Comets composition

Figure 17 The two main comet compositional groups. Plots of C2, C3, and NH relative to CN emission from the A Heam et al. (1995) study. The filled points are Jupiter family comets, the group with depleted C2 and C3 and...

S4 [90,91] (Table 16.2). A typical comet composition is based on the relative amounts of the different gas species measured in the coma of comets from the Oort cloud, an immense spherical cloud surrounding the solar system and extending from approximately 5000 to 100,000 AU. About half of the short-period comets arc from the Kuiper Belt, a disk-shaped region containing many small icy bodies extending beyond Neptune s orbit from 30 to 50 AU, which are depleted in carbon-chain molecules. The other short-period comets have the typical composition of Oort cloud comets. So far, carbyne molecules detected in the coma of active comets are very short chains unlike the longer chains characteristic of natural carbyne crystals such as chaoite in the Murchison meteorite [20,67]. 

For example — color-change stars with meal-powder-like comet composition as first effect. 

It has recently been suggested that the comets also went through a number of subtle, but important, evolutionary processes in the Oort cloud and the Kuiper belt. Thus, their present nature is probably not the original one, as was previously thought (Stern, 2003). The assumption that the material which comets contain is in the same state as it was when the solar system was formed must be revised or modified. The evolutionary mechanisms to which they were subjected are likely to have changed their chemical composition. 

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... 

Observe the chemical composition of the atmospheres and surfaces of comets, planets and satellites. 

The comet structure model proposed in Figure 6.16 shows clearly that the observation of molecules from Earth must be limited to those molecules present within the coma of the comet, and whilst they originate in part from the structure and composition of the nucleus the molecular observations are of the coma chemistry only. The coma observations will remain until we send a probe to land on the surface of a comet and report back the composition of the core. The Rosetta mission will do just this and we shall see the composition directly from the data it recovers, if successful. 

Some energy is reflected from the body, controlled by a property called the Albedo. Objects with larger albedos reflect more of the incoming light whereas lower albedos absorb more light. Clearly, albedo is a function of wavelength and this must be known for the surface composition of the comet. 

A sample of a comet has not yet been captured, but the elemental composition of a comet has been predicted. How do you think scientists can determine the composition of a distant celestial body, such as a comet ... 

Extraterrestrial materials consist of samples from the Moon, Mars, and a variety of smaller bodies such as asteroids and comets. These planetary samples have been used to deduce the evolution of our solar system. A major difference between extraterrestrial and terrestrial materials is the existence of primordial isotopic heterogeneities in the early solar system. These heterogeneities are not observed on the Earth or on the Moon, because they have become obliterated during high-temperature processes over geologic time. In primitive meteorites, however, components that acquired their isotopic compositions through interaction with constituents of the solar nebula have remained unchanged since that time. 

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). 

The Earth and other planetary bodies have been heavily modified by planetary-scale differentiation, smaller scale melting and the resulting chemical fractionations, collisions that mix material with different histories, and other processes. Samples of these materials are thus not suitable for determining the solar system composition. More primitive objects, such as comets and chondritic meteorites, have compositions more similar to the composition of... 

Elements and molecules emit and absorb photons with characteristic energies. As a result, measurements of stars, comets, or other luminous bodies with a spectrograph, which permits the output to be measured as a function of wavelength, reveal numerous emission or absorption lines (Fig. 4.1). These lines can be used to infer the compositions of the objects. The first spectroscopic measurements of the Sun, stars, and other luminous objects were made in the last half of the nineteenth century. However, it wasn t until the late 1920s that relatively accurate elemental abundances for the Sun and the stars were determined (see Box 4.1). 

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. 

Haack, H. and McCoy, T. J. (2004) Iron and stony-iron meteorites. In Treatise on Geochemistry, Vol. 1. Meteorites, Comets, and Planets, ed. Davis, A. M. Oxford Elsevier, pp. 325-345. This excellent review summarizes the compositions of iron meteorites. 

In this chapter we will consider the cosmochemistry of ice-bearing planetesimals. We will focus first on comets, because more is known about their chemistry than of the compositions of objects still in the Kuiper belt and Oort cloud. We will then explore asteroids whose ices melted long ago, and we will briefly consider some larger icy bodies, now represented by satellites of the giant planets. The importance of ice-bearing planetesimals to cosmochemistry stems from their primitive compositions, which have remained largely unchanged because of hibernation in a frozen state. 

Depending on conditions, frozen substances in comet nuclei can be crystalline ices, amorphous ices, and clathrate hydrates (compounds in which cages in the water-ice lattice can host guest molecules). Compositions of the ices and associated organic materials in comets have been determined from both telescopic and spacecraft observations. Spectral line measurements of gases in a comet s coma allow the identification of molecules and radicals. An inherent difficulty in spectral measurements is that volatiles in the coma are commonly broken... 

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