Illuminated granular water ice shows ‘dust’ emission

Context. Water ice in micro-granular form is the most common volatile in comets, and its behaviour when approaching the Sun must be understood before cometary activity can be properly modelled. Aims. To assess the properties of granular water ice, we investigated its evolution under illumination in a cryogenic high-vacuum environment. Methods. We produced a sample of water ice consisting of micrometre-sized particles, placed it inside a thermal vacuum chamber, and exposed it to high-intensity visible/near-infrared (VIS/NIR) illumination. Due to the energy absorption within the NIR bands of the ice, the sample is locally heated, which causes evaporation close to the surface. The total mass loss of the irradiated sample was measured using a scale and the surface temperatures were recorded with an infrared camera. Furthermore, we used several cameras to observe surface changes and ejected solid particles. Results. We derived the mass loss due to water-ice sublimation from the spatially resolved surface temperatures. This mass loss amounts to 68%–77% of the total mass loss. The remaining fraction (between 23% and 32%) of the mass is ejected in solid particles, which can be seen by the naked eye. Conclusions. The self-ejection of water-ice grains can be explained by a geometrical model that describes the sublimation of the icy constituents of the sample, taking into account the size distribution of the water-ice particles and the volume filling factor (VFF) of the sample. According to this model, solid ice particles are emitted when they (or the particle cluster they belong to) lose contact with the sample due to the faster evaporation of a smaller connecting ice grain. We discuss the possible relevance of this process for cometary dust activity.