Estimate of Water and Hydroxyl Abundance on Asteroid (16) Psyche from JWST Data

Abstract

Our understanding of solar system evolution is closely tied to interpretations of asteroid composition, particularly the M-class asteroids. These asteroids were initially thought to be the exposed cores of differentiated planetesimals, a hypothesis based on their spectral similarity to iron meteorites. However, recent astronomical observations have revealed hydration on their surface through the detection of 3 μm absorption features associated with OH and potentially H₂O. We present evidence of hydration due mainly to OH on asteroid (16) Psyche, the largest M-class asteroid, using data from the James Webb Space Telescope (JWST) spanning 1.1–6.63 μm. Our observations include two detections of the full 3 μm feature associated with OH and H₂O resembling those found in CY-, CH-, and CB-type carbonaceous chondrites, and no 6 μm feature uniquely associated with H₂O across two observations. We observe 3 μm depths of between 4.3% and 6% across two observations, values consistent with hydrogen abundance estimates on other airless bodies of 250–400 ppm. We place an upper limit of 39 ppm on the water abundance from the standard deviation around the 6 μm feature region. The presence of hydrated minerals suggests a complex history for Psyche. Exogenous sources of OH-bearing minerals could come from hydrated impactors. Endogenous OH-bearing minerals would indicate a composition more similar to E- or P-class asteroids. If the hydration is endogenous, it supports the theory that Psyche originated beyond the snow line and later migrated to the outer main belt.