3 μm Spectroscopic Survey of Near-Earth Asteroids

Near-Earth Asteroids (NEAs) are excellent laboratories for processes that affect airless body surfaces. S-complex (including V-type) NEAs were not expected to contain OH/H₂O on their surfaces because they formed in the anhydrous regions of the solar system and their surface temperatures are high enough to remove these volatiles. However, a 3 μm feature typically indicative of OH/H₂O was identified on other seemingly dry bodies in the inner solar system, raising the question of how widespread volatiles may be on NEAs. We observed 29 NEAs using both prism (0.7–2.52 μm) and LXD_short (1.67–4.2 μm) modes on SpeX on NASA’s IRTF in order to accurately characterize asteroid spectral type and the 3 μm region. Eight of the observed NEAs have a 3 μm absorption feature at >1σ (three of which are present to >2σ), and they exhibit four identified band shape types. Possible sources for OH/H₂O on these bodies include carbonaceous chondrite impacts and/or interactions with protons implanted by solar wind. Characteristics such as composition and aphelion appear to play an important role in the delivery and/or retention of OH/H₂O, as all eight NEAs with an absorption feature are S-complex asteroids and six enter the main asteroid belt. Additionally, perihelion, size, albedo, and orbital period may play a minor role. Our observations determined that nominally anhydrous, inner solar system bodies, and therefore near-Earth space in general, contain more OH/H₂O than previously expected. The identified trends should help predict which NEAs that have not yet been observed might contain OH/H₂O on their surfaces.