Space Sci Research Articles

Comment on “Electrostatic compressive and rarefactive shocks and solitons in relativistic plasmas occurring in polar regions of pulsar”

The aim of this comment is to show the solution of the KdVB equation used by Shah et al. (Astrophys. Space Sci. 335:529–537, 2011, doi:10.1007/s10509-011-0766-y) is not correct. So, the numerical results that are predicted in this manuscript should not be helpful for further investigations in a plasma laboratory. For this reason, we have employed the Bernoulli’s equation method to obtain the correct form of analytical solution to this equation, which is appropriate for the study of electrostatic compressive and rarefactive shocks and solitons in relativistic plasmas occurring in polar regions of pulsar.

The global surface composition of 67P/CG nucleus by Rosetta/VIRTIS. (I) Prelanding mission phase

From August to November 2014 the Rosetta orbiter has performed an extensive observation campaign aimed at the characterization of 67P/CG nucleus properties and to the selection of the Philae landing site. The campaign led to the production of a global map of the illuminated portion of 67P/CG nucleus. During this prelanding phase the comet’s heliocentric distance decreased from 3.62 to 2.93 AU while Rosetta was orbiting around the nucleus at distances between 100 to 10 km. VIRTIS-M, the Visible and InfraRed Thermal Imaging Spectrometer – Mapping channel (Coradini et al., [2007] Space Sci. Rev., 128, 529–559) onboard the orbiter, has acquired 0.25–5.1 µm hyperspectral data of the entire illuminated surface, e.g. the north hemisphere and the equatorial regions, with spatial resolution between 2.5 and 25 m/pixel. I/F spectra have been corrected for thermal emission removal in the 3.5–5.1 µm range and for surface’s photometric response. The resulting reflectance spectra have been used to compute several Cometary Spectral Indicators (CSI): single scattering albedo at 0.55 µm, 0.5–0.8 µm and 1.0–2.5 µm spectral slopes, 3.2 µm organic material and 2.0 µm water ice band parameters (center, depth) with the aim to map their spatial distribution on the surface and to study their temporal variability as the nucleus moved towards the Sun. Indeed, throughout the investigated period, the nucleus surface shows a significant increase of the single scattering albedo along with a decrease of the 0.5–0.8 and 1.0–2.5 µm spectral slopes, indicating a flattening of the reflectance. We attribute the origin of this effect to the partial removal of the dust layer caused by the increased contribution of water sublimation to the gaseous activity as comet crossed the frost-line. The regions more active at the time of these observations, like Hapi in the neck/north pole area, appear brighter, bluer and richer in organic material than the rest of the large and small lobe of the nucleus. The parallel coordinates method (Inselberg [1985] Vis. Comput., 1, 69–91) has been used to identify associations between average values of the spectral indicators and the properties of the geomorphological units as defined by (Thomas et al., [2015] Science, 347, 6220) and (El-Maarr et al., [2015] Astron. Astrophys., 583, A26). Three classes have been identified (smooth/active areas, dust covered areas and depressions), which can be clustered on the basis of the 3.2 µm organic material’s band depth, while consolidated terrains show a high variability of the spectral properties resulting being distributed across all three classes. These results show how the spectral variability of the nucleus surface is more variegated than the morphological classes and that 67P/CG surface properties are dynamical, changing with the heliocentric distance and with activity processes.

Possible Estimation of the Solar Cycle Characteristic Parameters by the 10.7 cm Solar Radio Flux

Two independent methods for estimating basic parameters of the solar cycle are presented. The first of them, the ascending-descending triangle method, is based on a previous work by Tritakis (Astrophys. Space Sci. 82, 463, 1982), which described how the fundamental parameters of a certain solar cycle could be predicted from the shape of the previous one. The relation between the two cycles before and after a specific 11-year solar cycle is tighter than between the two cycles belonging to the same 22-year solar cycle (even-odd cycle). The second is the MinimaxX method, which uses a significant relation in the international sunspot number between the maximum value of a solar cycle and its value 2.5 or 3 years (depending on the enumeration of the even or odd cycle) before the preceding minimum. The tests applied to Cycles 12 to 24 indicate that both methods can estimate the peak of the 11-year solar radio flux at a high confidence level. The data used in this study are the 10.7 cm solar radio flux since 1947, which have been extrapolated back to 1848 from the strong correlation between the monthly international sunspot numbers and the adjusted values of the 10.7 cm radio flux.

