Hale-Bopp

Figure 6.17 Tail structures of Hale-Bopp (left) and Halley (right) (1997/3/10, 4 38-5 00, Mt. Ho-Huan, Taiwan). (A colour reproduction of this figure can be seen in the colour section). (Reproduced by permission of Wei-Hao Wang, Institute for Astronomy, University of Hawaii)...

The recent close-encounter, especially with comet Hale-Bopp, focused the attention of the ground-based telescopes in all regions of the electromagnetic spectrum to produce the molecular inventory shown in Table 6.4. Chemical network models for... 

Table 6.4 Molecular inventory for Hale-Bopp observed at o, as 1 AU...

The next most likely possibility is cometary delivery of the atmosphere but again there are some problems with the isotope ratios, this time with D/H. The cometary D/H ratios measured in methane from Halley are 31 3 x 10-5 and 29 10 x 10-5 in Hayuatake and 33 8 x 10-5 in Hale-Bopp, whereas methane measurements from Earth of the Titan atmosphere suggest a methane D/H ratio of 10 5 x 10-5, which is considerably smaller than the ratio in the comets. The methane at least in Titan s atmosphere is not exclusively from cometary sources. Degassing of the rocks from which Titan was formed could be a useful source of methane, especially as the subnebula temperature around Saturn (100 K) is somewhat cooler than that around Jupiter. This would allow volatiles to be more easily trapped on Titan and contribute to the formation of a denser atmosphere. This mechanism would, however, apply to all of Saturn s moons equally and this is not the case. 

Hale-Bopp Comet from the Oort Cloud with a short period, discovered in 1995 when it was 7AU from the Sun. 

Earth and Planetary Science Letters 167 71-79 Bockelee-Morvan D. and Crovisier J. (2002). Lessons of Comet Hale-Bopp for Coma Chemistry Observations and Theory. Earth, Moon and Planets 89 53-71. 

Dust, representing less-volatile materials, is liberated from the comet and propelled outward by the expanding jets of gas. Appreciable quantities of dust are sometimes released for example, the dust production rate from comet Hale-Bopp exceeded its gas production rate by a factor of five. Comet particles range in size from submicron dust to small rocks. Direct particle impacts onto spacecraft suggest that solid particles in the coma can be of at least millimeter size. 

Cometary activity occurring at great distance from the Sun (corresponding to temperatures <100 K) is probably controlled by ices more volatile than H20. For example, comet Hale-Bopp exhibited emission of highly volatile CO at great solar distances. Trapped CO was presumably released by crystallization of amorphous ice or sublimation of ice crystals at very low temperatures. 

Table 12.1 Chemistry of comet Hale-Bopp, based on production rates of molecules, relative to water...

Spectra of comet Hale-Bopp, showing features attributable to silicate minerals, (a) Profile of fine structure in the 10 silicate emission feature a peak at 11.2 and a shoulder at 11.9 are due to olivine, and a slope change at 9.2 results from pyroxene, (b) Expanded infrared spectrum exhibiting a number of sharp peaks due to magnesian olivine and pyroxene. The region of (a) is bounded by a small box. Modified from Crovisier et al. (2000) and Hanner and Bradley (2003). 

Bockelee-Morvan, D. and Crovisier, J. (2002) Lessons of comet Hale-Bopp for coma chemistry. Earth, Moon, and Planets, 89, 53-71. 

Crovisier, J. plus 13 coauthors (2000) The thermal infrared spectra of comets Hale-Bopp and... 

Among the goals for the Stardust mission was identifying the origin of the crystalline silicates in comets, whose presence in comets had been known from observations of comets Halley and Hale-Bopp (as reviewed in Bockelee-Morvan... 

Figure 6.5 Comparison of the 10 pm Si-O stretching bands of a GEMS-rich IDP and astronomical silicates. (A) Chondritic IDP L2008V42A. Profile derived from transmittance spectrum. (B) Comet Halley (Campins Ryan 1989). (C) Comet Hale-Bopp (Hayward et al. 2000). (D) Late-stage Herbig Ae/Be star HD 163296 (Sitko et al. 1999). The structure at 9.5 qm in (B), (C), and (D) is due to telluric O3. Figure from Bradley et al. (1999).

Both the analysis of Comet Wild 2 dust particles (Brownlee et al. 2006) and infrared spectroscopy of dust particles from comets Halley, Hale-Bopp, and Tempel 1 (Lisse et al. 2006) have shown that crystalline silicates are common constituents of comets. The presence of crystalline silicates in comets indicates that the crystallization of amorphous interstellar silicates occurred in the comet-forming region... 

Water and other volatiles could have been supplied to Earth by comets and asteroids as part of the late veneer. The arguments for and against this hypothesis have recently been reviewed by Drake (2005). The D/H ratio measured in three comets to date is 2 x higher than on Earth, suggesting that comets could not have supplied more than 50% of Earth s water (Robert 2001). However, these comets may not be representative of objects colliding with the early Earth. If the Ar/H20 ratio measured in comet Hale-Bopp is typical, comets would have delivered 2 x 104 times more Ar than is presently found in Earth s atmosphere if they were the main source of Earth s water (Swindle Kring 2001). Consideration of the abundances of noble metals and noble gases led Dauphas Marty (2002) to estimate that comets contributed <1% of the Earth s water. It is unlikely that carbonaceous chondrites supplied most of the late veneer since these objects have different Os isotope ratios than Earth s mantle,... 

While many asteroids may have previously been comet-like, even active comets are somewhat asteroid like. The traditional concept of comets as dirty snowballs has been modified by some authors to consider them to be more like frosty rocks, because they contain more rock than ice. The dust gas production rate from the spectacularly active comet Hale-Bopp exceeded its gas production rate by a factor of 5 (Jewitt and Matthews, 1999). In considering the water contents of comets, it is interesting to consider that some comets may have ice abundances similar to the bound water content of hydrated silicate-rich asteroids. At least in some cases, comets and asteroids might have similar capacities for carrying water to other solar system bodies. 

Tidal breakup is not, however, the major source of fragmentation of comets. Most fragmentation occurs somewhat mysteriously far from massive bodies in the absence of tidal stress. Many comets including S4 LINEAR, Hale-Bopp, Wilson, Kohoutek, West, and Ikeya-Seki have been seen to fragment far from the Sun and Jupiter. Remarkable insight into the disintegration of comets has come from the study of the Kreutz family of sungrazing comets, a family of comets that pass within a few solar radii of the Sun. 

High levels of CO emission from Hale-Bopp at 7 AU (Jewitt et al., 1996) as well as transient... 

In addition to the nuclear source there is also clearly a distributed source above the nucleus, in the coma itself. Direct spacecraft measurements showed that two-thirds of the CO was released by Halley in the coma not directly from the nucleus (Eberhardt, 1999 Greenberg and Li, 1998). Detailed infrared observations of Hale-Bopp also showed that half of the CO was from a distributed source (Di Santi et al., 2001). These observations also showed that the distributed source turned on between 1.5 AU and 2 AU inbound and turned off at 2.2 AU outbound. The source tum-on was evidently triggered by a thermal threshold. 

The UV discovery of argon in Hale-Bopp has been implicated as evidence that the interior of that comet had not been heated to —35-40 K (Stem et al., 2000). Very low interior temperatures are also implied by the measured ortho to para ratios in NH3 (Kawakita et al., 2001) and water (Mumma et al., 1987). If correctly interpreted, these data imply that the comets studied were never even moderately heated in their interiors. 

Table 3 Relative production rates of molecules in comet Hale-Bopp (C/195 Ol).

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