Compaction experiments on ice-silica particle mixtures: Implication for residual porosity of small icy bodies

To evaluate the residual porosity of small icy bodies, we performed compaction experiments on ice-silica mixtures and studied the effects of silica content, temperature, and compaction time scale on residual porosity. To simulate the compositions of real icy bodies, we used ice-silica mixtures with different silica volume fractions (0-0.29). The mixtures were compacted at a constant compression speed of 0.2 or 2.0 mm/min and the temperature was set to -10 degrees C or a lower temperature (from -55 to -67 degrees C). For the -10 degrees C case, the mixtures were compressed to pressures of 30 MPa, while the lower temperature measurements were compressed to 80 MPa. In both cases, the residual porosity was found to be larger for higher silica fractions. At -10 degrees C and 30 MPa, the residual porosity varied from 0.01 to 0.14 for silica fractions of 0-0.29, whereas for the -55 to -67 degrees C and 80 MPa case, the corresponding residual porosities were 2-10 times larger. A two-layer model was proposed to calculate the compaction curves of ice-silica mixtures from the curves of the corresponding pure materials. We estimated the residual porosity of small icy bodies using this two- layer model. From our calculations, we expect that icy bodies with diameters smaller than 700 km have residual porosity larger than 0.3 when the temperature is lower than -55 degrees C.