The L-band Main-belt and NEO Observations Program (LMNOP): X-complex and Small C-complex Objects

Low-albedo objects dominate the main asteroid belt and include objects that seem to straddle the line between asteroids and comets (as traditionally conceived). Dynamical models suggest these low-albedo objects may have formed among and beyond the giant planets, only to be transported to their current orbits during a period of planetary migration. Many of these low-albedo objects are observed to have hydrated minerals and/or volatiles based on measurements in the 3-µm spectral region.

We present 118 observations of 80 low-albedo asteroids taken with the IRTF SpeX instrument from 2002-2013. These observations cover objects, mostly in the B, C, and X classes, with albedos < 0.1 that are neither in the Ch spectral class nor both in the C complex and > 200 km diameter. We find strong correlations between spectral class and 3-µm spectral shape, as was previously found for the Ch class. However, while the correlation for the Ch class was with the “Pallas-type” spectral shapes seen in meteorites, we see the vast majority of objects in the B and X spectral classes are not Pallas-types, and are either Themis-type or some related type. This is further evidence that 0.5-2.5-µm spectral slopes are reflective of composition, not merely a result of space-weathering processes. In addition, Themis-like spectra are seen in objects orbiting within the middle asteroid belt and in objects as small as 50-100 km size, providing additional constraints on the minerals that are responsible and the histories of their parent bodies.