Comets silicates

The oxygen isotopic compositions of comet silicate grains (Fig. 12.10) resemble those of chondrites, suggesting that they formed within the solar nebula (McKeegan et al., 2006). Only one grain was found with anomalous oxygen isotopes, demonstrating its extrasolar... 

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). 

The interiors of planets, moons, and many asteroids either are, or have been in the past, molten. The behavior of molten silicates and metal is important in understanding how a planet or moon evolved from an undifferentiated collection of presolar materials into the differentiated object we see today. Basaltic volcanism is ubiquitous on the terrestrial planets and many asteroids. A knowledge of atomic structure and chemical bonding is necessary to understand how basaltic melts are generated and how they crystallize. Melting and crystallization are also important processes in the formation of chondrules, tiny millimeter-sized spherical obj ects that give chondritic meteorites their name. The melting, crystallization, and sublimation of ices are dominant processes in the histories of the moons of the outer planets, comets, asteroids, and probably of the Earth. 

Fine structure on the silicate feature provides more specific mineralogical information. A small bump at 11.2 pm on the 10 pm feature (Fig. 12.4a), observed for several long-period comets, is generally interpreted as indicating crystalline olivine. Another shoulder at... 

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). 

Also, observations of crystalline silicates in the comets Halley (Swamy etal. 1988) and Tempel 1 (Harker et al. 2005) suggest that a large fraction of cometary dust was processed or formed in the hot inner regions of the protoplanetary disk and transported to the region where the comets formed. 

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... 

As the peaks observed near 33 and 69 p.m are due to phonon transitions in the crystalline silicate lattice, the ISO observations are an unambiguous indication of the presence of crystalline silicates in cometary dust. More recently, crystalline olivine and orthopyroxene have been identified in Comet C/2001 Q4 (NEAT) via 10 pm observations (Wooden et al. 2004). There is considerable controversy concerning the origin of these crystalline grains. 

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).

A detailed comparison of the predictions of these models with direct observations of the minerals present in dust from the primitive asteroids and comets provides a critical test of the accuracy of the presumed starting composition. For example, the effect of raising the C/O ratio in the initial composition results in some of the oxide and silicate minerals being replaced by carbides and carbonates (as discussed by Lodders 2003 Ebel 2006 and references therein). 

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).

The earliest detailed studies of silicate dust in protoplanetary disks targeted those brightest in the mid-infrared, where high quality spectra could be obtained even by severely flux-limited observations. Cohen Wittebom (1985) reported the earliest detection of crystalline silicate emission from the environment of young stars and interpreted it as evidence for dust having been transformed from its pristine state in the interstellar medium to the material known to be contained in the comets and perhaps primitive meteorites. Interestingly, this observation and explanation pre-dated the evidence that young stars are surrounded by disks and not by spherical envelopes. 

Silicates with olivine composition (MgxFe(i x))2Si04 are common in chondrites, comets, IDPs, and in protoplanetary disks. The Mg-rich end-member of the olivine family is forsterite, also often termed as Foioo the Fe-rich end-member is fayalite (Foo). The interstellar medium contains a similar concentration of the FeO- and MgO-rich silicates (see Chapter 2). Correspondingly, amorphous silicate grains frequently have similar magnesium and iron abundances in protoplanetary disks, in cometary dust, and in chondritic IDPs. In stark contrast, crystalline dust is almost always dominated by Mg-rich grains in protoplanetary disks (e.g. Malfait et al. 1998 Bouwman etal. 2008), comet tails (e.g. Crovisier el al. 1997 Wooden et al 2004 Harker et al. 2005 Lisse et al. 2006), in the most primitive and least processed chondritic matrices, and IDPs (for a review, see Wooden et al. 2007). 

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... 

The sizes of the cometary dust grains vary from less than a micron to probably several centimeters. Infrared observations near 10 pm show the silicate spectral features. In addition, there seems to be a black ingredient presumed to be carbon. Due to different accelerations from the solar radiation pressure, the larger particles follow the comet close in its orbit and are more concentrated to the orbital plane. They become sometimes visible in the anti-tails , narrow spikes which point towards the Sun by an effect of projection when the Earth crosses the comet s orbital plane. Non of the meteorites found so far on Earth seem to be of cometary origin. However, very fluffy micron sized interplanetary dust grains (Brownlee particles) which have been collected by high flying aircraft are possibly cometary debris. 

Hill H. G. M., Grady C. A., Nuth J. A., Hallenbeck S. L., and Sitko M. L. (2001) Constraints on nebular dynamics and chemistry based on observations of annealed magnesium silicate grains in comets and disks surrounding Herbig Ae/Be stars. Proc. Natl. Acad. Sci. 98, 2182-2187. 

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