ASTEROID MACROPOROSITY AS A CONSTRAINT ON METEORITIC ANALOGS: MODELING RYUGU

Macroporosity, as opposed to reflectance spectra and albedo, serves as a whole-body property capable of setting constraints on meteoritic compositional analogs for asteroids. Insight into meteorite-asteroid genetic relationships aid in the elaboration of models for the evolution of the early solar system, and have implications for the origins of Near Earth Asteroids and the context in which returned asteroidal samples will be analyzed. Here we physically model the structure of C-type “rubble-pile” Ryugu (NEA 162173), a likely proxy for other “rubble-piles”, as a granular aggregate to set a constraint on its macroporosity, then calculate a corresponding constituent rock density to be compared to sampled meteorite bulk densities. Our small-scale random particle packing experiments include the effects of container SA/V ratios, “edge-effects”, and particle mixtures on measured macroporosities.

Models were constructed according to different exponents for the power law describing cumulative boulder size distribution (N=CD^-𝛼, where C is a constant and D is boulder diameter) observed by the Hayabusa2 spacecraft in 2018. With 𝛼 values ranging from -2 to -4, the measured macroporosities range from 26.5% to 33.9%. Using a bulk density of 1.19±0.02 g/cm³ for Ryugu, these values correspond to rock densities in g/cm³ between 1.6 and 1.8. It is important to note that these are upper limits to Ryugu’s macroporosity, and thus constituent rock density, due to additional void spaces along container walls and the omission of finer analog particles. These values sit between the mean rock densities of the CI (1.57g/cm³) and CM (2.27g/cm³) meteorite classes, lowest of all CC. Based on rock density, CI meteorites appear to be the closest analogs for Ryugu under the assumption that Ryugu has a single composition. This does not necessarily mean Ryugu’s rocks are similar to CI meteorites in all respects, but implies that chondrules, as the densest component of chondrites, are rare or absent on this C-type asteroid.