The measured nitrogen-to-carbon ratio in comets is lower than for the Sun, a discrepancy which could be alleviated if there is an unknown reservoir of nitrogen in comets. The nucleus of comet 67P/Churyumov-Gerasimenko exhibits an unidentified broad spectral 2 reflectance feature around 3.2 µm, which is ubiquitous across its surface. Based on laboratory experiments, we attribute this absorption band to ammonium salts mixed with dust on the surface. The depth of the band indicates that semi-volatile ammonium salts are a substantial reservoir of nitrogen in the comet, potentially dominating over refractory organic matter and more volatile species. Similar absorption features appear in the spectra of some asteroids, implying a compositional link between asteroids, comets and the parent interstellar cloud.

The ESA Rosetta mission 1 has acquired unprecedented measurements of comet 67P/Churyumov-Gerasimenko's (hereafter 67P) nucleus surface, whose composition, as determined by in situ and remote sensing instruments including VIRTIS (Visible, InfraRed and Thermal Imaging Spectrometer) 2 appears to be made by an assemblage of ices, minerals, and organic material 3. We performed a refined analysis of infrared observations of the nucleus of comet 67P carried out by