Remote Sensing of Near-Earth Asteroids Using near Infrared (3 µm) Spectroscopy

Abstract

Created during the collapse of the solar nebula, asteroids remain as relatively unaltered fragments of the primordial Solar System. The near-Earth asteroid (NEA) population is a relatively easily reachable subset of these widely varying fragments, making them excellent laboratories for testing near-Earth processes that affect airless bodies. Most airless bodies in the inner Solar System are nominally anhydrous as their formation location and lack of an atmosphere generally preclude any form of hydration. The discovery of hydroxide (OH) and/or water (H₂O) on several such bodies raised several questions regarding the ubiquity of water in near-Earth space and its sources. In this dissertation, I investigate the sources of and the various mechanisms by which OH/H₂O is delivered to and/or retained by NEA surfaces to characterize the OH/H₂O budget of near-Earth space to aid in determining the source of Earth’s water.

I began by determining the prevalence of OH/H₂O in the NEA population by conducting a 5-year, near-infrared (NIR) spectroscopic survey of these bodies. I used the SpeX NIR spectrometer on NASA’s Infrared Telescope Facility (IRTF) to determine asteroid composition and analyze the 2-4-µm spectral region, where absorption features related to O-H bonds are found. Of the 29 NEAs observed, 8 likely contain OH/H₂O on their surfaces, proving OH/H₂O is pervasive in near-Earth space. Three of those 8 NEAs were observed such that I could conduct a detailed analysis of their band depth variations to further understand the OH/H₂O delivery mechanisms. My results indicate that OH/H₂O presence is dependent on asteroid composition and aphelion, as S-complex NEAs that enter the Main Asteroid Belt are more likely to possess a 3-µm feature, and that OH/H₂O appears to be spatially dependent on those asteroids for which the spectral feature of interest was found.

I completed my investigation into OH/H₂O delivery mechanisms by conducting diurnal and annual studies on (101955) Bennu, the target of NASA’s OSIRIS-REx mission. I compared spectra collected with the OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS) at different times of day and different points in its orbit to test Bennu’s hydration feature for temporal variations I hypothesize would be linked to a secondary source of OH/H₂O. I found clear diurnal and possible annual variations most easily explained by a secondary OH/H₂O delivery mechanism, likely solar wind hydrogen implantation. The combined results across all my investigations strongly suggest that the near-Earth OH/H₂O budget is significantly higher than previously believed and that the mechanisms by which it is delivered are as active now as they would have been in the past.