P33A-1423
Influence of the Viscosity Stratification on the Density Structure and the Geoid: Application to Venus
We have developed a numerical code to simulate the thermal convection in the mantle of the planets. The material of the mantle is approximated by the uncompressible newtonian fluid. The equations are solved by semi-spectral method. We have two versions of the code – 2D and 3D version. 3D version simulates thermal convection in a classical Boussinesq approximation in models with radially-dependent viscosity. 2D version uses extended Boussinesq approximation and general laterally variable viscosity. We use this code to study the internal dynamics of Venus. Important though indirect information about the structure of this planet provide the topography and the geoid. We use the results of Pauer et al., 2006, who analyze the Venusian geoid and topography and solve the inverse problem for viscosity. We use their best fitting viscosity models and carry out several simulations of mantle convection. We concentrate on the question how the geoid spectra computed in our convection models correspond with those of the observed data and how far is the Venusian mantle density structure from a simplified 2-D approximation used by Pauer et al. Further we study the effect of temperature-dependent viscosity and extended Boussinesq approximation.
P33A-1424
Venus' Chasmata and Earth's Spreading Centers: A Topographic Comparison
Like the Earth, Venus has a global rift system, which has been cited as evidence of tectonic activity, despite the apparent lack of Earth-style plate tectonics. Both systems are marked by large ridges, usually with central grabens. On Earth, the topography of the rifts can be modeled well by a cooling half-space and the spreading of two divergent plates. The origin of the topographic signature on Venus, however, remains enigmatic. Venus' rift zones (termed "chasmata") can be fit by four great circle arcs extending 1000s of kilometers. The Venus chasmata system measures 54,464 km, which when corrected for the smaller size of the planet, nearly matches the 59,200-km total length of the spreading ridges determined for Earth. As on Earth, the chasmata with the greatest relief (7 km in just a 30-km run for Venus) represent the most recent tectonic activity. We use topographic profiles to look for well-understood terrestrial analogs to Venusian features. Focusing on mid-ocean ridge systems on Earth, we examine the variation along individual ridges, or rises, due to the gradual change in spreading rate (and thus cooling times). We then analyze the difference between fast and slow ridges, and propose that this technique may also be used to pick plate boundaries along spreading centers (SAM/AFR vs. NAM/AFR, e.g.). These profiles are then compared to those for Venus' rifts. Topographic profiles are based on the Magellan (Venus) and ETOPO5 (Earth) data sets. Long wavelength features appear similar to spreading systems on Earth, suggesting a deep, thermal cause. Short wavelength features, such as rift troughs and constructional edifices, are quite different, however, as expected from the vastly different surface conditions. Comparison of topographic profiles from Venus and Earth may lend insight into tectonic features and activity on our sister planet.
P33A-1425
The Frequency-Area Distribution of Volcanic Units on Venus: Implications for the Evolution of the Planet.
The areas of volcanic units on Venus have been measured on the 1:5000000 geological maps published by NASA/USGS. These data were used to obtain a frequency-area distribution. The cumulative frequency area distribution of 1544 units cover six orders of magnitude from the largest unit (30 x 106 km2) to the smallest (20 km2). The probability distribution function has been calculated. The medium and large volcanic units correlate well with a power-law (fractal) relation for the dependence of frequency on area with a slope of -1.56. There are fewer small units than the expected values provided by the power-law relation. Our measurements cover 21.02 % of the planetary surface, 3.59 % of the study area was found to be tessera terrain and is excluded from this study of volcanism. The measurements were restricted to areas where geological maps have been published. The analysis was performed on two independent areas of the planet, with a near complete coverage of published maps. In both areas the largest volcanic unit covers a significant portion of the surface (58.75 % and 63.64 %, respectively). For the total measured volcanic units (excluding tessera), these two largest units (that could correspond to the same unit or not) cover the 61.18 % and they are stratigraphically superimposed on older volcanic units which cover 3.37 % of the area. The remaining area (35.45 %) is occupied by younger volcanic units stratigraphically superimposed on the large volcanic unit(s). These results are based on the independent mapping of a large number of geologists with different ideas about the geodynamical evolution of Venus and different criteria for geological mapping. Despite this fact, the frequency-area distribution of the mapped volcanic units supports a catastrophic resurfacing due to the emplacement of the largest unit(s) followed by a decay of volcanism. Our data for the frequency-area distribution of volcanic units provide new support for catastrophic resurfacing models. It is difficult to make our observations compatible with equilibrium, steady-state resurfacing models.