Dissipative gravitational collapse of an (an)isotropic star

We develop a framework to study the effects of pressure anisotropy on the evolution of a collapsing star dissipating energy in the form of radial heat flux. In this construction, the star begins its collapse from an initial static configuration described by Paul and Deb (Astrophys. Space Sci. 354:421, 2014) solution in the presence (or absence) of anisotropic stresses. The form of the initial static solution, which is a generalization of Pant and Sah (Phys. Rev. D 32:1538, 1985) model, complies with all the requirements of a realistic star and provides a simple method to analyze the impacts of anisotropy onto the collapse.

‘Effective’ collisions in weakly magnetized collisionless plasma: importance of Pitaevski’s effect for magnetic reconnection

In this paper we revisit the paradigm of space science turbulent dissipation traditionally considered as myth (Coroniti, Space Sci. Rev., vol. 42, 1985, pp. 399–410). We demonstrate that due to approach introduced by Pitaevskii (Sov. J. Expl Theor. Phys., vol. 44, 1963, pp. 969–979; (in Russian)) (the effect of a finite Larmor radius on a classical collision integral) dissipation induced by effective interaction with microturbulence produces a significant effect on plasma dynamics, especially in the vicinity of the reconnection region. We estimate the multiplication factor of collision frequency in the collision integral for short wavelength perturbations. For waves propagating transverse to the background magnetic field, this factor is approximately $({\it\rho}_{e}k_{x})^{2}$ with ${\it\rho}_{e}$ an electron gyroradius and where $k_{x}$ is a transverse wavenumber. We consider recent spacecraft observations in the Earth’s magnetotail reconnection region to the estimate possible impact of this multiplication factor. For small-scale reconnection regions this factor can significantly increase the effective collision frequency produced both by lower-hybrid drift turbulence and by kinetic Alfvén waves. We discuss the possibility that the Pitaevskii’s effect may be responsible for the excitation of a resistive electron tearing mode in thin current sheets formed in the outflow region of the primary X-line.

Relativistic model of anisotropic charged fluid sphere in general relativity

In this present paper, we present a class of static, spherically symmetric charged anisotropic fluid models of super dense stars in isotropic coordinates by considering a particular type of metric potential, a specific choice of electric field intensity $E$ and pressure anisotropy factor $\Delta$ which involve parameters $K$ (charge) and $\alpha$ (anisotropy) respectively. The solutions so obtained are utilized to construct the models for super-dense stars like neutron stars and strange quark stars. Our solutions are well behaved within the following ranges of different constant parameters. In the absence of pressure anisotropy and charge present model reduces to the isotropic model Pant et al. (Astrophys. Space Sci. 330:353–359, 2010). Our solution is well behaved in all respects for all values of $X$ lying in the range $0< X \leq 0.18$ , $\alpha$ lying in the range $0 \leq \alpha \leq6.6$ , $K$ lying in the range $0< K \leq 6.6$ and Schwarzschild compactness parameter “ $u$ ” lying in the range $0< u \leq 0.38$ . Since our solution is well behaved for a wide ranges of the parameters, we can model many different types of ultra-cold compact stars like quark stars and neutron stars. We have shown that corresponding to $X=0.088$ , $\alpha=0.6$ and $K=4.3$ for which $u=0.2054$ and by assuming surface density $\rho_{b} = 4.6888 \times 10^{14}~\mbox{g/cm}^{3}$ the mass and radius are found to be $1.51~M_{\varTheta}$ and 10.90 km respectively. Assuming surface density $\rho_{b} = 2 \times 10^{14}~\mbox{g/cm}^{3}$ the mass and radius for a neutron star candidate are found to be $2.313~M_{\varTheta}$ and 16.690 km respectively. Hence we obtain masses and radii that fall in the range of what is generally expected for quark stars and neutron stars.

The porosity of the upper lunar regolith

The porosity of the upper centimeter or so of the lunar regolith strongly affects several properties that are commonly studied remotely. Hence, it is important to determine its value. We have reanalyzed the data of Ohtake et al. (Ohtake et al. [2010]. Space Sci. Rev., 154, 57–77), who used spacecraft and laboratory reflectance measurements of the Moon by Kaguya Multiband Imager instruments and an Apollo sample to infer a lunar regolith porosity of 74–87%. Our analysis was augmented by using Lunar Reconnaissance Orbiter Wide and Narrow Angle Camera images. We confirm the Ohtake et al. (Ohtake et al. [2010]. Space Sci. Rev., 154, 57–77) estimate and refine it to 83±3%. However, depending on the validity of key assumptions, this value could be a lower limit, so that the actual porosity could be somewhat higher. Even though the magnetic resonance index of the sample indicates that it is mature, it is appears to be optically less mature than a standard photometric site near the sample collection site.

