Plasma drilling on Martian ice: Enabling efficient deep subsurface access to Mars' polar layered deposits

摘要

Mars' polar layered deposits (PLD) contain the best historical record of the Amazonian time period. Deep sub-surface access at Mars' poles is challenging due to the restriction of Mars' atmospheric condition. Traditional terrestrial mechanical drilling methods using drilling fluids are not feasible in these conditions and the melting probe approach is very inefficient due to heat loss. For ice in Mars' polar condition, the increase in surface area due to cracks would greatly reduce its thermal conductivity. This will lead to a higher thermal efficiency and an easier access to polar layered deposits during melting or sublimation. The preliminary results with pulsed plasma ice drilling, using localized electric breakdown with electric energy input per pulse up to 80 J, were promising under both Mars and Earth conditions. To simulate Mars' polar condition, a vacuum vessel system was designed and implemented with a pulsed discharge circuit, ice plate, controlled liquid nitrogen cooling, and CO2 gas feed -in. A simplified heat conduction model was established for estimating the heat conduction at various porosities for homogenous water ice and CO2 gas mixture using COMSOL under two scenarios including constant thickness of mixture layer and constant thermal mass of the mixture layer under the heat source. The simulation results for both scenarios indicated more reduction of heat loss with an increasing porosity. Experimental studies of thermal conductivity measurement of granular ice under the Earth condition inside a freezer using a custom thermal needle probe with known porosity proved the concept that voids/cracks in ice can reduce the thermal conduc-tivity. Based on simulation and experimental results, the classic melting probe model is extended with porous ice-CO2 mixture to estimate the power consumption on Martian ice at varying descending speed. Plasma drilling with limited power is promising for accessing Mars' PLD and can also be expanded to planetary exploration in searching for life on other planets.