Evidence of the complex structure of asteroid 21 Lutetia

We discuss the results of the analysis of three sets of observations of asteroid 21 Lutetia—spectrophotometry, simultaneous BVR photometry, and spectrometry—which show that the asteroid is not a monolithic body. The frequency analysis of the B-V and V-R color indices and the V values, which were obtained from simultaneous BVR measurements in 2004 and calculated from the spectrophotometric observations performed in 2000 (the synthetic values and the color indices), allowed us to demonstrate that the known rotation period of 8.^h172 of the asteroid does not exist at all. At a rather high confidence level, six new periods were found: 2.^h0, 2.^h93, 16.^h8, 1.^d25, 3.^d25, and 60^d. During spectral observations with a 1.25-m telescope at the southern laboratory of the Sternberg Astronomical Institute in Nauchnyi (Crimea) in 2004, the spectra of two components spaced 2.8″ apart were registered. In the short-wavelength spectral range, quick variations of the reflectance of the components were observed. They show the changes in their spectral types from S to C. The analysis of the synthetic values of the color indices determined from the spectrophotometric observations in 2000 confirmed the presence of quick spectral variations. We conclude that asteroid 21 Lutetia is a complex satellite system. This statement is confirmed by the analysis of data published in different sources.     

On the nature of unidentified cometary emission lines

The nature of unidentified cometary emission lines is discussed. A model of ice particles in cometary halos as a mixture of frozen polycyclic aromatic hydrocarbons (PAHs) and acyclic hydrocarbons is considered. The properties of frozen hydrocarbon particles are described and 5–7% of the unidentified cometary emission lines are considered as the photoluminescence of frozen hydrocarbons. The positions of unidentified emission lines in the spectrum of Comet 19P/Borrelly are compared with the positions of quasi-lines in the photoluminescence spectra of PAHs that were dissolved in acyclic hydrocarbons at a temperature of 77 K and that constitute a polycrystalline solution.     

Capture of comets from the Oort cloud into Halley-type and Jupiter-family orbits

This paper analyzes the capture of comets into Halley-type and Jupiter-family orbits from the nearparabolic flux of the Oort cloud. Two types of capture into Halley-type orbits are found. The first type is the evolution of near-parabolic orbits into short-period orbits (with heliocentric orbital periods P < 200 years) as a result of close encounters with giant planets. This process is followed by a very slow drift of cometary orbits into the inner part of the Solar System. Only those comets may pass from short-period orbits into Halley-type and Jupiter-family orbits, which move in orbits with perihelion distances q < 13 au. In the second type of capture, the perihelion distances of cometary orbits become rather small (< 1.5 au) during the first stage of dynamic evolution under the action of perturbations from the Galaxy, and then their semimajor axes decrease as a result of diffusion. The capture takes place, on average, in 500 revolutions of the comet about the Sun, whereas in the first case, the comet is captured, on average, after 12500 revolutions. The region of initial orbital perihelion distances q > 4 au is found to be at least as important a source of Halley-type comets as the region of perihelion distances q < 4 au. More than half of the Halley-type comets are captured from the nearly parabolic flux with q > 4 au. The analysis of the dynamic evolution of objects moving in short-period orbits shows that the distribution of Centaurs orbits agrees well with the observed distribution corrected for observational selection effects. Hence, the hypothesis associating the origin of Centaurs with the Edgeworth-Kuiper belt and the trans-Neptunian region exclusively should be rejected.     

Note on the structure of comet nuclei

The recent developments in cometary studies suggest rather low mean densities and weak structures for the nuclei. They appear to be accumulations of fairly discrete units loosely bound together, as deduced from the observations of Comet Shoemaker–Levy 9 during its encounter with Jupiter. The compressive strengths deduced from comet splitting by Öpik and Sekanina are extremely low. These values are confirmed by theory developed here, assuming that Comet P/Holmes had a companion that collided with it in 1892. There follows a short discussion that suggests that the mean densities of comets should increase with comet dimensions. The place of origin of short-period comets may relate to these properties.     

