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References to his computational algorithms are marked by the letter A throughout the text. Investigations of this bombardment are important because these processes may modify the com- position and other properties of the surface. Cometary particles most likely aggregated through condensation of wa- ter and other volatiles in cold K and highly diluted regions in the protosolar nebula Taylor, The range of conductivities adopted for dust in comet models varies over a wide range, reflecting the uncertainty about the real nature of refractory materials and their structure in comets.

From an observational point of view, the distinction between asteroids and comets is not as clear as it was in the past. Skip to main content.

The interaction between gases may therefore be neglected. Re- sults are given for two ice to pore ratios: From Cohen et al. The best spatial resolu- tion 2 pixels is about m at the top part of the image as obtained by the Halley Multicolour Camera on the Giotto spacecraft Courtesy H. Basic Equations orders of magnitude lower. Fortunately, the flux of initially trapped gas dominates in the interior of the nucleus, while the H 2 O flux becomes dominant in a very thin outer layer of the nucleus, where most of the sublimation occurs.


Benkhoff and Spohn a,b showed that in a porous matrix heat transport into the interior by the vapour phase is more effec- tive than heat conduction by the matrix. A different approach to modeling the structure of the dust mantle is to assume that the ice sublimates freely at the nucleus surface, carrying with it the smaller than critical size dust particles, while the larger particles are left behind.

Only after many collisions of fast molecules overtaking slow molecules, many collision mean free paths away from the surface, will the gas again obey a full but 0 drifting Maxwell velocity distribution with a definable temperature T. For suf- ficiently high impurity content the phase transition to crystalline ice can be endothermic.

Cometary material is highly porous. In these experiments, water vapour and gases such as CO, CO2and Ar are codeposited at low temperatures, and amor- phous water ice is obtained trapping the other gases inside. The model results contained in the book are complemented by chapters about the nucleus in general. Samples that were stored isothermally for some tens of hours at temperatures around K gile also undergone significant texture modifications that resulted in increased compressive strength, and thermal conductivity Spohn et al.

We expect that comets will be fragile and compressible relative to compact material because of their high porosity. The resolution gradually degrades to- ward the edges of the frame Courtesy Deep Impact Project.

Sekanina a, Fanale and Salvail The Structure of Comet Nuclei tering necks between dust grains making larger aggregates, which we will call dust particles.

Light-curve analyses can be used also to search for precession and multi-axes spin of a comet nucleus. By introducing differentiation to describe a continuously varying medium, and in the case in which tem- perature variations can be neglected in comparison with pressure variations, Eq. Thus, volatiles will tend to sublimate from convex particle surfaces and tend to recondense on concave areas where particles are in contact with flle other.

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The pressure of gas produced in the interior of the nucleus is also determined by the Clausius-Clapeyron equation at the appropriate temperature, but the gas may be heated in the pores of the matrix material as it escapes.

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We approximtaion conclude that the dust-to-gas ratios observed in comet comae are a result of comet nucleus evolution. Introduction — Observational Overview orbit of Neptune in a disk centered on the ecliptic.

Cubic close packing of spheres. However, in order to obtain an analytical expression for the thermal conductivity of a heterogeneous material, one must adopt a model for its microstructure.

Dust entrainment in the coma has been considered by Huebner For smaller nuclei, spin-up processes are more effective than for large nu- clei such as Chiron.

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Effective Thermal Conductivity 75 Horai,and references therein. Thus, in contrast to most comet nucleus models, we believe that the thermal conductivity of at least the uppermost layers that were heated repeatedly in numerous perihelia is much higher than expected. Work such as that by Ehrenfreund et al.

The trapping mechanism depends on different parameters such as the size of the molecules of the trapped gas, the polariz- ability of the gas, and mechanical blocking of the channels by overlaying ice accumulated during the deposition.

Size and Composition 13 Table 2. The geometry is distorted, as the nu- cleus bulk is represented by a central, isothermal point mass and only the outermost 2 m around the equator are shown.

The Solar System was probably formed by collapse from an interstellar molecular cloud. Several models have been developed, both for the interpretation of observations, and for studies con- nected with space missions.

On the surface, the interstices between them become too small to allow the escape of particles with smaller radii, even if these are smaller than the critical radius.

In this way a large amount of small particles are trapped as well and contribute to the formation of the mantle.

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