Analysis And Comparison Of Different Estimations Of Mars' Polar Motion

We have summed up three recent modellings of Mars' polar motion (Hilton, 1992; Yoder and Standish, 1997; Zhang, 1998) from which we have studied in particular the Chandler wobble (CW) of the planet and expressions of its period T_CW. We add a fourth study (Kubo, 1991) which deals with the polar motion on the Earth, but whose the results can easily be applied to Mars. To conpare the different expressions of the period T_CW from the authors above, we have chosen common parameters of the planet coming from recent data and we have calculated values of the period corresponding to these parameters after having determined from analytical developments the moments of inertia of the core and of the mantle. We show how the different formula proposed by the authors lead to different values of T_CW and we study in particular the contribution of somc parameters such as the core radius, the mass of the core and the Love number of these values.     

Escape Of Planetary Atmospheres Including The Effects Of Rotation And Orbital Motion Of The Planet

The problem of escape of planetary atmospheres is discussed taking into account the effects of planet's rotation and planet's orbital momentum. The expressions for the rates of loss of mass and angular momentum per unit area and the total rates of loss of mass and angular momentum for the whole planetary surface are obtained. These expressions are, then, applied to study the escape of the constituents of the atmospheres of Mars and Venus. This revised version was published online in July 2006 with corrections to the Cover Date.     

Observational Constraints On Surface Characteristics Of Comet Nuclei

Direct observations of the nuclear surfaces of comets have been difficult; however a growing number of studies are overcoming observational challenges and yielding new information on cometary surfaces. In this review, we focus on recent determinations of the albedos, reflectances, and thermal inertias of comet nuclei. There is not much diversity in the geometric albedo of the comet nuclei observed so far (a range of 0.025 to 0.06). There is a greater diversity of albedos among the Centaurs, and the sample of properly observed TNOs(2) is still too small. Based on their albedos and Tisser and invariants, Fernández et al. (2001) estimate that about 5% of the near-Earth asteroids have a cometary origin, and place an upper limit of 10%. The agreement between this estimate and two other independent methods provide the strongest constraint to date on the fraction of objects that comets contribute to the population of near-Earth asteroids. There is a diversity of visible colors among comets, extinct comet candidates, Centaurs and TNOs. Comet nuclei are clearly not as red as the reddest Centaurs and TNOs. What Jewitt (2002) calls ultra-red matter seems to be absent from the surfaces of comet nuclei. Rotationally resolved observations of both colors and albedos are needed to disentangle the effects of rotational variability from other intrinsic qualities. New constraints on thermal inertia of comets are consistent with previous independent estimates. The thermal inertia estimates for Centaurs 2060 Chiron and 8405 Asbolus are significantly lower than predicted by thermal models, and also lower than the few upper limits or constraints known for active, ordinary nuclei.     

Development Of The Hamiltonian Function Using The Jacobi System Of Reference

In this paper we describe two different methods to expand the second term of the planetary Hamiltonian function. The Jacobi system of coordinates is adopted leading to a unique evaluation of the Hamiltonian. Previous analytical or semi-analytical planetary theories suffer from the drawback of computing the perturbation function for each planet, which is quite cumbersome. The inclinations of planets are referred to a common fixed plane and the longitudes to a common origin. This is necessary when we deal with n > 2 planets. The treatment is straightforward, and no complexities appear throughout the analysis. This revised version was published online in July 2006 with corrections to the Cover Date.     

An Overview Of The Origin Of Life: The Case For Biological Prospecting On Mars

Studies of the Earth's earliest biosphere have suggested a close coupling between the evolution of early life forms and the physical and chemical evolution of the planetary surface. From a biological perspective there were many similarities between early Earth and early Mars. This has led to the idea that an origin of life event may have occurred on Mars, leading to the development of microbial life. Various theories have been advanced to explain the origin of life on Earth, and these are reviewed with relevance to Mars. If traces of past or present biogenic activity are to be found on Mars, then the most likely place to prospect is several kilometers below the surface where liquid water might be stable. Such prospecting may best lend itself to human exploration. This revised version was published online in July 2006 with corrections to the Cover Date.     

