Observable effects of convection and gravity waves on the Venus condensational cloud     

Kevin McGouldrick  Owen B. Toon 《Planetary and Space Science》2008,56(8):1112-1131

We present results of a simple two-dimensional model investigating the observable effects that convective motions and gravity waves can have on the condensational Venus cloud. Gravity waves have been observed in the Venus atmosphere in the form of temperature scintillations in the Magellan and Pioneer Venus occultation data. Multiple in situ probes and long-duration remote observations indicate the presence of convective motions in the Venus clouds. Dynamical studies by others have suggested that gravity waves can exist in the stable regions of the Venus atmosphere above the middle clouds and beneath the middle clouds, and likely are triggered by flow past sub-cloud plumes caused by convective overshooting. We find that a simplified treatment of convective kinematics generates variation in the Venus condensational cloud consistent with the observed variability of optical depth and brightness temperature. Specifically, we find that the downdraft regions in our simulated convective cell exhibit a decrease in cloud optical depth of around Δτ∼10. The brightness temperature ranges from about 460 K in the downdraft regions of the simulated convective cells, to about 400 K in the simulated updrafts. We also find that gravity waves launched by obstacles (such as overshooting convective plumes) near the cloud base exhibit horizontal wavelengths comparable to the separation between convective cells, and generate variations in brightness temperature that should be observable by instruments such as VIRTIS on Venus Express. However, a more robust treatment of the atmospheric dynamics is needed to address adequately these interactions between the clouds and the mesoscale dynamics.  相似文献   

Monitoring Venus’ mesospheric winds in support of Venus Express: IRAM 30-m and APEX observations     

E. Lellouch  G. Paubert  A. Moullet 《Planetary and Space Science》2008,56(10):1355-1367

We report on direct wind measurements in Venus’ mesosphere (90-115 km), performed in support of Venus Express, and based on CO millimeter observations. Most observations, sampling the CO(2-1) and CO(1-0) lines, were acquired with the IRAM 30-m telescope, over four distinct periods: (i) Summer 2006; (ii) May-June 2007, in association with the coordinated ground-based campaign; (iii) August 2007 inferior conjunction and (iv) September 2007. In the latter period, additional measurements (CO(3-2)) were obtained with the APEX 12-m telescope. Overall, the measurements indicate a large body of temporal variability of the Venus mesospheric field, but general features emerge: (i) winds strongly increase with altitude within the mesosphere, by a factor of 2-3 over a decade in pressure; (ii) many, but not all, of our observations can be viewed as the superposition of zonal retrograde and subsolar-to-antisolar (SSAS) flows of comparable speeds, typically 30-50 m/s near 0.1 mbar () and 90-120 m/s near 0.01 mbar () (iii) the wind field was very stable over three consecutive observing days in May-June 2007, but much more variable on a similar time base in August 2007 (iv) at a resolution, the nightside wind field appears very complex, with evidence that the SSAS flow does not reach high latitudes, and possible evidence for additional meridional winds. Our Summer 2006 observations, which sample Venus’ dayside, seem to suggest that a prograde zonal flow is superimposed to the SSAS circulation for this period. This surprising result, which implies a pre-midnight convergence of the wind field, requires confirmation, and fruitful comparisons may be obtained from the analysis of motions in the O₂ emission images, as observed by Venus Express.  相似文献   

A comparison between terrestrial, Cytherean and lunar impact cratering records     

David W. Hughes 《Monthly notices of the Royal Astronomical Society》2002,334(3):713-720

A comparison between the terrestrial, Cytherean and lunar cratering records indicates that the large craters (diameters   D > D ₀)  on these surfaces all have cumulative numbers that are proportional to   D ^-2.59±0.05  . Atmospheres have a negligible effect on the formation of   D > D ₀  craters. It is shown that this limiting diameter is  45±3 km  in the case of Venus, and  21.0±1.5 km  in the case of Earth. In this large-diameter range, there are about  1.51±0.34  times more craters, per unit area, on Venus than on the Earth, and about  1350±310  times more craters, per unit area, on the Moon than on the Earth.  相似文献   

