Rosetta is revealing new secrets about the composition of comet 67P/Churyumov–Gerasimenko

An international scientific team involving researchers from Université Paris-Sud has just identified the elemental composition of dust from comet 67P/C-G. These measurements indicate one of the most carbon-rich and least-altered materials ever explored. This cometary carbon is mainly found in the form of macromolecular organic matter, and hence it is mainly in this form that it could have been brought to the early Earth by comets.

On the left, the surface of the comet's nucleus seen by the Rosetta probe. Condensed ice under the surface escaping from the depths of the comet when it is warmed up as it approaches the sun. The gas emissions created produce small particles of matter that can be collected and analysed inside the Rosetta probe. © ESA /Rosetta /MPS for OSIRIS Team MPS /UPD /LAM /IAA /SSO /INTA /UPM /DASP /IDA

The European space probe, Rosetta, flew around comet 67P/Churyumov-Gerasimenko's (67P/C-G) nucleus for two years and examined it down to the last detail. More than a year after the end of the mission in September 2016, it has provided us with a wealth of information about this comet and its activity.  
The appearance of its nucleus is now known almost down to the last detail. Its surface is very dark, lacking in ice, changing and has a complex geomorphology (faults, cliffs, chasms, etc.), however the detail of its chemical composition, and in particular, the quantification of its carbonaceous content, supposed to be at least partly responsible for this dark colour, remained the subject of debate and hypotheses up until now. 


A year after having demonstrated that the comet dust contained a macromolecular-structured carbonaceous material, the scientific team for COSIMA, one of the on-board scientific instruments, was able to determine the average elemental composition of this dust and hence deduce that this macromolecular carbonaceous material represents more than half of the mass of these comet particles. The other half is mainly made up of anhydrous silicate minerals. COSIMA’s measurements, which have been spread out throughout the two years of the Rosetta mission, also demonstrate that all the dust analysed has a similar composition, regardless of its collection date, size or morphology, which suggests that it is a general property of the comet’s entire nucleus.


On the right, a picture of COSISCOPE, the COSIMA instrument's microscope designed and manufactured at IAS. This image shows a collection target (1 cm x 1 cm) with tiny fragments of the nucleus up to a millimetre in size that have impacted it. All this dust is made up of an intimate mixture of 50/50 (in mass) of silicate minerals and organic material. © on the right: ESA /Rosetta /MPS for COSIMA Team MPS /CSNSM /UNIBW /TUORLA /IWF /IAS /ESA /BUW /MPE /LPC2E /LCM /FMI /UTU /LISA /UOFC /vH&S

A form of preserved memory of our solar system


Comets like 67P/C-G (or previously Halley’s comet) are therefore among the most carbon-rich objects in the solar system. The results obtained from COSIMA's measurements provide an abundance ratio of carbon to silicon (C/Si), which is very close to the solar ratio and indicates the lack of significant hydration in the mineral phases. This information demonstrates the comet’s primitive nature. The comet has preserved the matter, which has accreted and which produced it, almost intact. Therefore, the comets are really a particular form of preserved memory of our solar system’s ancient history.


The nature of cometary carbon delivered to the early Earth


Furthermore, besides the wide molecular diversity observed for organic matter detected in the gas phase, these gases and sublimated ice which are the source of it, only represent a very small fraction of the comet’s total matter. Most of the comet's matter is made up of this intimate mixture of minerals and semi-refractory carbonaceous material measured in the dust. 

Hence, COSIMA's results demonstrate that most of comet P67/C-G's organic matter is in the form of macromolecular carbonaceous matter.  
Consequently, if comets did play a role in the emergence of life on our planet, in particular by supplying carbon-rich matter, it would mainly have been brought in this complex macromolecular form. 


On the left: the average elemental composition of comet P67/C-G’s dust particles. On the right: the average distribution by mass of minerals and organic material in this dust. Credit: ESA /Rosetta /MPS for COSIMA Team MPS /CSNSM /UNIBW /TUORLA /IWF /IAS /ESA /BUW /MPE /LPC2E /LCM /FMI /UTU /LISA /UOFC /vH&S.

Bardyn A., Baklouti D., Cottin H., Fray N., Briois C., Paquette J., Stenzel O., Engrand C., Fischer H., Hornung K., Isnard R., Langevin Y., Lehto H., Le Roy L., Ligier N., Merouane S., Modica P., Orthous-Daunay F.-R., Rynö J., Schulz R., Silén J., Thirkell L., Varmuza K., Zaprudin B., Kissel J. and Hilchenbach M. (2017) Carbon-rich dust in comet 67P/Churyumov-Gerasimenko measured by COSIMA/Rosetta. Month. Not. Roy. Astr. Soc. DOI:10.1093/mnras/stx2640

COSIMA, a mini physical chemistry laboratory
The COSIMA mass spectrometer is one of the instruments on board the Rosetta probe. It is managed by the Germans (MPS Göttingen) with significant French instrumental and scientific contribution, involving in particular researchers from the Institut d'Astrophysique Spatiale [Institute of Space Astrophysics] -IAS(CNRS/UPSud) and the Centre de Sciences Nucléaires et de Sciences de la Matière [Matter and Nuclear Science Centre] - CSNSM (CNRS/UPSud). COSIMA is a kind of mini physical chemistry laboratory collecting comet dust particles close to comet 67P/Churyumov-Gerasimenko's nucleus, detecting them and taking images of them with the COSISCOPE microscope, made at IAS (co-PI level of responsibility), and analysing them by secondary ion mass spectrometry combined with time of flight (ToF-SIMS).


Contact: Donia Baklouti – Institut d’Astrophysique Spatiale -IAS (UPSud/CNRS) -