On the darkness of Ryugu and other dark small bodies

arising from: C. Potiszil et al. Nature Astronomy https://doi.org/10.1038/s41550-024-02372-y (2024)

As observed by the Hayabusa2 spacecraft, the surface of the asteroid Ryugu is extremely dark, which led Potiszil et al. to predict a very high organic content (14.9–59.3 vol% C)¹. This prediction was shown to be largely overestimated by analysed returned samples, and this discrepancy is explained by Potiszil et al. in ref. ² by textural and photometric effects, as well as space weathering on organic matter. Rather, we argue that mineralogy plays a key role in the optical properties, and that an approach where ‘albedo’ is a linear function of organic content does not account for the nonlinearity of radiative transfer in complex media. More specifically, fine-grained Fe-bearing opaques are likely important darkening agents of primitive small bodies, cometary dust and possibly interstellar grains.