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I am trying to develop an alien species that could live on Titan and at first I supposed it would be easy with the species being able to use yellow, red and near-infrared wavelengths of light to see.

But then realised that the windows in Titan's atmosphere aren't as conveniently placed as those of Earth's and that blind spots could be caused by the absorption lines of certain chemicals, the most annoying absorption lines being those of the liquid on Titan's surface.

My question is if there is a window for both Titan's atmosphere and Titan's liquid? Would this result in a liquid with an appearance similar to water to our eyes or result in a liquid that would appear almost opaque?

A world where "water" is opaque sounds like an fascinating story writing opportunity due to how terrifying it is as a concept but I have no idea what liquid methane/ethane is like.

Water in Visible light compared to near-infrared

Also, if the liquid is opaque how does it effect a creature's ability to see at all since most eyes contain liquids. Would I have to copy the rattlesnake solution to the problem of infrared and also how would a creature's eyes (or some other vision apparatus) need to change to see near infra-red?

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  • $\begingroup$ The athmosphere of Titan is, as far as I know, a chemically very complex and not completely understood phenomenom. There is alot of nitrogen, methane and argon, but also a load of other, currently not even identified stuff floating around. The lakes on Titan seem to be nearly perfectly clear. But how all of that adds up for an answer to your question, i do not know. $\endgroup$ – DarthDonut Jul 12 '18 at 6:54
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There are three ideas we need to deal with: absorbance, transmittance, and reflection. (good starting point here.) Let's think of fluids approaching a strainer.

  • Absorabance is a measurement of how much the strainer will capture and hold (light won't pass through it, but it won't reflect off of it, either).

  • Transmittance is a measurement of how well the strainer lets things pass (light passes through the material without stopping).

  • Reflection is a measurement of how well stuff will bounce off the strainer (light bounces off the material).

I'm also fond of the cherry analogy. Somebody throws a cherry at you. Did you duck, letting the cherry pass by (transmittance), catch the cherry in your mouth and swallow (absorbance), or catch it and spit out the seed (reflection)? Don't laugh, metaphors are rarely perfect.

Simplistically, water and glass have high transittance, low absorbance, and little reflection when looked at straight on. (Change your angle of view, though... now all the physics about light transitioning between mediums kicks in).

A common green leaf has high absorbance of red wavelengths, high reflectance of blue and green wavelengths, and low to no transmittance. Which means you see green, don't see red, and the ground beneath the leaf is dark.

Water's absorbance of IR is very good

Water aborbs IR. No reflection, no IR on the bottom of the pool, it's opaque.

Before I continue, remember to separate "how we see it" from "how it would be seen." We "see" the green reflected from a leaf, so it looks opaque (and green). We don't see the red absorbed by the leaf so it's "invisible" (meaning, it doesn't look red, if only looking in the red spectrum, it would look black).

It's helpful when dealing with aliens to think of what they "see" in their spectrum being symbolically identical to what we "see" in our spectrum. The short wavelength is blue, the medium wavelength is green, the long wavelength is red. They would symbolically (or perceptually) have the same kind of differentiation. What we'd call red they'd call blue and what we can't see they'd call red.

Why is this important? If you chase that "water absorbs IR" link you'll see that the deeper you go into IR, the less absorbant and more transmittant water becomes.

Which means, to a denizen of Titan (I believe there's enough of the IR spectrum there to do this, correct me if I'm wrong), a gently undulating surface of water would look remarkably like a lake full of low viscosity tar or oil would to us — black with bright reflections at the longer wavelengths.

As for the atmosphere, depending on what's actually in it (good point DarthDonut), they might see fog were we see clearly. They'll see clouds just like we do, but what's causing the clouds is a very different matter.

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  • $\begingroup$ Thanks JBH for the great explanation! I had a suspicion that the IR would cause problems though makes it interesting to describe! $\endgroup$ – Merlin Rowlands Jul 14 '18 at 11:33
  • $\begingroup$ Though how would a native inhabitants would adapt to having opaque "water" will require some thought. Supposedly they would be terrified of being ambushed from the water in the same way crocodiles and pythons do. $\endgroup$ – Merlin Rowlands Jul 14 '18 at 11:42
  • $\begingroup$ Possibly. Consider how creatures act in muddy areas. Although, if you grew up with it... water being as basic as it is. Your tears and saliva would be opaque. You'd drink it all the time (assuming similar biology here, but as common as water is for life, maybe that's not unfair). It would be magic the way you'd bathe in this opaque stuff that evaporated into nothing. $\endgroup$ – JBH Jul 14 '18 at 14:36
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JBH's answer is excellent, so this is more of an adjunct to it than a totally separate answer.

There are organisms on Earth that have very interesting vision, including in the IR spectrum. I'm thinking mainly of Mantis shrimp, who have all sorts of options for seeing things. As JBH's answer points out, vision (in the general sense of the word) depends on what light actually makes it into the eye, and so the chemical absorption is less important than the absolute amount of light that is present to "see", through any mechanism. The major issue would be sensitivity to differences in wavelength, so that differences in shade could be determined.

So the darker the environment, the less likely "eyes" are to be well formed, or even present. Compare with the deep sea or deep cave complexes on Earth, where eyeless creatures abound.

Depending on your interests and needs, you can also be looser with "vision". Echolocation can give a solid impression of the precise dimensions of physical objects in 3-dimensional space, though color becomes irrelevant in that case.

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  • $\begingroup$ Thanks for the help Upper_case! Would echolocation be able to pass through the surface of the liquid that well? I'm pretty sure that fishing bats use echoes to detect the ripples in the surface created by fish rather than the fish themselves. $\endgroup$ – Merlin Rowlands Jul 14 '18 at 11:45
  • $\begingroup$ @MerlinRowlands Honestly I'm not sure. I suspect it would still work somewhat (when you put your head underwater, you can still hear sounds from above the surface, but that works better with low-frequency sounds which is the opposite of echolocation). So maybe a bit below the surface, but not by very much? We could get a good answer on Physics.SE, I think. $\endgroup$ – Upper_Case Jul 14 '18 at 17:08
  • $\begingroup$ I probably will go and ask sounds like the oceans of these worlds will have radically different creatures to our own. Since here on Earth almost everything non-mammalian can't use sound and hence depend on vision instead which can be seen also in bioluminescence so it sounds like one would have to redesign these sea creatures for their new surroundings. $\endgroup$ – Merlin Rowlands Jul 16 '18 at 9:47

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