共查询到20条相似文献，搜索用时 31 毫秒
1.
Pedro Machado David Luz Thomas Widemann Emmanuel Lellouch Olivier Witasse 《Icarus》2012,221(1):248-261
The most significant aspect of the general circulation of the atmosphere of Venus is its retrograde super-rotation. A complete characterization of this dynamical phenomenon is crucial for understanding its driving mechanisms. Here we report on ground-based Doppler velocimetry measurements of the zonal winds, based on high resolution spectra from the UV–Visual Echelle Spectrograph (UVES) instrument at ESO’s Very Large Telescope. Under the assumption of predominantly zonal flow, this method allows the simultaneous direct measurement of the zonal velocity across a range of latitudes and local times in the day side. The technique, based on long slit spectroscopy combined with the high spatial resolution provided by the VLT, has provided the first ground-based characterization of the latitudinal profile of zonal wind in the atmosphere of Venus, the first zonal wind field map in the visible, as well as new constraints on wind variations with local time. We measured mean zonal wind amplitudes between 106 ± 21 and 127 ± 14 m/s at latitudes between 18°N and 34°S, with the zonal wind being approximately uniform in 2.6°-wide latitude bands (0.3 arcsec at disk center). The zonal wind profile retrieved is consistent with previous spacecraft measurements based on cloud tracking, but with non-negligible variability in local time (longitude) and in latitude. Near 50° the presence of moderate jets is apparent in both hemispheres, with the southern jet being stronger by ～10 m/s. Small scale wind variations with local time are also present at low and mid-latitudes. 相似文献
2.
The launch of the Soviet space probes Vega 1 and Vega 2 to explore Venus, including its atmosphere, and flyby Halley??s comet, a rare guest in the inner Solar System, added a vivid page to the history of space exploration. This paper is dedicated to Designer General Vyacheslav M. Kovtunenko. 相似文献
3.
4.
P. Rosenblatt S.L. Bruinsma I.C.F. Müller-Wodarg B. Häusler H. Svedhem J.C. Marty 《Icarus》2012,217(2):831-838
On its highly elliptical 24 h orbit around Venus, the Venus Express (VEX) spacecraft briefly reaches a periapsis altitude of nominally 250 km. Recently, however, dedicated and intense radio tracking campaigns have taken place in August 2008, October 2009, February and April 2010, for which the periapsis altitude was lowered to the 186–176 km altitude range in order to be able to probe the upper atmosphere of Venus above the North Pole for the first time ever in situ. As the spacecraft experiences atmospheric drag, its trajectory is measurably perturbed during the periapsis pass, allowing us to infer total atmospheric mass density at the periapsis altitude. A Precise Orbit Determination (POD) of the VEX motion is performed through an iterative least-squares fitting process to the Doppler tracking data, acquired by the VEX radioscience experiment (VeRa). The drag acceleration is modelled using an initial atmospheric density model (VTS3 model, Hedin, A.E., Niemann, H.B., Kasprzak, W.T., Seiff, A. [1983]. J. Geophys. Res. 88, 73–83). A scale factor of the drag acceleration is estimated for each periapsis pass, which scales Hedin’s density model in order to best fit the radio tracking data. Reliable density scale factors have been obtained for 10 passes mainly from the second (October 2009) and third (April 2010) VExADE campaigns, which indicate a lower density by a factor of about 1.8 than Hedin’s model predicts. These first ever in situ polar density measurements at solar minimum have allowed us to construct a diffusive equilibrium density model for Venus’ thermosphere, constrained in the lower thermosphere primarily by SPICAV-SOIR measurements and above 175 km by the VExADE drag measurements (Müller-Wodarg et al., in preparation). The preliminary results of the VExADE campaigns show that it is possible to obtain with the POD technique reliable estimates of Venus’ upper atmosphere densities at an altitude of around 175 km. Future VExADE campaigns will benefit from the planned further lowering of VEX pericenter altitude to below 170 km. 相似文献
5.
We study the equilibrium points and the zero-velocity curves of Chermnykh’s problem when the angular velocity ω varies continuously and the value of the mass parameter is fixed. The planar symmetric simple-periodic orbits are determined numerically and they are presented for three values of the parameter ω. The stability of the periodic orbits of all the families is computed. Particularly, we explore the network of the families when the angular velocity has the critical value ω = 2√2 at which the triangular equilibria disappear by coalescing with the collinear equilibrium point L1. The analytic determination of the initial conditions of the family which emanate from the Lagrangian libration point L1 in this case, is given. Non-periodic orbits, as points on a surface of section, providing an outlook of the stability regions, chaotic and escape motions as well as multiple-periodic orbits, are also computed. Non-linear stability zones of the triangular Lagrangian points are computed numerically for the Earth–Moon and Sun–Jupiter mass distribution when the angular velocity varies. 相似文献
6.
