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

Abstract

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.

arXiv comment11 pages, 5 figures, and 2 tables. Published in ApjL

References in corpus (9)

• The Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope I. Overview of the instrument and its capabilities
• Modal abundances of CAIs: Implications for bulk chondrite element abundances and fractionations
• The OSIRIS-REx Thermal Emission Spectrometer (OTES) Instrument
• Main Belt Asteroids with WISE/NEOWISE: Near-Infrared Albedos
• The OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS): Spectral Maps of the Asteroid Bennu
• New polarimetric and spectroscopic evidence of anomalous enrichment in spinel-bearing Calcium-Aluminium-rich Inclusions among L-type asteroids
• Evidence for OH or H2O on the surface of 433 Eros and 1036 Ganymed
• The Formation of Jupiter, the Jovian Early Bombardment and the Delivery of Water to the Asteroid Belt: The Case of (4) Vesta
• Spectral reflectance analysis of type 3 carbonaceous chondrites and search for their asteroidal parent bodies