Hydration Features on Near-Earth Objects: Integrating New Data with Prior Results

Near-Earth objects (NEOs) are excellent laboratories for testing processes that affect airless bodies, as well as informing us about Solar System history. Though most NEOs are nominally anhydrous because they formed inside the Solar System frost line and their surface temperatures are high enough to remove volatiles, a 3-micron feature typically indicative of OH/H2O has been identified on several such bodies. Possible sources for OH/H2O on these bodies include carbonaceous chondrite impactors or interactions with protons implanted by solar wind. The MIT-Hawaii Near-Earth Object Spectroscopic Survey (MITHNEOS) began its 3-micron observation campaign of NEOs in 2022 and has obtained spectral data of 15 predominantly nominally anhydrous (i.e., mostly S-complex or V-type) targets using NASA’s Infrared Telescope Facility’s (IRTF) near-infrared spectrometer, SpeX. Spectra were collected using both prism (0.7-2.52 micron) and LXD_short (1.67-4.2 micron) modes to accurately characterize asteroid spectral type and the 3-micron region. Four of the 15 NEOs observed exhibit a 3-micron feature, exhibiting band shapes similar to those identified in a previous NEO survey (McGraw et al. 2022), which found a trend between hydration band presence and large aphelion (i.e., Q > 2.06 AU). Combining our new observations with the pre-existing database of NEO 2-4-micron data revealed that band depth increases with decreasing orbital inclination and that all NEOs with hydration bands have i < 27 degrees with most having i < 14 degrees. We find that NEOs with low inclination and large aphelia are the most likely bodies in near-Earth space to possess surficial OH/H2O.

💡 Research Summary

The paper presents new 2–4 µm spectroscopic observations of fifteen near‑Earth objects (NEOs) that are nominally anhydrous, primarily S‑complex and V‑type asteroids. The observations were carried out with NASA’s Infrared Telescope Facility (IRTF) using the SpeX instrument in both prism (0.7–2.52 µm) and long‑cross‑dispersed short (LXD_short, 1.67–4.2 µm) modes. This dual‑mode approach allowed the authors to (i) classify each target’s mineralogy in the 0.7–2.5 µm region and (ii) search for the diagnostic 3 µm absorption band that signals the presence of hydroxyl (OH) or water (H₂O) on the surface.

Data reduction followed the same pipeline used in earlier NEO 3‑µm surveys (e.g., Rivkin et al. 2018; McGraw et al. 2022). After standard extraction, telluric correction, and merging of orders, a thermal tail—thermal emission that becomes significant beyond ~2.8 µm for sun‑lit NEOs—was modeled and subtracted using the Near‑Earth Asteroid Thermal Model (NEATM). The authors then defined a linear continuum from 2.45 µm (the red edge of the 2‑µm band for S‑complex asteroids) to 4.0 µm, extrapolating beyond the atmospheric water‑vapor gap (2.45–2.85 µm). Band depth was measured at 2.9 µm, the shortest reliable wavelength outside the telluric window, and also at 2.95, 3.00, 3.05, and 3.10 µm to characterize band shape. A detection was considered “potential” if the depth exceeded the 1σ uncertainty and “definitive” if it exceeded 2σ.

Four of the fifteen NEOs displayed a 3‑µm absorption feature: (161989) Cacus, (756998) 2024 CR9, 1998 HH49, and 2006 WB. Cacus, 2024 CR9, and 1998 HH49 are S‑complex objects with band depths of 3.3 ± 2.4 %, 11.7 ± 8.6 %, and 17.4 ± 10.9 % respectively—these are considered potential detections (1σ). 2006 WB, classified as an Xc‑type, shows a definitive detection (9.9 ± 3.4 % at 2σ). The band shapes fall into three categories, with 2024 CR9 and 1998 HH49 sharing a similar profile, while Cacus and 2006 WB exhibit distinct morphologies.

A key result emerges from orbital analysis. All four hydrated NEOs have orbital inclinations i < 27°, and the majority have i < 14°. Moreover, they all possess aphelion distances Q > 2.06 AU, confirming the trend reported by McGraw et al. (2022) that larger aphelia correlate with hydration. By combining the new dataset with the existing 2–4 µm NEO database, the authors find that band depth increases as inclination decreases, establishing “low‑inclination, high‑aphelion” as the strongest predictor of surface OH/H₂O among near‑Earth objects.

The paper discusses four plausible sources of OH/H₂O on inner‑Solar‑System bodies: (1) native phyllosilicates, (2) exogenous cometary material, (3) exogenous carbonaceous impactors, and (4) solar‑wind proton implantation. For S‑complex and V‑type asteroids, native phyllosilicates are unlikely because these taxonomic classes lack the requisite hydrated minerals. The observed shallow, bowl‑shaped 3‑µm bands are consistent with either solar‑wind implantation (which typically yields band depths ≤ 5 %) or the delivery of carbonaceous material via low‑velocity impacts. The orbital correlation—low i and large Q—supports a hybrid scenario: objects that spend more time at higher heliocentric distances (large Q) are more likely to encounter carbonaceous debris from the main belt, while low inclination enhances the cumulative effect of solar‑wind implantation due to longer exposure to the solar wind flux.

By expanding the sample size by roughly 50 % and confirming the inclination–aphelion relationship, the study provides a statistically robust framework for predicting which NEOs are likely to host surface hydration. This has practical implications for future resource utilization (e.g., in‑situ water extraction), planetary defense (hydrated regolith may affect impact dynamics), and mission planning (selection of landing sites with potential volatiles). The detection of hydration on nominally anhydrous S‑ and V‑type bodies also challenges the simplistic view that these asteroids are completely dry, suggesting that surface processes such as solar‑wind implantation and continual delivery of exogenous material are more pervasive than previously thought.