Detection of Molecular H₂O on Nominally Anhydrous Asteroids

We used the FORCAST instrument on SOFIA to obtain mid-infrared spectra (4.9-13.7 mu 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 mu m features indicative of hydration by McAdam et al. We report a detection of a 6 mu 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 mu g g-1 and 448 +/- 209 mu g g(-1), consistent with values found on the sunlit Moon by SOFIA+FORCAST.