[meteorite-list] The Chemical Diversity of Comets Dates Back to their Origin (Comet 73P/Schwassmann-Wachmann 3)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 11 Jul 2007 19:08:07 -0700 (PDT)
Message-ID: <200707120208.TAA14906_at_zagami.jpl.nasa.gov>

Observatoire de Paris
Paris, France

Contact:
Nicolas Biver, Observatoire de Paris, LESIA, and CNRS
Tel: 33 1 45 07 78 09 Fax: 33 1 45 07 71 44

Dominique Bockelee-Morvan, Observatoire de Paris, LESIA, and CNRS
Tel: 33 1 45 07 76 05 Fax: 33 1 45 07 71 44

Jacques Crovisier, Observatoire de Paris, LESIA, and CNRS
Tel: 33 1 45 07 75 99 Fax: 33 1 45 07 71 44

11 July 2007

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.

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 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, and 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.

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,
and H2S (by a factor of 10 for some molecules) while normal in HCN, CH3CN,
HNCO and 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.

References

Compositional homogeneity in the fragmented comet 73P/Schwassmann-Wachmann
3,
Dello Russo, N. Vervack Jr., R.J., Weaver, H.A., Biver, N., Bockelee-Morvan,
D., Crovisier, J., Lisse, C.M., 2007,
Nature, 12 July 2007

Comparison of the chemical composition of fragments B and C of comet
73P/Schwassmann-Wachmann 3 from radio observations,
Biver, N., Bockelee-Morvan, J. Boissier, P. Colom, J. Crovisier, A.
Lecacheux, D.C. Lis, B. Parise, K. Menten and the Odin Team, 2006,
BAAS 38, 484-485

IMAGE CAPTIONS:

[Figure 1:
http://www.obspm.fr/actual/nouvelle/jul07/comet-f1.jpg (762KB)]
Comet 73P/Schwassmann-Wachmann 3 as seen by the Spitzer Space telescope on
May 4-6 2006 (Caltech/IPAC, NASA)

[Figure 2:
http://www.obspm.fr/actual/nouvelle/jul07/comet-f2.jpg (76KB)]
Abundances measured by infrared spectroscopy in fragments B (in red) and C
(in green). The range of measured abundances from the overall comet
population (except the disintegrating comet C/1999 S4 (LINEAR) shown in
blue) is shown in pink. From Dello Russo et al. (2007).

[Figure 3:
http://www.obspm.fr/actual/nouvelle/jul07/comet-f3_en.jpg (56KB)]
Abundances relative to H2O measured with IRAM 30-m, CSO and APEX radio
telescopes. From Biver et al. (2006)
Received on Wed 11 Jul 2007 10:08:07 PM PDT