Hydroxylated Mg-rich Amorphous Silicates: A New Component of the 3.2 μm Absorption Band of Comet 67P/Churyumov–Gerasimenko

The VIRTIS imaging spectrometer on board Rosetta has shown that the nucleus surface of comet 67P/Churyumov–Gerasimenko (67P/CG) is characterized by a broad absorption band at around 3.2 μ m. The feature is ubiquitous across the surface and its attribution to (a) specific material(s) has been challenging. In the present Letter, we report an experimental investigation showing that the interaction of hydrogen atoms with Mg-rich amorphous silicates determines the formation of hydroxyl groups. The resulting IR spectrum exhibits a broad feature around 3.2 μ m similar to that of comet 67P/CG. Hapke’s radiative transfer model was employed to estimate an upper limit contribution of 65% of hydroxylated silicates to the observed cometary band intensity. The presence of a hydroxylated fraction in silicates on the cometary surface would represent an evolutionary link between primitive objects of the solar system and dust in the interstellar medium (ISM), where silicate grains can be hydroxylated after having interacted with hydrogen atoms. This link is consistent with the detection of the aliphatic organics in 67P/CG that also originate in the ISM.