Abstract:
The cosmic dust particles found in space are mainly porous aggregates of smaller grains. Theoretically, these
aggregates are replicated using fractal geometry, assuming a cluster of spheres. Although the light scattering
response of cosmic dust aggregates has been thoroughly studied using clusters of spherical grains in the past few
decades, the effect of irregularities on the surface of each grain in an entire aggregate has mostly been neglected.
We introduce, for the first time, a visually realistic cosmic dust model that incorporates a mixture of rough fractal
aggregates (RFA) and agglomerated debris (Solids) to replicate the unusual polarization–phase curve observed in
the case of the interstellar comet 2I/Borisov at multiple wavelengths. The authenticity of the RFA structures has
been verified by replicating light scattering results of circumstellar dust analogs from the Granada Amsterdam
Light Scattering Database. We demonstrate that the light scattering response from the RFA structures has a very
close resemblance to the experimental values. Finally, we model the observed polarization–phase curve of the
interstellar comet 2I/Borisov using a mixture of RFA and solid particles. The best-fit data indicate the presence of
a higher percentage of porous RFA structures (80%) owing to the fact that the comet carries a higher percentage of
small and highly porous pristine cosmic dust particles. Further, the model indicates that the unusually steep
polarimetric slope and the high dust-to-gas ratio in newer comets are mainly due to a higher porous-to-compact
ratio
