The chemical composition of comets

Before the arrival of Rosetta near the comet 67P/Churyumov-Gerasimenko, our knowledge of the chemical composition of volatile material of comets, i.e. that of the parent molecules, came mainly from the study at distance of comet C/1995 O1 Hale-Bopp and of some others, especially Halley’s comet and the recent very active comets C/2012 F6 Lemmon and C/2014 Q2 Lovejoy. All these comets arrived from the Oort cloud, and have rather similar chemical compositions; the differences, if any, are not necessarily significant because the chemical composition of the ejected gas varies with time for the same comet according to the circumstances of its illumination by the Sun. The composition of the dust and other non-volatile materials was known from samples brought back by the Stardust mission or collected by aircraft flying at high altitude.

The analysis of the gas by the instruments of the Rosetta orbiter and of its lander Philae has considerably improved the situation. The observed comet 67P/Churyumov-Gerasimenko comes from the Kuiper belt and was “captured” by Jupiter. All molecules viewed from the ground in previous comets have been found, plus some others. But the abundance of these molecules relative to the water varies greatly with solar illumination. For example, the CO and CO2 ices sublimate more easily than the water ice so that their abundance relative to water varies between 2.5 percent and 80 percent depending on the circumstances. More detailed studies are needed before we know whether there are differences of abundance according to the origin of comets.

Comète C/1995 O1 Hale-Bopp - 2

La comète Hale-Bopp photographiée dans la nuit du 10 au 11 avril 1997 par Daniel Crussaire.

Crédit : Daniel Crussaire / Observatoire de Paris

Anyway, we find in all of these comets the same molecules as in the interstellar medium, confirming that their material is essentially unprocessed interstellar matter. A very surprising discovery was the presence of the oxygen molecule O2 in the ejecta of comet 67P, with a rather large abundance, about 4 percent, of water. This molecule, rare in the interstellar medium, seems to have been present at the formation of the comet, which poses still unsolved problems.

The analysis of the free dust and of the surface of the nucleus of comet 67P confirms the presence of silicates and of organic materials. These materials, as well as the parent molecules ejected by the comet, contain some of the building blocks of living molecules, including amino acids such as glycine, and it is quite possible that they were used in the creation of life on Earth when they were brought by the fall of comets.