Interaction of particles in the near field and opposition effects in regolith-like surfaces

The explanation of the opposition effects observed in brightness and polarization in different celestial bodies and laboratory samples is still far from being complete. The shadow hiding and coherent backscattering mechanisms are mentioned most frequently in this connection. In the present work, we consider one more scattering mechanism—the interaction of particles in the near field—and its influence on the brightness and polarization of light scattered by ensembles of particles at small phase angles. First, we analyze two manifestations of this mechanism: the field inhomogeneity in the vicinity of the scatterers and the shielding of particles by each other at distances compared with their sizes. Then, we use the model regolith described as an ensemble of clusters as constituents and compare the contributions of the coherent backscattering and the near-field effect to the intensity and polarization of light when the porosity of the ensemble is varied. The modeling confirms that the phase dependences of the intensity and polarization of light scattered by complex structures in the backscattering domain is mainly caused by these two mechanisms. The coherent backscattering works more effectively in sparse media, while the near-field effect manifests itself in more compact ensembles of wavelength-sized particles. However, it is difficult to distinguish quantitatively their contributions, even in models of simple structures. A number of observations, especially of moderate- and low-albedo objects, can be explained only by invoking the near-field effect.     

Laboratory simulation of the physical processes occurring on and near the surfaces of comet nuclei

Abstract— Laboratory comet simulation experiments are discussed in the context of theoretical models and recent ground-based and spacecraft observations, especially the Giotto observations of P/Halley. The set-up of various comet simulation experiments is reviewed. A number of small-scale experiments have been performed in many laboratories since the early 1960s. However, the largest and most ambitious series of experiments were the comet simulation experiments known as KOSI (German = Kometen Simulation). These experiments were prompted by the appearance of Comet P/Halley in 1986 and in planning for the European Space Agency's Rossetta mission that was originally scheduled to return samples. They were performed between 1987 and 1993 using the German Space Agency's (DLR) space hardware testing facilities in Cologne. As with attempts to reproduce any natural phenomenon in the laboratory, there are deficiencies in such experiments while there are major new insights to be gained. Simulation experiments have enabled the development of methods for making comet analogues and for exploring the properties of such materials in detail. These experiments have provided new insights into the morphology and physical behavior of aggregates formed from silicate grains likely to exist in comets. Formation of a dust mantle on the surfaces and a system of ice layers below the mantle caused by chemical differentiation have been identified after the insolation of the artificial comet. The mechanisms for heat transfer between the comet's surface and its interior, the associated gas diffusion from the interior of the surface, and compositional, structural, and isotopic changes that occur near the surface have been described by modeling the experimental results. The mechanisms of the ejection of dust and ice grains from the surface and the importance of gas-drag in propelling grains have also been explored.     

Some consequences of a phase transition of water ice on the heat balance of comet nuclei

We consider spheres of water ice of about 1 km in radius moving on three different orbits with a common perihelion distance of 8 AU. As evaporation is negligible in these cases, we call them inactive ice bodies. The surface temperature has been numerically calculated for two extreme situations: (1) The spheres are composed of amorphous ice with a heat conduction to the interior presumed to be negligible. (2) The spheres are composed of compact hexagonal ice with a heat conduction coefficient known from laboratory experiments. Whereas in case 1 the temperature is an unambiguous function of heliocentric distance, in case 2 we observe a thermal “hysteresis” and the maximum temperature has a phase lag with respect to perihelion. The perihelion temperature depends on the eccentricity of the orbit. The case of active ice bodies is also discussed. We come to the conclusion that an ice body moving on the orbit of Tempel 2 must contain crystalline ice and the variations of the surface temperature must be smoothed out in an important way. In the case of Halley's orbit, we suppose that the center of the ice body still contains large amounts of amorphous ice.     