Helium on Mars and Venus: EUVE observations and modeling     

Vladimir A. Krasnopolsky  G. Randall Gladstone 《Icarus》2005,176(2):395-407

Long-exposure spectroscopy of Mars and Venus with the Extreme Ultraviolet Explorer (EUVE) has revealed emissions of He 584 Å on both planets and He 537 Å/O⁺ 539 Å and He⁺ 304 Å on Venus. Our knowledge of the solar emission at 584 Å, eddy diffusion in Mars' upper atmosphere, electron energy distributions above Mars' ionopause, and hot oxygen densities in Mars' exosphere has been significantly improved since our analysis of the first EUVE observation of Mars [Krasnopolsky, Gladstone, 1996, Helium on Mars: EUVE and Phobos data and implications for Mars' evolution, J. Geophys. Res. 101, 15,765-15,772]. These new results and a more recent EUVE observation of Mars are the motivation for us to revisit the problem in this paper. We find that the abundance of helium in the upper atmosphere, where the main loss processes occur, is similar to that in the previous paper, though the mixing ratio in the lower and middle atmosphere is now better estimated at 10±6 ppm. Our estimate of the total loss of helium is almost unchanged at 8×¹⁰²³ s⁻¹, because a significant decrease in the loss by electron impact ionization above the ionopause is compensated by a higher loss in collisions with hot oxygen. We neglect the outgassing of helium produced by radioactive decay of U and Th because of the absence of current volcanism and a very low upper limit to the seepage of volcanic gases. The capture of solar wind α-particles is currently the only substantial source of helium on Mars, and its efficiency remains at 0.3. A similar analysis of EUV emissions from Venus results in a helium abundance in the upper atmosphere which is equal to the mean of the abundances measured previously with two optical and two mass spectrometers, and a derived helium mixing ratio in the middle and lower atmosphere of 9±6 ppm. Helium escape by ionization and sweeping out of helium ions by the solar wind above the ionopause is smaller than that calculated by Prather and McElroy [1983, Helium on Venus: implications for uranium and thorium, Science 220, 410-411] by a factor of 3. However, charge exchange of He⁺ ions with CO₂ and N₂ between the exobase and ionopause and collisions with hot oxygen ignored previously add to the total loss which appears to be at the level of 10⁶ cm⁻² s⁻¹ predicted by Prather and McElroy [1983, Science 220, 410-411]. The loss of helium is compensated by outgassing of helium produced by radioactive decay of U and Th and by the capture of the solar wind α-particles with an efficiency of 0.1. We also compare our derived α-particle capture efficiencies for Mars and Venus with observed X-ray emissions resulting from the charge exchange of solar wind heavy ions with the extended atmospheres on both planets [Dennerl et al., 2002, Discovery of X-rays from Venus with Chandra, Astron. Astrophys. 386, 319-330; Dennerl, 2002, Discovery of X-rays from Mars with Chandra, Astron. Astrophys. 394, 1119-1128]. The emissions from both disk and halo on Mars agree with our calculated values; however, we do not see a reasonable explanation for the X-ray halo emission on Venus. The ratio of the charge exchange efficiencies derived from the disk X-ray emissions of Mars and Venus is similar to the ratio of the capture efficiencies for these planets. The surprisingly bright emission of He⁺ at 304 Å observed by EUVE and Venera 11 and 12 suggests that charge exchange in the flow of the solar wind α-particles around the ionopause is much stronger than in the flow of α-particles into the ionosphere.  相似文献   

Oblique incidence hypervelocity impacts on rock     

M. J. Burchell  L. Whitehorn 《Monthly notices of the Royal Astronomical Society》2003,341(1):192-198

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Long-term evolution of the spin of Venus: I. theory     

Alexandre C.M. Correia  Jacques Laskar 《Icarus》2003,163(1):1-23

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Tremolite decomposition on Venus II. Products, kinetics, and mechanism     

Natasha M. Johnson  Bruce Fegley Jr. 《Icarus》2003,164(2):317-333

We present revised tremolite powder thermal decomposition kinetics using previous and newly acquired data from longer time (years instead of months) and lower temperature experiments (<1073 K). We also present kinetic results for decomposition of millimeter- to centimeter-sized tremolite grains. Natural tremolite samples were heated at ambient pressure in flowing CO₂ or N₂ gas from 1023-1238 K. The tremolite decomposition products are a physical mixture of two pyroxene solid solutions (with the bulk composition Dp₅₉En₄₁), a silica polymorph, and water vapor. Decomposition rates were calculated by using the mass loss of the heated samples. Tremolite crystals and crystalline powder decompositions follow different but related Avrami-Erofe'ev (nucleation and growth) kinetic models. The rate equations for thermal decomposition of tremolite crystalline powder and the larger crystal grains are log₁₀k_powder (h⁻¹)=18.69(±0.19)−23,845(±833)/T and log₁₀k_crystal (h⁻¹)=19.82(±0.07)−25,670(±916)/T. The associated apparent activation energies are 456(±16) kJ mol⁻¹ and 491(±18) kJ mol⁻¹, respectively. We propose a decomposition mechanism and suggest that decomposition and dehydroxylation occur simultaneously. The rate-limiting step is proposed to be structural rearrangement of the amphibole structure to the two pyroxenes and silica. This step and the overall decomposition rate are predicted to be independent of pressure from 1 to 100 bars. These kinetic analyses strengthen our previous conclusion (Johnson and Fegley, 2000, Icarus 146, 301-306) that if hydrous minerals, such as tremolite, formed on Venus during a wetter past, then these minerals could still exist at current conditions on Venus' surface today.  相似文献   

