Coherent backscattering and opposition effects observed in some atmosphereless bodies of the solar system

The results of photometric and polarimetric observations carried out for some bright atmosphere-less bodies of the Solar system near the zero phase angle reveal the simultaneous existence of two spectacular optical phenomena, the so-called brightness and polarization opposition effects. In a number of studies, these phenomena were explained by the influence of coherent backscattering. However, in general, the interference concept of coherent backscattering can be used only in the case where the particles are in the far-field zones of each other, i.e., when the scattering medium is rather rarefied. Because of this, it is important to prove rigorously and to demonstrate that the coherent backscattering effect may also exist in densely packed scattering media like regolith surface layers of celestial bodies. From the results of the computer modeling performed with the use of numerically exact solutions of the macroscopic Maxwell equations for discrete random media with different packing densities of particles, we studied the origin and evolution of all the opposition phenomena predicted by the coherent backscattering theory for low-packing-density media. It has been shown that the predictions of this theory remain valid for rather high packing densities of particles that are typical, in particular, of regolith surfaces of the Solar system bodies. The results allow us to conclude that both opposition effects observed simultaneously in some high-albedo atmosphereless bodies of the Solar system are caused precisely by coherent backscattering of solar light in the regolith layers composed of microscopic particles.     

Migration of small bodies in the solar system

We investigate several parts of the process of migration of small bodies to the Earth from the asteroid and transneptunian belts. The obtained characteristic times up to collisions of near-Earth objects with the Earth are less than those obtained by other scientists.     

Observations of distant solar system bodies

Multicolour VRI photometry and astrometry of one Centaur and seven Edgeworth-Kuiper objects were obtained. One object, 1994 JQ₁ may be as red as 5145 Pholus, the reddest minor planet previously known. The Centaur 1995 DW₂ has more moderate colour indices, similar to main-belt asteroids. Seven R-magnitudes were obtained for 1994 JS, 1995 FB₂₁, and 1995 GY₇. No light variation above the expected noise is evident, apart from a few outliers. A total of 47 astrometric positions were obtained for the eight objects. The four nights of observations with the ESO New Technology Telescope covered 0.52 square degrees. Two previosuly unknown object, 1995 FB₂₁ and 1995 GY₇, were discovered. We estimate the density of Edgeworth-Kuiper objects brighter than R = 24 to 5.3 per square degree of sky near the ecliptic.     

Shape of small solar system bodies

The morphometric parameters are examined for the shape of fragments of ordinary chondrites, iron meteorites, S- and C-class stony asteroids, metallic asteroids, and icy small bodies of the Solar System. All small Solar System bodies are shown to have, depending on their composition and, hence, physical and mechanical properties, a specific shape that is unique to a given composition. C-class asteroids, the strength of which is almost three times less than that of S asteroids, differ from the latter in their less elongated shape. No systematic change is observed in the morphometric parameters (increased roundness or sphericity) of small bodies of differing compositions depending on their mass, which suggests that the hypothesis of creep in small Solar System bodies is unlikely to be true. The absence of creep confirms that, regardless of their composition, all small Solar System bodies are solid elastic bodies having an ultimate strength (tensile strength and compressive strength) and a yield strength.     

Gravitational potential of solid bodies in the solar system

The gravitational potential of a solid body is expanded without approximation in any moving reference frame, in terms of harmonic coefficients relative to fixed axis in the body.     

Astrometric baseline observations of solar system bodies

The use of ground-based and space baseline observations of Solar System bodies is considered. Baseline observations allow one to determine the distance to observed objects and (in some cases) the parameters of their orbital motion. Certain results of baseline observations of near-Earth asteroids and the results of model analysis of spacecraft observations are presented.     

Examination of light curves of the system RR Lyncis

The eclipsing binary system RR Lyncis is examined on the basis of six light curves analysed by means of iterative optimization techniques. An optimal resultant set of geometric elements for the system is obtained. It is established that there is third light in the system although there are model inadequacies, suggestive of intrinsic variability or circumstellar matter. It is proposed that a third star accounts for this third light and plausible distance limits for such an object are considered.     