Magnetic confinement, magnetohydrodynamic waves and smooth line profiles in active galactic nuclei

In this paper, we show that if the broad-line region clouds are in approximate energy equipartition between the magnetic field and gravity, as hypothesized by Rees, there will be a significant effect on the shape and smoothness of broad emission-line profiles in active galactic nuclei. Linewidths of contributing clouds or flow elements are much wider than their thermal widths, because of the presence of non-dissipative magnetohydrodynamic waves and their collective contribution produce emission-line profiles broader and smoother than would be expected if a magnetic field were not present. As an illustration, a simple model of isotropically emitting clouds, normally distributed in velocity, is used to show that smoothness can be achieved for less than ∼8×10⁴ clouds and may even be as low as a few hundred. We conclude that magnetic confinement has far-reaching consequences for observing and modelling active galactic nuclei.     

Numerical simulation of high-velocity impact ejecta following falls of comets and asteroids onto the Moon

High-velocity comet and asteroid impacts onto the Moon are considered and the material masses ejected after such impacts at velocities above the second-cosmic velocity for the Moon (2.4 km/s) are calculated. Although the results depend on a projectile type and the velocity and angle of an impact, it has been demonstrated that, on average, the lunar mass decreases with time. The Moon has lost about 5 × 10¹⁸ kg, that is, about one-hundredth of a percent of its mass, over the last 3.8–3.9 billion years. The ejection of lunar meteorites and lunar dust, rich in ³He, is considered as well. The results of the study are compared to the results of earlier computations and data on lunar meteorites.     

The effect of internal inhomogeneity on the activity of comet nuclei - Application to Comet 67P/Churyumov-Gerasimenko

We present thermal evolution calculations of inhomogeneous asymmetric initial configurations of a spherical model of Comet 67P/Churyumov-Gerasimenko, using a fully 3-dimensional numerical code. The initial composition is amorphous H₂O ice and dust, in a “layered-pile” configuration, where layers differing in ice/dust ratio and thermal properties extend over a fraction of the surface area and about 10 m in depth and may overlap. We analyze the effect of one such layer, as well as the combined effect of many layers, randomly distributed. We find that internal inhomogeneities affect both the surface temperature and the activity pattern of the comet. In particular, they may lead to outbursts at large heliocentric distances and also to activity on the night-side of the nucleus. The rates of ablation and depths of dust mantle and crystalline ice outer layer as functions of longitude and latitude are shown to be affected as well.     

Fluidization and multiphase transport of particulate cometary material as an explanation of the smooth terrains and repetitive outbursts on 9P/Tempel 1

The Deep Impact mission discovered repetitive outbursts on Comet 9P/Tempel 1 and the presence of several smooth terrains on its surface. We present new measurements of the extent of the smooth terrains, the slopes along their centerlines, and the areas of their likely source regions. Our analysis of these features indicates that they are <700 orbits old and probably the result of an ongoing process. The implications of the recently found locations of the source regions of the repetitive outbursts are also analyzed. We propose that the origins of these phenomena are in the different regimes of fluidization and gas transport in a weakly bound particulate mixture of ice and dust above an assumed amorphous/crystalline H₂O phase change boundary where CO and/or CO₂ gas is released. The depth of this boundary is estimated to lie between 30 and 100 m below the surface. The smooth terrains are visualized as occurring about once every ∼70 orbits at random locations of the nucleus where a spurt in CO production occurs over a limited region of the phase change boundary. The weak (tensile strength ) crystalline and dust overburden is locally ruptured and fluidized by the CO gas pressure and is then extruded onto the surface at speeds of ∼0.003-0.03 m/s, well below the escape velocity of 1.3 m/s. Once on the surface a base pressure of only 2.5 Pa is required to ensure fluidization of the extruded material and it can remain fluidized for typically ∼20 h against diffusive loss of CO. As the material accelerates down the local topography it deflates due to diffusive gas loss. The flow becomes increasingly viscous until it is no longer fluidized at which point it quickly halts forming a terminal scarp. The mean speed of the laminar flow is estimated at 0.3 m/s for an emplacement time of ∼3 h. Topographic features on the flow >0.3 m in size should become fully relaxed during the emplacement time explaining the smooth texture seen in the images. In contrast, the repetitive outbursts require a gas-laden reservoir to have formed in the vicinity of the phase change boundary well below their preferred location. We visualize the outbursts to be the result of either spouting or bubble transport to the surface where the release of gas is diurnally modulated by either thermal stresses or H₂O sublimation back pressure. The source region for the i2 smooth terrain is found to coincide with an H₂O-ice rich area and we propose that the process of elutriation, i.e., the separation of different classes of particulates depending on their drag properties, occurs in the fluidized material as it flows up to and through the surface. In this way the material becomes enhanced in H₂O crystals relative to siliceous and carbonaceous particulates.     