Sublimation of ice–tholins mixtures: A morphological and spectro-photometric study

Sublimation, the direct transition from solid to gas phase, is a process responsible for shaping and changing the reflectance properties of many Solar System surfaces. In this study, we have characterized the evolution of the structure/texture and of the visible and near-infrared (VIS–NIR) spectral reflectance of surfaces made of water ice mixed with analogues of complex extraterrestrial organic matter, named tholins, under low temperature (<−70°C) and pressure (10−5mbar) conditions. The experiments were carried out in the SCITEAS simulation setup recently built as part of the Laboratory for Outflow Studies of Sublimating Materials (LOSSy) at the University of Bern (Pommerol, A. et al. [2015a]. Planet. Space Sci. 109–110, 106–122). As the water ice sublimated, we observed in situ the formation of a sublimation lag deposit made of a water-free porous (>90% porosity) network of organic filaments on top of the ice. The temporal evolution of the tholins and water ice spectral features (reflectance at the absorption bands wavelengths, red slope, from 0.40 to 1.90μm) are analyzed throughout the sublimation of the samples. We studied how different mixtures of tholins with water (0.1wt.% tholins as coating or inclusions within the water particles), and different ice particle sizes (4.5±2.5 or 67±31μm) influence the morphological and spectral evolutions of the samples. The sublimation of the ice below the mantle produces a gas flow responsible for the ejection of mm to cm-sized fragments of the deposit in outbursts-like events. The results show remarkable differences between these samples in term of mantle structure, speed of mantle building, rates and surface area of mantle ejections. These data provide useful references for interpreting remote-sensing observations of icy Solar System surfaces, in particular the activity of comet nuclei where sublimation of organic-rich ices and deposition of organic-dust particles likely play a major role. Consequently, the data presented here could be of high interest for the interpretation of Rosetta, and also New Horizons, observations.

Charged analogue of Vlasenko–Pronin superdense star with variable cosmological term

The set of three static spherically symmetric solutions of the Einstein–Maxwell field equations by Maurya and Gupta, Astrophys. Space Sci.333, 149 (2011) are modified by introducing the variable cosmological term. Motivated by Tiwari et al, Indian J. Pure Appl. Math.31, 1017 (2000), some particular values of the cosmological term are taken to obtain well-behaved solutions of the Einstein–Maxwell field equations. All the results given by Maurya and Gupta can be obtained as particular cases of our solutions by choosing a cosmological term equal to zero.

Water equivalent hydrogen estimates from the first 200 sols of Curiosity’s traverse (Bradbury Landing to Yellowknife Bay): Results from the Dynamic Albedo of Neutrons (DAN) passive mode experiment

The Dynamic Albedo of Neutrons (DAN) experiment on the Mars Science Laboratory (MSL) rover Curiosity is designed to detect neutrons to determine hydrogen abundance within the subsurface of Mars (Mitrofanov, I.G. et al. [2012]. Space Sci. Rev. 170, 559–582. http://dx.doi.org/10.1007/s11214-012-9924-y; Litvak, M.L. et al. [2008]. Astrobiology 8, 605–613. http://dx.doi.org/10.1089/ast.2007.0157). While DAN has a pulsed neutron generator for active measurements, in passive mode it only measures the leakage spectrum of neutrons produced by the Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) and Galactic Cosmic Rays (GCR). DAN passive measurements provide better spatial coverage than the active measurements because they can be acquired while the rover is moving. Here we compare DAN passive-mode data to models of the instrument’s response to compositional differences in a homogeneous regolith in order to estimate the water equivalent hydrogen (WEH) content along the first 200 sols of Curiosity’s traverse in Gale Crater, Mars. WEH content is shown to vary greatly along the traverse. These estimates range from 0.5±0.1wt.% to 3.9±0.2wt.% for fixed locations (usually overnight stops) investigated by the rover and 0.6±0.2wt.% to 7.6±1.3wt.% for areas that the rover has traversed while continuously acquiring DAN passive data between fixed locations. Estimates of WEH abundances at fixed locations based on passive mode data are in broad agreement with those estimated at the same locations using active mode data. Localized (meter-scale) anomalies in estimated WEH values from traverse measurements have no particular surface expression observable in co-located images. However at a much larger scale, the hummocky plains and bedded fractured units are shown to be distinct compositional units based on the hydrogen content derived from DAN passive measurements. DAN passive WEH estimates are also shown to be consistent with geologic models inferred from other MSL instruments, which indicate that fluvial/lacustrine activity occurred at certain locations (e.g., Yellowknife Bay).

