Detection of a 2.85 micrometer Feature on 5 Spinel-rich Asteroids from JWST

Ground-based observations of `Barbarian' L-type asteroids at 1 to 2.5-$\mu$m indicate that their near-infrared spectra are dominated by the mineral spinel, which has been attributed to a high abundance of calcium-aluminum inclusions (CAIs) -- the first solids to condense out of the protoplanetary disk during the formation of the Solar System. However, the spectral properties of these asteroids from 2.5 to 5-$\mu$m, a wavelength region that covers signatures of hydrated minerals, water, and organics, have not yet been explored. Here, we present 2 to 5-$\mu$m reflectance spectra of five spinel-rich asteroids obtained with the NIRSpec instrument on the James Webb Space Telescope. All five targets exhibit a $\sim$ 2.85-$\mu$m absorption feature with a band depth of 3-6$\%$ that appears correlated in strength with that of the 2-$\mu$m spinel absorption feature. The shape and position of the 2.85-$\mu$m feature are not a good match to the 2.7-$\mu$m feature commonly seen in carbonaceous CM meteorites or C-type asteroids. The closest spectral matches are to the Moon and Vesta, suggesting commonalities in aqueous alteration across silicate bodies, infall of hydrated material, and/or space weathering by solar wind H implantation. Lab spectra of CO/CV chondrites, CAIs, as well as the minerals cronstedtite and spinel, also show a similar feature, providing clues into the origin of the 2.85-$\mu$m feature.