Evolution of Micro-Granular Porous Dust-Ice Mixtures Under Solar Irradiation: Implications for Cometary Dust Activity

As comets draw closer to the Sun, they begin to emit small solid particles, producing the familiar dust coma and tail. The cohesive forces binding these micrometer-sized grains typically reach about 1 kPa, while the gas pressure generated by sublimating volatiles usually remains only a few pascals. This stark difference presents a major challenge: any plausible dust release model must explain how such strong cohesion between particles can be overcome.

Cohesion can be effectively reduced only under particular conditions. One such scenario involves the formation of weakly bound porous aggregates — cometary "pebbles" — composed of fine micron-sized dust grains assembled into millimeter-sized structures. This concept aligns well with in situ findings from the COSIMA, MIDAS, and GIADA instruments. A theoretical model proposed by Skorov and Blum (2012) describes the removal of these porous aggregates through sublimation-driven gas flow, and subsequent laboratory experiments have confirmed the validity of this mechanism.

Earlier models, including related approaches, generally assumed the presence of a dry, porous dust crust situated above an icy substrate, with sublimating gases accumulating pressure at the ice-dust boundary. In these models, it is primarily fragments of the crust — rather than individual grains — that are detached and ejected into space. More recently, however, new models have emerged, focusing instead on the possibility of releasing primary dust particles or small aggregates (Schuckhart & Blum, 2025).

In this study, we extend this perspective by investigating the evolutionary dynamics of a densely packed, heterogeneous porous layer consisting of non-volatile dust grains intermixed with icy inclusions, subjected to solar heating. The absorption of solar radiation triggers the sublimation of the embedded ice, which progressively weakens the mechanical integrity of the porous structure and drives its spatial reorganization.

To simulate these processes, we develop a novel computational framework based on the DEM that explicitly represents the granular structure of the heterogeneous material (Reshetnyk et al. 2025). Our study examines systems composed of both solid particles and porous aggregates, considering arrangements of both monodisperse and polydisperse spheres. Two formation scenarios are analyzed: layers resulting from initial deposition and those compacted under external mechanical loads. Particular attention is devoted to the phenomenon of jamming, where compression immobilizes the particles into a dense, stable configuration.

Using tools from percolation theory and graph analysis, we map the pathways leading to material disintegration. Our results demonstrate that the sublimation of internal ice can promote the detachment and ejection of individual dust grains or small clusters from the cometary surface. Unlike traditional approaches that focus on the global tensile strength of the dust layer, our model emphasizes localized release mechanisms, offering new insights into the origin of fine dust observed in cometary environments. Additionally, we find that some of the ejected aggregates may retain residual ice, potentially serving as a prolonged source of both dust and water vapor — a result consistent with observations of comet 67P/Churyumov-Gerasimenko.

 

References:

Reshetnyk, V. et al. 2025. Key structural characteristics of porous layers in diffusion modelling: A study on polydispersity, shape, and hierarchy. PSS 260. doi:10.1016/j.pss.2025.106078

Schuckart , C. and J. Blum 2025. A&A, doi: 10.1051/0004-6361/202553750

Skorov, Y. and J. Blum. 2012. Dust release and tensile strength of the non-volatile layer of cometary nuclei. Icarus 221, 1–11. doi:10.1016/j.icarus.2012.01.012