. Firn Aquifers: Water-laden snow on the polar ice sheets

Abstract
Satellite data from passive and active microwave sensors have been used to detect and map firn aquifers- water-satured layers within the porous upper layers of a glacier. The long-term freeze-thaw condition of the deep firn is determined from the evolution of the residual melt signal in the microwave data in the autumn, winter, and spring months. Our algorithm performed well at tracking the full extent of aquifers around the Greenland coastline, where they had been partially constrained by airborne radar data after being discovered in 2011. Applying the algorithm to the Antarctic continent, several areas were identified with signals indicative of seasonal or multi-annual aquifers. A team of NSIDC and CIRES scientists visited two of the sites in December of 2018. Using a hot-ring coring system, the team drilled into the upper firn on the Wilkins Ice Shelf and encountered a well-developed aquifer at 12 meters depth. The aquifer extended over the entire area surveyed, and likely covers the entire Wilkins Shelf based on our data, and airborne data from prior years. A second site on the George VI Ice Shelf, with less robust indications of a perennial aquifer, had multiple thick (20 cm) ice layers in the firn but no liquid water. The presence of an aquifer on Wilkins Ice Shelf, which has experienced several ice shelf disintegration episodes similar to those of the Larsen B Ice Shelf, suggests that aquifers can support hydrofracture-driven enhanced fracturing. Thus, areas of other ice shelves on the Antarctic coast where future conditions might approach those currently present on the Wilkins Ice Shelf would be susceptible to hydrofracture, rapid ice shelf retreat, and subsequent faster flow from feeder glaciers.