A Critical review of theoretical models of negatively polarized light scattered by atmosphereless solar system bodies

About a dozen physical mechanisms and models aspire to explain the negative polarization of light scattered by atmosphereless celestial bodies. This is too large a number for the reliable interpretation of observational data. Through a comparative analysis of the models, our main goal is to answer the question: Does any one model have an advantage over the others? Our analysis is based on new laboratory polarimetric and photometric data as well as on theoretical results. We show that the widely used models due to Hopfield and Wolff cannot realistically explain the phase-angle dependence of the degree of polarization observed at small phase angles. The so-called interference or coherent backscattering mechanism is the most promising model. Models based on that mechanism use well-defined physical parameters to explain both negative polarization and the opposition effect. They are supported by laboratory experiments, particularly those showing enhancement of negative polarization with decreasing particle size down to the wavelength of light. According to the interference mechanism, pronounced negative branches of polarization, like those of C-class asteroids, may indicate a high degree of optical inhomogeneity of light-scattering surfaces at small scales. The mechanism also seems appropriate for treating the negative polarization and opposition effects of cometary dust comae, planetary rings, and the zodiacal light.     

Collective resonant phenomena on small bodies in the solar system

For both asteroids and meteor streams, and also for comets, resonances play a major role for their orbital evolutions but on different time scales. For asteroids both mean motion resonances and secular resonances not only structure the phase space of regular orbits but are mainly at the origin for the inherent chaos of planet crosser objects.For comets and their chaotic routes temporary trapping into orbital resonances is a well known phenomenon. In addition for slow diffusion through the Kuiper belt resonances are the only candidates for originating a slow chaos.Like for asteroids, resonances with Jupiter play a major role for the orbital evolution of meteor streams. Crossing of separatrix like zones appears to be crucial for the formation of arcs and for the dissolution of streams. In particular the orbital inclination of a meteor stream appears to be a critical parameter for arc formation. Numerical results obtained in an other context show that the competition between the Poynting-Robertson drag and the gravitational interaction of grains near the 2/1 resonance might be very important in the long run for the structure of meteor streams.     

Ultraviolet observations of small bodies in the solar system by OAO-2

Broadband filter photometry from 2100 to 4300 Å has been obtained by OAO-2 for the following objects: The Galilean satellites; Titan; the rings of Saturn; and three asteroids. Agreement with independent ground-based photometry in the region of overlap is good. The previously known decrease in reflectivity from visual to ground-based ultraviolet wavelengths continues to 2590 Å for all these objects. Europa's reflectivity continues to decline towards 2110 Å, and the rings' reflectivity levels off from 2590 to 2110 Å. Other targets were too faint at 2110 Å to be measured reliably by OAO-2.The low ultraviolet albedo of Titan has important implications for that planet's atmospheric structure (Caldwell, Larach, and Danielson, 1973; Danielson, Caldwell, and Larach, 1973; Caldwell, 1974b). The ultraviolet reflectivity of Saturn's rings is suggestive of a two-component system, one being pure H₂O particles. The ultraviolet albedos of the Galilean satellites are consistent with existing upper limits for atmospheric abundances, but require either that former estimates of the fractional coverage of H₂O frost are too high, an unlikely circumstance, or that the frost has been darkened by some external agent in the space environment.     

