Hydroxylated Mg-rich Amorphous Silicates: A New Component of the 3.2 μ m 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 3.2μm. The feature isubiquitous across the surface and its attribution to(a)specific material(s)has been challenging. In the presentLetter, we report an experimental investigation showing that the interaction of hydrogen atoms with Mg-richamorphous silicates determines the formation of hydroxyl groups. The resulting IR spectrum exhibits a broadfeature around 3.2μm similar to that of comet 67P/CG. Hapke’s radiative transfer model was employed toestimate an upper limit contribution of 65% of hydroxylated silicates to the observed cometary band intensity. Thepresence of a hydroxylated fraction in silicates on the cometary surface would represent an evolutionary linkbetween primitive objects of the solar system and dust in the interstellar medium(ISM), where silicate grains canbe hydroxylated after having interacted with hydrogen atoms. This link is consistent with the detection of thealiphatic organics in 67P/CG that also originate in the ISM.