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Alright, so I've been meaning to write out a planet with golden/yellow fields and a violet / purple ocean.

What I was thinking about is having the water of this world is so clean and nourishing that its very color is violet, as a sort of representation of its alkaline nature. I was thinking a possible explanation, taking from the real world from the likes of Rio da Prata in Brazil and many others where the water is so filtered and clean, it's crystalline in appearance. Anyway, I'm not sure if the same reason as to why a water would be clear would work to explain why it's purple.

The thing I'm mainly going for is that this ocean is clean. Maybe it doesn't have to be H2O per se, but that it serves the same purpose, at the very least to the local species (which I was hoping would be human, but if that's not possible, I'll have to deal with it.) Perhaps a high concentration of certain minerals and calcium carbonates could go about into explaining this 'phenomenon?'

I think I once heard about microbes and/or marine life that purify the water by emitting some sort of radiance of some property, but I've been trying to fact-check if that was something real or not, and I couldn't find anything. If that's just something I daydreamed, could that suffice as a valid reasoning? Maybe creatures unique to the planet, not necessarily microbes that do so, and such high quantity in these alien lifeforms have allowed the oceans to turn violet?

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  • $\begingroup$ If my "science" is completely bogus, please call me out on it or correct me. I don't want to go off on baseless assumptions and delusions. Anyway, thank you, everyone. $\endgroup$ – Primicerius Kaine Apr 20 '19 at 9:46
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    $\begingroup$ Welcome to Worldbuilding. If you have a minute, please take the tour and have a look at the help center. I've edited your question to concentrate on the main problem. If you have another question, please open a new post for that. Also note that fishing for ideas is off topic here. You should ask practicable and reasonably scoped questions that have exactly one right or best answer. If you get such a best answer, please honor the efforts of the poster by accepting it, as described in the tour. $\endgroup$ – Elmy Apr 20 '19 at 10:09
  • $\begingroup$ Is your water clean, alkaline or nourishing? Those are three mutually contradictory things. $\endgroup$ – Mike Scott Sep 22 '20 at 12:50
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Change the Wavelengths Absorbed

To understand how we can make water purple, we first need to understand why it is blue.

Water is blue as it is absorbs longer wave length lights such as reds, oranges and yellows but shorter wave lengths such as blue is reflected, making the water appear blue (or sometimes a more green colour).

If you changed the physical properties of the water to make it better at absorbing these longer wave lengths of light, it would start to appear purple.

Note that, like our own blue water, it may appear to be clear in small quantities. Also, it would only appear to be purple if the water was clean as contaminates, such as mud, would make it seem more of a brown colour.

Changing what wave lengths of light water absorbs or reflects well should not have too much an impact on evolution. The eyes of aquatic or amphibious life may be slightly changed to account for the different colour and underwater plants may be different colours. Beyond this though i can’t see it having a huge effect on life, especially on terrestrial or arial life.

Changing Perceptions

A possible alternative to the above is to simply change how colour is perceived. As user TheDyingOfLight pointed out, altering the physical properties of water may have unintended side effects.

Instead, i propose simply changing how your humans perceive light. Rather than seeing the colour blue, as we humans do, they perceive the water to be purple. You may be able to achieve this by either altering the numbers of rods and cones in the eye or by changing how the brain interprets the visual information.

Whilst this does not actually make the water purple, as you wanted, for all intents and purposes it is purple. As long as you are only viewing the world from the eyes of these humans, your water will be purple. “A rose by any other name (or in this case colour) is still a rose”. One thing to note though is your humans would also see the sky as purple, not a huge issue but its just something to think about.

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  • $\begingroup$ You propose to change something rooted in fundamental physical properties. I feel that this would cause a cascade of secondary and tertiary effects, going way beyond the intended effect. $\endgroup$ – TheDyingOfLight Apr 20 '19 at 19:09
  • $\begingroup$ @TheDyingOfLight Perhaps, it would still achieve the desired effect though, making it a perfectly valid answer. $\endgroup$ – Liam Morris Apr 20 '19 at 19:23
  • $\begingroup$ While that is right, the absorption spectrum of a material depends on its structure. This www1.lsbu.ac.uk/water/water_vibrational_spectrum.html article shows only part of this complex problem. A materials chemistry also alters with its structure. So you wouldn´t have water anymore if you changed the absorbed wavelength. You suggest using a different chemical substance for the oceans and the question asked for water oceans. $\endgroup$ – TheDyingOfLight Apr 20 '19 at 19:32
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    $\begingroup$ That is pretty much my the genetics option form my answer. Thanks for having a civil discussion. $\endgroup$ – TheDyingOfLight Apr 20 '19 at 19:45
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    $\begingroup$ @TheDyingOfLight I assure you that i came to that conclusion completely independently of your answer, i suppose the phrase “great minds think alike” may apply here or “every great idea has already been though of before” [paraphrased] $\endgroup$ – Liam Morris Apr 20 '19 at 19:50
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I see three approaches to this problem.