On the survivability of an enantiomeric excess of amino acids in comet nuclei during the decay of26Al and other radionuclides

We present a computer model calculation for the racemization of a possible excess of amino acids in the icy fraction of comet nuclei bring about by ionizing radiation released during the decay of²⁶Al,⁴⁰K,²³⁵U,²³⁸U and²³²Th. The model takes into account a total of 110 chemical reactions, of which 91 are needed to explain the radiation chemical processing of the major constituents of comet nuclei (Navarro-Gonzálezet al., 1992) and 19 are necessary to simulate the radiolysis of glycine/alanine mixtures in aqueous solutions (Navarro-Gonzálezet al., 1994 and 1996). It is predicted that an enantiomeric excess of alanine would not be destroyed by radioracemization during the decay of embedded radionuclides. Nevertheless, this enantiomeric excess could be attenuated by the formation of racemic amino acids in the interior of comet nuclei as a result of the radiation-induced polymerization of HCN.     

The Morphology of Small Craters and Knobs on the Surface of Jupiter's Satellite Callisto

Using the images of Callisto's surface acquired at 15-km resolution by the Galileo spacecraft during its C21 orbit, we studied the morphology of craters with diameters of less than 1–2 km and knobs. By analogy with other regions of Callisto that have been studied, these craters and knobs are thought to be formed by the sublimation degradation of the rims of larger craters that are also present in the region under study. The small craters closely resemble similar-sized lunar craters and, by analogy with the latter, are also divided into morphological classes. The depths of 42 craters of different morphological classes are estimated using shadow lengths visible in the craters. The fractions of the craters of different classes in the subpopulation are determined as a function of the crater diameter. Evidence has been obtained that larger craters degrade at a slower rate than smaller ones. The mean thickness of the mantle of dark material (40 m) is estimated from the sizes of the craters ejecting the blocks of the basement ice material. The shape of the knob shadows shows that the knobs are heights of mostly conical form with slopes whose steepness is close to the angle of repose. Analysis has shown that the observed landforms and material units of the region under investigation have been formed during two successive stages of the geologic history of Callisto. Large craters, knobs, and the mantle of dark material were formed mostly at the end of the period of heavy meteorite bombardment. The leading processes of this period are impact cratering, the sublimation of Callisto's crustal ice with the accumulation of residual non-icy material, and downslope mass movement. The next stage, which continues until the present time, involved the formation of the subpopulation of small (<1–2 km) craters. This formation was accompanied by the impact reworking of the upper portion of the dark mantle. The key processes occurring at this stage are impact cratering and downslope mass movement. The mean intensity of resurfacing at this stage is much lower than at the preceding stage.     

Formation and evolution of the chaotic terrains by subsidence and magmatism: Hydraotes Chaos, Mars

The origin of the martian chaotic terrains is still uncertain; and a variety of geologic scenarios have been proposed. We provide topographic profiles of different chaos landscapes, notably Aureum and Hydraotes Chaos, showing that an initial shallow ground subsidence occurred at the first step of the chaos formation. We infer that the subsidence was caused by intrusion of a volcanic sill; which could have produced consequent melting as well as release of ground water from disrupted aquifer. Signs of a volcanic activity are observed on the floor of Hydraotes Chaos, a complex and deep depression located at the junction of three channels. The volcanic activity is represented by small, 0.5 to 1.5 km diameter, rounded cones with summit pits. The cone's size and morphology, as well as the presence of possible surrounding lava flows, suggest that they are primary volcanic cones similar to terrestrial cinder cones. The identification of volcanic activity on the deepest chaos, where the lower crustal thickness and the faults/fractures system contributed to the magma rising, reveals that magmatic activity, proved by the cones, and possibly help by structural activity, has been a major factor in the formation of chaotic terrains.