Plausibility of purple oceans

Question

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?

Answer 1

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.

Answer 2

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

Answer 3

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.

Answer 4

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).

Answer 5

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.