How do you find crevasses on Antarctic glaciers, using a satellite in space?
Moving around on Antarctica’s ice cover doesn’t come without risk; technology is helping us out.
Since the earliest days of Antarctic exploration, crevasses have proved to be a big problem. Many of the first explorers wrote of tragic tales of losing men as the snow unexpectedly opened up beneath them. Douglas Mawson on his absolutely horrendous-sounding expedition in 1912–1913 witnessed his friend Belgrave Ninnis, six of their strongest sled dogs, most of their food, and the main tent disappear into a glacial chasm, never to be seen again. Edgar Evan’s fall into a crevasse on the tragic Terra Nova Expedition’s unsuccessful return journey from the South Pole caused injuries that would seal his fate. Unfortunately, there have been many more incidents of this nature since then. The danger hidden below remains ever-present for modern Antarctic operators. We’ve spent a bit of time working out how satellites can help us out.
The issue with a lot of the crevasses is that they are hidden, to the naked eye they are invisible. That’s why thinking of early explorers wandering around is truly terrifying, they were literally wandering through glacial minefields. To those accustomed to glacial travel in the mountains, this is just part of the deal, and the risk can be managed by roping together, the idea being that one of the other team members will arrest the fall if another team member were to open up a crevasse. But when we start talking about large vehicle convoys, aircraft landings, and setting up field camps, this becomes impractical. This is where technology can really start to help us out.
Surface crevasses form when the fracture toughness of the ice is exceeded. This happens if there is a large enough change in the velocity of the ice across a given area, or the ice interacts with obstacles below and adjacent to it. This results in voids of air forming as the ice deforms. These voids can be tens of meters across. How deep they go depends on ice properties, but the ones I’ve encountered go deep enough - over 50 meters. The open voids may reach the surface, or they may remain enclosed below the overlying snowpack. If they do reach the surface, commonly, driven by wind, snow will accumulate at the edges and eventually build a snow bridge across this space. Depending on their thickness and snow properties, these bridges can be strong, supporting many tonnes, or weak, in which case they can fail under a relatively small loading.
We recently published a paper using the German Aerospace Center’s TerraSAR-X satellite to detect crevasses in Antarctica. This approach isn’t new per se, but we have attempted to identify the key imaging parameters when trying to successfully achieve this from space. A bit on satellites. Most of the satellites used for Antarctic observations are in what is called a polar orbit. They fling around Earth in a near-North-South pass which takes around 90 minutes per orbit. The satellite we have been using, TerraSAR-X houses a system called a synthetic-aperture-radar, or SAR.
From 500 km above, the SAR system fires out X-band microwave energy at a pre-determined target area on the Earth’s surface, some of this energy is reflected back to the satellite and the complex imaging and processing systems are able to construct an image from the data. You can think of it like the radar firing a pulse of light at the Earth, except the light is in a different invisible frequency to the visible light we are accustomed to. Because the light energy is of a different frequency it interacts with the Earth’s surface slightly differently from other light. In fact even before we get to the surface there is a very important interaction that doesn’t happen…the microwave energy isn’t impacted by clouds, so we can see the surface even if it is cloudy. This is very useful as there are lots of clouds over Antarctica. Its interaction at the snow surface is also very important, the dry snow over much of Antarctica doesn’t stop all the energy right at the surface, some of it manages to continue into the snowpack until it is all absorbed or reflected. Turns out some of this energy makes its way over ten meters into the snowpack. The crevasses below the surface are highly reflective to that incident radar energy and appear as bright lines in the processed imagery.
We found out a few cool things and the short of it is we can gather a lot of information about crevasse hazards using satellite radar. We were able to identify crevassing buried beneath the surface, undetectable to the naked eye, and visual imaging satellite platforms. It is essentially our completely remote, minesweeping technology. This approach is allowing activities on glacial terrain to be made safer for science parties heading into the deep field in Antarctica.
We are now able to confidently detect crevasse fields in locations years ahead in the planning stages of science events. There are still uncertainties and there is more to learn, but it really frames the problem in an entirely new way. With this new framing and the ability to look almost anywhere we want, hopefully, we can better manage the risk in what is an inherently dangerous place.
Thinking back to the days of those early explorers, the situations they found themselves in, with no satellites or other technologies to assist them is quite something, or maybe, in their case, ignorance truly was bliss.
