Celestia Forums

What is it that makes the ocean blue? Since water is transparent, i gather it is something external, like the sunlight or something...?
For example, since Mars is red, would an ocean there still be blue? Or would it be red, or perhaps brownish gray?

-rthorvald

The sky (Ozone in particular) gives that blue color to oceans. When the sky is grey, the sea looks all grey too...

Vincent

Vincent wrote:The sky (Ozone in particular) gives that blue color to oceans. When the sky is grey, the sea looks all grey too...

Ok. So, the color will reflect the color of the atmosphere, then. But what of Mars? It looks red, but i have seen photos from the surface where it appears blue...

-rthorvald

The Martian sky is pinkish (salmon) colored primarily from dust in the sky.

For an excellent essay on sky colors, take a peek at: http://www.orionsarm.com/whitepapers/sk ... orlds.html

...john...

There's no liquid water on Mars surface... The red color comes from iron oxyd on its surface.

Am I wrong ?

@+
Vincent

Vincent wrote:There's no liquid water on Mars surface... The red color comes from iron oxyd on its surface.

Hm, this was a hypothetical example. As we have no real ones to compare with.

-rthorvald

Vincent wrote:The sky (Ozone in particular) gives that blue color to oceans. When the sky is grey, the sea looks all grey too...

First, I don't think ozone in particular has much to do with it. Rayleigh scattering can be performed by anything with an electric dipole moment.

Second, while you're mostly seeing reflection of the sky when you look at the surface of the water at a shallow angle, in general there's more to it than reflection of the sky. Water also preferentially absorbs long wavelengths, so the ocean would look blue from space even if the air didn't scatter blue light.

Matt McIrvin wrote:Water also preferentially absorbs long wavelengths, so the ocean would look blue from space even if the air didn't scatter blue light.

This can easily be seen when diving under water. The deeper you go the more blueish the light will be.
And that's also a reason why you find these intense red colored inhabitants of deeper sea. Red becomes black and helps to hide.

maxim

maxim wrote:

Matt McIrvin wrote:Water also preferentially absorbs long wavelengths, so the ocean would look blue from space even if the air didn't scatter blue light.

This can easily be seen when diving under water. The deeper you go the more blueish the light will be.

Until of course the light is completely absorbed after a few tens of metres tops, in which case there's no illumination at all.

And that's also a reason why you find these intense red colored inhabitants of deeper sea. Red becomes black and helps to hide

One of the most amazing things I've seen was on "The Deep" episode of the BBC's "Blue Planet" series (which is an utterly brilliant documentary series about the oceans, definitely a must see). Quite aside from the flippin' underwater LAKE sitting on the seafloor (actually a separate body of hypersaline water, but it had a surface and ripples and everything. Quite mindblowing), they had a deep sea creature that hunted prey using what effectively was a biological searchlight that emitted red light. It had a short range in the water because red light is absorbed quickly, but of course since that was the case, none of the creatures could actually SEE in red light - except for this hunter. So it could "paint" its prey with red light that only it could see, and use that to catch its completely oblivious food.

Nature is just so damn cunning sometimes - it's no wonder that some people think it had to be designed... but personally I think that assumption takes away all the wonder from things that did in fact occur naturally.

I belive the oceans on Earth are blue due to the following reasons:
1 - Pure water is indeed blue (or indigo or violet).
2 - Blue and Violet light suffers more scattering in water than yellow and red light, making some of the blue light return to our eyes, while the red/yellow part of the visible spectrum gets absorbed by the water.
3 - Our atmosphere is blue (due mainly to scaterring), and a part of the blue light scattered by our atmosphere is reflected by the seas.

So, even on Mars (due to reasons 1 and 2), oceans would look possibly bluish or dark blue, unless the water is dusty.

Here follows an interesting link with absorption spectrum data for pure water in the visible range:
http://omlc.bme.ogi.edu/spectra/water/

Found this:

http://www.badastronomy.com/bad/misc/blue_sky.html

And something to play with:

http://asd-www.larc.nasa.gov/Outreach/g ... d_sky.html

Nachor wrote:Found this:

http://www.badastronomy.com/bad/misc/blue_sky.html

And something to play with:

http://asd-www.larc.nasa.gov/Outreach/g ... d_sky.html

Have a look earlier in this dept. You'll find plenty of discussions about the sky colors and their physical origin by people who /really/ know...

Bye Fridger