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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 Komle 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.

Melting probes as vehicles to explore terrestrial ice sheets have been designed and applied successfully since the early 1960’s. Later on, in the 1990’s, various proposals were made to apply such probes also as a means to explore ice sheets on other bodies of the solar system, e.g. Jupiter’s icy satellite Europa or the ice caps of Mars. For this type of subsurface probes the name cryobot has become common. We review both early developments and more recent efforts to develop probes for application in planetary environments, i.e. under low pressures and low temperatures. The current state of art as well as the pros and cons of the different concepts hitherto considered are described. While many tests with various probes have been done in terrestrial environments, experiments under low surface pressure conditions are rare. Therefore, we report here on lab tests with a simple melting probe under the range of pressure and temperature conditions that would be encountered on the surface of Mars and compare them with corresponding tests under a much lower gas pressure, possibly representative for icy satellites. The contribution of evaporation during the melting and its variation with surface pressure is also considered. All surface pressure measurements that have been performed on Mars up to now indicate a surface pressure above the water triple point pressure (612 Pa). This means that water ice always transforms into the liquid phase when warmed up to 0°C, before it evaporates into the ambient atmosphere. The temporary existence of the liquid phase around the heated tip of the cryobot allows good thermal conductance between probe and surrounding ice, which is an important pre-requisite for efficient melt penetration. Our experiments indicate that under all possible Mars surface pressures the liquid phase is present when the probe is heated up. This finding confirms experimentally that a probe as it was proposed by Paige (1992) for in situ exploration of the Mars north polar layers would work in the expected way, although the penetration velocity must be expected be lower than under Earth pressure conditions. A test with the same probe, but under an almost two orders of magnitude lower gas pressure than on Mars, still indicates the temporary existence of the liquid phase in the contact region between the probe and the surrounding ice.

Two laboratory test series were performed with the aim of ensuring the proper functionality of the key sampling mechanisms installed aboard the Mars rover ExoMars, currently scheduled for launch in 2020 by the European Space Agency ESA. In order to facilitate the chemical analysis of the Martian ground accessible to the ExoMars drill, the retrieved drill cores must first be milled. This task is performed by a crushing station (CS), which delivers the milled product to a dosing device (PSDDS). From there the material is distributed further to the various analysis instruments mounted on the rover. The first test series was performed with a mock-up of crushing station and dosing device under simulated Martian pressure and temperature conditions. As a worst-case scenario, crushing of frozen soil mixtures was performed and the milling products were collected in the dosing station before being further distributed. In the second test series, granular analogue materials equivalent to the milled products obtained in previous tests were stored for periods of several days in the input funnel of the dosing device. The set-up included a regulation valve through which water vapour was streamed into the vacuum chamber to create a water vapour-saturated atmosphere. The purpose of this series of tests was to investigate if the presence of water can cause cementation of the samples, and how this subsequently affects the operation of the crushing and distribution devices. Our results indicate that the milling device works very well with the current design both for loose and for hard block-like materials, e.g., chunks of frozen soil. It was also found that milled material, when subjected to a water-saturated atmosphere, does not experience any cementation.