P33A-1426
Is the interior of Venus dry due to a Mega-collision?
It is claimed that the interior of Venus is comparatively dry since even though it is hotter than Earth its rocks are estimated to be stronger, rather than weaker; i.e. there is more water in the interior of Earth to weaken rocks. The high strength of Venus' near surface rocks is argued from slow relaxation of craters and the high correlation between topography and gravity. How is it that Venus' interior is much drier than Earth? The final stage of planetary formation involves impacting of planetary embryos. The leading idea for Earth-Moon formation involves a large glancing impact. Mars is also proposed to have suffered a major collision. I propose that Venus is dry since it formed through an even larger head-on collision of two nearly equal proto-planets. Both bodies would have been totally disrupted though numerical simulations demonstrate that head-on collisions produce no satellites. The disruption would mix iron and water allowing rapid reactions, releasing hydrogen from the water. At low pressures it would be released to hydrogen gas which would be light enough to escape the assembling planet, and possibly released to iron hydride at very high pressures which would sink to the core. As a result the silicate part of the final assembled planet would be virtually devoid of water. A simple test will be to search for hydrated minerals on the surface of Venus, which are predicted to be stable if they exist. If this hypothesis is correct they should be rare.
P33A-1427
Stellar Occultation of the Ultraviolet Nitric Oxide Nightglow with SPICAV on Board Venus Express
Ultraviolet nightglow have been detected on Venus for the first time by Barth and al., in 1968, from mariner 5, then identified like to be nitric oxide nightglow by Feldmann and al., and by Stewart and Barth, in 1979, with Pioneer. SPICAV (SPectroscopy for the Characteristics of the Atmosphere of Venus), currently in fly on board Venus Express, also see them. We descibe here a direct model allowed to reproduce this nitric oxide nightglows. It is a first approach in a better understanding of the dynamic phenomena of the venusian thermosphere. This nightglows are due to radiative recombinaison process. On the dayside of the planet, we have nitrogen and oxygen atoms created by UV disssociation of N2, CO2 and O2. This atoms are transported on the nightside, where they recombine themselves and emit an ultraviolet radiation. Thus NO nightglow are tracers of the descending branch of the solar, anti-solar circulation in the thermosphere of Venus. The model using geometric parameters who are fixed for each observation, simulate the travel of a ray of light inside the spectrometer and reproduce the spectra of nightglows. We model a nitric oxide layer, controled by his borderline altitudes and his brightness. Wa have to adjust the parameters of the layer to fit the data. The results, in the making, already are very supporting. Thus, fit the ultraviolet nitric oxide nightglows, as a tracer of the venusian thermospheric circulation, will lead to a better understanding of the dynamic phenomena in the same region of this atmosphere.
