"32 cm"3 s"1

coma. Collisional reactions between neutrals are therefore important only very close to the nucleus.

The ionic chemistry is more complex. Ions are mainly produced by photoionisation, photodissociative ionisation, electron impact and charge transfer. Photodissociative ionisation by extreme UV photons produces ions in an excited state, which then dissociate quickly. This process can be efficient in the inner coma, because extreme UV photons can penetrate deeply into the coma. Electrons produced by ionisation processes in the inner coma are involved in a number of reactions, like electron ionisation, electron dissociation and electron dissociative recombination. Positive ion charge transfer reactions are also important in the inner coma. Solar wind particles, however, do not penetrate into the cometopause (see Sect. 3), and the reactions involving these species are therefore most important at large nucleocentric distances, where solar wind particle densities are the highest. An overview of the relative importance of the various ionic reactions can be found in [194].

5.1 Chemistry of Some Frequently Observed Species NH, NH2, NH3

The formation of NH and NH2 by ammonia (NH3) photodissociation is well established now. However, in earlier measurements (e.g., [78,216]) of NH2 and NH, their production rates were consistently below the mass-spectrometer data of NH3 for comet Halley (see detailed discussion in [11]). This was still the case in first comparisons of NH and NH2 production rates to direct radio observations of NH3 in comet Hale-Bopp [23,187]. With the recently revised NH2 g-factors [128-130], the agreement is however improved.

The chemistry of the carbon-bearing radicals is complex (Fig. 44). The main parent molecule of the C2 radical is C2H2 (ethane). C2H2 had been proposed to form the parent molecule of the well-known C2 radical [120] by the reaction C2H2 + hv ^ C2H, followed by C2H + hv ^ C2 (see also discussion in [46]). Abundance ratios of C2 and of C2H2 in comet Hale-Bopp [187,207] are consistent with acetylene playing a major role as parent molecule for C2. However, additional parents are likely. For example, C2H6 (acetylene) can lead to the formation of C2 through decay to C2H4, which subsequently dissociates to C2H2, again resulting in C2. Another parent species detected is HC3N [35,146], which dissociates into C2H + CN, again leading to the C2 radical. It has been shown [105] that the production rates of the main C2 parent species can be derived from observations using a chemistry scheme involving not only photoreactions, but also electron impact reactions (Fig. 44).

No parent molecule of C3 has been detected yet. The radical is most likely formed by C-bearing molecules like C3H4, or more complex species (see discussion in [105]). Because of the expected weak spectral emission lines of such

Fig. 44. Overview of the main reaction pathways for the formation and destruction of C2 and C3 radicals [105]

complex organic molecules, a direct detection of C3 parent molecules probably has to wait for future in situ observations.

CN, HCN, HNC, H2CO, CO, Extended Coma Sources

A question still under debate is whether photodissociation of HCN is the only process forming CN radicals. While it was shown that HCN is sufficient to explain the observations of CN in comet Hale-Bopp beyond rh = 3 AU, it is still unclear whether additional sources of CN exist in comets near perihelion. The arguments for and against HCN being the only CN source steam from disagreements in their production rates, analysis of spatial CN profiles and comparisons on the spatial CN distribution in optical images with 2D maps of the HCN radio signal. The result is inconclusive so far. Recent evidence for an additional parent of the CN radical also comes from measurements of the 15N/14N isotopic ratio in high-resolution optical spectra [10,121]. This ratio is significantly different to the value measured for HCN in comet Hale-Bopp (see also Sect. 6.4). Speculations about the nature of the additional CN source range from sublimation of icy-dust-particles in the coma to HCN polymers on dust grains. Recently, laboratory experiments have been set-up to investigate the formation of CN from HCN polymers experimentally (e.g., [85]). For a recent review on the problem of CN sources, see [84].

CO is a relatively abundant parent ice in cometary nuclei. However, in-situ and ground-based measurements [68] of comet P/Halley have shown an extended source for CO (or distributed source, depending on naming convention) in the coma in addition to its nucleus source (Fig. 45). The coma source peaked at about 104 km from the nucleus and accounted for about half of the CO observed in comet Halley.

Fig. 45. Indication for an extended source for CO by the NMS spectrometer on board Giotto [70]

Was this article helpful?

0 0

Post a comment