István Horváth 《Astrophysics and Space Science》2009,323(1):83-86
The BeppoSAX Catalog has been very recently published. In this paper we analyze—using the Maximum Likelihood (ML) method—the
duration distribution of the 1003 GRBs listed in the catalog with duration. The ML method can identify the long and the intermediate
duration groups. The short population of the bursts is identified only at a 96% significance level. MC simulation has been
also applied and gives a similar significance level; 95%. However, the existence of the short bursts is not a scientific question
after the Compton Gamma-Ray Observatory’s observation. Our minor result is this well-known fact that in the BeppoSAX data
the short bursts are under-represented, mainly caused by the different triggering system. Our major result is the identification
of the intermediate group in the BeppoSAX data. Therefore, four different satellites (CGRO, Swift, RHESSI and BeppoSAX) observed
the intermediate type Gamma-Ray Burst. 相似文献
7.
Solar System Research - The paper considers the most significant relativistic effects in the rotational dynamics of Neptune’s satellites (Triton (N1), Naiad (N3), Thalassa (N4), Despina (N5),... 相似文献
8.
The mass and radius of our closest neighbour Venus are only slightly smaller than those of the Earth indicating a similarity in composition. However, the lack of self-sustained internal magnetic field in Venus points to a difference in the core structure. The theory of tricritical phenomena has recently been used to study solidification at the high pressures and temperatures of the Earth, revealing how the Earth’s core works. This theoretical approach is here applied to Venus. While keeping Venus’ mantle density similar to the Earth’s, one obtains the gravitational acceleration g inside Venus, its moment of inertia factor, the size, pressure and density of its core, together with the planet’s temperature profile. Mainly due to the temperature difference between the core–mantle boundary and surface being 21% smaller than on the Earth, and the 11.5% smaller gravitational acceleration, Venus’ Rayleigh number Ra parameterizing mantle convection is only 54% of the Earth’s, offering a possible explanation for the present lack of plate tectonics on Venus. The theory as discussed predicts that Venus is molten at the centre, with temperature about 5200 K, and has 8 mol.% impurities there, slightly more impurities than in the Earth’s inner core boundary fluid. These impurities are likely to be a combination of MgO and MgSiO3. 相似文献
9.
The giant planetary magnetospheres surrounding Jupiter and Saturn respond in quite different ways, compared to Earth, to changes in upstream solar wind conditions. Spacecraft have visited Jupiter and Saturn during both solar cycle minima and maxima. In this paper we explore the large-scale structure of the interplanetary magnetic field (IMF) upstream of Saturn and Jupiter as a function of solar cycle, deduced from solar wind observations by spacecraft and from models. We show the distributions of solar wind dynamic pressure and IMF azimuthal and meridional angles over the changing solar cycle conditions, detailing how they compare to Parker predictions and to our general understanding of expected heliospheric structure at 5 and 9 AU. We explore how Jupiter’s and Saturn’s magnetospheric dynamics respond to varying solar wind driving over a solar cycle under varying Mach number regimes, and consider how changing dayside coupling can have a direct effect on the nightside magnetospheric response. We also address how solar UV flux variability over a solar cycle influences the plasma and neutral tori in the inner magnetospheres of Jupiter and Saturn, and estimate the solar cycle effects on internally driven magnetospheric dynamics. We conclude by commenting on the effects of the solar cycle in the release of heavy ion plasma into the heliosphere, ultimately derived from the moons of Jupiter and Saturn. 相似文献
10.
The far-reaching gravitational force—in the approximation of Newton’s law of gravitation—is described by a heuristic model with hypothetical massless particles propagating at the speed of light in vacuum and transferring momentum and energy between physical entities through interactions on a local basis. The model has some similarities with the impact theory presented by Nicolas Fatio de Duillier to the Royal Society in 1690. Objections raised against this idea are dispelled by invoking the Special Theory of Relativity, considering non-local interactions, and replacing the shielding concept by a secular mass increase of massive bodies. Some consequences and applications of the model are discussed. 相似文献
11.