P33A-1428
VENUSIAN UPPER HAZE PROPERTIES: DETECTION OF A MULTIMODAL DISTRIBUTION AT HIGH ALTITUDE BY SPICAV/SOIR
The SPICAV suite of instruments is composed of three separate channels: UV (110 to 320 nm), near-IR (0.7 to 1.7 μm) and the SOIR channel (2.3 to 4.4 μm). On specific opportunities, all three channels have operated simultaneously during solar occultations and have provided almost contiguous spectral information from 0.11 to 4.4 μm. Occultation observations provide several advantages, in particular it does not require cross-calibrating the channels together as scientific analysis is based on relative measurements; i.e. atmospheric transmissions which are the ratio of spectra obtained at a given altitude where the atmosphere produces some attenuation onto that collected outside the atmosphere where the sun can be observed free of any absorption. Haze opacities are readily retrieved using Beer-Lambert's law and vertical distribution from 65 to 120 km is inferred using regular onion peeling technique. Over the interval covered by SPICAV/SOIR, the spectral behavior of haze particles can be fully and robustly evaluated since the size parameter varies by more than one order of magnitude. Extraction of extinction coefficients have been performed for all three channels, allowing derivation of size distribution parameters. Details on the observations made for each channel will be presented. Profiles exhibit peculiar wavy structures that suggest gravity wave vertical propagations or localized destruction processes. Size distribution results will be discussed, in particular the possibility for a multi-modal distribution potentially implying different processes of formation and destruction at work in the mesosphere of Venus.
P33A-1429
Venus Mesospheric Water Vapour From VIRTIS-H VEX Dayside Measurements
The Venus-Express mission works in orbit around Venus since April 11th 2006. VIRTIS-H is a high resolution (1-3 nm) IR channel of the VIRTIS mapping spectrometer. Venus nadir dayside spectra measured with VIRTIS-H have been used to derive simultaneously the altitude of the cloud tops and water abundance in the mesosphere. A line-by-line multiple scattering code has been used to fit the CO2 bands at 2.48 micron and H2O bands at 2.56 micron. Clouds have been simulated with "mode 2" particles of 75 percent H2SO4 and exponential vertical profile in the mesosphere with the scale height of 4 km. Highest quality data has been selected from 8 orbits with local times from 10 to 15 h, and a special attention has been given to the regions near the local noon. We obtained an average cloud top altitude of 75 +/-1 km for low latitudes regions, equator ward of 30 degrees. Results for higher latitude are under examination. Water vapour abundance in low latitudes regions is found to be about 1 ppm at 75 km, with weak variations in the selected orbits and up to 2 -3 ppm in the others, besides with no indication of enhancement observed in 80s from Pioneer Venus observations. Our measurements of cloud top altitude and H2O abundance agree well with other VEX observations: VIRTIS-M and SPICAV, respectively.
P33A-1430
The Venus Neutral Atmosphere from the Radio Science Experiment VeRa on Venus Express
The Venus Express Radio Science Experiment VeRa is sounding the Venus neutral atmosphere and ionosphere using the spacecraft radio subsystem in the oneway radio link mode at X-band (8.4 GHz) and S- band (2.3 GHz). An Ultrastable Oscillator (USO) provides a high quality onboard frequency reference source for the derivation of electron density profiles in the ionosphere and profiles of pressure, temperature and neutral number density of the neutral atmosphere. Radial profiles of neutral number density derived from the occultations cover the altitude range 40 to 90 km, which are converted to vertical profiles of temperature and pressure. The polar orbit of Venus Express provides the opportunity to study the atmosphere at all planetocentric latitudes under varying illumination conditions. Five occultation seasons could be covered so far during the Venus Express mission resulting in a data set of more than 150 profiles of the neutral atmosphere. The thermal structure is investigated with regard to the latitudinal and temporal variability. A distinct cold collar region could be observed on both hemispheres. The tropopause altitude increases in this latitude region while the tropopause temperature shows a strong decrease. Profiles of static stability are found to be latitude-dependent and nearly adiabatic in the middle cloud region.
P33A-1431
The Structure of the Venus Ionosphere
The radio science experiment Vera on Venus Express (VEX) sounds the ionosphere of Venus during so- called occultation seasons. An Ultrastable Oscillator connected to the onboard transmitters allows the transmission of stabilized dual-frequency one-way radio signals which makes it feasible to record the ingress and the egress from occultation as well. Half of the observations of each occultation season is at day, the other half at nighttimes. The ingress location makes a latitudinal cut through one hemisphere from equatorial to polar regions during the occultation season and back while the egress location stays at high polar latitudes of the opposite hemisphere. Seventy occultations with about 140 electron density profiles have been observed. The entire structure of the ionosphere can be observed: from the base at 115 km, a well established double layer daytime structure V1 and V2 (main peak) at 130 km and 150 km, respectively, a bulge in the topside, the well established ionopause feature. The behaviour of peak densities and peak altitudes of both V1 and V2 as a function of solar zenith angle as seen with VEX will be compared with past observations, models and conclusions.