Asymmetric kinetic equilibria: Generalization of the BAS model for rotating magnetic profile and non-zero electric field

Finding kinetic equilibria for non-collisional/collisionless tangential current layers is a key issue as well for their theoretical modeling as for our understanding of the processes that disturb them, such as tearing or Kelvin Helmholtz instabilities. The famous Harris equilibrium [E. Harris, Il Nuovo Cimento Ser. 10 23, 115–121 (1962)] assumes drifting Maxwellian distributions for ions and electrons, with constant temperatures and flow velocities; these assumptions lead to symmetric layers surrounded by vacuum. This strongly particular kind of layer is not suited for the general case: asymmetric boundaries between two media with different plasmas and different magnetic fields. The standard method for constructing more general kinetic equilibria consists in using Jeans theorem, which says that any function depending only on the Hamiltonian constants of motion is a solution to the steady Vlasov equation [P. J. Channell, Phys. Fluids (1958–1988) 19, 1541 (1976); M. Roth et al., Space Sci. Rev. 76, 251–317 (1996); and F. Mottez, Phys. Plasmas 10, 1541–1545 (2003)]. The inverse implication is however not true: when using the motion invariants as variables instead of the velocity components, the general stationary particle distributions keep on depending explicitly of the position, in addition to the implicit dependence introduced by these invariants. The standard approach therefore strongly restricts the class of solutions to the problem and probably does not select the most physically reasonable. The BAS (Belmont-Aunai-Smets) model [G. Belmont et al., Phys. Plasmas 19, 022108 (2012)] used for the first time the concept of particle accessibility to find new solutions: considering the case of a coplanar-antiparallel magnetic field configuration without electric field, asymmetric solutions could be found while the standard method can only lead to symmetric ones. These solutions were validated in a hybrid simulation [N. Aunai et al., Phys. Plasmas (1994-present) 20, 110702 (2013)], and more recently in a fully kinetic simulation as well [J. Dargent and N. Aunai, Phys. Plasmas (submitted)]. Nevertheless, in most asymmetric layers like the terrestrial magnetopause, one would indeed expect a magnetic field rotation from one direction to another without going through zero [J. Berchem and C. T. Russell, J. Geophys. Res. 87, 8139–8148 (1982)], and a non-zero normal electric field. In this paper, we propose the corresponding generalization: in the model presented, the profiles can be freely imposed for the magnetic field rotation (although restricted to a 180 rotation hitherto) and for the normal electric field. As it was done previously, the equilibrium is tested with a hybrid simulation.

The DISR imaging mosaic of Titan’s surface and its dependence on emission angle

The DISR (Descent Image/Spectral Radiometer) imagers took about 300 images during the descent of the Huygens probe in Titan’s atmosphere. We combined them into a photometrically calibrated mosaic of Titan’s surface within 100km of the Huygens landing site with a resolution ranging from 1km at 100km distance from the landing site to 50cm near the landing site. We analyzed how the reflectivity of each location varies with the changing phase angle and emission angle during the descent. We found strong variations with the emission angle but no significant variation of the surface phase function. The latter is possibly obscured by the diffuse nature of illumination by Titan’s sky at visible wavelengths. We constructed a map of this emission angle dependence, which represents a measure of surface reflectivity and roughness of the terrain. Titan’s surface probed by the images shows various terrain types with a small correlation between reflectivity and roughness. We propose that variations in the emission angle dependence might be correlated with average tilt angles of the surface. We detected elevated terrain in the bright highland north of the landing site as published by Soderblom et al. (Soderblom, L.A. et al. [2007]. Planet. Space Sci. 55, 2015–2024), but their high ridges in the lakebed appear mostly flat in our analysis. We integrated the color information from the DISR spectrometers into our mosaic to create a color map. We also compared the features seen in maps of the reflectivity, the emission angle dependence, and color to features seen in Cassini RADAR images. Finally, we provide a refined version of the images from Huygens after landing down to a resolution of 2mm.

