  The nucleus of comet 67P/Churyumov-Gerasimenko exhibits a broad spectral
reflectance feature around 3.2 $\mu$m, which is omnipresent in all spectra of
the surface, and whose attribution has remained elusive since its discovery.
Based on laboratory experiments, we have shown that most of this absorption
feature 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 a
major reservoir of nitrogen in the comet, which could dominate over refractory
organic matter and volatile species. These salts may thus represent the
long-sought reservoir of nitrogen in comets, possibly bringing their
nitrogen-to-carbon ratio in agreement with the solar value. Moreover, the
reflectance spectra of several asteroids are compatible with the presence of
NH4+ salts at their surfaces. The presence of such salts, and other
NH4+-bearing compounds on asteroids, comets, and possibly in proto-stellar
environments, suggests that NH4+ may be a tracer of the incorporation and
transformation of nitrogen in ices, minerals and organics, at different phases
of the formation of the Solar System.
