Understanding Asteroids by the Distribution of their Hydrated Minerals

A recent large survey of low-albedo asteroid spectra in the 3-μm region show a range of hydrated compositions, with one large group similar to the CM chondrites and a second large group showing evidence of icy and/or ammoniated minerals and spectrally similar to Comet 67P (Rivkin et al. 2022). In addition, a small fraction of objects show no evidence of hydrated minerals, and a very small number of the largest objects appear spectrally similar to the dwarf planet Ceres. Interestingly, the work also shows that existing asteroid taxonomic classes are, with rare exceptions, poor predictors of hydrated composition, suggesting that the use of those classifications as proxies for formation location may vastly underestimate the amount of material transported to the asteroid belt from the transneptunian region.While we now have a good sense of the rough abundances of the most populous groups, we do not understand how those groups relate to one another or the variety of compositions within each group, in part because these compositions are only roughly constrained at best. We do not know whether 3-micrometer band shape is consistent within an asteroid family or if it can be used as a way of identifying interlopers, and thus we also do not know whether original parent bodies are homogeneous in their hydrated minerals or not.We propose a series of tasks to better understand the nature of low-albedo asteroids via targeted observations of collisional family members, relic planetesimals, and background objects in the 3-micrometer region, compositional modeling, development of a taxonomy for 3-micrometer spectra, and consideration of physical properties. The main objective of this work is to determine the distribution of and relationship between the groups with different hydrated compositions in the original low-albedo planetesimal population, as well as to determine or better constrain the hydrated compositions themselves. The wider applications of the proposed work include insights into volatile delivery to the inner solar system, the representativeness of the meteorite collection (and how returned samples relate to low-albedo objects more generally), and non-ice compositions throughout the entire solar system. The work proposed also follows several of the recommendations for Strategic Research in the newly-released Planetary Science Decadal Survey (National Academies of Sciences, Engineering, and Medicine 2022).