Inertia in Friedmann Universes with variable G $G$ and Λ $\varLambda$

In light of the recent interest in dynamical dark energy models based on a cosmology with varying gravitational and cosmological parameters $G$ and $\varLambda$ , we present here a model of inertia in a type of Friedmann universe with $G = G_{0}(A/A_{0})^{\sigma}$ ; $A$ being the dimensionless scale factor, that was recently studied by Singh et al. (Astrophys. Space Sci. 345:213, 2013). The proposed Machian model of inertia utilizes the curved space generalization of Sciama’s law of inertial induction, which is based on the analogy between the retarded far fields of electrodynamics and those of gravitation, and expresses the total inertial force $F= -ma$ on an accelerating mass $m$ in terms of contributions from all matter in the observable Universe. We show that for a varying Friedmann model with $\sigma=-3/2$ , inertial induction alone can account for the total inertial force on the accelerating mass. We then compare this cosmological model with current observational constraints for the variation of $G$ .

DIVISION G COMMISSION 21: GALACTIC AND EXTRAGALACTIC BACKGROUNDS RADIATION

Commission 21 was one of the oldest and smallest in the IAU yet one which underwent the most evolution since its inception. It began in 1955 as Light of the Night Sky (Lumière du Ciel Nocturne) under the Presidentship of Jean Dufay (University Observatory at Lyon). As the name suggested, in the years before space observations, its focus was on observations of atmospheric light which, at the time, was the domain of astronomers. Thus the early proceedings of the Commission were dominated by reports of the daytime and nighttime emission as observed from different locations and different conditions. Our knowledge of the Earth's atmosphere is still shaped by these early observations (Meier R.R., 1991, Space Sci. Rev. 58, 1). Members of Commission 21 were key to organizing interational collaborations to set up atmospheric stations at different latitudes, including in the Souther hemisphere and in ensuring a consistent calibration between the instruments. The Commission also bought in results from Soviet scientists in an era where communication was limited by both politics and language.

A fast iterative method for Chandrasekhar’s H $\boldsymbol{H}$ -functions for general laws of scattering

This work shows that notable acceleration of the speed of calculating Chandrasekhar's H-functions for general laws of scattering with an iterative method can be realized by supplying a starting pproximation produced by the following procedure: (i) in the cases of azimuth-angle independent Fourier components, values of the isotropic scattering H-function given by an accurate yet simple-to-apply formula, in particular, the one by Kawabata and Limaye (Astrophys. and Space Sci. Vol. 332, 365-371, 2011 DOI 10.1007/s10509-010-0512-x; see also Astrophys. and Space Sci. Vol. 348, 601, 2013 DOI 10.1007/1009-013-1589-9, for erratum), and (ii) for azimuth-angle dependent Fourier components, an already obtained solution of the next lower order term. The paper has been published in Astrophys. and Space Sci. Vol. 358, 32-38 (2015) DOI 10.1007/s10509-015-2434-0, and the final publication is available at link.springer.com.

Effects of manifold correction methods on chaos indicators

The manifold approach of Nacozy et al. (Astrophys Space Sci 14:40–51, 1971), the approximate velocity correction method of Wu et al. (Astron J 133:2643–2653, 2007), and the velocity scaling method of Ma et al. (New Astron 13:216–223, 2008a) are some of the available manifold correction methods. They have been highly successful at maintaining invariant integrals in two-body problems and the Sun–Jupiter–Saturn system. This paper discusses their efficiency on chaos indicators. Because the planar circular restricted three-body problem involves the Jacobi constant $$C_\mathrm{j}$$ and chaotic phenomena, it is preferable to check the numerical performances of manifold corrections. First, we find that a low-order algorithm combined with manifold corrections can greatly improve the precision of the Jacobi constant $$C_\mathrm{j}$$ . Then, numerical experiments show that these manifold correction methods have the same performance in Poincare sections, Lyapunov exponents, fast Lyapunov indicators, smaller alignment indices, and relative finite time Lyapunov indicators. Moreover, manifold corrections not only allow for the use of larger step sizes compared to low-order algorithms without correction but also save substantial computation time compared to the high-order algorithm RKF7(8). In particular, the velocity scaling method of Ma et al. (2008a) lends itself to practical application in long-term integration.

