The deuterium abundance of water derived by in situ measurements in comet Halley [13, 69] and by remote sensing at radio wavelengths in comet Hyakutake and Hale-Bopp is given in Table 6. The values are significantly higher than the standard mean ocean water values (SMOW) on Earth (Fig. 49). The abundances of D/H in comets suggest incorporation of interstellar material during comet formation. The difference to SMOW implies, that probably only a minor fraction of water on Earth comes from impacting comets. A detailed discussion on possible formation scenarios can be found in [34] and [157], and references given in Table 6.

7 Dust Particles

The cometary dust component is visible as a prominent dust tail in many comets (Fig. 1). The dust particles can be seen because they scatter the solar light efficiently. In Sect. 3, we already discussed the dynamics of dust particles in the inner coma and in the dust tail. Here, an overview of the nature of the dust particles is given.

Fig. 49. The D/H ratio in comets and solar system objects [34]
Fig. 50. Interplanetary dust particles have irregular shape and low density. This particle is about 10 |lm in length. (NASA/JPL)

It is generally believed that dust particles are irregular and porous particles, very much like the interplanetary dust particles collected at the top of the Earth atmosphere (Fig. 50). At cold temperatures, under the conditions found in the interstellar medium, silicate minerals form in the amorphous state. Formation at higher temperatures or heating of the dust grains to temperatures above —1000K will lead to crystalline silicates. The proportion of amorphous to crystalline silicates in dust grains will therefore give us some information on whether interstellar grains may have survived the conditions in the pre-planetary disc and about the temperature regime in the disc where the grains formed which have then been incorporated into comets. The chemical composition of the dust grains will show the homogeneity of the refractory cometary material. The study of dust grains is therefore an important component in the puzzle that needs to be completed to understand the formation of comets.

7.1 Composition

In situ data for the chemical composition of cometary dust grains were available only from the Giotto spacecraft (ESA) until recently. New information is expected from the analysis of the grain samples returned to Earth by the Stardust mission (NASA). The in situ analysis of dust grains in the coma of comet Halley showed in general three types of grains with about equal relative particle abundance [140]:

- Particles similar to CO chondrites: Na, Mg, Si, Ca, Fe (—35%).

- Particles consisting mainly of light atoms: C, H, O, N (so-called CHON-particles) (-30%).

- Particles consisting of silicates, but also light elements (mixture of the two cases) (-35%).

For individual particles, however, large variations in composition have been found [94]. The density of dust particles was estimated to — 1gcm~3, consistent with their expected fluffy nature [94].

CHON particles are believed to consist mainly of organic material. However, a clear identification has been difficult so far. It is often proposed that these organic particles may form part of the extended coma sources discussed for some of the gas species (see Sect. 5).

Information on the chemical and mineralogical composition of cometary dust grains can also be obtained from spectral observations. Spectra in the infrared wavelengths range show prominent emission features (Fig. 51). Around 10 |m stretching vibrations of Si-O bonds in silicates produce a well-known emission feature. Additional emissions are present at longer wavelengths, e.g. at 16 |m and 35 |m caused by bending modes. Fortunately, the atmospheric window around 10 | m allows to study this feature in many comets in ground-based observations. At longer wavelengths, we need space telescopes.

The silicate emission features in Hale-Bopp and many other comets have been compared to spectral models and laboratory spectra of silicates to identify the emission peaks in terms of mineralogical composition. A combination of crystalline and amorphous grains, consisting mainly of pyroxene and olivine, is found to match best with the observed spectra (e.g., [159]). Thus, cometarydust grains are a mixture of material condensed at low temperatures (amorphous) and of material processed by high temperatures (crystalline). This composition probably results from mixing processes in the proto-planetary disc.

It is interesting to note that no strong 10 | m silicate emission feature has been detected in short-period comets yet [102]. Whether this is caused by compositional differences of comets belonging to different dynamical classes remains, however, unclear so far. Nevertheless, this finding is, among others, a strong motivation for future spectral observations of cometary silicate features.

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Fig. 51. ISO SWS spectrum of comet Hale-Bopp [57]. Several silicate emission features are seen superimposed on a black body spectrum

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Fig. 51. ISO SWS spectrum of comet Hale-Bopp [57]. Several silicate emission features are seen superimposed on a black body spectrum

7.2 Size Distribution

To derive the size distribution of dust particles again in situ measurements are ideal. Giotto measured the dust particles at comet Halley. Figure 52 shows the resulting size distribution [153]. The diagram shows the number of detected dust particles versus their radius and mass after assuming a mean density of 1 gcm~3 for the dust particles. We note that:

• Most particles have small radii.

• The dust mass is concentrated in a few large particles.

At large radii, only very few particles were detected by Giotto, and beyond 10~3 m radius, the distribution is just a simple extrapolation. The dominance of small dust particles in number is also supported by data of the Stardust mission obtained in the coma of comet Wild 2 [93,204].

If the exact size distribution of dust particles in a cometary coma can not be determined observationally, as it is usually the case, it is typically expressed by a function like [101]:

Here, N is a scaling factor, a the particle radius, and a0 the lower size limit. Figure 53 shows an example of size distributions for different constants M and N, which determine the position of the maximum and the slope of the distribution.

As in situ observations of comets are rare, many attempts are made to derive the dust size distribution from ground-based observations. This can be made by using infrared observations of the thermal emission of cometary dust grains in comparison to light scattered at optical wavelengths, by polarization

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