Detection of Molecular H2O on Nominally Anhydrous Asteroids

Abstract

We used the FORCAST instrument on SOFIA to obtain mid-infrared spectra (4.9–13.7 μ m) of four S-type asteroids: (7) Iris, (11) Parthenope, (18) Melpomene, and (20) Massalia. Three of these four silicate-rich asteroids (Iris, Melpomene, and Massalia) were observed to have 3 μ m features indicative of hydration by McAdam et al. We report a detection of a 6 μ m feature that is unambiguously attributed to molecular water on two asteroids, Iris and Massalia, with peak heights of 4.532% ± 0.011% and 4.476% ± 0.012%, respectively. We estimate the abundance of molecular water based on these peak heights to be 454 ± 202 μ g g ^−1 and 448 ± 209 μ g g ^−1 , consistent with values found on the sunlit Moon by SOFIA+FORCAST.

Keywords

• Asteroids
• Infrared spectroscopy
• Surface composition