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We think of oceans and large lakes as being blue.

How can I get other colours in a way that is reasonably compatible with what we know from terrestial bio-geo-chemistry?

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  • $\begingroup$ What are you looking for here, an itemized list? That's either TB (there's a book's worth of chemical compounds that are liquid at room temperature and not blue), or POB, or quite possibly both. Please narrow it down for us. $\endgroup$ – Gryphon Feb 23 '19 at 19:19
  • $\begingroup$ Also, possibly relevent Chemistry.SE question that was closed as TB: chemistry.stackexchange.com/questions/30976/… Interestingly, the answer mentions Francium as an element that is almost liquid at room temperature. I do not want to know how someone determined the melting point of Francium. $\endgroup$ – Gryphon Feb 23 '19 at 19:20
  • $\begingroup$ @Gryphon I'm just looking for one example or more, provided the example hasn't been said by another answerer. $\endgroup$ – SealBoi Feb 23 '19 at 19:32
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    $\begingroup$ I have voted to re-open it as, while broad, it has solicited interesting answers. $\endgroup$ – Sherwood Botsford Feb 24 '19 at 14:07
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    $\begingroup$ I don't see why this is closed? This is essentially asking for a list of colors that can be made for bodies of water that harbor life and that occur naturally. $\endgroup$ – Crettig Feb 24 '19 at 21:47
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Red Algae

The easiest way to get a non-green/blue ocean is perhaps to use a life form. Just make sure your ocean has the ideal nutrients for them.

enter image description here

Salt Lake in Turkey Turns Red Because of Algae Bloom

https://abcnews.go.com/International/salt-lake-turkey-turns-red-algae-bloom/story?id=32590572

enter image description here

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Ammonia has a tendency to dissolve alkaline metals. This answer claims this would make the oceans very blue, this page suggests brown. The argument for blue is that solvated electrons make solutions deep blue; when the concentration is high ($[H]>3$M) the colour goes copper, which might be the source for the brown. Now, that is a pretty concentrated solution, so I would expect blue unless there were other substances adding colour.

Liquid nitrogen, helium and hydrogen are pretty colourless, but liquid oxygen is sky-blue (and under pressure and low temperature forms a lot of colourful crystal phases). Ozone is darker blue. Hydrogen perioxide is apparently a watery transparent blue. Of course, a planet with liquid oxidant oceans may have some habitability issues. I suspect that the same dispersion effects that make water blue will make liquid nitrogen, helium and hydrogen oceans look blueish.

Hydrocarbon have plenty of opportunities to acquire colour. Liquid methane is again colorless, and might look very clear on worlds like Titan where atmospheric methane has already filtered out the wavelengths that tend to be absorbed. However, the atmospheric haze consisting of photochemical smog is clearly yellowish and likely soluble in the methane. I would expect most hydrocarbon ocean planets to have oceans coloured by more complex tholin molecules, that would tend towards the red-yellow side. In extreme cases, maybe black.

Sulphuric acid is again colorless, but does tend to turn yellowish when contaminated with iron ions. And again organic matter and other substances can make it yellow, red, or black.

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Water isn't blue. Or not very. Water appears blue-ish because most of the light you are seeing is reflected sky. It's not the same blue, because some of it is absorbed instead, so you are in effect getting dimmed skylight, or light blue mixed with black.

[Showing variable colour by semi-specular reflection[1] Here the surface is calm enough that there is only small amounts of surface scattering.

View at a sharper angle At the top of the picture you see a mottle of blue and dark bands. The blue bands are the back side of the wave. Reverse the light path from your eye, the water is at a shallower angle, so reflection dominates. The front side of the wave is show more light from the interior of the water.

Look at the reflection of the bow of the canoe. This area has a poor view of skylight, so the colour here is a mix of some bottom algae, and red light reflected from the canoe.

Smoky day

On a smoky overcast day, the water looks like hammered metal.

Water can be coloured by what's in it. Suspended clay gives it a brown colour. Dissolved tannic acid (peat bogs) along with micron sized peat particles make it the colour of strong tea in your cup, black if more than a foot deep.

Shallow windy water Stormy shallow water. In the shallow water at the end of this lake wave action has lifted some of the bottom sediment int suspension, giving the water a brownish cast.

Rapid

*Smooth water in foreground and background shows bluish tinge of sky. White water is light scattered off of entrained air. The light brown is organics from the peat bogs that is the source for the water.

With algae in it water is green. There can be a substantial difference in green depending on which algae. Some algae are red -- hence red tide.

Runoff water from a glacier is often a milky gray initially. As it gets far enough and slow enough for larger rock flour particles to settle, it takes on a blue cast. The particles that remain are are close in size to the wave length of blue light, so it's scattered more.

Abraham Lake *Abraham Lake is coloured by rock flour. The water here is only about 30 miles from it's source on the Columbia Ice Fields in Jasper Park.

Crater Lake on cloudy day

Several comments have mentioned Crater Lake. On a day of broken overcast it doesn't appear particularly blue. I suspect much of it's reputation is due to being at high elevation, so the sky also is an extraordinary blue.

So now you have several mechanisms to colour water:

  • Suspended solids
  • Dissolved solids
  • Micro-organisms.
  • Sky reflections.

There is no reason for any of these to not work with other liquids.

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  • $\begingroup$ Water appears blue-ish because most of the light you are seeing is reflected sky That is a fantastic statement - which would be even more fantastic if you could supply a link to a reasonably scholarly reference to substantiate it. (I am in no way saying you're wrong, I'm just hoping for the improvement). $\endgroup$ – JBH Feb 23 '19 at 21:23
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    $\begingroup$ "Water appears blue-ish because most of the light you are seeing is reflected sky" is a myth. scientificamerican.com/article/why-does-the-ocean-appear $\endgroup$ – Robyn Feb 23 '19 at 21:34
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    $\begingroup$ Water is blue because it preferentially absorbs longer wavelengths of light. Go look at Crater Lake sometime. The water there is incredibly clear, incredibly pure, and incredibly blue, even on cloudy days. $\endgroup$ – Mark Feb 23 '19 at 22:04
  • $\begingroup$ @JBH I have also heard the statement about the reflected sky somewhere, and found it to be true afterwards. The "color" of any water surface is dominated by what that water surface reflects, followed by what chemicals/particles are found at/directly under the surface. In the absense of these things, water is just almost fully transparent. Yet, because it still does absorb some tiny amounts of light like any other material, deep bodies of pure water would look just black without the sky reflecting off their surfaces. $\endgroup$ – cmaster - reinstate monica Feb 23 '19 at 22:04
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    $\begingroup$ @cmaster, see my note above. Crater Lake is a deep body of pure water, and it looks intensely blue under virtually all conditions. $\endgroup$ – Mark Feb 23 '19 at 22:07

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