Antarctic and arctic meltwater is "bad" because it's dark, but why is transparent liquid on white stuff so dark?

Question

The Washington Post's Antarctic heat wave melted 20 percent of an island’s snow cover in days, caused melt ponds to proliferate includes the figure below of meltwater ponds on top of snow/ice.

The article is a good read, but my question is one of optics, namely what makes a pond of meltwater (essentially pure water) on top of something so white become so dark?

It increases sunlight absorption and heating, which in this case is "bad" but what's the physics or phenomena behind this that makes these so dark in the first place?

If I understand correctly, even shallow ponds or puddles of water can significantly reduce albedo, increase absorption and induce further melting, sometimes in a "runaway" manner. So I don't think it's bulk absorption by the water or contaminants, I think there's something more interesting going on.

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Melt ponds on the Antarctic Peninsula’s George VI Ice Shelf on Jan. 19, 2020. (NASA)

Answer 1

Water has lowest EM absorption in the blue part of light spectrum and increases rapidly towards both UV and red parts of spectrum.

As a result in visible light water is blue. Same goes for the ice as it has very similar absorption spectrum.

While there is a lot of white in the picture, all of it is a thin snow cover on top of blue ice. Once the snow melts you get a blue water on top of thick layer of transparent blue ice that combined absorb most of the light making ponds looks exceptionally dark. Water would be so much lighter in colour if, for example, instead of transparent ice, you'd have sand on the bottom.

Answer 2

"Transparent" is not the same as "white" : white bodies reflect most of the light while transparent bodies let the light though. Once the light enters into water, it may need to travel a long way before it has a chance to go out, and that long travel path provides more opportunities for absorption.

Water absorbs light by itself much more than snow, but contamination makes the situation even worse. If there's a dirt particle in the snow, it has to be located within the top few centimetres to get any significant energy from sunlight: only 20% of light penetrate snow deeper than 5 cm . In clear water, light penetrates much deeper (25% of energy at 10 m depth), so all dirt particles within that volume will readily dissipate heat when exposed to sunlight.

If you wonder why snow is white, consider this: dry snow is a mix of ice (refractive index 1.31) and air (refractive index 1), so there's a lot of possible angles at which light will be reflected from the boundary (partially or totally), and a lot of boundaries as well. Wet snow is a mix of ice (refractive index 1.31) and water (refractive index 1.33), so those boundaries barely affect passing light. Solid ice or water have a single boundary with air and no internal boundaries at all.

Answer 3

Water (solid or liquid) has some absorbtion. It is rather low for pure water for visible or near-visible light and that's why water it is considered transparent. But only to an extent - few meters of water look blue and few hundred meters look black, esp. if you are UNDER those few hundred meters.

Then we have snow. Snow has an abundance of optical boundaries in all directions, so it scatters the light well. The light passes only few milimeters water in total before it gets reflected or refracted out of the snow mass. You don't absorb much light in a few mm of water.

If you compress or melt the snow, optical boundaries disappear and the light is allowed to go deep in the water body where it gets chance to be absorbed.

p.s. Water is not the only substance that looks white when powdered. Most powdered substances tend to look white-ish, even if in their solid form they are pretty much not white and just partially transparent. See sulfur, copper sulfate and so on.