COSIMA, a Rosetta instrument, sheds light on the composition of Comet 67P

The European space probe Rosetta orbited the nucleus of Comet 67P/Churyumov-Gerasimenko for two years and studied it in great detail. More than a year after the mission ended in September 2016, it has already provided us with a wealth of information about this comet and its activity: its size, its mass, and the nature and proportions of some of the molecules present in gaseous form in the comet’s coma (water vapor, carbon dioxide and carbon monoxide, oxygen, and a multitude of small organic molecules—that is, molecules composed mainly of carbon, hydrogen, nitrogen, and oxygen). The appearance of its nucleus is now known in almost every detail. Its surface is very dark, contains very little ice, is ever-changing, and exhibits a complex morphology (fissures, cliffs, chasms, etc.), but the details of its chemical composition—and in particular, the quantification of its carbon content, which is believed to be at least partly responsible for this dark color—had until now remained a subject of debate and speculation.

COSIMA is an instrument aboard the Rosetta spacecraft. It is led by Germany (MPS Göttingen) with significant French contributions to both the instrument and the science. The instrument’s mission was to collect sub-millimeter-sized dust particles ejected by the comet’s nucleus, photograph them (using the COSISCOPE microscope, for which the IAS is responsible), and then analyze a portion of them using time-of-flight secondary ion mass spectrometry (TOF-SIMS). One year after demonstrating that the comet’s dust contained carbonaceous material with a macromolecular structure, the COSIMA scientific team was able to determine the average elemental composition of this dust and thus conclude that this macromolecular carbonaceous material accounts for nearly half the mass of these cometary particles. The other half consists mainly of anhydrous silicate minerals. COSIMA measurements taken throughout the two-year Rosetta mission also show that all the dust particles analyzed have a similar composition, regardless of when they were collected, their size, or their morphology, suggesting that this is a property common to the entire cometary nucleus.