Research Article

Ammonium salts are a reservoir of nitrogen on a cometary nucleus and possibly on some asteroids

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Science  13 Mar 2020:
Vol. 367, Issue 6483, eaaw7462
DOI: 10.1126/science.aaw7462

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Ammonium salts on comet 67P

The distribution of carbon and nitrogen in the Solar System is thought to reflect the stability of carbon- and nitrogen-bearing molecules when exposed to the heat of the forming Sun. Comets have a low nitrogen-to-carbon ratio, which is contrary to expectations because they originate in the outer Solar System where nitrogen species should be common. Poch et al. used laboratory experiments to simulate cometary surfaces and compared the resulting spectra with comet 67P/Churyumov-Gerasimenko. They assigned a previously unidentified infrared absorption band to nitrogen-containing ammonium salts. The salts could contain enough nitrogen to bring the comet's nitrogen-to-carbon ratio in line with the Sun's.

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Structured Abstract

INTRODUCTION

Comets and asteroids preserve information on the earliest stages of Solar System formation and on the composition of its building blocks. The nature of their solid material can be investigated by analyzing the sunlight scattered by their surfaces. The nucleus of comet 67P/Churyumov-Gerasimenko (hereafter 67P) was mapped by the Visible and InfraRed Thermal Imaging Spectrometer, Mapping Channel (VIRTIS-M) on the Rosetta spacecraft from 2014 to 2015. The nucleus appeared almost spectrally uniform from 0.4 to 4 μm, characterized by a low reflectance of few percent, a reddish color, and an unidentified broad absorption feature around 3.2 μm, which was ubiquitous throughout the surface. The darkness and the color of comet 67P could be due to a mixture of refractory organic molecules and opaque minerals. Although water ice may contribute to the 3.2-μm absorption, it cannot explain the entire feature.

RATIONALE

Semivolatile compounds of low molecular weight, such as carboxylic (−COOH)–bearing molecules or ammonium (NH4+) ions, have been proposed as potential carriers of the 3.2-μm absorption feature. To test these hypotheses, we performed laboratory experiments to measure the reflectance spectra of these compounds mixed in a porous matrix of submicrometric opaque mineral grains, under simulated comet-like conditions (170 to 200 K, <10−5 mbar).

RESULTS

The 3.2-μm absorption feature is consistent with ammonium salts mixed with the dark cometary surface material. We attribute additional absorption features to carbonaceous compounds and traces of water ice. Several ammonium salts can match the absorption feature equally well: ammonium formate, ammonium sulfate, or ammonium citrate. A mixture of different ammonium salts could be present.

Ammonium salts at the surface of comet 67P could have been synthesized through acid-base reactions of ammonia (NH3) with the corresponding acid molecules in solid ices. That reaction may have occurred in the interstellar medium, in the protoplanetary disk, or during the sublimation of the ices in the cometary nucleus.

The depth of the band suggests that the cometary surface contains an upper limit of ~40 weight % (wt %) of ammonium salts, but the exact concentration remains unknown. If the amount of ammonium salts is higher than ~5 wt %, they constitute the dominant reservoir of nitrogen in the comet, containing more nitrogen than the refractory organic matter and the volatile species, such as NH3 and N2. Consequently, the abundance of nitrogen in this comet is closer to that of the Sun than previously thought.

CONCLUSION

Ammonium salts may dominate the reservoir of nitrogen in comets. Their presence in cometary dust may explain increases of gas-phase NH3 and HCN observed in some comets when close to the Sun, which could be caused by the thermal dissociation of ammonium salts. Several asteroids in the Main Belt, Jupiter’s Trojan asteroids, and its small moon Himalia have similar spectra to that of comet 67P, with a broad spectral absorption feature at 3.1 to 3.2 μm, which we suggest could also be due to ammonium salts. The dwarf planet Ceres has ammoniated phyllosilicates on its surface, which may have formed from ammonium ions inherited from outer Solar System objects with compositions similar to that of comet 67P. The presence of these salts on comet 67P, and possibly on other primitive Solar System bodies, suggests a compositional link between asteroids, comets, and the proto-solar nebula.

Comparison of ammonium formate spectrum with the average spectrum of comet 67P.

The average reflectance spectrum of comet 67P (black line) and the spectrum of a mixture of ammonium formate (NH4+ HCOO) with opaque grains measured in the laboratory under comet-like conditions (blue line). Also shown are views of the 4-km-diameter comet nucleus (Credit: ESA/Rosetta/NAVCAM–CC BY-SA IGO 3.0; http://creativecommons.org/licenses/by-sa/3.0/igo) and the 48-mm-diameter laboratory sample.

Abstract

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.

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