Presentation #505.04 in the session Observing Physical Properties of NEOs Part I.
Near-Earth Asteroids (NEAs) are excellent laboratories for processes that affect the surfaces of airless bodies. Most 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, such as the Moon and Vesta, and more recently on the NEAs Eros and Ganymed. Six other NEAs have been identified that exhibit or potentially exhibit a 3-μm feature, but Ganymed and Eros have been observed the most. Possible sources for OH/H2O on these bodies include carbonaceous chondrite impacts or interactions with protons implanted by solar wind.
We observed Eros and Ganymed multiple times using SpeX on NASA’s IRTF. Spectra were collected using both prism (0.7-2.52 μm) and LXDshort (1.67-4.2 μm) modes in order to accurately characterize asteroid spectral type and the 3-μm region. We have made 8 observations of Eros and 6 observations of Ganymed as part of this ongoing project. Including observations made by Rivkin et al. (2018) of these two objects, band depths range from -0.4±3.3% to 7.9±7.4% for Eros and from 0.0±1.5% to 6.2±2.1% for Ganymed.
Various characteristics, including sub-solar latitude, heliocentric distance, and observing parameters, were investigated to determine the cause of the band depth variance for these two NEAs. Time from perihelion has a loose correlation with band depth, but no other determining factors have been identified, though studies are ongoing. Understanding the factors controlling these varying band depths will enable further understanding of the processes by which OH/H2O is delivered and/or retained on NEA surfaces.