P33A-1432
The ionosphere of Venus: Venus Express radio-science data and model comparison
Ionospheric profiles of Venus are regularly measured by the Venus Express radio science experiment (VERA) since April 2006. A few profiles characterized by different solar zenith angles and latitudes are analysed and successfully compared with a one-dimensional coupled fluid-kinetic ionosphere model. We explain in details an original method which consists in retrieving some neutral parameters from such a comparison. This method is based on a study that was carried out for the terrestrial ionosphere at high-latitude (Blelly et al. 1996).
P33A-1433
Observations of Nonlinear Ionopause Waves in 32Hz Venus Express Magnetometer Data
The solar wind interacts directly with the ionosphere of Venus. This is due to the lack of an intrinsic planetary magnetic field. This is significantly different to the terrestrial case where the ionosphere is protected by the Earth's magnetic field. The shear velocity profile at the boundary between the solar wind and Venusian ionosphere can lead to the formation of nonlinear waves along the boundary. High temporal resolution (32Hz) magnetic field data collected by Venus Express is used to analyse the structure, location and rate of occurrence of nonlinear waves on the ionopause. The implications of these observations, with respect to mass loading of the solar wind with ionospheric material and the redistribution of dayside ionospheric plasma to maintain the nightside ionosphere are discussed.
P33A-1434
Tropospheric CO on Venus: Abundance and Dynamics
The two and a half years of data accumulated by Venus Express since April 2006 have led to an improved understanding of the bulk global cloud layer and the deep atmosphere composition and dynamics on Venus. In particular, global maps of tropospheric CO retrieved at the 2.32 micron 2-0 band by the VIRTIS spectrometer reveal details of the latitudinal extent of the Hadley circulation, and previously unseen diurnal variability. An optimal estimation spectral fitting algorithm is used to retrieve a global map of CO at 35km, which shows an equator-to-pole increase from 23 to 32 ppm with an uncertainty of 2 ppm. A faster band-ratio technique yields a very similar estimate of the abundance of CO and can be applied to a much larger data set, allowing meridional and temporal variability to be examined. The subsequent analysis of 1450 VIRTIS image cubes reveals more complex features in the tropospheric CO distribution, bringing a new perspective on the meteorology of the lower atmosphere.
P33A-1435
Gravity waves in Venus upper atmosphere revealed by CO2 Non Local Thermodynamic Equilibrium emissions
The imaging capabilities of VIRTIS-M infrared spectrometer on board Venus Express mission are used to analyze perturbations of CO2 non local thermodynamic equilibrium emissions in the thermosphere of Venus. These emissions at 4.3µm wavelength originate from the 110-140 km altitude range and are sensitive to density perturbations. They present wave like perturbations of about 0.5% root mean square amplitude of background signal with wavelengths in the 90-400 km range. The horizontal phase velocities are similar in magnitude and direction from one orbit to the other, with averages of 70 m/s westward and 30 m/s northward. The geographical wave distribution and the orientation of wavefronts demonstrate that the polar vortex at work in the cloud layer is the source of these gravity waves. The large westward zonal phase velocity in the 9h-15h local time range favours the super-rotation dynamics within 30 degrees from south pole, and at least up to 115 km altitude. Because the maximum altitude and sensitivity of these emissions to density perturbations is not quantified, a quantitative estimate of the upward energy transfer and its effect on the atmosphere circulation cannot be done. However, this study demonstrates the strong influence of the polar vortex on the Venus atmosphere circulation up to the thermosphere.
