Substantial amounts of ammonium salts have been identified in the surface material of the comet 67P/Churyumov-Gerasimenko, researchers report, likely revealing the reservoir of nitrogen that was previously thought to be “missing” in comets. The chemical and isotopic composition of the Sun is thought to reflect that of the solar nebula from which it was born. Comets, which are thought to have condensed long ago in the cold outer reaches of the solar nebula, far from the heat of the forming Sun, should contain the same primordial material. However, the nitrogen-to-carbon ratio (N/C) measured in comets, including Comet 67P, is lower than for the Sun, when it should be the same. The reason for this nitrogen deficiency is unknown. Spectroscopic measurements of Comet 67P’s surface by the Rosetta spacecraft show a ubiquitous, yet unidentified, infrared absorption feature centered around 3.2 micrometers (μm). While several materials, including nitrogen-bearing molecules, have been suggested as the source of the unknown spectral feature, a lack of reference data for plausible compounds has prevented its attribution. Oliver Poch and colleagues used laboratory experiments to simulate various possible comet surface compositions and measured their resulting reflectance spectra, attempting to reproduce the 3.2 μm feature observed on Comet 67P. Among the candidate compounds the authors tested, several nitrogen-containing ammonium salts could reproduce the comet’s unidentified infrared absorption bands. Based on these new findings, Poch et al. conclude that the ammonium ions (NH 4 + ) in these compounds are likely the source for the 3.2-μm feature. The authors identify the semi-volatile ammonium salts as a substantial reservoir of nitrogen, the abundance of which could be high enough to account for the observed N/C discrepancy between comets and the Sun.
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Science