Eight-color maps of Titan’s surface from spectroscopy with Huygens’ DISR

During the descent of the Huygens probe in Titan’s atmosphere, the Descent Imager/Spectral Radiometer (DISR) acquired spectra of 3660 locations within 250km of the landing site. Each spectrum consisted of 200 resolution elements between 480 and 960nm wavelength. With the help of radiative transfer models, contributions from the atmosphere and surface were separated. In eight methane windows, the data were combined into a map of Titan’s surface reflectivity with 250km diameter near the landing site. Principal component analysis revealed three significant components, a brightness component that is consistent with a mosaic based on DISR imaging of much higher spatial resolution, a spectral slope component, and a spectral curvature component. The brightness component has stronger contrasts at longer wavelengths, or brighter areas have a larger spectral slope, consistent with previous results (Keller et al. [2008]. Planet. Space Sci. 56, 728–752). The second component corresponds to small differences in spectral slopes that are not correlated with features seen before except for an area with unusual high spectral slope found by the same authors and confirmed here. Our map of the second component gives another important parameter in characterizing and understanding Titan’s surface. The third principal component is somewhat noisy and describes variation in the spectral curvature that have never seen before at similar wavelengths. These variations require processes to differentiate surface spectra. To extend this work to longer wavelengths, 62 spectra from 850 to 1600nm wavelength were investigated too, although the much lower number of spatial resolution points revealed only two significant components in the principal component analysis. They correlate with the first two components found in the shorter wavelength data. We also compare our results with an observation by Cassini’s Visible Imager/Mapping Spectrometer (VIMS) that imaged part of our investigated area with 4096 spatial resolution elements. Both data sets are complementary. DISR data extend to about 1500nm wavelength while most surface features are seen in the VIMS data beyond 1500nm.

Anomalous Earth flybys of spacecraft

A small deviation from the $1/r$ potential is expected for the gravitational interaction of extended bodies. It is explained as a consequence of a recently proposed gravitational impact model (Wilhelm et al. in Astrophys. Space Sci. 343:135–144, 2013) and has been applied to anomalous perihelion advances by Wilhelm and Dwivedi (New Astron. 31:51–55, 2014). The effect—an offset of the effective gravitational centre from the geometric centre of a spherical symmetric body—might also be responsible for the observed anomalous orbital energy gains and speed increases during Earth flybys of several spacecraft. However, close flybys would require detailed considerations of the orbit geometry. In this study, an attempt is made to explain the anomalous Earth flybys of the Galileo, NEAR Shoemaker and Rosetta spacecraft.

Titan’s meridional wind profile and Huygens’ orientation and swing inferred from the geometry of DISR imaging

The altitude and zonal motion of the Huygens probe descending through Titan’s atmosphere was determined early under the assumption of no meridional motion (Bird et al. [2005]. Nature 438, 800–802). By comparing images taken during the descent, Karkoschka et al. (Karkoschka et al. [2007]. Planet. Space Sci. 55, 1895–1935) determined the meridional motion of Huygens, which was generally much smaller than its zonal motion. Here, we present a comprehensive geometrical analysis of all images taken during the descent that is four times more accurate than the previous study. The result is a meridional wind profile across Titan’s troposphere with northward winds by up to 0.4m/s with an average of 0.1m/s above 1km altitude, and southward winds below, peaking at 0.9m/s near 0.4km altitude. The imaging data extend down to 0.22km altitude, although additional information came from the horizontal impact speed near 0.8m/s southward (Schröder et al. [2012]. Planet. Space Sci. 73, 327–340). There is a region between 5 and 8km altitude with no significant meridional wind. In the stratosphere, the average meridional wind was 1.2±1.5m/s northward, and zero meridional motion is possible down to 15km altitude. We present the difference between the zonal speeds of Huygens and the wind that was ignored in previous publications and amounts to up to 7m/s. We determined the three rotational angles of Huygens for the times of each exposure that showed surface features. During 26 exposures, the swing speed of Huygens was fast enough to smear images. Inferred swing speeds were up to 20°/s during the calm phase of the descent, consistent with up to 40°/s swings reported before during the rough phase. The improved geometric calibration of images allowed identification of many features also seen in Cassini radar images. This comparison yields the location of the Huygens LandingSite as 192.34±02° West and 10.47±0.02° South.

Role of conharmonic flatness in Friedmann cosmology

In this paper, investigation of conharmonically flat FRW space-time is considered in presence of perfect fluid as a matter source. To get the deterministic model, we have employed the condition between vacuum density Λ and the Hubble parameter H as Λ=3βH2, where β is a constant (Astrophys. Space Sci. 314:83, 2008). Physical features of the model are discussed in detail. An estimate of the statefinder and snap parameters is also carried out for the model. The observed values of these parameters agree ΛCDM model. We see that the model is in agreement with recent observations.