  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 reflectance feature around 3.2 micrometers, which
is ubiquitous across its surface. On the basis of laboratory experiments, we
attribute this absorption band to ammonium salts mixed with dust on the
surface. The depth of the band indicates that semivolatile 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.
