Deployment and Recovery of an Ice-Melt Probe at the Greenland Summit Using an Anti-Freeze Borehole Fluid

Ice melt probes are attractive candidates for deploying instruments within and beneath ice sheets at logistical costs much lower than those of conventional drilling, and are also a prospective technology for accessing subglacial oceans in the outer solar system. Classical ice melt probes descend into ice using electrical power supplied from the ice surface, via wires that spool out from the descending vehicle. Water in the melt hole above the probe refreezes, thus preventing recovery of the probe and instrumentation, as well as any samples of englacial meltwater or subglacial waters. The University of Washington Ice Diver was first developed as a classical melt probe and proven in the Greenland ablation zone to 400 m depth. We have, on the basis of modeling by Hills et al. (Annals of Glaciology 1-5, 2020), since investigated the use of anti-freeze injected into the melt hole above a descending probe to prevent refreezing while avoiding clogging the hole with slush. In May/June 2021 at the Greenland Summit, we used a modified version of the Ice Diver to test the simplest antifreeze approach indicated by modeling. From the ice sheet surface to 65 m depth, melt water produced by the probe flowed laterally into surrounding firn and did not accumulate above the probe; below that depth we observed melt water retention in the hole. We injected methanol (our antifreeze) 1.5 m above the top of the descending probe at a rate specified by modeling for the particular diameter of the probe (8 cm), descent rate, and ice temperature (-31C). We proceeded to a depth of 103 m below the ice sheet surface, continually feeding the tether down to the probe from the ice surface though the methanol/water mixture in the hole. We encountered no resistance from slush or refrozen ice as we recovered the probe through the liquid-filled hole. Moreover, 36 hours after probe recovery and we again lowered the probe again, without obstruction, to the previous depth before recovering it again. We have thus validated the simplest anti-freeze strategy indicated by our modeling, in the field, at temperatures characteristic of large parts of the Greenland and Antarctic ice sheets. Following presentation of details and data from this experience, we discuss prospects for a second generation method that would reduce the volume of anti-freeze required by a factor of 4 or more.