Dust, comet

Comet A small (typically 1-10 km in diameter) interplanetary body that mostly consists of ice, other frozen materials, and dust. Comets originate from the outer solar system (compare with asteroid and meteoroid). 

The dust-comet halo represents an extreme case of an extended and sparse planetaiy atmosphere. Its optical thickness is veiy small. Therefore, the role of multiple scattering is small too. 

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 cometary coma The coma and the nucleus form the head of the comet the streams of dust and gas released by the comet form a very large, extremely tenuous atmosphere called the coma, which can have a spread up to around 104—105 km. The coma is not developed when the comet is a long way from the sun, but when it comes closer (at around 5 AU), the ice mixture begins to sublime and is ejected as a gas stream. Dust particles are entrained at a velocity of around one kilometre per second. 

The mass of Halley s Comet is about 1014kg, and thus its mean density is only 200 kg/m3. The rate of loss of material has been estimated as 5,000 kg/s. The nucleus is loosely packed and exhibits point craters and chasms from which gas and dust escape. These emissions consist mainly of water vapour (—80% by volume) as well as 6% CO, < 3% C02, -2.5% CH4, -1.2% NH3 and < 6% N2 (Flechtig and Keller, 1987). At the point where Giotto came nearest to the comet, the estimated amount of water being ejected was close to 15,000 kg/s, while that of dust particles was between 6,000 and 10,000 kg/s. Ions derived from water were detected in the... 

Until a few years ago, it was considered impossible that biomolecules or their precursors could have survived the huge temperatures which would have been generated when comets hit the Earth. Today it seems possible that about 0.1% of such substances could have remained unchanged. A comet with a diameter of around 3 km may contain around 1027 dust particles. If 0.1% were to reach Earth unchanged, there would still be 1024 intact particles around 1 mm in size. 

Stardust February 7, 1999, saw the start of NASA s Stardust mission the cometary probe, the first mission to collect cosmic dust and return the sample to Earth, has a time-of-flight mass spectrometer (CIDA, Cometary and Interstellar Dust Analyser) on board. This analyses the ions which are formed when cosmic dust particles hit the instrument s surface. In June 2004, the probe reached its goal, the comet 8 IPAVild 2, getting as close as 236 km The CIDA instrument, which was developed at the Max Planck Institute for Extraterrestrial Physics in Garching (near Munich), studied both cometary dust and interstellar star dust. 

The interstellar dust was shown to contain quinone derivatives as well as oxygen-rich condensed aromatic compounds the quinones were present in both hydrated and carboxylated form. Very little nitrogen was present in the compounds detected. The cometary material, however, contained condensed nitrogen heterocycles. Hardly any oxygen was detected in the solid phase of the cometary dust it possibly evaporates from the tail of the comet in the form of water or oxidized carbon compounds. The authors assume that these analytical results could lead to a reconsideration of the current biogenesis models (Kissel et al 2004 Brownlee, 2004). 

Mission Deep Impact In July 2005, NASA steered a projectile, about 370 kg in weight, at the comet 9F/Tempel (dimensions 4x4x14km), in order to obtain more exact information on its structure and composition. The impact was visible from Earth the Rosetta spacecraft discussed above also sent pictures to Earth. The dust/ice ratio determined after the impact is very probably greater than unity, so that comets are probably icy dustballs rather than (as had previously been surmised) dirty snowballs . The density of the cometary nucleus, which seems to consist of porous material, is roughly equal to that of ice. The impact set free around 19 GJ of... 

Fig. 3.9 Greatly simplified representation of the path taken by the material under study, beginning with nucleosynthesis and ending with laboratory analysis. Circumstellar dust (a component of the primeval presolar nebula) which was contained in asteroids or comets came to Earth in meteorites and was then available for exact study (Lugmair, 1999)...

Figure 6.16 Structure of the comet showing the nucleus, the coma and two tails - an ion tail and a dust tail...

Molecular inventory An extension of the idea of molecular processing on dust grains in the interstellar medium to the surface of a comet... 

The early period of the Earth s history, known geologically as the Hadean, is associated with huge bombardment of the surface of the planet by meteors and comets. The sequence of events in the Earth s formation is shown in Figure 7.3, starting with the Hadean. Any volatile materials on the surface of dust grains or planetesimals deposited on the Earth will be removed and become part of the atmosphere, or more generally the volatile component inventory of the planet. 

The circumstances producing an e.e. on the early Earth could well have been capture and amplification of one of the fluctuational mechanisms outlined above, although present evidence favors the meteoric infall of chiral molecules synthesized in interstellar dust douds. The total flux of organics on the early Earth, delivered to Earth both by way of comets and other bolides, and synthesized by terrestrial mechanisms... 

Greenberg, J. M. Chirality in interstellar dust and in comets Life from dead stars in Physical Origin of Homochirality in Life Cline, D. B., Ed. American Institute of Physics Woodbury, NY, 1996, pp 185-210. 

Isotope variations found in extraterrestrial materials have been classified according to different processes such as chemical mass fractionation, nuclear reactions, nucleosynthesis, and/or to different sources such as interplanetary dust, solar materials, and comet material. Various geochemical fingerprints point to the reservoir from which the planetary sample was derived and the environment in which the sample has formed. They can be attributed to a variety of processes, ranging from heterogeneities in the early solar nebula to the evolution of a planetary body. For more details the reader is referred to reviews of Thiemens (1988), Clayton (1993, 2004), and McKeegan and Leshin (2001). 

Cosmochemistry is the study of the chemical composition of the universe and the processes that produced those compositions. This is a tall order, to be sure. Understandably, cosmochemistry focuses primarily on the objects in our own solar system, because that is where we have direct access to the most chemical information. That part of cosmochemistry encompasses the compositions of the Sun, its retinue of planets and their satellites, the almost innumerable asteroids and comets, and the smaller samples (meteorites, interplanetary dust particles or IDPs, returned lunar samples) derived from them. From their chemistry, determined by laboratory measurements of samples or by various remote-sensing techniques, cosmochemists try to unravel the processes that formed or affected them and to fix the chronology of these events. Meteorites offer a unique window on the solar nebula - the disk-shaped cocoon of gas and dust that enveloped the early Sun some 4.57 billion years ago, and from which planetesimals and planets accreted (Fig. 1.1). 

When the elements are ejected from the stars where they were produced, they are in the gas phase. Subsequently, they combine in various chemical compounds and most condense as solids. The nature of those compounds and their behavior in the various environments encountered on their way to becoming part of the solar system can, in principle, be determined from the basic chemical properties of the elements. Evaporation and condensation are also important in the solar system and have played a defining role in determining the properties of planets, moons, asteroids, and the meteorites derived from them, comets, dust... 

We will now describe each of the various kinds of meteoritic samples available for cosmochemical investigation, progressing from primitive materials to samples from differentiated bodies. Presolar grains extracted from meteorites have already been described in Chapter 5, and interplanetary dust particles (IDPs) and returned comet samples will be described in Chapter 12. 

The solar system formed from a well-mixed collection of gas and dust inherited from its parent molecular cloud. The bulk composition of this material, as best we can know it, is given by the solar system abundances of elements and isotopes (Tables 4.1 and 4.2). From this bulk material, the planets, asteroids, and comets formed, each with its own unique composition. The processes that produced these compositions separated, or fractionated, elements and isotopes from one another. By studying these elemental and isotopic fractionations, we can potentially identify the processes that separated the elements and can leam about the physical conditions involved. This is particularly important for understanding the early solar system, because its processes and conditions are not directly observable. 

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