On the sputter alteration of regoliths of outer solar system bodies

This paper discussed several processes that are expected to occur when the porous regoliths on bodies without atmospheres in the outer Solar System are subjected to energetic ion bombardment. The conclusions reached in many of the papers involving sputtering published in the planetary literature are qualitatively or quantitatively incorrect because effects of soil porosity have been neglected. It is shown theoretically and experimentally that porosity reduces the effective sputtering yield of a soil by more than an order of magnitude. Between 90 and 97% of the sputtered atoms are trapped within the regolith, where they are fractionated by differential desorption. Experiments indicate that more volatile species have higher desorption probabilities. This process is the most important way in which alteration of chemical and optical properties occurs when a regolith is sputtered. When a basic silicate soil is irridiated these effects lead to sputter-deposited films enriched in metallic iron, while O, Na, and K are preferentially lost. The Na and K are present in the atmosphere above the sputtered silicate in quantities much greater than their abundances in the regolith. Icy regoliths of SO₂ should be enriched in elemental S and/or S₂O. This prediction is supported by the probable identification of S₂O and polysulfur oxide bands in the IR spectra of H-sputtered SO₂ reported by M. Moore (1984, Icarus 59, 114–128). When porous mixtures of water, ammonia, and methane frosts are sputtered, the loss of H and surface reactions of C, N, and O in the deposits should produce complex hydrocarbons and carbohydrates, some of which may be quite dark. Such reactions may have played a role in the formation of the matrix material of carbonaceous chondrites prior to agglomeration.     

Modelling the outcomes of high-velocity impacts between small solar system bodies

We present a self-consistent numerical algorithm aimed at predicting the outcomes of high-velocity impacts between asteroids (or other small bodies of the solar system), based on a set of model input parameters which can be estimated from the available experimental evidence, and including the possible gravitational reaccumulation of ejected fragments whose velocity is less than a suitably defined escape velocity. All the fragment mass distributions are modelled by truncated power laws, and a possible correlation between fragment ejection velocity and mass is taken into account in different ways, including a probabilistic one. We analyze in particular the effectiveness of the gravitational reaccumulation process in terms of different choices of the collisional parameters and the assumed relationship between fragment speed and mass. Both the transition size beyond which solid targets are likely to reaccumulate a large fraction of the fragment mass and the collision energy needed to disperse most of the fragments are sensitive functions of the assumed fragment velocity versus mass relationship. We also give some examples of how our algorithm can be applied to study the origin and collisional history of small solar system bodies, including the asteroid 951 Gaspra (recently imaged by the Galileo probe) and the asteroid families.     

Analysis of the light curves of the eclipsing system V548 Cygni

The eclipsing binary V548 Cygni was observed photoelectrically in yellow and blue light at Ege University Observatory in 1975. A total of 857 observations were obtained of each colour. Three times of minima were obtained and new light elements were derived. The orbital analysis were made by the methods of Kitamura and Wood. The system has a third body whose total luminosity is greater than that of secondary companion.     

Synthetic analysis of light curves of the close binary system UZ Puppis

BV light curves obtained by Bookmyer (1985) have been analysed with Yamasaki's synthetic technique. As some small difference exists between the light maxima, analysis has been made for the first and the second half of light curve, separately. The result indicates that system parameters deduced from the second half give a better fitting with smaller O-C residuals. These parameters reveal that UZ Pup is contact to the Roche lobe.     

Very small bodies in the solar system: The Earth's atmosphere as detector

Particles of mass less than about 1 gm are a minor fraction of the total matter impinging on the Earth averaged over millennia time scales. However, these particles dominate during a single particular year and produce the most obvious evidence of incoming extra-terrestrial matter in the form of ablation trails in the atmosphere which are visible at night as meteors.Observations of meteors give astronomical information on the composition, structure, and cometary associations of the particles. The composition is deduced from optical spectra of meteors, whilst telescopic studies of the trails during formation give information on the physical structure of the particles. Any cometary associations are deduced from measurement of meteor orbits determined photographically, using television, or by radar.Meteors occur in the atmosphere at heights from about 70 to 120 km. Optical observations are restricted to night-time and usually under conditions of low moonlight. A typical television based detector can record +8^M meteors with a sporadic rate of 15–20 per hour and velocities accurate to about 3%. The luminosity of the trail is strongly dependent on the velocity of the meteoroid (to about the third power).Radar observations of meteors are unrestricted by weather or time of day, and can readily detect meteors at least two orders of magnitude smaller in mass than those detectable optically. Again the observations are heavily biased toward the higher velocities as the electron line density varies approximately asV ^3.5. However, the higher the velocity of the meteoroid the greater the height of the meteor trail, and the reduced probability of radar detection due to rapid diffusion of the trail. Thus radar observations tend to select meteors in the intermediate velocity range 30–40 km s^–1.     

