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The nucleus of comet 67P/Churyumov-Gerasimenko exhibits a broad spectralreflectance feature around 3.2 $\mu$m, which is omnipresent in all spectra ofthe surface, and whose attribution has remained elusive since its discovery.Based on laboratory experiments, we have shown that most of this absorptionfeature is due to ammonium (NH4+) salts mixed with the dark surface material.The depth of the band is compatible with semi-volatile ammonium salts being amajor reservoir of nitrogen in the comet, which could dominate over refractoryorganic matter and volatile species. These salts may thus represent thelong-sought reservoir of nitrogen in comets, possibly bringing theirnitrogen-to-carbon ratio in agreement with the solar value. Moreover, thereflectance spectra of several asteroids are compatible with the presence ofNH4+ salts at their surfaces. The presence of such salts, and otherNH4+-bearing compounds on asteroids, comets, and possibly in proto-stellarenvironments, suggests that NH4+ may be a tracer of the incorporation andtransformation of nitrogen in ices, minerals and organics, at different phasesof the formation of the Solar System.

astrophysics and space science proceedings

Identification of Ammonium Salts on Comet 67P/C-G Surface from Infrared  VIRTIS/Rosetta Data Based on Laboratory Experiments. Implications and  Perspectives