Unknown organic molecules on a comet, a fairly varied surface structure but a rather homogeneous structure in depth, organic compounds gathered in clumps and not dispersed in the ice … These are some of the results issued from the first data of Philae on the surface of comet “Tchouri”. As part of the ESA Rosetta mission, this work has involved researchers from CNRS, Aix-Marseille University, Joseph Fourier University, the University of Nice Sophia Antipolis, the UPEC, the UPMC, the University of Paris-Sud, the University of Toulouse III – Paul Sabatier and the UVSQ, with the support of CNES. They are published in a set of eight articles, on July 31, 2015, within the journal Science. These results in situ, very rich in new information, show some differences with previous observations of comets and existing models.
The Rosetta cometary rendezvous mission offered, thanks to the landing of the Philae module, an exceptional opportunity: that of the in situ study of a cometary nucleus (from its surface to its internal structure), 67P/Cheryumov-Guerasimenko (alias Churii). It is likely to advance the understanding of these small celestial bodies that bear witness to the origins of the solar system. The measurements made from 12 to 14 November 2014 (during the 63 hours following its separation from Rosetta) by the ten instruments of the Philae lander complemented the observations made by the Rosetta orbiter1. And its eventful arrival on the comet was even a source of additional information.
New organic molecules
Twenty-five minutes after Philae’s initial contact with the nucleus of the comet, COSAC (Cometary sampling and composition experiment) carried out a first chemical analysis, in “sniffer” mode, i.e. by examining the particles that had entered the apparatus passively. These particles probably come from the dust cloud produced by Philae’s first contact with the ground. Sixteen compounds could be identified, divided into six classes of organic molecules (alcohols, carbonyls, amines, nitriles, amides and isocyanates). Among them, four were detected for the first time on a comet (methyl isocyanate, acetone, propionaldehyde and acetamide).
These molecules are precursors of molecules important for life (sugars, amino acids, DNA bases, etc.). But the possible presence of these more complex compounds could not be unambiguously identified in this first analysis. Moreover, almost all the molecules detected are potential precursors, products, assemblies, or by-products of each other, which gives an insight into the chemical processes at work in a cometary nucleus and even in the collapsing protosolar cloud in the early days of the solar system.
Piles of organic matter from the start
The cameras of the CIVA (Comet infrared and visible analyser) experiment revealed that the terrain near the final landing site of Philae is dominated by dark agglomerates that are likely to be large grains of organic molecules. Since the materials of comets have been very little modified since their origins, this means that in the early days of the solar system, organic compounds were already agglomerated in the form of grains, and not just as small molecules trapped in ice as previously thought. It is such grains that, when introduced into the planetary oceans, could have favoured the emergence of life.
Varied terrains hiding a rather homogeneous interior
COSAC identified a large number of nitrogen compounds, but no sulphur compounds, contrary to what was observed by the ROSINA instrument aboard Rosetta. This may indicate that the chemical composition differs depending on the sampled location.
In addition, the mechanical properties of the soils could be deduced from Philae’s rebound “accommodation”. The lander first hit the surface at a place called Agilkia, and then bounced several times before reaching the site called Abydos. Philae’s trajectory and the data recorded by its instruments show that Agilkia is composed of granular material over about 20 centimetres, while Abydos has a hard surface.
On the contrary, the interior of the comet appears more homogeneous than predicted by the models. The CONSERT radar experiment (Comet nucleus sounding experiment by radio transmission) gives, for the first time, access to the internal structure of a cometary nucleus. The propagation time and the amplitude of the signals having crossed the upper part of the “head” (the smaller of the two Chouri lobes) show that this portion of the nucleus is globally homogeneous, on a scale of tens of metres. These data also confirm that the porosity is high (75 to 85%), and indicate that the electrical properties of the dust are similar to those of carbonaceous chondrites.
A tormented surface
The CIVA-P (P for Panorama) experiment, composed of seven microcameras, took a panoramic image (360°) of the final landing site of Philae. It revealed that the fractures already spotted at large scales by Rosetta are also found down to the millimetre scale. These fractures are formed by thermal shock, due to the large temperature differences that the comet experiences during its course around the sun.
Details on the location and orientation of Philae
This panoramic image, in which a foot or antenna appears in places, also revealed Philae’s position. He lies in a hole of his own size, lying on his side (with only two feet by three in contact with the ground), and surrounded by walls that make it difficult for him to get solar power and to communicate with Rosetta.
The CONSERT instrument, with three periods of line-of-sight observations between the Rosetta probe and Philae, determined the area (150 metres by 15 metres) where Philae is located. This facilitated the reconstruction of Philae’s trajectory between the first contact site, Agilkia, and the final landing site, Abydos. Then, using the signals that passed through the interior of the comet, CONSERT reduced the uncertainty on the location of Philae (at the edge of the region called Hatmehit) to a band of 21 meters by 34 meters.
Together with the four other published papers (e.g. on the magnetic and thermal properties of Chouri), these first measurements on the surface of a comet renew the picture we had of these small bodies in the solar system.
The images are available from the CNRS photo library, firstname.lastname@example.org.
French Laboratories involved
- l’Institut d’astrophysique spatiale (CNRS/Université Paris Sud)
- l’Institut de chimie de Nice (CNRS/Université Nice Sophia Antipolis)
- l’Institut Fresnel Marseille (CNRS/Aix-Marseille Université/Ecole Centrale Marseille)
- l’Institut méditerranéen d’océanographie (CNRS/Université de Toulon/IRD/Aix-Marseille Université)
- l’Institut de planétologie et astrophysique de Grenoble (CNRS/UJF)
- l’Institut de recherche en astrophysique et planétologie (CNRS/Université Toulouse III – Paul Sabatier)
- le Laboratoire d’astrophysique de Marseille (CNRS/Aix-Marseille Université)
- le Laboratoire atmosphères, milieux, observations spatiales (CNRS/UPMC/UVSQ)
- le Laboratoire interuniversitaire des systèmes atmosphériques (CNRS/UPEC/Université Paris Diderot)
- le Laboratoire de planétologie et géodynamique de Nantes (CNRS/Université de Nantes/Université d’Angers)
1 Voir le communiqué de presse du 10 décembre 2014, « Rosetta : les premiers résultats de l’instrument ROSINA » et celui du 21 janvier 2015, « Tchouri sous l’œil de Rosetta ».
- 67P/Churyumov-Gerasimenko surface properties, as derived from the first CIVA-P in situ panoramic images, J-P. Bibring et al.,Science, 31 juillet 2015. DOI : 10.1126/science.aab0671
- Properties of the 67P/Churyumov-Gerasimenko interior revealed by CONSERT radar, W. Kofman et al.,Science, 31 juillet 2015. DOI : 10.1126/science.aab0639
- Organic compounds on comet 67P/Churyumov-Gerasimenko revealed by COSAC mass spectrometry, F. Goesmann et al.,Science, 31 juillet 2015. DOI : 10.1126/science.aab0689
- The landing(s) of Philae and inferences about comet surface mechanical properties, J. Biele et al.,Science, 31 juillet 2015. DOI : 10.1126/science.aaa9816
- Jérémie Lasue : email@example.com