On The Low Frequency Quasi Periodic Oscillations Of X-Ray Sources

Based on the interpretation of the twin kilohertz Quasi Periodic Oscillations (kHz QPOs) of X-ray spectra of Low Mass X-Ray Binaries (LMXBs) ascribed to the Keplerian and the periastron precession frequencies at the inner disk respectively, we ascribe the low frequency (0.1–10 Hz) Quasi Periodic Oscillations (LFQPO) and HBO (15–60 Hz QPO for Z sources or Atoll sources) to the periastron precession at some outer disk radius. It is assumed that both radii are correlated by a scaling factor of 0.4. The conclusions obtained include: All QPO frequencies increase with increasing accretion rate. The theoretical relations between HBO (LFQPO) frequency and the kHz QPO frequencies are similar to the measured empirical formula.     

The Effect Of Spin-Gravity Interaction On The Wave Lengths Of Photons Moving In The Earth's Gravitational Field

It is pointed out that there exist real discrepancies between the used theories and experimental measurements in two phenomena on astronomical and laboratory scales. Those two phenomena are concerned with the motion of spinning particles in gravitational fields. A thurough review is presented on different theoretical possibilities of searching for a feasable solution for the existing puzzling discrepancies. One theoretical possibility; in which one may find an interpretation for these discrepancies; is exposed. This possibility is based upon taking into considerations, spin- gravity interaction when spinning particles are moving in gravitational fields. The obtained results; due to such possibility; are applied to calculate the effect of the Earth's gravitational field on the wave lengths of photons carrying information between two points having different gravitational potential. This revised version was published online in July 2006 with corrections to the Cover Date.     

On Pulsar Electrodynamics in Rotating Frames

In this paper we have considered a rotating, perfectly conducting sphere and have calculated the electric and magnetic field distributions measured by the rotating observer using the anholonomic approach. The calculations have been done for the following two cases: (1) rotating charged spherical shell and (2) a uniformely magnetized sphere. We have shown that in the limiting situation (a/c)² 1 and 1, the magnetic field distribution is the same for both observers, inertial and noninertial. The expressions obtained for the electric field components in the rotating frame have been compared with the corresponding expressions in the inertial frame, where the observer is at rest. Some of the results are in agreement with Post's approach to noninertial electrodynamics.     

Spectroscopy Of Comet Hale-Bopp In The Visible/Near Infrared: Modeling Of Dust Properties

Previous analysis of imaging spectroscopy data in the 0.4–1 μm spectral range of comet Hale-Bopp, have shown the presence of two regions on the sunward and antisunward sides of the nucleus exhibiting different continuum emission (Bellucci, 1998, hereafter paper I). In this work we present the modeling of the continuum emission in terms of size distribution and composition of the dust grains. The spectra are fitted by micron sized olvine grains. A mechanism to explain the spatial gradient is also proposed. This revised version was published online in July 2006 with corrections to the Cover Date.     

A Study Of Internal Structure Models And Dynamical Parameters Of Ganymede

Jupiter's satellite Ganymede is the largest natural satellite in the Solar System. As a result of the close encounter of Ganymede by the Galileo spacecraft in June and September 1996, the second zonal J₂ and the second sectorial C₂₂ Stokes parameters are now well determined (Anderson et al., 1996). Using the updated geodetic parameters, we have constructed a group of models for the internal structure of Ganymede, and have estimated some dynamical parameters for these models. A comparison with the Moon is made. The conclusion that can be drawn from this study is that, whereas Ganymede at present is in a state of hydrostatic equilibrium, this is certainly not the case for the Moon. This revised version was published online in July 2006 with corrections to the Cover Date.     

On The Internal Structure Of Starless Cores. Physical and Chemical Properties of L1498 and L1517B