Main Belt Comets: A new class of small bodies in the solar system

Comets and asteroids have traditionally been considered two distinct separate populations of small bodies in the solar system, according to their different dynamical, observational, and compositional characteristics.The discovery of a new class of objects, the so-called, Main Belt Comets (MBCs), exhibiting a clear cometary activity but having at the same time orbits indistinguishable from the ones of asteroids in the Main Belt provided further evidence that asteroids and comets, rather than two distinct separate classes, represent the end-members of a continuum of small bodies, with compositions from the very rocky to the very icy.Their study is nowadays deepening our knowledge of the formation mechanisms of the solar system and of the distribution of volatile materials in the protoplanetary disk.In this paper the present knowledge of MBCs is reviewed in terms of physical properties derived from observations, dynamical studies about the origin and formation, thermal modeling of the nuclei, investigations about the activation mechanisms, and the eventual contribution to the presence of water on our planet. An overview of the large-scale surveys dedicated to their discovery and of the detection techniques used so far is also given. Moreover, open question and indications for future observations and modeling are outlined.     

Dipolar magnetic moment of the bodies of the solar system and the Hot Jupiters

Context

The planets magnetic field has been explained based on the dynamo theory, which presents as many difficulties in mathematical terms as well as in predictions. It proves to be extremely difficult to calculate the dipolar magnetic moment of the extrasolar planets using the dynamo theory.

Objective

The aim is to find an empirical relationship (justifying using first principles) between the planetary magnetic moment, the mass of the planet, its rotation period and the electrical conductivity of its most conductive layer. Then this is applied to Hot Jupiters.

Method

Using all the magnetic planetary bodies of the solar system and tracing a graph of the dipolar magnetic moment versus body mass parameter, the rotation period and electrical conductivity of the internal conductive layer is obtained. An empirical, functional relation was constructed, which was adjusted to a power law curve in order to fit the data. Once this empirical relation has been defined, it is theoretically justified and applied to the calculation of the dipolar magnetic moment of the extra solar planets known as Hot Jupiters.

Results

Almost all data calculated is interpolated, bestowing confidence in terms of their validity. The value for the dipolar magnetic moment, obtained for the exoplanet Osiris (HD209458b), helps understand the way in which the atmosphere of a planet with an intense magnetic field can be eroded by stellar wind. The relationship observed also helps understand why Venus and Mars do not present any magnetic field.     

Meridian observations of solar system bodies with the struveertel instruments of the Pulkovo Observatory

Two Struve-Ertel instruments were used for the daytime observations of the Sun, Mercury, Venus and Mars at Pulkovo from 1956 to 1976. The FK4 equinox and equator corrections were derived. Both the instruments were installed in 1983–1986 at the Kislovodsk Station of the Pulkovo Observatory. The atmospheric dispersion and lateral refraction have been estimated at the Station.     

Trails of solar system minor bodies on WFC/ACS images

In this paper, we analyse very short arcs of minor bodies of the Solar System detected on Hubble Space Telescope (HST) Wide Field Channel ACS images. In particular, we address how to constrain the Keplerian orbital elements for minor body detections, illustrating the method for two objects. One of the minor bodies left 13 successive trails, making it the most well-sampled object yet identified in the HST archive. Most interestingly, we also address the problem of ephemeris prediction and show that in the particular case of HST very short arcs the confinement window for subsequent recovery is significantly reduced to a narrow linear region, that would facilitate successive observations.