Space Chronicles on Ice #12
by Don Pettit
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Dec 17, 2006:
Feasting one's eyes on the robin egg hue of blue ice presents a natural beauty that rivals a rainbow. If Iris, the goddess of the rainbow, had a twin sister, she would surely be the goddess of blue ice. Considering the total extent of the east Antarctic ice sheet, these blue ice regions are rather small, extending for only a few kilometers and sometimes, tens of kilometers in any direction. However, compared to the length scale of a human, these areas seem to swallow up your being in surrounded chromatic awe.
The color of blue ice is the result of two complementary processes. First, the color of water molecules in either liquid or solid form is intrinsically blue. Water has an absorbance in the near infrared part of the spectrum, the part that is adjacent to the red but that we can not see. The tail of this infrared absorption extends into the visible region, preferentially absorbing the red end of the spectrum. If the path length is sufficiently long, the transmitted light through both water and ice becomes blue. The problem arises in that this absorbance is so weak, it takes multi-meters of path length in either pure water or ice to impart a noticeable blue hue. Also, since only the transmitted light is blue, to see this effect the observer would have to be buried below the ice looking up towards the source of light.
The second process that makes ice look blue is light scattering. Light, if traveling through a pure medium, moves forward with no obstructions. If there are particles, if there is junk in the medium, then a fraction of the light is scattered and thus redirected into angles other than the forward direction. We see this in theaters with the projected spot light. In clean air, the beam is not seen, but when the stage smoke is added, the beam becomes visible due to the scattering of light.
Glacier ice is like a theater set after the stage smoke has been released. It is filled with small bubbles and other particulate debris that provide sites for light scattering. Thus when sunlight falls onto glacier ice, a fraction is scattered in other directions outside of the forward path. This light may be scattered many times, like a pinball bouncing off bumpers, and instead of racking up high points, it effectively increases the path length traveled by the ray. This process, for lack of a better name, is known as multiple scattering. Of the light that penetrates the glacier ice, a small fraction, after multiple scatterings, exits out in directions that we can see. Since these rays of light have traveled long distances within the ice due to multiple scattering, the weak absorbance of water results in this redirected light appearing blue.
Where it might take multi-meters of ice with the observers having to be below looking up to see blue, multiple light scattering effectively collapses the equivalent path length to less than a meter of ice and redirects the light backwards so that it can be seen while standing on the surface.
So in short, the blue nature of ice coupled with multiple scattering allows an observer on the glacier surface to be treated with robin’s egg blue.
Blue ice regions are a natural part of the glacier anatomy. According to the construct of man, glaciers have two distinct regions, a region where snow accumulates and eventually becomes compacted into ice and an ablation region where the ice wastes away by melting, breaking off into the sea, or sublimation. The ablation region is typically downhill from the accumulation region, and under the effects of gravity, the ice moves slowly, perhaps a few meters per year, from the accumulation to ablation regions. The ablation regions are typically near the continental edge by the ocean. This slow, icy movement causes the glacier to calve off icebergs which then become entities of their own. It is not unusual to see blue ice in these fringe regions near the continental edge. This blue ice though, is often times covered with a layer of snow white, masking the true color and intrinsic beauty.
Blue ice deep in the continental interior is different. Here we are in the heart of the accumulation zone yet these blue ice regions somehow represent small pockets of wasting, of ablation, of anti-growth. Like some diseased tissue found among the healthy, these interior blue ice regions represent a process that is opposite of the growing force. The blue ice is typically wedged up alongside of a nanatak or other mountainous obstacle. Perhaps these obstacles impede the downward flow of the ice and then these stagnate regions begin to waste away, to ablate, and become blue ice.
No matter the reason, small areas of blue ice are found among the healthy growing ice tissue in the continental interior. These blue ice regions then can be fed new ice as in the "conveyor belt model," or become isolated regions and simply waste away as in the "stranded ice model." And both are affected by the katabatic winds. Regardless of the mental construct, these interior blue ice regions concentrate any rocks that are contained within, including meteorites. Hence, the attraction to this remote region by scientists in search of extraordinary bits of rubble left over as construction debris from when our solar system was built.