We have characterized the internal structure of two close-to-round starless cores in Taurus, L1498 and L1517B, setting constraints on the initial conditions of star formation and on models of core condensation. Our analysis is based on high angular resolution observations in at least two transitions of NH₃, N₂H⁺, CS, C³⁴S, C¹⁸O, and C¹⁷O, together with maps of the 1.2 mm continuum. For both cores, we derive radial profiles of constant temperature and constant turbulence, and density distributions close to those of non-singular isothermal spheres. Using a Monte Carlo radiative transfer model, we derive abundance profiles for all species and find a pattern of strong chemical differentiation. NH₃ has a higher abundance toward the core centers while N₂H⁺ has a constant abundance over most of the cores. Both C¹⁸O and CS (and isotopomers) are strongly depleted in the core interiors, most likely due to their freeze out onto cold dust grains. Concerning the kinematics of the dense gas, we find (in addition to constant turbulence) a pattern of internal motions at the level of 0.05 km s^?1. These motions seem correlated with asymmetries in the pattern of molecular depletion, and we interpret them as residuals of core contraction. Their distribution and size suggest that core formation happens in rather irregular manner. A comparison with supersonic turbulence models of core formation shows that our observed cores are much more quiescent than allowed by these models.     

Analysis Of The Disappearing Filament And Flare Of 7 May 1992

By using Yohkoh soft X-ray data, H filtergrams, and radio data, the activation of the disappearing filament and the flare eruption on 7 May 1992 have been studied. Main conclusions are as follows: (1) the emergence of new magnetic flux tends to affect the pre-existing X-ray loops, which usually appear in arcades spanning H filament, changing the magnetic environment of the filament, and then enhance the current in the filament. Therefore newly emerging flux plays a fundamental role in the destabilization of this filament. (2) According to the H data and the rising motion of the filament, the corresponding current variation in the filament has been calculated. It seems that the current interruption may be a possible trigger mechanism for this filament disappearance. (3) The magnetic field strength and the energy flux of energetic electrons in the source region of microwave bursts have been estimated by using the microwave spectrum. During the main phase, the mean magnetic strength and the energy flux of energetic electrons are about 300–400 G and 1×1011 erg cm–2 s –1, respectively. (4) The energy provided by reconnection of the current sheet and the total energy of the current filament are estimated and we show that there is enough energy stored in the filament to feed the 7 May, 1992 flare.     

On the relationships between impact crater diameters and projectile kinetic energy

By reforming a series of expressions derived by Öpik (1969), we have shown explicitly the dependence of impact crater diameter on projectile kinetic energy. Comparisons between this reformed version of Öpik's theory and the models of Gault (1973) and Oberbeck and Aoyagi (1972) have demonstrated good agreement between experiment and theory over seventeen orders of magnitude of projectile kinetic energy.     

Images Of Polarization And Colour In The Inner Coma Of Comet Hale-Bopp

On March 31 and April 1, 1997, simultaneous photometry and polarimetry of comet Hale-Bopp's dust was conducted with the two-channel focal reducer of the Max-Planck-Institute for Aeronomy attached to the 2 m telescope of Pik Terskol Observatory (Northern Caucasus). Interference filters at642 nm and 443 nm selected red and blue narrow-band continuum windows. The observations have been averaged over the one hour of timethe comet could be observed. The polarization maps cover an area of about1 arcmin² around the nucleus. The values of polarization degree measured close to the nucleus agree very well with observations obtained with aperture polarimetry. They are lower than in the surrounding coma by about 1%. In our field of view the polarization increases along the sun-comet line from the solar to the antisolar side by about 3%. The dust shells are visible in the polarization images. The polarization in the shells is higher by 1 to 2%and this increase is higher in the red than in the blue range. Therefore the ratio of red to blue polarization (≈ 1.2) increases in the shells by ≈ 0.03. In principle, the polarization excess in the shells, the ratio of red/blue polarization and the higher integrated polarization as compared to other comets can be explained by an excess of particles of radius of about 0.1 μm. Such particles, however, are subject to strong radiation pressure and will be pushed back into the tail before they reach the observed location of the shells. Real Rayleigh particles cannot explain the observed increase in the ratio of red/blue polarization. One therefore cannot exclude the possibility that the excess polarization in the shells is caused by fluffy aggregates via effects which are presently not well understood. The colour map shows features not well related to intensity and polarization, perhaps another dust shell of a different particle size. This revised version was published online in July 2006 with corrections to the Cover Date.     

