A Large 3-micron Spectroscopic Survey of Mid-Outer Main Belt Asteroids

We previously measured spectra of 35 mid-outer Main Belt asteroids spanning the 2.5 < a < 4.0 AU region and identified four spectral groups on the basis of the shape and band center of the 3-µm feature (Takir and Emery 2012, Takir et al. 2015). We have now increased the total number of the studied mid-outer Main Belt asteroids to ~100 objects to further investigate the distribution of these asteroids in terms of their hydration state. The new observations include asteroids with different classes (e.g., C, B, P, D, G, and T), diameters (~100–400-km), and families/groups (Hygiea, Themis, Cybele, and Hilda). Asteroids were observed using NASA IRTF and SpeX spectrograph/Imager and its two modes, the Prism (0.7-2.52 µm) and long-wavelength cross dispersed (LXD: 1.9-4.2 µm) between 2015 and 2020. We selected our targets from four heliocentric (semi-major axis) bins: 2.5 < a < 2.9 AU, 2.9 < a < 3.3 AU, 3.3 < a < 3.6 AU, and 3.6 < a < 4.0 AU.

We classified the newly-observed asteroids according to Takir and Emery (2012)’s 3-µm grouping. Most of the observed asteroids selected from the first and second bin were found to be consistent with the ‘sharp’ 3-µm group, reflectance decreases with decreasing wavelength into the 2.50-2.85 μm spectral region obscured by Earth’s atmosphere. The 3-µm band shape of the sharp group is inferred to be due to the presence of phyllosilicates on the surfaces. We did not find any strong correlation between the band depth at 3 µm and the heliocentric distance. Our observations have also revealed that 15 mid-outer Main Belt asteroids with a 3-µm band that is nearly identical in shape and position, a broad absorption with maximum depth ~3.15 µm, to that found in the spectra of Ceres and comet 67P/Churyumov-Gerasimenko. All of these asteroids are predominantly concentrated in the second and third semi-major bins. No new asteroid was found to have a rounded 3-µm band shape, in which reflectance increases with decreasing wavelength shortward of about 3.07 µm. Some of the objects that were previously found to be part of the 'rounded' group appear to be more consistent with the group that is similar to Ceres.

All final spectra (Prism and LXD) of the observed mid-outer Main Belt asteroids have been prepared for submission to PDS Small Bodies Node. This study was funded by NASA Solar System Observations Program (NNX17AJ24G).

References: Takir, D. and Emery, J. (2012) Icarus 219, 641–654. Takir, D. et al. (2015) Icarus 257, 185–193.