Why Icebergs are Blue
By Mark Dubovoy
Perhaps it is because of my training as a scientist, or perhaps my innate curiosity to understand how things work, or simply my aversion to spreading incorrect concepts, superstitions or plain lies about things; the fact is that I get quite annoyed when I observe seemingly well educated and seemingly rational people spread false myths.
A typical example is the old false myth that “according to the laws of Physics the bumble bee can’t fly, therefore, the fact that it flies is some kind of unexplained miracle”. The fact is that a simple calculation based on the weight of the insect, the wing shape and the muscle strength shows without a doubt that the bumble bee should easily be able to fly. One can use either this simple model or extremely sophisticated models, but they all conclude that the bumble bee can easily fly, as they should.
In other words, the fact that the bumble bee can fly is not some sort of mysterious miracle, it is a simple consequence of the basic laws of Biology and Physics.
As to why this myth, and others, continue to be propagated by even intelligent people, I leave the reader to figure out. I personally find it quite disturbing, particularly when these kinds of lies are presented to young children as facts.
This brief introduction brings me to the subject at hand. I have just returned from the latest Luminous Landscape trip to Antarctica. The trip was fantastic, and the opportunities to capture amazing images were endless.
During the trip, I heard a number of people talking about the amazing colors of some ice formations, and I asked some of the guides, as well as some folks that had been there before and studied Icebergs why some Icebergs are blue. Unfortunately, the answers given were a bunch of false myths. I immediately figured out that this was the case because of my background as a Physicist, and I also suspected what the real answer was (it turns out I was right), but I wanted to wait until our return to do a little research and check my facts.
So, without further ado, let me mention the false myths first:
1: Icebergs turn blue because of pressure that compacts the ice and dislodges the air trapped inside. Wrong, in fact the hollow areas with the most air (crevices and holes) are the bluest.
2. Old Icebergs are blue, young ones are not. Wrong. Often times ancient Icebergs are white and very young ones are deep blue.
3. Icebergs are blue because of refraction. Wrong. For those of you who do not remember what refraction is, think of what happens when you submerge part of a straight object like a pencil in water. The object appears to bend at the surface. There is some refraction in glaciers, but its effect is negligible.
4.Glaciers are blue because of light scattering, the same phenomena that make the sky blue. Wrong again. Without going into a full explanation, the sky looks blue because of an effect called Rayleigh Scattering. Raleigh scattering is produced by Electron-Photon interactions in the gases in the upper atmosphere where there is little or no Oxygen left. This is completely different from anything that happens with liquid water at sea level.
Now, let’s explain what is really happening:
We have all seen snow crystals. They are quite beautiful and as you all know, they have very interesting shapes with many facets pointing in all different directions. You can think of the facets as tiny mirrors. Most icebergs are formed by many years of falling snow. As such, they are composed of huge numbers of these crystals, so when the Iceberg is hit by light, all these tiny mirrors reflect the light with the result that the iceberg looks the same color as the incident light. If illuminated by white sunlight it will reflect diffuse white light. If it is illuminated by, say, yellow light at sunset, it will look yellow. Some icebergs have certain areas that were formed by freezing ocean water instead of snow. These areas are usually full of tiny air bubbles. The bubble surfaces also act as tiny mirrors, so these areas also give an overall white appearance in full sunlight.
The bluish streaks in some Icebergs are caused by the the refreezing of meltwater which fills crevasses formed in the glacier (that creates the Iceberg) as it creeps over land. In other cases, the crevasses or holes (full of air) turn a deep blue. We will explain why this happens below.
Old Icebergs go through hundreds of thousands of years of compression and sometimes ongoing thawing and refreezing of the ice. As you go deeper and deeper into a glacier, the weight of the ice and snow above increases dramatically. As this pressure increases, the air that was originally trapped by the falling snow is forced out. As this happens, the reflective surfaces of our "snowflakes" get “crunched together” and in some cases can completely disappear. Since most of the reflective surfaces within this kind of iceberg have been eliminated, light hitting this iceberg no longer "bounces" off of millions of tiny mirrors. Instead, light is forced to travel through the Iceberg and penetrate deep enough to either find some internal surface to reflect back from or penetrate all the way through the iceberg.
In view of the above, it behooves us to look into what happens when sunlight travels through seawater, frozen or not.
The water molecule in its free state has three fundamental vibration modes (symmetrical stretch, symmetrical bend and asymmetrical bend). As most of you remember (or should remember) from High School Physics, the energy of a Photon is directly related to the frequency of light in that Photon. In other words, Blue (high frequency) Photons carry much more energy than red (low frequency) Photons, with Orange, yellow and green (in that order) in between.
It is a well known fact that lower frequency Photons have precisely the right amount of energy to excite the vibrational modes of the water molecule. Thus, as white light travels through water, the red/orange/yellow components of light get absorbed by the water and cause the water molecules to vibrate, while high frequency (blue) photons continue to travel through the water. The net result is a slight increase in the water temperature (molecular vibrations translate directly into temperature), and the white light turning blue as it travels deep into the water.
Anyone who has scuba dived or snorkeled in the ocean is thoroughly familiar with these conditions: The water has a thin warmer layer on top and as one descends into the water, after several meters in depth everything looks blue. In fact, the main reason why underwater photographers use flash is to keep the light source close enough to the subject to preserve the lower frequency colors.
Blue water is the only known example of a natural color caused by vibrational transitions. In most other cases, color is caused by the interaction of photons of light with electrons.
Snow and Ice
This same spectral selection occurs when light is forced to travel deep through snow or ice. The next time you go skiing, try poking a hole in the snow with your pole. Make sure the hole is at least one meter deep. Since you are forcing the light to travel down the walls of the hole a considerable distance before it hits the bottom and reflects back, the result is that the hole not only looks darker, but it also looks blue. This is what causes the blue color associated with the depths of crevasses in glaciers and Icebergs.
Now that we understand the basic cause of the blue color, it becomes self evident that the shape of the Iceberg, the chemical composition of the sea water including all the substances dissolved in it, the angle at which it is illuminated, whether the surface has melted and re-frozen and many other such factors will have a strong influence on its color.
As an example, that directly contradicts some of the myths mentioned above, here is an unmanipulated image of a relatively young Iceberg that I took on this trip. It appears that the surface has melted and re-frozen a number of times. There are many air gaps in the Iceberg and no discernible areas of extreme pressure. I believe that the color, helped by the angle of the incident light, is stunning.
Another example of an image that also proves the above mentioned myths are false is the following:
This was a truly massive formation. I would estimate the height to be about the same as a 20-story building. There are signs everywhere of multiple layers, stratification, very old age and extreme pressure. Yet everything looks white, except for the crevasses and holes.
In this case, the extreme compaction and the angle of the light hitting a vertical wall (cross section with lots of lined up molecular “mirrors”) actually works very well to produce a great white reflection.
Thus, we have examples of a young formation with no extreme pressure that is blue, and a massive very old formation with extreme pressures that is white.
I sincerely hope that this sheds some light (pun intended) on some of the phenomena that help produce these amazing visual treats.
The more I reflect on this trip to Antarctica, the more I realize its impact and beauty. It is one of the most magnificent and unique places on the planet.
As such, I leave you with one last image from the trip that appropriately represents “reflections”.
Mark Dubovoy is a Scientist, Venture Capitalist and Photographer. He is a contributor to this site and to other Photographic publications. His photographs can be found in private collections and in the permanent collections of several major museums in the United States, Mexico and Japan. Mark holds M.A. and Ph.D. degrees in Physics from UC Berkeley.
January, 2009 – Copyright Mark Dubovoy