The chemical diversity of comets dates back to their origin

The chemical diversity of comets dates back to their origin
Extrait du Observatoire de Paris centre de recherche et enseignement en astronomie et
astrophysique relevant du Ministère de l'Enseignement supérieur et de la Recherche.
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The chemical diversity of
comets dates back to their
origin
Date de mise en ligne : dimanche 1er juillet 2007
Observatoire de Paris centre de recherche et enseignement en astronomie et
astrophysique relevant du Ministère de l'Enseignement supérieur et de la
Recherche.
Copyright © Observatoire de Paris centre de recherche et enseignement en astronomie et astrophysique relevant du Ministère de l'Enseignement
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The chemical diversity of comets dates back to their origin
A well-studied comet appears to be uniform in chemical composition. Scientists from Johns
Hopkins University and LESIA of Paris Observatory studied two fragments from the
disintegrating comet 73P/Schwassmann-Wachmann 3. The fragments reveal a range of
different depths, and they were found very similar in icy composition. This goes against the
theory that outer layers of comet nuclei are heavily processed by solar radiation, making
their outsides chemically different to their insides.
The composition of cometary ices provides key information on the chemical and physical properties of the outer
solar nebula where comets formed, 4.6 Gy ago. About two dozen molecules released from the sublimation of
nucleus ices have been identified in cometary atmospheres, mainly by infrared and microwave spectroscopy.
Chemical diversity is observed both in the class of Oort cloud comets and within the Jupiter-family comet population
consisting of short-period comets formed in the Kuiper belt. This remarkable diversity can be attributed to several
factors including differences in the chemical and physical environments in comet-forming regions, chemical
evolution during their long storage in the Oort cloud and Kuiper belt, and thermal processing by the Sun when
entering the inner Solar System. This latter mechanism, which may deplete the outer layers of comet nuclei in the
most volatile species, is invoked to explain the low CO abundances measured in Jupiter-family comets, while
several Oort cloud comets exhibit high CO abundances.
To determine the relevance of these factors, measurements of the chemistry as a function of depth in cometary
nuclei are critical. Cometary nuclei are fragile objects and often experience fragmentation (e.g., D/1993 F2
Shoemaker-Levy 9 that collided Jupiter in 1994) and eventually full disintegration (C/1999 S4 (LINEAR) in 2001).
Fragmenting comets expose formerly buried material. The chemical analysis of several fragments provide a
sensitive test for chemical heterogeneity within a comet nucleus.
Figure 1 : Image des fragments de 73P/Schwassmann-Wachmann 3 obtenue en mai 2006 par le télescope
spatial Spitzer. Crédit Caltech/IPAC, NASA. Cliquer sur l'image pour l'agrandir
Comet 73P/Schwassmann-Wachmann 3 is a Jupiter-family comet that split into at least five fragments during its
1995 apparition. Fragmentation pursued at its 2006 apparition : 68 named fragments were identified (Fig. 1). The
very close approach of the comet to Earth in May 2006 (0.07 AU) allowed the volatile inventory of the main
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The chemical diversity of comets dates back to their origin
fragments B and C (of hundreds of meters size) to be determined with detail.
The observations presented in Nature journal were conducted in the 2.8-4.7 microns range with the high resolution
spectrometers CSHELL and NIRSPEC of NASA IRTF and Keck 2 telescopes, respectively. Ro-vibrational lines of
H2O, CH3OH, HCN, H2CO, C2H2, et C2H6 were detected in the two fragments. The relative abundances between
the two bodies are found to be remarkably similar to each other when compared to the diversity in chemistry within
the overall comet population (Fig. 2). Complementary investigations performed in the microwave range using the
IRAM 30-m, the Caltech Submillimeter Observatory (CSO), the Atacama Pathfinder Experiment (APEX) and Odin
telescopes show that this similarity of composition extends to HNCO, CH3CN, H2S and CS (Fig. 3). These results
provide strong evidence that the nucleus of 73P/Schwassmann-Wachmann 3 is chemically homogeneous and that
its composition primarily reflects formative conditions and not evolutionary processing from numerous close
passages to the Sun.
Figure 2 : Abondances mesurées par spectroscopie infrarouge dans les fragments B (en rouge) et C (en
vert). Les abondances mesurées dans les autres comètes s'échelonnent dans les domaines indiqués en
rose. Source : Dello Russo et al. (2007) Cliquer sur l'image pour l'agrandir
This finding is especially interesting when considering the deviant composition of this comet. Our measurements
show that comet 73P/ Schwassmann-Wachmann 3 is strongly depleted in CH3OH, H2CO, C2H2, C2H6, NH3, et
H2S (by a factor of 10 for some molecules) while normal in HCN, CH3CN, HNCO et CS contents relative to water
(Fig. 3). Jupiter-family comets with depleted volatile abundances in carbon-chain molecules are common. The
chemical diversity observed in the population of comets formed in the Kuiper Belt (as well as Oort cloud comets)
would then be primordial. Whether this chemical diversity reflects different nebular formation regions, the timing of
comet formation in the early stages of the Solar System or another as yet unidentified effect remains to be seen.
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