P33A-1436
Effects of planetary-scale waves on temporal wind variations in the Venusian thermosphere
In recent years, the importance of planetary-scale waves for dynamics of the Venusian upper atmosphere has been recognized. For example, Forbes and Knopliv [2007] suggested propagations of planetary-scale waves originated in the cloud deck to the thermosphere from reanalysis of the Magellan spacecraft data. In addition, recent simulation studies suggest importance of tidal waves for the superrotation in the Venusian cloud deck [Takagi and Matsuda, 2007]. Venus Climate Orbiter (VCO), which will be launched in 2010 as the second Japanese planetary mission, is expected to provide precious information about upward propagating planetary scale-waves which can't be obtained by Venus Express because of the polar orbit and the close-up observations. In order to understand effects of the planetary-scale waves propagating from the cloud top on the thermospheric circulation, we have developed a new general circulation model (GCM) which includes about 80-200 km altitude region. The GCM solves the primitive equations for momentum, energy and composition. The solar EUV heating, NIR heating and 15μm Radiative cooling are considered. We also consider O, CO and CO2 as the major composition of the Venusian mesosphere and thermosphere. The horizontal and vertical resolutions are 10° in longitude, 20° in latitude, and 0.5 scale height in altitude, respectively. In this study, we perform GCM simulations with use of global distributions of planetary-scale waves taking into account the recent simulation results [e.g., Takagi and Matsuda, 2007] and the past observations [e.g., Del genio and Rossow, 1990]. We will also develop a method for GCM simulations with the VCO data.
P33A-1437
Complex Permittivity Model of Venus Atmosphere and Implications for Design of Imaging Altimeter and INSAR Orbiters
To design altimeter and interferometric SAR (InSAR) systems for measuring Venus' topography, the effects of Venus' atmosphere on the signals need to be investigated. These radar systems are envisioned to operate at X-band, and therefore, a model of Venus atmosphere permittivity profile at X-band is required and has been developed in this work. The effect of signal propagation through this atmosphere and its implication in designing the altimeter and the InSAR instruments are also investigated. The model was constructed for the complex dielectric constant of the atmosphere. Using relations between permittivity and polarization of polar material, the real part of the atmosphere dielectric constant was obtained by calculating the total polarization of the mixture of known atmospheric components including CO2, N2, H2O, SO2, H2SO4, CO, and OCS. The contribution of each atmospheric component to the mixture polarization was calculated based on given temperatures and component densities in the mixture. For each atmospheric component, the polarization was modeled as a function of frequency, temperature, and pressure based on available information in literature. Imaginary part of the atmospheric dielectric constant was calculated by superposing the measured absorptions of mixture components. The temperature and pressure dependences of absorption of each component were modeled according to measurement data and published information. Hence, based on several datasets inferred or directly measured from previous explorations of Venus, the complex dielectric constant profile has been constructed. The validity of the atmosphere permittivity model has been verified by comparing simulation results with measurement data of Venus atmosphere, e.g., from nadir refractivity and absorption measured by the Magellan mission for a portion of the profile. Using this simulated dielectric constant profile, the X-band electromagnetic wave propagation in Venus atmosphere has been modeled, in particular for phase delay and ground pixel center shift of prospective altimeter and InSAR systems. Sensitivity of those quantities to perturbations in atmospheric profile has been investigated as well. The results indicate that radar signal processing and image formation schemes can tolerate at least up to 10% uncertainty in our knowledge of the atmospheric permittivity profile, therefore holding promise that such systems can be successful in producing accurate surface topography for Venus.
