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71.
Thierry Montmerle Philippe Claeys Muriel Gargaud Purificatión López-García Hervé Martin Robert Pascal Jacques Reisse Franck Selsis 《Earth, Moon, and Planets》2006,98(1-4):299-312
This concluding chapter is divided into two main parts. The first part is a summary of the main facts and events which constitute the present body of knowledge of the chronology of life in the solar system, in the form of “highlights” in astronomy, geology, chemistry and biology. The second part raises the interrogation “Is life universal?”, and tries to provide answers based on these facts and events. These answers turn out to differ widely among the various disciplines, depending on how far they feel able to extrapolate their current knowledge. 相似文献
72.
Baltis Vallis is a 6800-km long canali-type channel on Venus. Canali have a unique combination of morphological characteristics: extraordinary length, a single main conduit, and a degree of similarity to terrestrial rivers. These characteristics have given rise to intensive discussions on whether the origin of canali is erosional or constructional. Cross-sectional profiles of such channels reveal the detailed morphology of the structure and enable us to distinguish between these two possible origins; however, canali are just several kilometers wide and are therefore too small for the construction of cross-sectional profiles from Magellan altimetry data. Instead, we propose a new method for reconstructing short-wavelength topography using brightness data from Synthetic Aperture Radar images. We apply Muhleman's backscattering function to the backscatter intensity calculated from the brightness of Magellan Full-Resolution SAR Map images. The estimated vertical error of this new method is less than 5 m for a distance of 1 km across the channel. We studied 120 sites along an approximately 6000 km extent of Baltis Vallis. The channel profiles reveal that in nearly 90% of these sites, the bottom surface of the channel is lower than the surrounding plains by 20-100 m. Clear levee structures and intra-channel ridges are recognized in about 30 and 25%, respectively, of the sites analyzed within Baltis Vallis. Most of the levees occur in the upper segment of Baltis Vallis, while intra-channel ridges are mostly confined to the region between 1500 and 3000 km downstream from the probable source. The average depth and width of the channel are 46 m (standard deviation: 16 m) and 2.2 km (standard deviation: 0.4 km), respectively, and the depth profile along the channel is highly undulatory. The groove-like morphology and paucity of levee structures indicates an erosional origin. Furthermore, the observed undulations in depth along the channel indicate that Baltis Vallis most likely formed by mechanical erosion. The observed morphological transition from levees to intra-channel ridges suggests that the channel-forming processes changed across an area located approximately 1500 km from the source. Carbonatite is the most likely candidate material for the low-viscosity fluid that formed Baltis Vallis. 相似文献
73.
The global circulation of the Venus atmosphere is characterized at cloud level by a zonal super rotation studied over the years with data from a battery of spacecrafts: orbiters, balloons and probes. Among them, the Galileo spacecraft monitored the Venus atmosphere in a flyby in February 1990 in its route toward Jupiter. Since the flyby was almost equatorial, published analysis of zonal winds obtained from displacements of cloud elements on images obtained by the SSI camera [Belton, M.J.S., and 20 colleagues, 1991. Science 253, 1531-1536] stop at latitudes 50° north and south. In this paper we present new results on Venus winds based on a reanalysis of an extended set of images obtained at two wavelengths, 418 nm (violet) and 986 nm (near infrared), that sense different altitude levels in the upper cloud. Our main result is that we have been able to extend the zonal wind profile up to the polar latitudes: 70° N and 70° S at 418 nm and 70° N at 986 nm. Binned and smoothed profiles are given in tabular form. We show that the zonal winds drop in their velocity poleward of latitudes 45° N and 50° S where an intense meridional wind shear develops at the two cloud levels. Our data confirm the magnitude of this shear, retrieved previously from radio occultation data, but disagrees with it in the latitudinal location of the sheared region. The new wind data can be used to recalibrate the zonal winds retrieved from the previous measurements of the temperature field and the cyclostrophic balance assumption. The meridional profiles of the zonal winds at the two cloud levels are used to assess the vertical wind shear in the upper cloud layer as a function of latitude and locate the most unstable region. 相似文献
74.
Submillimeter line observations of CO in the Venus middle atmosphere (mesosphere) were observed with the James Clerk Maxwell Telescope (JCMT, Mauna Kea) about the May 2000, February 2002 superior and July 1999, March 2001 inferior conjunctions of Venus. Combined 12CO and 13CO isotope spectral line measurements at 345 and 330 gHz frequencies, respectively, provided enhanced sensitivity and vertical coverage for simultaneous retrievals of atmospheric temperatures and CO mixing ratios over the altitude region 75-105 km with vertical resolution 4-5 km. Supporting millimeter 12CO spectral line observations with the Kitt Peak 12-m telescope (Steward Observatories) provide enhanced temporal coverage and CO mixing sensitivity. Implementation of CO/temperature profile retrievals for the 2000, 2002 dayside (superior conjunction) and 1999, 2001 nightside (inferior conjunction) periods yields a first-time definition of the vertical structure and diurnal variation of a low-to-mid-latitude mesopause within the Venus atmosphere. At the times of these 1999-2002 observations, the Venus mesopause was located at a slightly lower level in the nightside (0.5 mbar, ∼87 km) versus the dayside (0.2 mbar, ∼91 km) atmosphere. Average diurnal variation of Venus mesospheric temperatures appears to be ≤ 5 K at and below the mesopause. Diurnal variation of Venus thermospheric temperatures increases abruptly just above the mesopause, reaching 50 K by the 0.01-mbar pressure level (∼102 km). Atmospheric temperatures above and below the Venus mesopause exhibited global-scale (≥4000 km horizontal) variations of large amplitude (7-15 K) on surprisingly short timescales (daily to monthly) during the 2001 nightside and 2002 dayside observing periods. Venus dayside mesospheric temperatures observed during the 2002 superior conjunction were also 10-15 K warmer than observed during the 2000 superior conjunction. A characteristic timescale for these global temperature variations is not defined, but their magnitude is comparable to previous determinations of secular variability in nightside mesospheric temperatures (Clancy and Muhleman, 1991). 相似文献
75.
