Line heat-source measurements of the thermal conductivity of porous H2O ice, CO2 ice and mineral powders under space conditions

Measurements of the thermal conductivity of porous loose mineral, porous H2O ice and porous CO2 ice samples under low temperatures (77 K < T < 300 K) and pressures (10−4 Pa < p < 105 Pa) are reported. The samples were selected to cover the end members of possible comet nucleus compositions and the ambient conditions were chosen to investigate the samples under space conditions. A transient technique is used for the measurements which is well suited for in situ application. The method is based on the line heat-source technique: a thin internally heated cylindrical sensor is inserted into the sample material. The thermal conductivity is deduced from the observed temperature rise in the sensor and the heating power applied. Depending on sensor dimensions, single experiment runs may be completed within a few minutes. The method proved to be accurate, fast and well suited for an application in the laboratory as well as in situ, e.g. on future comet nucleus or Mars missions. A thermal probe (MUPUS-PEN) which employs the experimental technique discussed here has been proposed for the ROSETTA surface science package “RoLand”. The thermal conductivity of the loose dunite sample is studied as a function of gas pressure. At low pressures, it is almost constant and close to 0.03 W m−1 K−1. At atmospheric pressure, the thermal conductivity is about one order of magnitude higher. Both domains are linked by a pressure region with a strong pressure dependency of the thermal conductivity. Three porous water ice samples with different pore sizes have been investigated. The results are in agreement with theoretical predictions (e.g. Steiner et al., 1991) and reveal a strong increase of the thermal conductivity at temperatures close to the sublimation temperature of water ice (≈ 200 K in vacuo). The increase is due to heat transport by pore filling vapour which is more effective in samples with large pore radii. The measured matrix conductivity is close to 0.02 W m−1 K−1, while maximum values for the effective (= matrix + vapour) thermal conductivity at high temperatures exceed 0.25 W m−1 K−1. Similar results are obtained for one porous CO2 ice sample.