Glaciology

Photo map
Map of part of Antarctica showing Transantarctic mountain range and major iceshelves.

Antarctica is not only a long way from the UK to go hunting meteorites – it’s also vast. Forty-four times larger than the UK at 14 million square kilometres and almost entirely covered in ice, searching for meteorites here brings new meaning to looking for a needle in a haystack. It’s a brutally cold, windy and unforgiving haystack. It requires months of planning, logistical and field support from the British Antarctic Survey and is physically and mentally hard on the field team.

So why search here in the first place? Well Antarctica offers scientific meteorite search teams a few advantages. First off, Antarctica is a dry desert – albeit a very cold one – with small amounts of snow, so the weathering processes that would normally destroy meteorites in the mid-latitudes are much reduced. The lack of vegetation is also a great help as meteorite falls are not hidden immediately by falling into a pile of leaves or into a forest. However one of the main strengths has to be the ice itself. Meteorites are typically dark in colour, darker than most terrestrial rocks, making them easier to spot when they lay on the bright white or blue coloured icy background.

Crevasses blue ice
Heavily crevassed icesheet region indicating very fast glacier flow. [Image: A R D Smedley]
Though this contrast in colour helps to track down meteorites, it doesn’t change the fact that Antarctica is a big place and meteorites are few and far between. Luckily Antarctica is a dynamic place: the ice sheet is not stagnant and flows both gradually and also in fast moving streams towards the edge of the continent. Here mountain ranges act as barriers to the outward flow. Sometimes they almost block the flow entirely, and sometimes barely poke their summits out of the 2 km thick ice sheet – when they are called nunataks (where the word is derived from the Inuit language). Either way they cause the ice to slow, to be forced upward, or to stagnate in the same way as an eddy might when a river meets a rock in its way. Combined with low snowfall and high winds the upwelling ice surface in these areas is scoured or ablated off. The surface then isn’t formed of the most recent snowfall but is ice: hard, compacted – and blue, not white — rising from deep within the ice sheet and that originally fell far away as snow.

Called Blue Ice Areas, these areas of upwelling ice are the secret to looking for meteorites in Antarctica. The same process that brings compacted old blue ice to the surface, brings with it meteorites that fell far off and concentrates them on the surface of the ice eddy…. waiting to be found. This process means that in a blue ice area we find meteorites from the wider region as well as those that directly fall onto it, and, with low weathering rates and slow moving ice, meteorites accumulate over tens of thousands of years.

So, the odds are slightly more in our favour now. We can focus on blue ice areas rather than the entire continent, but we still have to find them (and beforehand, not by skiddoing back and forth over the entire ice sheet). Not all blue ice areas are created equally either, so we have to do our best to pick those that we think will give us the best chances.

How do you find a blue ice area – and for that matter how do discriminate between a “good” i.e., productive one and “bad” one? As the old saying goes: data, data, data. Specifically satellite data, plus some physics. In terms of finding blue ice areas we are blessed with some wonderful, open access, data sets and by using two key characteristics of blue ice areas – their colour and compactness – means we can use satellite imagery and reflectance to distinguish between snow (covering most of the continent) and the blue ice we are interested in (covering perhaps 1%).

What makes a blue ice area a good source of meteorites is a little more complex and something that is not well understood. We think it depends on the overall flow speed of the ice, the detailed flow of ice around the mountains, its geometry and extent, the prevailing weather conditions, and more. But here again we can try and get a handle on many of these variables from satellite data and when combined with a model of all these processes, we can hypothesis likely candidate areas.

Ice Flow Map
Antarctic iceflow map where red colours represent far flowing ice streams and showing locations of collected meteorites (pins) and names of mountain regions. Arrows denote major motions of ice flow from the continent interior to the Southern Ocean. [Image: A R D Smedley]