Cloud brightness distribution and turbulence in Venus using Galileo violet images     

J. Peralta  R. Hueso  A. Sánchez-Lavega 《Icarus》2007,188(2):305-314

Venus cloud covered atmosphere offers a well-suited framework to study the coupling between the atmospheric dynamics and the structure of the cloud field. Violet images obtained during the Galileo flyby from 12 to 17 February 1990 have been analyzed to retrieve the zonal power spectra of the cloud brightness distribution field between latitudes 70° N and 50° S. The brightness distribution spectra serve as a diagnostic of the eddy kinetic energy spectrum providing indirect information about the distribution of energy along different spatial scales. We composed images covering a full rotation of the atmosphere at the level of the UV contrasted clouds obtaining maps of almost 360° that allowed us to obtain the brightness power spectra from wavenumbers k=1 to 50. A full analysis of the spectrum slope for different latitude bands and ranges of wave numbers is presented. The power spectra follow a classical law k−n with exponent n ranging from −1.7 to −2.9 depending on latitude and the wavenumber range. For the whole planet, the average of this parameter is −2.1 intermediate between those predicted by the classical turbulence theories for three- and two-dimensional motions (n=−5/3 and n=−3). A comparison with previous analysis of Mariner 10 (in 1974) and Pioneer Venus (in 1979) shows significant temporal changes in the cloud global structure and in the turbulence characteristics of the atmosphere.  相似文献   

Beta Regio, Venus: Evidence for uplift, rifting, and volcanism due to a mantle plume   总被引：1，自引：0，他引：1  

Alexander T. Basilevsky  James W. Head 《Icarus》2007,192(1):167-186

Geophysical data have led to the interpretation that Beta Regio, a 2000×25000 km wide topographic rise with associated rifting and volcanism, formed due to the rise of a hot mantle diapir interpreted to be caused by a mantle plume. We have tested this hypothesis through detailed geologic mapping of the V-17 quadrangle, which includes a significant part of the Beta Regio rise, and reconnaissance mapping of the remaining parts of this region. Our analysis documents signatures of an early stage of uplift in the formation of the Agrona Linea fracture belts before the emplacement of regional plains and their deformation by wrinkle ridging. We see evidence that the Theia rift-associated volcanism occurred during the first part of post-regional-plains time and cannot exclude that it continued into later time. We also see evidence that Devana Chasma rifting was active during the first and the second parts of post-regional-plains time. These data are consistent with uplift, rifting and volcanism associated with a mantle diapir. Geophysical modeling shows that diapiric upwelling may continue at the present time. Together these data suggest that the duration of mantle diapir activity was as long as several hundred million years. The regional plains north of Beta rise and the area east and west of it were little affected by the Beta-forming plume, but the broader area (at least 4000 km across), whose center-northern part includes Beta Regio, could have experienced earlier uplift as morphologically recorded in formation of tessera transitional terrain.  相似文献   

An investigation of possible causes of the holes in the condensational Venus cloud using a microphysical cloud model with a radiative-dynamical feedback     

Kevin McGouldrick  Owen B. Toon 《Icarus》2007,191(1):1-24

Near-infrared observations of the nightside of Venus reveal regions of high brightness temperatures. These regions of high brightness temperatures are caused by the localized evaporation of the middle and lower cloud decks, which are about 50 to 60 km above the surface of the planet. We simulate the Venus condensational middle and lower cloud deck with the University of Colorado/NASA Ames Community Aerosol and Radiation Model for Atmospheres (CARMA). Our simulated clouds have similar characteristics to the observed Venus clouds. Our radiative transfer model reproduces the observed temperature structure and atmospheric stability structure within the middle cloud region. A radiative-dynamical feedback occurs which generates mixing due to increased absorption of upwelling infrared radiation within the lower cloud region, as previously suggested by others. We find that localized variations in temperature structure or in sub-grid scale mixing cannot directly explain the longevity and optical depth of the clouds. However, vertical motions are capable of altering the cloud optical depth by a sufficient magnitude in a short enough timescale to be responsible for the observed clearings.  相似文献