Planetary atmospheres are complex dynamical systems whose structure, composition, and dynamics intimately affect the propagation of sound. Thus, acoustic waves, being coupled directly to the medium, can effectively probe planetary environments. Here we show how the acoustic absorption and speed of sound in the atmospheres of Venus, Mars, Titan, and Earth (as predicted by a recent molecular acoustics model) mirror the different environments. Starting at the surface, where the sound speed ranges from ∼200 m/s for Titan to ∼410 m/s for Venus, the vertical sound speed profiles reveal differences in the atmospheres' thermal layering and composition. The absorption profiles are relatively smooth for Mars, Titan, and Earth while Venus stands out with a noticeable attenuation dip occurring between 40 and 100 km. We also simulate a descent module sampling the sound field produced by a low-frequency “event” near the surface noting the occurrence of acoustic quiet zones. 相似文献
76.
Asteroid 2002 VE68, a quasi-satellite of Venus 总被引:1,自引:0,他引:1
S. Mikkola R. Brasser P. Wiegert K. Innanen 《Monthly notices of the Royal Astronomical Society》2004,351(3):L63-L65
77.
Laura Schaefer 《Icarus》2004,168(1):215-219
Chemical equilibrium calculations of volatile metal geochemistry on Venus show that high dielectric constant compounds of lead and bismuth such as PbS (galena), Bi2S3 (bismuthite) or Pb-Bi sulfosalts condense in the venusian highlands and may be responsible for the low radar emissivities observed by Magellan and Pioneer Venus. Our calculations also show that elemental tellurium is unstable on Venus' surface and will not condense below 46.6 km. This is over 30 km higher than Maxwell Montes, the highest point on Venus' surface. Elemental analyses of Venus' highlands surface by laser induced breakdown spectroscopy (LIBS) and/or X-ray fluorescence (XRF) can verify the identity of the heavy metal frost on Venus. The Pb-Pb age of Venus could be determined by mass spectrometric measurements of the Pb207/Pb204 and Pb206/Pb204 isotopic ratios in Pb-bearing frosts. All of these measurements are technologically feasible now. 相似文献
78.
C. Z. Zhang 《Earth, Moon, and Planets》1996,75(2):135-139
By means of the comparison between dynamical figures of Venus and the Earth, we confirmed that the nonhydrostatic moment difference ratios of the two celestial bodies are approximately equal. The hydrostatic flattening of the external equipotential surface of Venus according to the theory of internal field is equal to about 6 × 10–8, but the observed value of Venusian equatorial flattening is equal to 6.8 × 10–6. The latter is two orders of magnitude more than the former. These facts indicate that Venus itself is far from hydrostatic equilibrium. 相似文献
79.
T. Tavenner E.F. Young M.A. Bullock J. Murphy S. Coyote 《Planetary and Space Science》2008,56(10):1435-1443
We present a map of the global mean lower cloud coverage of Venus. This map is the average of 35 nights of 2.26 μm night side observations taken at NASA's Infrared Telescope Facility on Mauna Kea, over the years spanning 2001-2007. The atmosphere of Venus is a very dynamic system, and the lower clouds are constantly changing [Crisp, D., Allen, D.A., Grinspoon, D.H., Pollack, J.B., 1991a. The dark side of Venus: near-infrared images and spectra from the Anglo-Australian Observatory. Science, 253, 1263-1266]. By studying average cloud coverage, the daily variations are suppressed in order to see the underlying persistent cloud pattern. We find a relatively thick but highly variable equatorial band of clouds (±20° in latitude) and more quiescent mid-latitude clouds that are less opaque on average, with persistent cloudiness near the poles. We show that there is enough variation between our daily observations or between observations taken in different months that they cannot be considered individually representative of the global mean. We also compare the cloud coverage map to the topography of Venus and find no definitive correlations with high altitude features. 相似文献
80.
A. Fedorov C. Ferrier S. Barabash C. Mazelle H. Gunell K. Brinkfeldt A. Grigoriev M. Yamauchi W. Baumjohann A.J. Coates D.R. Linder K.C. Hsieh J.-J. Thocaven H. Koskinen T. Sales P. Riihela N. Krupp J. Luhmann S. Orsini A. Mura M. Maggi P. Brandt K. Szego R.A. Frahm J.R. Sharber P. Bochsler 《Planetary and Space Science》2008,56(6):812-817
We have an unique opportunity to compare the magnetospheres of two non-magnetic planets as Mars and Venus with identical instrument sets Aspera-3 and Aspera-4 on board of the Mars Express and Venus Express missions. We have performed both statistical and case studies of properties of the magnetosheath ion flows and the flows of planetary ions behind both planets. We have shown that the general morphology of both magnetotails is generally identical. In both cases the energy of the light (H+) and the heavy (O+, etc.) ions decreases from the tail periphery (several keV) down to few eV in the tail center. At the same time the wake center of both planets is occupied by plasma sheet coincident with the current sheet of the tail. Both plasma sheets are filled by accelerated (500-1000 eV) heavy planetary ions. We report also the discovery of a new feature never observed before in the tails of non-magnetic planets: the plasma sheet is enveloped by consecutive layers of He+ and H+ with decreasing energies. 相似文献