Elastic properties of amorphous and crystalline ice films

Water is the basic ingredient of comets and interstellar dust. It is believed that the cometary ices were originally formed as amorphous ices that have since been reprocessed. In order to model cometary history it is therefore necessary to know the physical properties of amorphous water ice because it is well established that the low temperature properties of amorphous solids differ greatly from those of their crystalline counterparts. In a series of laboratory experiments the elastic properties (shear modulus and internal friction) of thin amorphous ice films are investigated. With the measurements of the elastic shear modulus it is shown that directly after evaporation some amorphous ice films are extremely porous with a porosity p of up to 0.6. The simultaneous measurement of the internal friction establishes the huge amount of local disorder which is present in these films. During annealing processes at temperatures below 100 K both the porosity and local disorder are greatly reduced. The impact of these findings is discussed on the thermal conductivity, which is the most important quantity needed for the thermal modelling of comets. It is concluded that the high porosity found in the ice films reduces the thermal conductivity by no more than two orders of magnitude.