Comet, Hyakutake

To account for these compositional features of comets, Bar-Nun et al. [23] experimentally showed a relation of the relative abundances of CO, Ar, N2, and CHj trapped in ice, versus temperatures, and inferred tliat the abundances of N2 and CO in comets were inherent from the gases trapped during ice forming in die Uranus-Neptune regions [24]. Notesco et al. [25] have performed a similar analysis for CH and C2H, successfully accounting for the proportions of these two gases observed in comet Hyakutake by Mumma et al. [22]. 

Acetylene has been observed in the atmospheres of Jupiter and Titan [33, 34] and more recently has been identified in significant abundance in comet Hyakutake [35]. Following the discovery of acetylene in Hyakutake, photochemical experiments have demonstrated [36] that this molecule is a likely precursor of C2, a widely observed component of comets. Acetylene itself may therefore be a ubiquitous constituent of comets. It has been proposed [37] that polymerization of acetylene in cometary impact on planetary atmospheres may be responsible for the formation of polycyclic aromatic hydrocarbons (PAHs) which may in turn be responsible for the colors of the atmospheres of Jupiter and Titan. Shock-induced polymerization of acetylene has been observed in the gas phase [38], and static high-pressure experiments have demonstrated polymerization of orthorhombic solid acetylene above 3 to... 

Krasnopolsky VA, Mumma MJ (2001) Spectroscopy of comet Hyakutake at 80-700 A First detection of solar wind charge-transfer emissions. Astrophys J 549 629-634 Krasnopolsky VA, Bowyer S, Chakrabarti S, Gladstone GR, McDonald JS (1994) First measurement of helium on Mars implications for the problem of radiogenic gases on the terrestrial planets, learns 109 337-351... 

Data from the laser-induced fluorescence of C2 radicals obtained in the laboratory during 193 nm photolysis of C2H2 have been used to explain band profiles of C2 in the nucleus of comet Hyakutake, observed by the Hubble Space Telescope. In conjunction with ab initio computations, the data have led to the proposal that photolysis of C2H2, in the laboratory and in comets, proceeds by a sequential mechanism, first producing C2H and then C2. Two excited electronic states of C2H have been identified and 2 11) through which photodissociation in the second step occurs. Measurements of the kinetics and translational energy release in the near-UV photodissociation of the allyl radical have indicated that allene formation is the dominant H-loss reaction channel. ... 

Millimetre astronomy has also been found to be a powerful tool for the physicochemistry of comets. This was fully demonstrated by the observations of two exceptional comets Hyakutake (1996) and... 

Figure 12 The 1io Oqo HDO line at 465 GHz, observed at the Caltech Submillimetre Observatory, in comet Hyakutake. Two lines of methanol are present in the same spectrum. Reproduced by permission from Crovisier J and Bockelee-Morvan D (1997) Comets at the submillimetric wavelength in ESA Symposium, Grenoble, France.

Hale-Bopp (1996-1997). The newly detected molecules in these two comets are CS, NH3, HNC, HDO, CH3CN, OCS, HNCO, HC3N, SO, SO2, HCCS, HCOOH, NH2CHO, CN, CO% HCO+ H3O+. This number is considerably bigger than the total number of molecules previously in comets. Figure 12 shows the detection of HDO and methanol in the comet Hyakutake. 

Observed abundances normalized to that of water ice (=100). W33A and NGC 7538-IRS 9 are two luminous protostars which span the observed range in interstellar ice composition. The abundances for the comets are an average of those observed for comets Hyakutake and Hale-Bopp. 

In late 1985, radar observations of comet Halley, which was much more active than IRAS-Araki-Alcock, yielded echoes with a substantial broadband component presumed to be from a large-particle swarm, but no narrowband component, a negative result consistent with the hypothesis that the surface of the nucleus has an extremely low bulk density. In 1996, Goldstone obtained 3.5-cm echoes from the nucleus and coma of comet Hyakutake (C/I996 B2). The coma-to-nucleus ratio of radar cross section is about 12 for Hyakutake versus about 0.3 for lAA. The radar signatures of these three comets strengthen impressions... 

Comsovici et al., 1998 [78] observed Comet Hyakutake C/1996 B2 during the period 26 Aprils May 1996 around perihelion searching for the 22 GHz neutral water line by using a fast multichannel spectrometer coupled to the 32 m dish of the... 

Cosmovid, C.B., MontebugnoU, S., Otfei, A., Pogrebenko, S., Cortighoni, S. The puzzling detection of the 22 GHz water emission line in Comet Hyakutake at perihelion. Planet. Space Sci. 46,467-470 (1998)... 

The recent ai roach of large comets such as IP/Halley, C71996 B2 Hyakutake, and C/1995 Ol Hale-Bopp to the Earth provided a good opportunity to investigate the detailed composition of cometary ices by various methods such as mass spectrometry, infrared spectroscopy, and radio emission. The composition of interstellar ices is compared with that of the cometary ices in Table 9.3. It is striking that cometary and interstellar ices have quite comparable relative molecular abundances. 

Fig. 4. Isotopic variations of hydrogen in the Solar System (adapted from Robert et al. 2000). The deuterium/ hydrogen ratio of different components is normalized to the D/H ratio of the Sun (as it was before deuterium burning), which is thought to represent H2 in the protosolar nebula. Numbers along the y-axis represent the numbers of cases. Terrestrial hydrogen is enriched in deuterium by a factor of about six relative to solar. Among Solar System objects analysed so far, carbonaceous chondrites, Antarctic micrometeorites (Engrand Maurette 1998) and chondruies from LL3 chondrites present a distribution of D/H values that centre around the terrestrial D/H ratio. Notably, comets analysed so far (Halley, Hale Bopp and Hyakutake, references given by Dauphas et al. (2000)) present D/H values about two times higher than the terrestrial value.

Table I. The main interstellar ice components (relative to H2O) compared to that inferred for several comets. Cometary references Halley (24) Hyakutake and Hale-Bopp (25). See Table 2 in Reference 6 for details.

Figure 4. Polarimetric color vs. phase angle for comet (+) IP/Halley, (o) C/1996 B2 (Hyakutake), and ( ) C/1995 01 (Hale-Bopp). The data are from [22, 28-31].

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