  • Star Type

Change the spectral class of the systems star. Make it put out most of its light in the purple instead of the green like our sun. This formula will give you your stars peak output in nm wavelength for a given temperature $T$ in Kelvin. Purple is between 380 and 420 nm.

$peak \space radiation = (2.898 \times 10^{-3}/T) \times 10^9$

The issue here is that peak radiation in the purple would mean significantly more UV and especially ionising UV radiation than our green peaking sun. This would strongly effect evolution. It would also shorten the time available for evolution, as the star would be big and short lived, maybe spectral class F or A.

Water appears blue because it reflects the longer visible wavelengths better than the shorter ones. So looking at its absorption spectrum [3] it would be purple and blue. In fact our own oceans are purple, but there is just too little purple light coming from the sun. So we make the sun hotter and bigger as discussed above, putting its peak output into the purple. This would get you purple oceans, but also severe risks of sunburn and free sterilisation of unprotected biomatter. Land life will have more issues than water life and might never develop. For humans this would be a dangerous planet. This video [1] goes into details on the subject.

  • Particles

The other approach would be to have particles or chemicals in the water. I'm not a chemist so I'm at a loss on what material to use.

My solution would be plankton or algae. While this might interfere with your clean water restriction it needn't. Have the stuff be discrete and permanent. A real world example would be this. [2] The source is crappy, but I couldn't find a better one. The reason for it is some kind of algae. Make it fluorescent to get an even cooler look.

  • Genetics

This is a clever work around but might not be worth the trouble. Have the inhabitants of the planet or the arriving humans perceive violet instead of blue with their eyes. Explain this with evolution or genetic engineering. Yet everything they see would look different so this will open a sack of problems.

EDIT: Found this page about waters interactions with light. It makes for an interesting and related read. [4]

[1] https://youtu.be/L9MNC45Jr6Q

[2] https://www.google.com/amp/s/www.breakingisraelnews.com/93005/watch-exodus-718-come-life-indonesian-river-turns-poisoned-blood-red/amp/

[3] https://images.app.goo.gl/94pvGLPfcdnUaj6T8

[4] http://www1.lsbu.ac.uk/water/water_vibrational_spectrum.html

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  • $\begingroup$ Water is not blue because it reflects the sky, that is a misconception. I explained why it is blue in my own answer earlier, here is the link i used: scientificamerican.com/article/why-does-the-ocean-appear $\endgroup$ – Liam Morris Apr 20 '19 at 15:06
  • $\begingroup$ Looking at waters absorbtion spectrum you are right. I'm gonna edit my answer. Yet your answer attempts to solve the issue by changing fundamental physical properties. Thats a very bad way of doing things. $\endgroup$ – TheDyingOfLight Apr 20 '19 at 15:23
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Most of the coloured waters are solutions of some minerals. These include the pool in Iceland. That would be the best answer.

Bacteria can synthesize chemicals for "food". Their "poop" can be a coloured substance. This is what happens in some of the pools of Yellowstone.

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Both are very easy. Fields can without doubt be golden-yellow on Earth too. Nothing to invent there.

Seas are only marginally more difficult (I am on record as saying otherwise some eighteen years ago, but the scenario is quite different).

You can have algae or cyanobacteria thriving in the photic zone (first, say, 50m of depth). These can be almost any colour you want. The fact that the most efficient way of dealing with your particular solar spectrum might yield a different colour isn't really an issue.

Also, the very concept of colour is tricky, as demonstrates the fact that in ancient times the Earth seas were described as purple in colour. Homer famously spoke of ἐπὶ οἴνοπα πόντον, "the wine-faced sea" (usually translated as upon the wine-dark sea and widely regarded as meaning the wine-coloured sea; elsewhere he describes the sea as ερυθρός, reddish-hued. Admittedly, Homer was blind, but still. Other authors spoke of the purple foamy waves. Some took it so far as to say that in ancient times, we didn't see the blue colour at all).

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Scientists think that 3 and a half billion years ago on Earth that the oceans were once purple. As chlorophyll hadn't been developed yet ancient photosynthesisers used other chemicals that had a purple pigment rather than a green one. There is one caveat in this hypothesis if you want your world to support intelligent life and a purple ocean. The purple photosynthesisers did not produce oxygen as a by-product, likely some other molecule, which would mean all life would be anaerobic, not a good matchup with intelligence.

There is another theory known as banded iron formations however. Banded iron formations are where a large amount of oxygen is produced all at once, coupled with lots of iron in the oceans. Usually this causes the iron to turn to rust, making the ocean red. But in theory a mild banded iron formation could make a purple ocean. There is a wrench here too, and that is that all banded iron formations are dated to the precambrian on Earth, and also that a lot of rust will create toxic oceans. However if iron were to be exposed from the mantle all at once, that could sustain a purple ocean for at least a few million years. There is no solution to the toxicity however, as rust will seep into freshwater rivers and lakes but creatures will evolve to persist and possibly even digest rust.

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