The Venus Express (VEX) mission has been in orbit to Venus for more than 4 years now. The Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument onboard VEX observes Venus in two channels (visible and infrared) obtaining spectra and multi-wavelength images of the planet that can be used to sample the atmosphere at different altitudes. Day-side images in the ultraviolet range (380 nm) are used to study the dynamics of the upper cloud at 66–72 km while night-side images in the near infrared (1.74 μm) map the opacity of the lower cloud deck at 44–48 km. Here we present a long-term analysis of the global atmospheric dynamics at these levels using a large selection of orbits from the VIRTIS-M dataset covering 860 Earth days that extends our previous work (Sánchez-Lavega, A. et al. [2008]. Geophys. Res. Lett. 35, L13204) and allows studying the variability of the global circulation at the two altitude levels. The atmospheric superrotation is evident with equatorial to mid-latitudes westward velocities of 100 and 60 m s?1 in the upper and lower cloud layers. These zonal velocities are almost constant in latitude from the equator to 50°S. From 50°S to 90°S the zonal winds at both cloud layers decrease steadily to zero at the pole. Individual cloud tracked winds have errors of 3–10 m s?1 with a mean of 5 m s?1 and the standard deviations for a given latitude of our zonal and meridional winds are 9 m s?1. The zonal winds in the upper cloud change with the local time in a way that can be interpreted in terms of a solar tide. The zonal winds in the lower cloud are stable at mid-latitudes to the tropics and present variability at subpolar latitudes apparently linked to the activity of the South polar vortex. While the upper cloud presents a net meridional motion consistent with the upper branch of a Hadley cell with peak velocity v = 10 m s?1 at 50°S, the lower cloud meridional motions are less organized with some cloud features moving with intense northwards and southwards motions up to v = ±15 m s?1 but, on average, with almost null global meridional motions at all latitudes. We also examine the long-term behavior of the winds at these two vertical layers by comparing our extended wind tracked data with results from previous missions. 相似文献
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13.
Robert S. French Mark R. Showalter Rafael Sfair Carlos A. Argüelles Myriam Pajuelo Patricio Becerra Matthew M. Hedman Philip D. Nicholson 《Icarus》2012,219(1):181-193
Image photometry reveals that the F ring is approximately twice as bright during the Cassini tour as it was during the Voyager flybys of 1980 and 1981. It is also three times as wide and has a higher integrated optical depth. We have performed photometric measurements of more than 4800 images of Saturn’s F ring taken over a 5-year period with Cassini’s Narrow Angle Camera. We show that the ring is not optically thin in many observing geometries and apply a photometric model based on single-scattering in the presence of shadowing and obscuration, deriving a mean effective optical depth τ ≈ 0.033. Stellar occultation data from Voyager PPS and Cassini VIMS validate both the optical depth and the width measurements. In contrast to this decades-scale change, the baseline properties of the F ring have not changed significantly from 2004 to 2009. However, we have investigated one major, bright feature that appeared in the ring in late 2006. This transient feature increased the ring’s overall mean brightness by 84% and decayed with a half-life of 91 days. 相似文献
14.
Mathieu Hirtzig Tetsuya Tokano Sébastien Rodriguez Stéphane le Mouélic Christophe Sotin 《Astronomy and Astrophysics Review》2009,17(2):105-147
Saturn’s satellite Titan is a particularly interesting body in our solar system. It is the only satellite with a dense atmosphere,
which is primarily made of nitrogen and methane. It harbours an intricate photochemistry, that populates the atmosphere with
aerosols, but that should deplete irreversibly the methane. The observation that methane is not depleted led to the study
of Titan’s methane cycle, starting with its atmospheric part. The features that inhabit Titan’s atmosphere can last for timescales
varying from year to day. For instance, the reversal of the north–south asymmetry is linked to the 16-year seasonal cycle.
Diurnal phenomena have also been observed, like a stratospheric haze enhancement or a possible tropospheric drizzle. Furthermore,
clouds have been reported on Titan since 1993. From these first detections and up to now, with the recent inputs from the
Cassini–Huygens mission, clouds have displayed a large range of shapes, altitudes, and natures, from the flocky tropospheric
clouds at the south pole to the stratiform ones in the northern stratosphere. It is still difficult to compose a clear picture
of the physical processes governing these phenomena, even though of lot of different means of observation (spectroscopy, imaging)
are available now. We propose here an overview of the phenomena reported between 1993 and 2008 in the low atmosphere of Titan,
with indications on the location, altitude, and their characteristics in order to give a perspective of our up-to-date understanding
of Titan’s meteorological manifestations. We shall focus mainly on direct imaging observations, from both space- and ground-based
facilities. All of these observations, published in more than 30 different refereed papers to date, allow us to build a precise
chronology of Titan’s atmospheric changes (including the north–south asymmetry, diurnal and seasonal effects, etc). Since
the interpretation is at an early stage, we only briefly mention some of the current theories regarding the features’ nature. 相似文献
15.
