Antarctic Megadunes: How the Dunes Were Formed
Ordinary snow dunes are formed by wind blowing sand or snow into large piles. But these Antarctic dunes are different. The scientists working on this project have a theory that the dunes were carved out of the ice by the unrelenting winds in the area. Even after spending time there and collecting data, it will take a second visit and extensive analysis to determine if the theories and models are valid.
The model was developed by Dr. Mark Fahnestock of the University of New Hampshire and Dr. Ted Scambos of the National Snow and Ice Data Center (NSIDC); both scientists were members of the Research Team. They propose that the dunes start as a disruption of the wind by small hills in the ice. These ice hills "probably represent hills and valleys in the bedrock, hundreds of meters below the surface," according to Ted Scambos. In this area, nearly all of the wind is due to katabatic (directional) flow. This flow is due to intense cooling of the lowest part of the atmosphere against the snow. The layer of air begins to slide downhill because it is more dense than the air above it. The katabatic winds begin to "bounce" over the hills of ice. Over time, this creates a rippling effect in the snow.
Snow Grain Size
Although no other group has been to this research site before, the megadunes team did have some information about the dune features from earlier work. American traverses of the Antarctic Plateau in the 1950's and 1960's crossed the dune pattern in several places without realizing there was an overall pattern, because they did not have the perspective of satellite images. Fortunately, these expeditions dug snow pits and made other measurements. The expeditions found extremely coarse (large) snow crystals, up to 2 cm (3/4 inch) across. Their studies provide most of the initial knowledge of snow grain size and snow accumulation for this region.
The megadunes researchers now think they know how snow crystals get to be this coarse. Almost no snow accumulates in the megadunes regions; what little snow falls there gets blown away by the relentless winds. This means that existing snow in the top layers stays near the surface for many years. Every year this snow goes through intense "thermal cycling" from the warmer temperatures of summer to the bitter cold of winter. This cycle is the warming and cooling of the upper few meters of snow during the seasonal changes. (Keep in mind that warm and cool are relative terms, since it is always far below freezing here, even in the height of Antarctic summer.)
The model assumes that when the uppermost snow warms in the summer, tiny amounts of water vapor leave the snow grains and enter the air between the grains. Then winter comes, and the snow pack cools tremendously, to -80°F (-62°C) or lower. Warmer air from summer is still present in the lower layers. Water vapor from the warm layer moves towards the colder snow at the surface and sticks to the snow grains there. It's similar to when you take a glass from the freezer — it becomes "frosty." This cycle repeats every year until the very slow process buries the snow grains — now very coarse — deep in the snow pack and out of reach of thermal cycling.
Radar data and satellite images show megadune features that suggest alternating fine grains and coarse grains, and alternating rough and smooth surfaces. The following features characterize the dunes:
- Upwind face: rough surface of meter-size sastrugi (snow ridges, like frozen waves on the ocean) with fine snow grains
- Downwind side: smooth surface of icy glaze covering very coarse snow crystals
Satellite radar data measures the "backscatter" or reflected energy from the top 2 to 20 meters (6.5 to 65 feet) of the snow surface. Radar sensors detect features that are near the size of radar waves, around one inch to one foot. The rough upwind face scatters more energy, making it appear bright in the radar images. The smooth surface and the coarse snow grains on the downwind face absorb more energy, making them appear dark.
To the human eye, snow is highly reflective. But if you have ever seen a glacier or looked into a hole made in deep snow, you know that ice can have a bluish color. The bigger the crystals of ice, the bluer they appear. Ted Scambos describes this by using the analogy of hard candy. "Take a piece of candy, any color, and pound it, really pulverize it into a powder. What color is it? It's white. That's because the light we see is bouncing off the surface of all the broken sugar grains rather than penetrating the inside of the sugar where the color is. Snow looks white because of tiny reflective edges, but ice looks blue because the light travels through the crystals for a longer distance before it comes back to you." The main absorption for snow is actually in the infrared, just beyond what the human eye can see. But satellite sensors can detect the extra infrared absorption and identify coarse snow.
The Wind in the Snow
A unique aspect of the megadunes may be airflow through the snow pack. This is a natural result of the coarse grain size in the snow and the constant directional wind. The team does not know what effect airflow might have on the snow chemistry, but their questions include: Will airflow mix up the chemistry of the snow across large regions? Will some areas act as filters, sifting all the particles out of the air and accumulating them? The researchers hope to test the ice for airflow and to measure the chemical composition of the ice.