The strength of outgassed porous dust aggregates

Context. Outgassing of dust-ice aggregates plays an important role on the surfaces of cometary nuclei as well as for snow-line crossings in protoplanetary disks. Aims. To assess the stability of desiccated dust aggregates, we measured the tensile strength of silica dust samples over a wide range of volume filling factors. Methods. We produced these silica dust samples over a wide range of volume filling factors by gently evaporating dust-ice mixtures with various dust-to-ice mass ratios under vacuum conditions. The tensile strengths of these samples were then measured using the standardized Brazilian disk test. Experiments were performed in a vacuum and at room temperature but were also compared to measurements in air at room temperature and in a vacuum at elevated temperatures. Results. For spherical amorphous silica dust, we find no influence of the environmental conditions (air, vacuum, or heating) on the measured tensile strength. However, for angular crystalline silica dust we see a strong increase in tensile strength in a vacuum compared to air and an even higher increase when the samples are heated in a vacuum. For the spherical silica dust samples, we find a characteristic increase in the tensile strength with decreasing particle size. The tensile strength of samples with identical particle sizes increases strongly with an increasing volume filling factor. Extrapolation of our data to a volume filling factor of 0.1 (90% porosity) shows that a tensile strength as low as 1 Pa can be reached. Conclusions. Numerical simulations show that evaporating water ice in the subsurface layers of comets can reach gas pressures of ~1 Pa. Thus, a desiccated dust layer with a 10% volume filling factor should be detachable and released into the cometary coma. Using a relation between the tensile strength and the critical fragmentation energy, we predict the break-up speed of dust aggregates in mutual collisions as a function of the volume filling factor. Furthermore, we discuss the susceptibility of the aggregates to ram pressure. These values are relevant for protoplanetary disk research and for meteoroids entering planetary atmospheres.