We revisit a set of symplectic variables introduced by Andre Deprit (Celest Mech 30, 181–195, 1983), which allows for a complete symplectic reduction in rotation invariant Hamiltonian systems, generalizing to arbitrary dimension
Jacobi’s reduction of the nodes. In particular, we introduce an action-angle version of Deprit’s variables, connected to the
Delaunay variables, and give a new hierarchical proof of the symplectic character of Deprit’s variables. 相似文献
16.
We investigated the motion of the perijove and ascending node of the 8th satellite of Jupiter, Pasiphae. The main perturbations by the Sun on the satellite permitted to use an intermediate orbit obtained by approximated solutions of differential equations previously transformed by the Von Zeipel method. The orbit is a non-Keplerian ellipse. The secular motion of the ascending node, argument of perijove, and essential periodic perturbations were taken into account. Using our theory we showed that the inclination and eccentricity of Pasiphae can acquire values by which the orbit becomes a librating one; but, within Pasiphae’s observation period, the motion of its perijove is circulating. Taking into account the results of our previous works on Pasiphae motion, we can conclude that the mean motion of the ascending node is similar for different values of the satellite inclination and eccentricity. But the mean motion of the perijove strongly depends on the orbit inclination and eccentricity, according to the Lidov–Kozai mechanism. 相似文献
17.
O. S. Shalygina V. V. Korokhin E. V. Shalygin Yu. I. Velikodskii 《Kinematics and Physics of Celestial Bodies》2008,24(4):201-208
Peculiarities in the use of CCD matrices for photopolarimetric observations are considered. Algorithms for processing the CCD photopolarimetric and polarimetric observations of Jupiter and other bright extended objects are described with an emphasis on the specifics of these photodetectors. We present new estimates of the north-south asymmetry parameter of polarization of Jupiter obtained from the observations in 2006 and 2007. The new data are in good agreement with the previous observations. 相似文献
18.
E. Ye. Khachikian 《Astrophysics》2010,53(1):1-17
As Victor Ambartsumyan, himself, noted, the concept of active galactic nuclei occupies a special place among his scientific
ideas. It was proposed more than half a century ago and was recognized by the U.S. National Academy of Sciences as revolutionary,
on a copernican scale. However, by no means all of its propositions were accepted at once by large parts of the astronomy
community. Nevertheless, as the American astrophysicist A. R. Sandage has written, “today, not one astronomer would deny the
mystery surrounding the nuclei of galaxies or that the first to recognize the rich reward held in this treasury was Viktor
Ambartsumian.” The purpose of this article is to acquaint the reader with the major stages in the formation and development
of the concept of active galactic nuclei and with some of the work on this topic done at the Byurakan and other astrophysical
observatories throughout the world. 相似文献
19.
Solar System Research - Based on the analysis of the data available in the literature on laboratory measurements of the dielectric characteristics of the lunar soil samples delivered to the Earth... 相似文献
20.
The equilibrium suggested as a buffer for CO2 in the Venus atmosphere, CaCO3 + SiO2 = CaSiO3 + CO2, cannot act as a buffer at the Venus surface/troposphere – the pressure–temperature slope of the equilibrium and that of the atmosphere (dry adiabat with significant greenhouse heating) do not provide buffering capacity (if indeed CaCO3 were present). Instead, perturbations to T or P(CO2) can produce catastrophic expansion or collapse of the atmosphere. This instability can be generalized to all devolatilization reactions that produce a radiatively active gas in a planetary atmosphere dominated by such gases, and gives a simple thermochemical criterion for whether a reaction could buffer such an atmosphere. Simple decarbonation reactions fail this criterion, suggesting that the abundance of CO2 in a CO2-dominated atmosphere cannot be buffered by chemical reactions with the surface; a similar conclusion holds for the abundance of H2O in an H2O-dominated (steam) atmosphere. Buffering of minor gases is more likely; a mineral buffer equilibrium for SO2 proposed for Venus, FeS2 + CO2 = Fe3O4 + SO2 + CO, passes the thermochemical criterion, as does a reaction involving Ca sulfate. These inferences can be generalized to atmospheres in ‘moist’ adiabatic equilibria, and to extrasolar Venus-like planets, and will help in interpreting the compositions of their atmospheres. 相似文献