Sun Cups on the Lemon Creek Glacier
October 05, 2011
The photo above showing a mottled glacier surface was taken on Lemon Creek Glacier, not far from Juneau, Alaska. It was snapped at 5:45 a.m. (local time) on July 17, 2011. The view is eastward over the Lemon Creek Glacier from the arete that's home to the Juneau Icefield Research Program (JIRP) -- Camp 17. Featured here is the zone of accumulation, where more snow falls than melts each year, near the head of the glacier. As of mid July, 2011, there was still almost 10 ft (3m) of new snow on top of last year's accumulation. Compaction of this snow, plus melting and refreezing had increased the density of the snowpack to approximately 500 kg/cu. m, as it slowly but steadily is transformed into solid, blue glacial ice -- 850 kg/cu. m. Note that the density of newly fallen snow is closer to 100 kg/cu. m.
The surface of the snowpack has developed a pockmarked texture through a process of uneven melting and evaporation of snow and ice from its surface. Meltwater collects in small, barely noticeable depressions that form on the surface as a result of wind action, or perhaps from melting enhanced by the presence of a bit of dark colored dust or particle. The presence of meltwater accelerates the melting process in the depression, while on the ridges between the depressions, evaporation is taking place. Melting requires only about 1/7 as much energy as evaporation, so over time the depressions deepen and widen to form what are known as "sun cups." Small sun cups can provide footholds for hikers on ascents or descents, but as they grow larger and deeper, the icy ridges and slushy depressions may become an impediment to easy travel, even on relatively flat surfaces.
The inset shows a close-up of the sun cups encountered later that morning as the students in the JIRP Pre-College program trekked out to their test pit on the ice. Their tracks and the pit they dug to sample the density can be seen near the right center of the photo.