Cometary nucleus

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... 

Determination of the chemical, mineralogical and isotopic compositions of volatiles and refractories in the cometary nucleus. 

In the 1950 s, three major concepts have brought d isive advances to the physics of comets Oort s model of a distant cometary cloud, Biermann s solar wind theory of tail formation and dynamics, and Whipple s icy conglomerate model of the cometary nucleus. The main ideas of these concepts shall be outlined briefly. 

The size of a cometary nucleus cannot be measured directly, since even in the largest telescopes it remains an unresolved point of light. Photometric brightness measurements of comets still far away from the Sun before a radiating halo has formed, together with a phase law and a plausible value for the albedo, yield diameters of the order of 1-20 km (Roemer ). Periodic comets are, on the average, smaller than new ones, since they lose about 0.1 % of their masses per revolution. 

Jewitt D. C. (2002) From Kuiper Belt object to cometary nucleus the missing ultrared matter. Astron. J. 123, 1039-1049. 

Meteor showers occur when Earth passes through the tube-like structure of meteoroids left in the wake of a comet. Such meteoroid tubes, or as they are more commonly called meteoroid streams, are formed after a comet has made many repeated passages by the Sun. Meteoroid streams are composed of silicate (i.e. rocky) grains that were once embedded in the surface ices of a parent comet. Grains are released from a cometary nucleus whenever solar heating causes the surface ices to sublimate. New grains are injected into the meteoroid stream each time the comet passes close by the Sun. 

The spatial distribution of circular polarization along the cut through the coma and nucleus in the solar and antisolar directions was investigated for comet S4 (LINEAR) [86]. Its maximum value reached up to 1%. At most times, the degree of circular polarization at the cometary nucleus was close to zero. The left handed as well as right handed polarization was observed over the coma although left-circularly polarized light was systematically observed in the sunward part of the... 

The international Rosetta comet rendezvous mission is designed to perform a detailed investigation of a comet in our solar system. As part of the core payload for this mission, the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) will determine the elemental, isotopic, and molecular composition of the atmospheres and ionospheres of comets as well as the temperature and bulk velocity of the gas and ions and the homogenous and inhomogenous reactions of gas and ions in the dusty cometary atmosphere and ionosphere [78]. More specifically, the global molecular, elemental, and isotopic composition and the physical, chemical and morphological character of the cometary nucleus will be determined. In addition, Rosetta will elucidate the processes by which the dusty cometary atmosphere and ionosphere are formed and characterize their dynamics as a function of time, heliocentric, and cometocentric positions. 

Since a cometary coma is nearly transparent at radio wavelengths, radar is much more capable of unambiguous detection of a cometary nucleus than are optical and infrared methods, and radar observations of several comets (see Table I) have provided useful constraints on nuclear dimensions. The radar signature of one particular comet (IRAS-Araki-Alcock, which came within 0.03 AU of Earth in May 1983) revolutionized our concepts of the physical nature of these intriguing objects. Echoes obtained at both Arecibo (Fig. 26) and Goldstone have a narrowband component from the nucleus as well as a much weaker broadband component from large particles ejected mostly from the sunlit side of the nucleus. Models of the... 

Radio astronomy of comets has made its most significant advances in the area of spectroscopy. Many so-called parent molecules (molecules which sublimate directly off the nucleus, in contrast to daughter molecules, which are products of the parents) have been observed at radio wavelengths through their rotation lines in the millimeter and submillimeter bands. Observations of these spectral lines give valuable information on the icy composition of the cometary nucleus, gas production rates, physical conditions in coma, and variation of these in time and heliocentric distance. 

The impact also showed that the cometary nucleus is extremely porous and that the ice was close to the surface but below a devolatilized layer with thickness of order the impactor diameter. The impact crater was about 100 m the impact itself did not change the orbital parameters of the object. The surface of the nucleus revealed layered structures, signs of outbursts, many of them correlated with the rotational period and also impact craters. A review about the results was given recently by A Hearn, 2008 [4]. 

Abstract. Cometary nuclei are composed mostly of water ice and mineral grains. When a comet passes near the Sun, several processes modify the subsurface layer of the nucleus which, according to theory, produce more cohesion and stratification. Presently a Rosetta mission is being prepared to comet 46P/Wirtanen. Plans are to land on the nucleus and penetrate its subsurface layer with a mechanical tool (experiment Mupus). Thus, understanding of the processes responsible for the evolution of a cometary nucleus is of key importance for the success of the mission and for interpretation of the results. This work is intended to estimate how quickly the process of grain sintering could modify the outer part of nucleus. The numerical model is adopted from previous work by Kossacki [1] however, at present the evolution of the cometary orbit is included. 

At large distances from the Sun, a cometary nucleus is cold and can evolve only owing to absorbtion of cosmic radiation. However, as the comet approaches the Sun, the surface of its nucleus is warmed, and various processes start modifying the properties of the cometary matter. Some of them tend to increase the cohesiveness of the material, but others have the opposite result. Cometary simulation experiments predict that the metamorphic processes increasing the strength of granular ice should dominate over those that reduce its cohesiveness [3],... 

The cometary nucleus is composed of materials of difierent volatility. Thus, when the surface of the nucleus is heated, its outer layer becomes depleted of the volatile components. This is because of sublimation followed by partial escape to space and partial migration toward the centre of the nucleus. Since the less volatile component of a cometary matter is mineral dust, the stratification process leads to the formation of a possibly very porous [6], [7], [2] dust cover on the surface of the nucleus. The dust mantle evolves in a very complex way. The sublimation of volatiles removes nonmineral components from the cometary matter below the dust layer, which makes the dust cover grow from the bottom. However, the sublimated vapor flowing out to space blows out the mineral grains. This results in the continuous loss of the mantle mass. Altogether, the escape of ice and mineral components of the cometary nucleus leads to the gradual decrease of its radius. 

It is assumed that the dust layer is eroding at the same rate as it is growing owing to the movement of its bottom (sublimation front of water ice) toward the center of cometary nucleus. The thickness of the dust mantle is therefore constant. The surface temperature is calculated according to the energy balance equation accounting for the absorption of solar radiation, the IR emission, and heat conduction through the dust mantle. 

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