"67P/C-G": A comet worth its salt!

On the  March 13, 2020
By comparing the spectra measured by the Rosetta probe's VIRTIS instrument with those of artificial comets produced at the Institut de Planétologie et d’Astrophysique de Grenoble (IPAG-CNRS/UGA), an international consortium of research bodies - including the CNRS, the universities of Grenoble, Paris and Aix-Marseille, and the Paris Observatory - has been able to identify ammonium salts on the surface of the comet 67P. This is the first time that compositions this rich in nitrogen have been detected on a comet. A discovery published in Science on 12 March 2020.
Comets and asteroids are the most primitive bodies in the solar system and have evolved very little since the formation of planets. They are in some ways archives of the solar system. Deciphering their composition could therefore enable us to better understand how planets are formed. Observing the Sun's light reflected by the surface of comets and asteroids is one way of estimating their composition, as the matter present on their surfaces absorbs this light at specific wavelengths. Between 2014 and 2016, the VIRTIS imaging spectrometer (Visible, InfraRed and Thermal Imaging Spectrometer) on the European Space Agency's Rosetta probe revealed that the surface of the nucleus of Comet 67P-Churyumov Gerasimenko is almost entirely uniform in terms of its composition. The comet's spectrum is very dark and reddish, due to the presence of complex carbon and opaque mineral compounds and has an absorption feature around the infrared wavelength of 3.2 µm linked to the presence of compounds whose nature was until now difficult to establish.

To determine which compounds are responsible for this absorption feature, researchers at the IPAG (CNRS/UGA) carried out laboratory tests to produce and measure the spectra of artificial comets. Fine particles of water ice containing grains of opaque minerals, as well as organic molecules and ammonium salts, were produced first. These ice particles were then placed in a simulation chamber reproducing the temperature (approx. -100°C) and space conditions found on the surface of a comet. Under these conditions the water ice sublimates, passing from a solid to a gaseous state, and the impurities (other molecules and minerals) form a porous residue of very fine grains with a texture analogous to the surface of a comet. By comparing the spectrum of light reflected by the surfaces of these artificial comets containing ammonium salts with those of the surface of the comet 67P measured by VIRTIS, researchers were able to identify these salts (for example, ammonium formate, NH4+ HCOO-) as being the main elements responsible for the absorption feature of 3,2 µm observed in the comet's nucleus.

It is the first time that nitrogen-rich ammonium salts have been observed in a comet nucleus. Most previous observations of comet dust and gases suggested that comets contained little nitrogen (relative to carbon) compared with the Sun, for reasons that were little understood. One possible explanation would be the existence of a still unidentified reservoir of nitrogen in comets. This new study suggests that ammonium salts could form this reservoir. Although the exact quantity of salt is still difficult to estimate from existing data, it is likely that these nitrate salts contain the largest percentage of nitrogen found on the comet 67P. The remaining nitrogen would be distributed in the organic matter of dust and the volatile compounds of ice. Furthermore, previous observations of increases in the concentration of gases NH3 and HCN emitted by certain comets traveling close to the Sun could be explained by the thermal dissociation of ammonium salts present in comet dust.

The implications of this discovery go far beyond comets alone. Several asteroids in the asteroid belt and several of Jupiter's asteroids, as well as its small moon Himalia, have similar spectra to the comet 67P, which could also be due to the presence of ammonium salts on their surfaces. Additionally, the surface of the dwarf planet Ceres is covered in ammoniated phyllosilicates which may have been formed from ammonium salts inherited from objects similar to the comet 67P.

Finally, the discovery of ammonium salts in a comet, the solar system's most primitive body, sheds new light on the still little understood incorporation and evolution of nitrogen, from the interstellar medium to small bodies and planets. These salts could play unexpected roles in the chemistry of nitrogen at each phase of the cosmic cycle of matter, in ice mantles of grains of pre-stellar and protoplanetary dust where these salts perhaps began forming, during the coagulation of grains to form planetesimals, or in the provision of nitrogen to planets and the potential development of prebiotic chemistry.

crédits : ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

The surface of the comet 67P is composed of a mixture of organic matter, minerals and ammonium salts. This image, taken on 25 March 2015 by the navigation camera of the Rosetta probe, shows the cometary nucleus approximately 4 km long. It is surrounded by jets composed of grains of dust projected by the gases emitted following heating of the ice present in the nucleus as the comet approaches the Sun.

image en haut à gauche ; crédits : ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0
Comparison of the spectrum of the artificial comet containing ammonium salt (in red) with the spectrum of the surface of the comet 67P (in black). The comet's core is approximately 4 km long (top-left image; credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0 http://creativecommons.org/licenses/by-sa/3.0/igo/). The artificial comet was produced in a laboratory in a recipient with a diameter of 5 cm (bottom-left image; credits: Poch et al., 2020).
[1] This work involves researchers from the IPAG (CNRS/UGA), the Laboratoire d’Astrophysique de Marseille (CNRS/Aix-Marseille University/CNES), the Physics of Ionic and Molecular Interactions laboratory (CNRS/Aix-Marseille University) and the Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique (CNRS/Paris Observatory – PSL/Sorbonne University/Université de Paris).
Published on  March 17, 2020
Updated on March 17, 2020