LINE HEAT-SOURCE MEASUREMENTS OF THE THERMAL-CONDUCTIVITY OF POROUS H2O ICE, CO2 ICE AND MINERAL POWDERS UNDER

Measurements of the thermal conductivity of porous loose mineral, poro us H2O ice and porous CO2 ice samples under low temperatures (77 K < T < 300 K) and pressures (10(-4) Pa < p < 10(5) Pa) are reported. The s amples were selected to cover the end members of possible comet nucleu s compositions and the ambient conditions were chosen to investigate t he samples under space conditions. A transient technique is used for t he measurements which is well suited for in situ application. The meth od is based on the line heat-source technique: a thin internally heate d cylindrical sensor is inserted into the sample material. The thermal conductivity is deduced from the observed temperature in the sensor a nd the heating power applied. Depending on sensor dimensions, single e xperiment runs may be completed within a few minutes. The method prove d to be accurate, fast and well suited for an application in the labor atory as well as in situ, e.g. on future comet nucleus or Mars mission s. A thermal probe (MUPUS-PEN) which employs the experimental techniqu e discussed here has been proposed for the ROSETTA surface science pac kage ''RoLand''. The thermal conductivity of the loose dunite sample i s studied as a function of gas pressure. At low pressures, it is almos t 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 dom ains are linked by a pressure region with a strong pressure dependency of the thermal conductivity. Three porous water ice samples with diff erent 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 t he sublimation temperature of water ice (approximate to 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 matri x conductivity is close to 0.02 W m(-1) K-1, while maximum values for the effective (= matrix + vapour) thermal conductivity at high tempera tures exceed 0.25 W m(-1) K-1. Similar results are obtained for one po rous CO2 ice sample. Copyright (C) 1996 Elsevier Science Ltd