Posts Tagged ‘blueice’

Whys and Wherefores

January 4, 2011

This one’s a bit late–sorry about that! I’ve gone for a longer post to make up for the tardiness.

Antarctica, as you may have noticed, is a long way away from almost everything. And even aside from being distant, it’s logistically tricky to get to. The Southern Ocean is stormy and mercurial at the best of times, every iota of fuel and supplies must be shipped in, and the preponderance of ice and snow presents its own special problems. So why have we made the trek back once again?

You may recall that last year we came down seeking types of ice that can’t be found anywhere else on Earth: sea ice so cold that the salts trapped within it can crystallize, and ice whose surface has sublimated away to leave behind a crust of the salt mirabilite. This year we’re once again seeking ice that exists nowhere else. In this case, we’re looking for ice that has formed from the accumulation of snow and eventually become re-exposed to the sun by sublimation without ever experiencing temperatures above freezing.

There are three main layers to the Antarctic ice sheet. At the surface is snow. Over time, as surface snow is buried by the accumulation of later years, it compacts and grows dense and hard. This deep-down, compacted snow is called firn. In the rest of the world “firn” simply means snow that has survived through the summer melt season, but in Antarctica, where there is no summer melt season except at a few places near the coast, firn is sometimes defined as snow that has reached a certain density (550 kilograms per cubic meter, which is the density at which simple rearrangement of ice particles gives way to more complicated densification processes.) The firn continues to grow denser under the weight of the snow and firn above it until the spaces between snow grains close off to become bubbles, and the firn becomes ice.

Blue ice stratigraphy

The ice flows under its own weight. In some places, the ice surface may flow into an area of the ice sheet where sun and wind vaporize (or sublimate, which means to turn directly from solid to vapor) the snow faster than it can accumulate. In this case, the snow on the surface will sublimate away, exposing the firn below and eventually the ice. And that firn and ice—exposed to the sun, yet never melted—is unique to Antarctica. These “blue ice zones” are of interest to meteorite hunters, because meteorites that may have been buried in the snow become exposed, and are easy to spot on the surface. My colleagues already found and collected some meteorites during their first trip out.

We’re interested for a different reason, though–we want to know how much light reflects off of these blue ice zones, because there would have been a lot of them on Snowball Earth. (Here’s a great article in the Antarctic Sun that talks about our research.) Many areas of Snowball Earth, particularly near the equator, would have been so dry that snow sublimated away faster than it could accumulate.

These areas of exposed blue ice, being much darker than snow, would have absorbed a lot of sunlight and had a significant effect on the planet’s balance of energy. So knowing exactly how much sunlight they absorb is important to people trying to model Snowball Earth.

So that’s why we’re here. Mostly, anyway–some of us, including myself, also have a keen interest in snow and want to do some side projects involving the microstructure of snow and bubbly ice. But more on that later!