falls far from where it evaporated, which I think will make your planet more ecologically homogeneous. Fewer sharp boundaries between forest and desert; every ecosystem will be larger and more blurred together with its neighbors.

Visibility

Also due to Rayleigh scattering, a thicker atmosphere makes faraway things harder to see, since the light that reflects off of them gets scattered. It's a negligible effect on Earth, most noticeable when looking at mountains near the horizon (they kind of seem to fade into the sky). But on your planet, it will be much more significant, not just because there's more atmosphere, but because everyone's flying.

The distance to the horizon is pretty much directly proportional to your height, which means flying things can potentially see a lot farther. I'd expect them to hit this visibility limit way more frequently than we do on Earth, to the extent that the horizon usually won't be visible unless they're very close to the ground. Instead, the ground will gradually fade away like there's a thin, blue-violet fog.

Answer 3

I am not going to cover the ground of other answers or comments, but I would expect that mirages and distortions due to vortecies in such a dense atmosphere to be more pronounced due to the higher refractive index. But, you could have species that have evolved corrective lens systems in their eyes that can partially correct for this.

Also, have you considered how this is going to not only affect visual effects but also problems such as drag on flyers? Flying may be easier but top speeds may be slower, just a thought and if top speeds are slower then the evolutionary pressure on high resolution, high refresh rate vision (eg a pidgeon sees twice as many images/frames per second as humans ~50 vs -20 frames per second) or distortion correcting vison may be lower.

How beings eveolved on this word will affect how they see,not just the density or the air at sea level.

At some altitudes light would behave just as it does at sea level on earth. But the effect of changing pressure on a set of eyes, could mean if you fly too high your eyes explode. Or those that do fly high have 2 sets of eyes and close one and open the others as they climb. They may not use squishy eyes like us that would be prone to deformation under large pressure changes, but have compound or soild sate biological crystal eyes.

How you see on a world is not just a function of the atmosphere, but at sea level it would be very different to earth, for a human.