ELASTIC PROPERTIES OF AMORPHOUS AND CRYSTALLINE ICE FILMS

Water is the basic ingredient of comets and interstellar dust. It is b elieved that the cometary ices were originally formed as amorphous ice s that have since been reprocessed. In order to model cometary history it is therefore necessary to know the physical properties of amorphou s water ice because it is well established that the low temperature pr operties of amorphous solids differ greatly from those of their crysta lline 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 mo dulus it is shown that directly after evaporation some amorphous ice f ilms are extremely porous with a porosity p of up to 0.6. The simultan eous measurement of the internal friction establishes the huge amount of local disorder which is present in these films. During annealing pr ocesses at temperatures below 100 K both the porosity and local disord er are greatly reduced. The impact of these findings is discussed on t he thermal conductivity, which is the most important quantity needed f or the thermal modelling of comets. It is concluded that the high poro sity found in the ice films reduces the thermal conductivity by no mor e than two orders of magnitude. Copyright (C) 1996 Elsevier Science Lt d