OH/H₂O on Near-Earth Asteroids
Presentation #509.02 in the session "Asteroids: NEOs Physical Properties 3 - volatiles and composition".
by L. McGraw, J. Emery, C. Thomas, A. Rivkin, and N. Wigton
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. Possible sources for OH/H2O on these bodies include carbonaceous chondrite impacts or interactions with protons implanted by solar wind.
We observed NEAs 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 40 observations of 26 NEAs as part of this ongoing project. Of those, at least 3 NEAs exhibit an absorption feature in the 3-µm region: (433) Eros, (1036) Ganymed, and (3122) Florence. All three have been observed multiple times and by multiple observers (e.g., Rivkin et al. [2018]. Icarus 304, 74-82). Five more NEAs exhibit a potential feature: (96590) 1998 XB, (214088) 2004 JN13, 2014 JO25, (1627) Ivar, and (163373) 2002 PZ39.
Four band shape types have been identified. Type 1 exhibits a wide, linearly increasing (from 2.85 µm) feature; the NEAs of this type are larger than 4.5 km in diameter, have perihelia greater than 1 AU, and are S-types. Type 2 exhibits a narrow, linearly increasing feature. Type 3 exhibits a narrow, zero-sloped feature that sharply increases to the continuum around 3.05 µm. The NEAs of this type are smaller than 1 km in diameter, have perihelia less than 1 AU, and are Q-types. Type 4 exhibits a wide, zero-sloped feature that sharply increases to the continuum around 3.2 µm. Understanding the similarities and differences between these groups will enable further understanding of the processes by which OH/H2O is delivered and/or retained on NEA surfaces.