P33A-1438
Remote Raman – LIBS Geochemical Investigation under Venus Atmospheric Conditions
The extreme Venus surface temperature (740K) and atmospheric pressure (93 atm) creates a challenging environment for future lander missions. The scientific investigations capable of Venus geochemical observations must be completed within several hours of the landing before the lander will be overcome by the harsh atmosphere. A combined remote Raman – LIBS (Laser Induced Breakdown Spectroscopy) instrument is capable of accomplishing the geochemical science goals without the risks associated with collecting samples and bringing them into the lander. Raman and LIBS are highly complementary analytical techniques where Raman spectroscopy is used to determine the sample molecular structure and LIBS is employed to quantitatively determine the elemental composition. Wiens et al. (2005) and Sharma et al. (2006) demonstrated that one can integrate both analytical techniques into a single instrument capable of planetary missions. Here, we will present data that demonstrates the utility of both Raman spectroscopy and LIBS under Venus conditions using separate instruments. All of the samples in these experiments were placed in a pressure vessel containing 93 atm of CO2 at 150°C and the vessel was placed 1.6m from the telescope. The elemental analysis was completed with a dual pulsed (DP) LIBS instrument employing two Nd:YAG lasers operating at 1064nm. These lasers were focused onto the sample surface and the emission was collected with a Catalina Scientific Echelle Spectrometer connected to an intensified charge coupled device (ICCD). These experiments involved probing several rock powder standards and minerals. The LIBS elemental analysis involved generating a partial least squares (PLS) model with the rock powder standards to quantitatively determine the major elemental abundance. The Raman spectra of minerals were collected up to 970 K at 9 m with a frequency doubled Nd:YAG laser operating at 532nm and the backscattered light was collected with a transmission spectrometer connected to another ICCD with 2 μs gate during daytime. A comparison of Raman spectra of gypsum (CaSO4.2H2O), dolomite (CaMg(CO3))2), olivine (Mg2Fe2-xSiO4) as a function of temperature shows that the Raman lines remains sharp and well defined even in the high temperature spectra. These time-resolved Raman measurements show that high temperature of minerals will not be a limitation and remote Raman spectroscopy would be a potential tool for rapidly exploring Venus surface mineralogy and surface processes.
P33A-1439
Nuclear Polar VALOR: An ASRG-Enabled Venus Balloon Mission Concept
In situ exploration of Venus is expected to answer high priority science questions about the planet's origin, evolution, chemistry, and dynamics as identified in the NRC Decadal Survey and in the VEXAG White Paper. Furthermore, exploration of the polar regions of Venus is key to understanding its climate and global circulation, as well as providing insight into the circulation, chemistry, and climatological processes on Earth. In this paper we discuss our proposed Nuclear Polar VALOR mission, which would target one of the polar regions of Venus, while building on design heritage from the Discovery class VALOR concept, proposed in 2004 and 2006. Riding the strong zonal winds at 55 km altitude and drifting poleward from mid-latitude this balloon-borne aerial science station (aerostat) would circumnavigate the planet multiple times over its one- month operation, extensively investigating polar dynamics, meteorology, and chemistry. Rising and descending over 1 km altitude in planetary waves - similar to the two VEGA balloons in 1985 - onboard instrumentation would accurately and constantly sample and measure other meteorological and chemical parameters, such as atmospheric temperature and pressure, cloud particle sizes and their local column abundances, the vertical wind component, and the chemical composition of cloud-forming trace gases. As well, when viewed with terrestrial radio telescopes on the Earth-facing side of Venus, both zonal and meridional winds would be measured to high accuracy (better than 10 cm/sec averaged over an hour). Due to three factors: the lack of sunlight near the poles; severe limitations on the floating mass-fraction available for a power source; and the science requirements for intensive and continuous measurements of the balloon's environment and movement, a long-duration polar balloon mission would require a long-lived internal power source in a relatively lightweight package. For our concept we assumed an Advanced Stirling Radioisotope Generator (ASRG). In return, this mission would provide two orders of magnitude more science data than expected from the original battery-powered VALOR concept, and could reduce measurement uncertainties by a factor of five. In addition to the science return, the secondary objective of this proposed mission would be to space qualify ASRGs through all mission phases and in various operating environments. Lifetime testing would be demonstrated using a second ASRG on the carrier that would keep operating after the in-situ element is delivered. Based on the results of this and another eight ongoing NASA funded studies, NASA will make a decision about the inclusion of ASRGs in the next Discovery AO, due in the summer of 2009.