Chemical Composition Diversity Among 24 Comets Observed At Radio Wavelengths

1. Chemical Composition Diversity Among 24 Comets Observed At Radio Wavelengths Nicolas Biver, Dominique Bockelée-Morvan, Jacques Crovisier, Pierre Colom and Florence Henry et al. Earth, Moon and Planets 90: 323-333,2002 Celestial Dynamic Presentation, 2009/06/17, Pan Wei-Hsiang

2. Outline • Introduction • Cometary Classification Scheme • Tisserand Parameter • Discussions • Chemical Abundances • CN-bearing • CO-bearing • Sulphur-bearing • Conclusion 1

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4. Cometary Classification Scheme • For a comet with a period greater than 200 years, its orbit determination would not have been accurate enough to prove the linkage. Thus the division has no physical justification. • Comets are defined as dynamically new is the first time that they have passed through the planetary system. • Carusi et al. (1987a) suggested defining the boundary between Jupiter-family and Halley-type comets along a line of constant Tisserand parameter, T=2. • It is an approximation to the Jacobi constant, objects with T > 3 cannot cross Jupiter’s orbit in the circular restricted case, being confined to orbits either totally interior or totally exterior to Jupiter. 3

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6. .001 .0001 Nearly-Isotropic Comets • Comets with T < 2, which are mainly comets from the Oort cloud, are designated nearly isotropic comets. Comets are defined as dynamicallynew that this is the first time that they have passed through the planetary system. • For the average change in 1/a caused by Jupiter alone is already 0.00048, it follows from this that comets with 1/a < 0.0001 are objects that have come for the first time from the larger cloud and will be denoted as new comets. (1951, Oort et al.) • Comets that have a small enough semi-major axis to be able to be trapped in a mean motion resonance with a giant planet are designated as Halley-type comets and those that have semi-major axes larger than this as external comets. • Pluto is in 3:2 mean motion resonance with Neptune, and thus the object with the largest semi-major axis that is known to be in a mean motion resonance. 5

7. Ecliptic Comets • Ecliptic comets can be further subdivided into three groups. Comets with 2 < T < 3 are on Jupiter-crossing orbits and dynamically dominated by that planet. We shall call these Jupiter-family comets. • Comets with T > 3 (not Jupiter-crossing) should not be considered members as the Jupiter family. A comet that has T > 3 and a < aJ is designated a Encke-type. Note that this combination of the T and a implies that the orbit of this object is entirely interior to Jupiter, i.e., the aphelion distance is less than aJ . • A comet that has T > 3 and a < aJ (orbit is exterior to Jupiter) is designated a Chiron-type. It is important to note that Chiron-type comets are probably the same as Centaur asteroids. The differences between an asteroid and a comet in this part of the solar system is not clear. 6

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9. CN-bearing Molecules • HCN was the first molecule firmly detected at millimeter wavelengths in a comet, in 1P/Halleyin 1985. It has subsequently been detected in 23 comets.Its measured abundance relative to water ranges between 0.08% and 0.25%– when a water outgassing rate is available – and is one of the least variable from comet to comet. For this reason,HCN is also often taken as a reference. • CH3CN (乙腈)has been detected in only 4 comets (including one marginal detection) since 1996, with an abundance relative to HCN between 0.08 and 0.23. • HNC has been detected in 5 comets since 1996, but its origin is puzzling. While production through chemical reaction in the coma of Hale–Bopp was invoked to explain its increasing abundance closer to the Sun, it cannot explain abundances relative to HCN in the range 6–17% observed in other comets. It also seems that there is a general trend of increasing HNC/HCN ratio for decreasing rhin all comets. 8

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11. CO-bearing Molecules • Methanol (CH3OH) has been detected in 17 comets since C/1989 X1 (Austin) in 1990. Several lines have often been simultaneously detected, enabling temperature measurements. Its abundance relative to water varies at least twice as much as for HCN:< 0.9% to 6.2%, with most comets (including all Jupiter-family comets) being in the 1.5–2.8% range. • Formaldehyde (H2CO): This molecule has been detected in 13 comets since 1989. We have assumed a distributed source with a Haser equivalent density distribution and a parent lifetime of about 10 000 s at rh= 1 AU, as suggested by many observations – this is equivalent to a parent scale-length of 8000 rh1.5for an average expansion velocity of 0.8 rh−0.5km s−1. Its abundance relative to water varies by one order of magnitude and a factor of 6 relative to HCN. Curiously, the comets that are abundant in formaldehyde are also abundant in methanol but owing to the long lifetime of CH3OH (about 77 000 s at 1 AU), it is not a likely parent molecule for H2CO. Large polymers thermally degraded on grains are a more likely parent source for formaldehyde. 10

12. Comet Hale-Bopp at 25.7AU (Gy. M. Szabo et al., 2008) Comet Hale-Bopp on 2007 October 21. The field of view is 5.7’ x 3.8’ ; north is up, east is to the left. CO-bearing Molecules • CO: other techniques (infrared and UV) are usually more sensitive to measure the CO abundance in comets close to the Sun. But this molecule can have a high abundance (CO/HCN> 85 in 4 long-period comets) above ≈ 10% relative to water close to the Sun and much higher further away. Far from the Sun, CO is the main driver of cometary activity and can be detected in the radio out to more than 14 AU. In 4 other comets (including short-period comets) we found CO/HCN below 40. Hence CO might not be the second most abundant species in these comets. 11

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14. Sulphur-bearing Molecules • Hydrogen sulphide (H2S) was first detected in comets in 1990 and has been detected in 11 comets. Its abundance relative to HCN varies by a factor 5 – and even by one order of magnitude with respect to water – but most comets have a ratio H2S/HCN between 4 and 6. Variations in H2S abundances seem completely uncorrelated with dynamical origin. • CS has been recently observed in the radio but was observed in comets in the UV well before 1996. So far we have assumed (in the 9 comets in which we detected CS) that it is the photo-dissociation product of CS2. However, it was found that the CS/HCN ratio decreased with heliocentric distance as rh−0.7. This trend is observed in some other comets and was previously observed in comet 1P/Halley. • This is puzzling as CS2 is a very volatile molecule and is not expected to sublimate less easily than H2O or HCN further away from the Sun. Taking out this trend with rh, the CS/HCN ratio is close to 0.8 at rh≈ 1 1AU in most comets. 13

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16. Conclusion • A larger sample of comets and a multidimensional analysis of the grouping of the comets according to their chemical composition are obviously needed. • Heliocentric trends (for HNC, CS and formaldehyde) should be further understood before any definitive conclusions can be drawn. • Although the sample is still sparse, there is no obvious correlation between the dynamical class and the chemical composition of comets. This is shown for the relative abundances of HCN and CH3OH. 15

17. References • 1951, Oort et al. Differences Between New And Old Comets • 1994, Harold F. Levison, Comet Taxonomy • 2002, Nicolas Biver et al., Chemical Composition Diversity Among 24 Comets Observed At Radio Wavelengths • 2007, Jacques Crovisier, Cometary Diversity And Cometary Families • 2008, Gy. M. Szabó, Cometary Activity At 25.7 AU Hale-bopp 11 Years After Perihelion • 2008, Jacques Crovisier et al. Radio Observations Of Jupiter-family Comets • 2008, Jewitt, The Kuiper Belt And Other Debris Disks 16