The Nature of Low-Albedo Small Bodies from 3-$\mu$m Spectroscopy: One Group that Formed Within the Ammonia Snow Line and One that Formed Beyond It

We present evidence, via a large survey of 191 new spectra along with previously-published spectra, of a divide in the 3-$\mu$m spectral properties of the low-albedo asteroid population. One group ("Sharp-types" or ST, with band centers $<$ 3 $\mu$m) has a spectral shape consistent with carbonaceous chondrite meteorites, while the other group ("not-Sharp-types" or NST, with bands centered $>$ 3 $\mu$m) is not represented in the meteorite literature but is as abundant as the STs among large objects. Both groups are present in most low-albedo asteroid taxonomic classes, and except in limited cases taxonomic classifications based on 0.5-2.5-$\mu$m data alone cannot predict whether an asteroid is ST or NST. Statistical tests show the STs and NSTs differ in average band depth, semi-major axis, and perihelion at confidence levels $\ge$98\%, while not showing significant differences in albedo. We also show that many NSTs have a 3-$\mu$m absorption band shape like Comet 67P, and likely represent an important small-body composition throughout the solar system. A simple explanation for the origin of these groups is formation on opposite sides of the ammonia snow line, with the NST group accreting H2O and NH3 and the ST group only accreting H2O, with subsequent thermal and chemical evolution resulting in the minerals seen today. Such an explanation is consistent with recent dynamical modeling of planetesimal formation and delivery, and suggests that much more outer solar system material was delivered to the main asteroid belt than would be thought based on the number of D-class asteroids found today.