Distinct Populations of Low-Albedo Asteroids Based on Their Hydrated Mineral Compositions

We present a compilation of over 200 low-albedo asteroid spectra covering the 2—4-µm region, including roughly 150 unpublished measurements of 86 objects as well as published spectra of Ch-class and large (D>200 km) asteroids. We separate these spectra via their most prominent absorption bands into “Pallas type” (PT) with band centers (BC) < 3.0 µm and “Not Pallas Type” (NPT) with BC > 3.0 µm. The NPT group is a superset of multiple groups identified by other authors including the “Ceres types” and “Themis types” (or “sharp” and "rounded"). The PT group is largely but not completely composed of Ch and Cgh asteroids, and most non-Ch/Cgh asteroids fall into the NPT group. We call particular attention to the fact that we do not have any meteorites with NPT spectra, and we note that several NPT objects bear a close spectral resemblance to the average spectrum of comet 67P. The NPT group is a significant one. BC can be determined in our sample for 65 of the 100 largest low-albedo, main-belt asteroids (D>120 km), with NPTs outnumbering PTs 35-30, with two additional objects showing no discernable band. The NPTs are the most numerous group among the 16 sampled objects in the Cybele and Hilda regions as well. However, both NPTs and PTs can be found throughout the main belt and Cybele/Hilda regions. Hypothesis testing shows that the NPT and PT groups are not statistically similar, to a high level of confidence. The average BCs for the groups are far from the defined dividing lines: 2.93 µm vs. 3.13 µm. Their band depths are different at a 99% confidence level (mean band depth of 20% for PT, 12% for NPT). Their average group densities are different with greater than 90% confidence (NPTs are 20% less dense than PTs), and NPTs are found at an average of 0.15 AU further from the Sun than PTs. We conclude that the PT and NPT asteroids represent distinct populations (with each group possibly also containing distinct populations). Given the association of the NPT objects Ceres, Cybele, and Themis with icy and/or ammoniated compositions and the similarity to 67P, we suggest that the NPTs were delivered to the asteroid belt from formation locations more distant than the PTs. We also note that there are many implications from this work for our understanding of dynamics and processes early in solar system history.