Melting probe experiments under Mars surface conditions – the influence of dust layers, CO2-ice and porosity

This paper reports about recent melting probe experiments under a pressure that is within the pressure regime at the surface of Mars. As compared to the results given in Kömle et al. (2018), the experiments were extended in several aspects: (i) A deeper ice sample was used in order to study the performance of the probe in the subsurface, (ii) dust layers were embedded inside the samples in order to study their influence on probe penetration, (iii) a surface cover of granular CO₂-ice was added, in order to study the performance of the probe in the presence of dry ice, and (iv) the performance of the melting probe in a highly porous ice layer (fresh bonded snow) was studied. In addition, the case of several heating cycles, interrupted by cooling periods, was considered. The experiments demonstrated that the melting probe concept remains successful also in the presence of embedded sand and dust layers, as long as such non-volatile layers are thin, say in the order of centimetres. Under Mars pressure there is always liquid water present in the melt hole, except for the fresh bonded snow case, where the melt water is absorbed by the surrounding snow. However, because the Mars surface pressure is typically just slightly above the water triple point pressure, this water is permanently boiling and there are strong convective motions which whirl up dust particles and remove them at least partially from the hot nose region. The presence of a porous CO₂-ice cover on the surface does not seriously hamper melting probe penetration, because CO₂-ice is much more volatile than water ice and the probe passes such a layer very fast when heated with the same power. In fresh bonded snow, which has very high porosity, the probe penetrates correspondingly faster than in compact ice. Finally, in the last experiment, it was demonstrated explicitly that the melting probe concept works also, when heating is applied in several cycles, interrupted by periods of no active heating.