Long-Term Variability Of The Optical Emission Lines In The Nuclear Spectrum Of The Seyfert Galaxy Ngc 3227

Fifty-three spectrograms in the optical region (3700–7300 Å) with the spectral resolution ～8 Å have been obtained for the Seyfert nucleus of the galaxy NGC 3227 with the 6-m telescope on January 1977, while the nucleus was in the historically important epoch of its extreme maximum brightness. Width of the slit was 1″, length of the box during the spectra measurements was 1.5″. Data obtained by us and those compiled from literature showed that profiles of the Balmer lines H_α, H_β and H_γ are different, evidencing that the gas emitting these lines is highly self-absorbed. It was shown that narrow components of the profiles revealed by Rubin and Ford kept their positions (radial velocities) over 25 years. The components showed intensity variations compared to the central one from minimum to maximum of the nucleus brightness. The same variations were observed by us earlier in the emission line profiles of the NGC 7469 nucleus spectrum. Narrow profile components can reflect long-lived flows or jets in the broad line region (BLR). Obtained facts evidenced that long-lived gas streams and flows causing narrow components of broad line profiles presented not only when BLR of accretion disc is strong but when BLR of accretion disc declined. Blue bump at radial velocity of ?5000 km/s in H_γ profile was revealed in spectra of high states of the nucleus, which disappeared in low state. One of the interpretations of this event can be in the framework of a model of one-sided or two-sided gas ejection during the high state of the nucleus, positive radial velocities of which being screened out by a circumnuclear disk.     

The Transverse Velocity Field Of An Euv Solar Prominence

Previous studies have shown that the measured velocity field in solar prominences exhibits a slightly different behaviour depending on the observational conditions, on the investigation method, and possibly on the type of prominence. Observations of prominences seen at the limb reveal strong downward motions, whereas upflows are detected as Doppler shifts in filaments on the disk. In order to shed new light on this point, we have investigated the mass motions in a solar prominence by using a new method for calculating the geometric distortion between subsequent images. Flows perpendicular to the line of sight have been determined in several layers of the prominence-corona atmosphere, using extreme ultraviolet (EUV) lines formed at different temperature levels (T=104–106 K). We show that the motions mainly have a vertical direction, oriented both upwards and downwards. The velocity pattern can change rapidly during time intervals exceeding 10–15 min. We also find that the measured velocity field shows a similar pattern in all the studied lines.     

Mixed Particle Stream Model Of The Main Peak Of 1998 Leonids

On the basis of the radar observational data of the Leonids obtained in China in November of 1998, we found that the characteristics of the meteor population index γ during the main peak could not be explained well with a one-component magnitude distribution. Moreover, according the visual observational data obtained in China and other districts of Asia, γ also presents the property of non-uniformity. In this paper, we set forth a mathematical model of mixed particle streams. From this, the observed phenomenon of rarity of visual meteors and abundance of radar meteors during the main peak is interpreted. This revised version was published online in July 2006 with corrections to the Cover Date.     

Reconstruction Of The Subsurface Structure Of The Marquez Impact Crater In Leon County,Texas, Usa,Based On Well‐Log And Gravity Data

Abstract— In Leon County, Texas, USA, the Marquez Dome, an approximately circular 1.2 km diameter zone of disturbed Cretaceous rocks surrounded by shallow dipping Tertiary sediments, has been interpreted by Gibson and Sharpton (1989) and Sharpton and Gibson (1990) as the surface expression of a buried complex impact crater. New gravity and magnetic anomaly data collected over the Marquez Dome have been combined with well‐log and seismic reflection information to develop a better estimate of the overall geometry of the structure. A three‐dimensional model constructed to a depth of 2000 m from all available information indicates a complex crater 13 km in diameter with an uplift in the center of at least 1120 m. The zone of deformation associated with the cratering event is limited to a depth of <1720 m. No impact breccias were recovered in drilling at two locations, 1.1 and 2 km from the center of the structure, and the central uplift may be the only prominent remnant of this impact into unconsolidated, water‐rich sediments. The magnetic anomaly field shows no correlation with the location and extent of the structure.     

On The Concept Of Periapsis In Hill’s Problem

The concept of closest approach is analyzed in Hill’s problem, resulting in a partitioning of the position space. The different behavior between the direct and retrograde motion is explained analytically, resulting in a simple estimate of the variation of Hill’s periodic and quasi-circular orbits as a function of the Jacobi constant. The local behavior of the orbits on the zero velocity surfaces and an analytical definition of local escape and capture in Hill’s problem are also given.