Astronomical silicates

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).

Examples of the dust cross-section distributions for astronomical silicate at different wavelengths and for different power-law particle size distributions are shown in Fig. 7.1. The 9.7 pm absorption cross-section distribution for particles with p = — 3.5 actually peaks at 1 pm, but is otherwise dominated by particles <3 pm. Thus, for this distribution, observations at 9.7 pm are not very sensitive to large particles and their presence cannot be ruled out. For only a slightly shallower size distribution with p = —3.0, the absorption cross-section suddenly becomes dominated by large particles, and particles smaller than 1 pm will not contribute much to the total opacity. At 1 mm, the picture is different. Here, the absorption... 

The 10 p.m feature of chondritic IDPs has been compared with the 10 p.m feature of astronomical silicates. No particular IDP IR class consistently matches the —10 p.m feature of solar system comets or silicate dust in the interstellar medium (Sandford and Walker, 1985). However, the —10 p.m features of CP IDPs composed mostly of GEMS and submicrometer enstatite and forsterite crystals generally resemble those of comets and late-stage Herbig Ae/Be stars in support of the hypothesis that some CP IDPs are of cometary origin (Figure 10). 

The second important source for the hydrosphere and the oceans are asteroids and comets. Estimating the amount of water which was brought to Earth from outer space is not easy. Until 20 years ago, it was believed that the only source of water for the hydrosphere was gas emission from volcanoes. The amount of water involved was, however, unknown (Rubey, 1964). First estimates of the enormous magnitude of the bombardment to which the Earth and the other planets were subjected caused researchers to look more closely at the comets and asteroids. New hypotheses on the possible sources of water in the hydrosphere now exist the astronomer A. H. Delsemme from the University of Toledo, Ohio, considers it likely that the primeval Earth was formed from material in a dust cloud containing anhydrous silicate. If this is correct, all the water in today s oceans must be of exogenic origin (Delsemme, 1992). 

A Dirty Silicate Story A friend of one of the authors is an astronomer—as well as a professional mineral dealer—who became interested in dirty silicates as candidates for interstellar dust. He therefore selected for determination of the blackest natural silicate mineral in his possession, the coal-black mineral hornblende, which contains a high concentration of impurities such as iron. A slice about 100 jam thick was polished, and transmission was measured in a recording spectrophotometer. The fact that appreciable light was transmitted for all near-infrared and visible wavelengths indicated that k was rather small. Calculations indeed confirmed that k was less than 10 4 between about 6 and 0.3 jum. And yet this was the blackest silicate in the possession of a professional collector. It is not easy to find A = 0.01 in the band gap region of... 

Figure 5.5 Winds in the solar nebula might be one of the possible processes responsible for the mixing of hot and cold components found in both meteorites and comets. Meteorites contain calcium-aluminum-rich inclusions (CAIs, formed at about 2000 K) and chondrules (formed at about 1650K), which may have been created near the proto-Sun and then blown (gray arrows) several astronomical units away, into the region of the asteroids between Mars and Jupiter, where they were embedded in a matrix of temperature-sensitive, carbon-based cold components. The hot component in comets, tiny grains of annealed silicate dust (olivine) is vaporized at about 1600 K, suggesting that it never reached the innermost region of the disk before it was transported (white arrows) out beyond the orbit of Pluto, where it was mixed with ices and some unheated silicate dust ( cold components). Vigorous convection in the accretion disk may have contributed to the transport of many materials and has been dramatically confirmed by the Stardust mission (Nuth 2001).

Fine structure on the silicate feamre was first seen in observations of comet Halley. It consisted of a small 11.2 pm bump on the silicate feature detected in high spectral resolution and good signal-to-noise ratio observations. This small bump has now been seen on the silicate feature of several comets and it is widely interpreted as evidence for the presence of olivine because IR studies of powdered olivine samples show fine stmcmre at 11.2 pm. In the astronomical literature this feature is considered to be crystalline olivine as compared to amorphous silicates that cannot produce the pronounced 11.2 pm bump on the overall 10 pm silicate feamre. 

The 11.2 pm fine structure on the Si-O silicate feature has provided interesting insight into the relationship between cometary and interstellar materials, because IR observations of silicates in the diffuse interstellar medium and molecular clouds do not show the feature (Molster et al., 2002a,b). Searches for the 11.2 pm fine structure towards the Galactic center indicates that less than 0.5% of interstellar silicates are crystalline (Kemper and Tielens, 2003). The crystalline olivine feature is, however, seen in certain astronomical objects, stars surrounded with disks. It has been seen in Beta Pictoris (Knacke et al., 1993) and Herbig Ae/Be stars... 

Waelkens et al., 1996) massive pre-main sequence stars surrounded with disks of dust and gas. Herbig Ae/Be stars even show transient gas features in their spectra that have been interpreted as comets falling into the star (Beust et al., 1994). The presence of the olivine feature in comets and circumstellar disk systems and the lack of it in interstellar and molecular clouds, the parental materials for star and planetary formation, is somewhat of a conundrum. A common astronomical interpretation is that interstellar grains are amorphous silicates and when warmed in a circumstellar disk environment, they anneal to produce crystalline materials. The other possibility is that olivine in comets and disks condenses from vapor produced by evaporation of original interstellar materials. 

There is additional astronomical information on cometary silicates that provides far more information than simply the presence of olivine. High-resolution and good signal-to-noise ratio IR spectra show additional fine strucffire on the 10 p,m silicate feature of bright LP comets. A small feature at 11.9 p.m is also due to olivine and a slope change at 9.2 pm and 9.3 pm is attributed to pyroxene and amorphous silicate with pyroxene composition (Manner and Bradley, 2003) (Figure 11). 

WatersL. B. F. M. andMolsterF. J. (1980) Crystalline silicates in space. In Highlights in Astronomy Volume 12 (ed. H. Rickman). Astronomical Society of the Pacific, San Francisco, pp. 48—51. 

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