Written by Nate Stewart
One of our daily goals up here on the ice sheet is to measure and track changes in the specific surface area of snow, or SSA. Specific surface area is the amount of surface area on the grains of snow in a unit volume of snow. In general, larger grains have less surface area per unit volume than smaller ones, but the complexity of the snow grain shape is important too. A round snow grain has much less surface area than a classic dendritic snowflake of similar size. In addition to surface albedo, snow chemistry, and snow density, we believe that greater understanding of SSA and its variation across the ice sheet is critical to understanding the extraordinary and unexpected melt that occurred up here last year.
Snow is incredibly effective at scattering and absorbing sunlight and, as a result, is critical to the energy budget of the Earth. The Greenland ice sheet is, in effect, a massive natural solar reflector. The ability of the ice sheet to reflect light is determined, among other things, by the size and the specific surface area of the very snow grains that make it up. The snow grains themselves are highly variable, ranging from rock solid melt-freeze layers to the light and fluffy dendritic snow crystals we attempt to emulate using paper and scissors. Unlike the elaborate branching dendrites characterized by their large SSA values, melt freeze layers have relatively small SSA and absorb much more solar radiation. Despite the importance of SSA to the Earth’s climate, very little is known about how SSA varies and to what degree this matters to the climate system itself. This is the very reason we are towing along a DUFISSS with us as we traverse the ice sheet.
DUFISSS is a special instrument developed by our friend and colleague Florent Domine, a snow scientist from Takuvik, the Arctic research group at the University of Laval in Quebec. DUFISSS (DUal Frequency Integrating Sphere for Snow SSA measurement) measures SSA using a novel optical system. The machine itself looks like a formidable little box with feet. Inside the box is an open sphere 15 cm in diameter situated above an opening just large enough for a hockey puck-sized snow sample. Once a snow sample is placed beneath the sphere it is illuminated from above by a 1310 nanometer beam blasting down from a laser diode. Reflected back is detectable flux of photons that bounce back into the sphere and are collected by a photodiode that converts light intensity into voltage. We are then able to read a voltage value on an LCD screen and use it to calculate SSA. The photodiode “collector” is calibrated using a set of known calibration standards each time we use DUFISSS in the field. They range from 5% reflectance, which looks as dark as soot, to 99% reflectance, which looks as bright as the brightest white dress shirt you could possibly imagine. Once calibrated, the SSA snow sampling method is quick and fun. We core the snow surface and the snow pack at 3 cm intervals using a stainless steel core the size of a small drinking glass. The core itself is lifted from the pit wall using a spatula and then injected into a hollow hockey puck-sized cup before it placement beneath DUFISSSS. The technique minimizes snow crystal deformation and solar heat-up during sampling – two insidious sources of error when sampling snow – and mobilizes two quintessential Canadian institutions, the spatula and hockey.
Historically, dogs have played a critical role in maintaining the morale of men on polar expeditions. Despite heinous weather, suboptimal sleeping conditions, and brutally hard work, their unconditional loyalty and ever-readiness to play has forever been a welcome source of lightheartedness and sincere companionship for weary men. In the notable absence of dogs on the SAGE traverse, DUFISSS has affectionately and faithfully filled this role for the team.
Tremendous thanks to Florent and his colleagues for their technical innovation and development of a polar field-capable SSA instrument. DUFISSS is a marvelous alternative to a number of vastly more time and material expensive pre-existing techniques (think vacuum containers, massive amounts of liquid nitrogen, x-ray machines, and methane gas . . . each of which would be dangerous, smelly, or downright catastrophic to move across a cold, remote, high altitude roller coaster of an ice sheet via snow machine and sled ).
Tomorrow night we call upon DUFISSS for the second-to-last time on our journey across Greenland. We are certain he’ll stand tall and laser diode the SSA out of anything we place beneath it.