Composition, Mineralogy, and Porosity of Multiple Asteroid Systems from Visible and Near-infrared Spectral Data
Sean S. Lindsay, Franck Marchis, Joshua P. Emery, J. Emilio Enriquez, Marcelo Assafin
Accepted to Icarus
Subjects: Earth and Planetary Astrophysics
We provide a taxonomic and compositional characterization of Multiple Asteroid Systems (MASs) located in the main belt (MB) using visible and near-infrared (0.45-2.5 um) spectral data of 42 MB MASs. The mineralogical analysis is applied to determine meteorite analogs for the MASs, which, in turn, are applied to the MAS density measurements of Marchis et al. (2012) to estimate the system porosity. The macroporosities are used to evaluate the primary MAS formation hypotheses. The visible observing campaign includes 25 MASs obtained using the SOAR telescope with the Goodman High Throughput Spectrometer. The infrared observing campaign includes 34 MASs obtained using the NASA IRTF with the SpeX spectragraph. The MASs are classified using the Bus-DeMeo taxonomic system. We perform a NIR spectral band parameter analysis using a new analysis routine, the Spectral Analysis Routine for Asteroids (SARA). The SARA routine determines band centers, areas, and depths by utilizing the diagnostic absorption features near 1- and 2-um. The band parameter analysis provides the Gaffey subtype for the S-type MASs; the relative abundance olivine-to-pyroxene ratio; and olivine and pyroxene modal abundances for S-complex and V-type MASs. This mineralogical information is applied to determine meteorite analogs. We determine the H, L, and LL meteorite analogs for 15 MASs with ordinary chondrite-like (OC) mineralogies. We observe an excess (10/15) of LL-like mineralogies. Of the MASs with LL-like mineralogies, seven are consistent with Flora family membership, supporting the hypothesis that the Flora family is a source of LL-like NEAs and LL chondrites on Earth. Using the measured densities of the meteorite analog and the MAS densities from Marchis et al. (2012), we estimate the macroporosity for 13 MASs and find that all estimated macroporosities are in agreement with formation hypotheses.
The overabundance of LL-like mineralogies in our S(IV) sample is explained by Flora family membership. Seven out of the ten MASs with OC-like mineralogies fall within the orbital parameter description of the Flora family, and all seven are analogous to the LL chondrites. This overabundance is consistent with studies of NEAs, which find a similar excess of LL-like mineralogies, strengthening the proposed connection between LL chondrites and the Flora family. These systems are also rubble-piles consistent with the Flora family forming via the catastrophic disruption suggesting a progenitor body with an LL-like mineralogy. Moreover, we note that two LL-like MASs consistent with the Eunomia family are also both LL-like suggesting the Eunomia family as a possible additional source for LL-like NEAs and LL chondrites on Earth.