Energy analysis of porous water ice under space-simulated conditions: results from the KOSI-8 experiment

A sublimation experiment (KOSI-8) was performed on a pure, porous water ice sample under conditions of low pressure and temperature and insolation by an artificial sun. Temperatures within the sample, gas flux from the surface, mass loss of the sample during the experiment and the irradiation input were measured for a detailed analysis of energy flow. A main feature of the analysis was the development of a convex temperature profile along the middle axis of the sample as a consequence of heat transfer by water vapor flow. The amount of the energy transport by the water was about 40% of the total heat flux available for heating the sample. To interpret the unusually high temperatures measured a few millimeters below the surface, it is assumed that the radiative energy input penetrates about 3 mm into the sample (solid-state greenhouse effect). To trace the flow of water vapor during the experiment, the top 10 mm of the ice was enriched with 10% HDO. Ice samples for isotopic analyses were taken from the post-insolation surface and from various depths. Isotopic enrichment was observed only at the surface, corresponding to a 2.3% admixture of recondensed vapor from the initially 10-mm-thick surface layer, to a depth of 19 mm below the original surface. No isotopic enrichment could be detected below a crust that had formed, i.e. 57 mm below the original surface. During the experiment an average of about 16 mm of the ice eroded from the surface. The weight of the sample was continuously monitored and a total mass loss of 1.75 ± 0.05 kg was measured between start and end of insolation. A comparison of the power balance of a pure, porous ice sample and an ice/dust sample shows that in either case the net flux of energy available for sublimation and warming of the sample is less than 20% of the insolation. In the case of a pure ice sample, most of the irradiation is reflected due to the high albedo; in the case of an ice/dust sample, thermal reradiation of the sample is very high because of the high surface temperatures, ≈ 350 K.