Numerical Investigation of a Path to Recoverable Melt Probes for Subglacial Lake Exploration

Abstract:

With over 300 known subglacial lakes in Antarctica, substantial exploration could be aided by the logistical efficiency of melt probes. The classical Philberth probes were left beneath the ice due to the hole freezing shut during the passage of the probe. Leaving probes behind in a subglacial lake could harm the ecosystem. Here we are motivated to engineer a recoverable melt probe while keeping the logistical efficiency. We present results of numerical calculations on how one method could work for maintaining an open hole, thus maintaining mechanically free connection from which the probe could be recovered from. The numerical model evaluates heat flow and hole radius according to the cylindrical statement of the Stefan problem. Freezing in is modeled around the melted out hole at various depths and temperatures relevant in Antarctica. Modeled times until the hole completely closes in without heat or antifreeze are, under the Antarctic surface temperature of -55 ^oC: an hour, and basal conditions of -5 ^oC: about 18 hours. Current design speeds of the Ice Diver range from 6.5 to 10 m/hr, meaning the probe may be meters away before significant freezing occurs. Modeled results suggest that the initial freezing acts to decrease the thermal gradient in the surrounding ice after the passage of the melt probe. It is thus beneficial to let the ice freeze in and then add antifreeze to minimize the volume needed to reach the eutectic solution. This will also reduce the formation of slush as has seen in other boreholes. Further modeling will help to demonstrate the role additional heating of the hole will play in conjunction with antifreeze for maintaining an open hole. To prevent contamination of subglacial lakes the warmer basal section may be kept open by heating alone due to slow basal freezing rates and ethanol may be used as antifreeze since it will be buoyant on the basal water.