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| Correlations between cloud thickness and sub-cloud water abundance on Venus |
| Past spacecraft observations of Venus have found considerable spatial and temporal variations of water vapour abundance above the clouds. Previous searches for variability below the clouds at 30-45 km altitude found no large scale latitudinal gradients, but lacked the spatial resolution to detect smaller scale variations. Here we interpret results from the VIRTIS imaging spectrometer on Venus Express, remotely sounding at near-infrared "spectral window" wavelengths, as indicating that the water vapour abundance at 30-40 km altitude varies from 22 to 35 ppmv (±4 ppmv). Furthermore, this variability is correlated with cloud opacity, supporting the hypothesis that its genesis is linked to cloud convection. It is also possible to fit the observations without requiring spatial variation of water abundance, but this places a strong constraint on the spectral dependence of the refractive index data assumed for the lower cloud particles, for which there is as yet no supporting evidence. |
| Publication date: 26 Jan 2010 |
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| Visible and near-infrared nightglow of molecular oxygen in the atmosphere of Venus |
| The Herzberg II system of O2 has been a known feature of Venus' nightglow since the Venera 9 and 10 orbiters detected its c(0)-X(v") progression more than 3 decades ago. We search for its emission at 400 nm-700 nm in spectra obtained with the VIRTIS instrument on Venus Express. Despite the weakness of the signal, integration over a few hours of limb observations of the planet's upper atmosphere reveals the unambiguous pattern of the progression. The selected data sample mainly the northern latitudes within a few hours of local midnight. The emission is ubiquitous on the nightside of Venus and can be discerned at tangent altitudes from 80 km to 110 km. The average emission vertical profiles of the c(0)-X(v") progression and the O2 a(0)-X(0) band, the latter from simultaneous near-infrared spectra, are quite similar, with their respective peaks occurring within ±1 km of each other. We conclude that the net yield for production of the c(0) state is low, ~1%-2% of the oxygen recombination rate, and that O(3P) and CO2 are the two likely quenchers of the Herzberg II nightglow, although CO cannot be ruled out. We also derive a value of 2.45 × 10-16 cm3 s-1 for the rate constant at which CO2 collisionally quenches the c(0) state. Our VIRTIS spectra show hints of O2 A'(0)-a(v3) emission but no traces of the O (1S-1D) green line at 557.7 nm. |
| Publication date: 23 Dec 2009 |
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| Disappearing induced magnetosphere at Venus: Implications for close-in exoplanets |
| The solar wind interaction with a planetary atmosphere produces a magnetosphere-like structure near the planet whether or not the planet has an intrinsic global magnetic field. In the case of planets like Venus or Mars, which have no global intrinsic magnetic field but possess a significant atmosphere, a magnetosphere is induced in the highly conducting ionosphere by the time-varying magnetic field carried by the solar wind. The induced magnetosphere at Venus and Mars is almost a "permanent" feature of the solar wind interaction. Here we report a Venus Express observation of the absence of the dayside part of the induced magnetosphere, when the interplanetary magnetic field (IMF) is nearly aligned with the solar wind flow. Using MHD simulations for this extreme IMF orientation, we examine the global interaction of the solar wind with Venus when the magnetic barrier disappears. Furthermore, we estimate the atmospheric loss under this extreme situation. While this solar wind aligned IMF interaction with a planet case is presently rare, and even rarer over solar system history, it might be an appropriate analogue of the interaction of a stellar wind with close-in exoplanet. Thus the solar wind interaction with Venus under this extreme condition might provide us a natural laboratory for studying the evolution of the atmospheres of "hot Jupiters" as well as close-in "terrestrial" planets. |
| Publication date: 27 Oct 2009 |
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| Altimetry of the Venus cloud tops from the Venus Express observations |
| Simultaneous observations of Venus by Visible and Infrared Thermal Imaging Spectrometer and Venus Monitoring Camera onboard the Venus Express spacecraft are used to map the cloud top altitude and to relate it to the ultraviolet (UV) markings. The cloud top altitude is retrieved from the depth of CO2 absorption band at 1.6 microns. In low and middle latitudes the cloud top is located at 74 ± 1 km. It decreases poleward of ±50° and reaches 63-69 km in the polar regions. This depression coincides with the eye of the planetary vortex. At the same latitude and hour angle, cloud top can experience fast variations of about 1 km in tens of hours, while larger long-term variations of several kilometers have been observed only at high latitudes. UV markings correlate with the cloud altimetry, however, the difference between adjacent UV dark and bright regions does not exceed several hundred meters. Surprisingly, CO2 absorption bands are often weaker in the dark UV features, indicating that these clouds may be a few hundred meters higher or have a larger scale height than neighboring clouds. Ultraviolet dark spiral arms, which are often seen at about ~70°, correspond to higher altitudes or to the regions with strong latitudinal gradient of the cloud top altitude. Cloud altimetry in the polar region reveals the structure that correlates with the thermal emission maps but is invisible in UV images. This implies that the UV optically thick polar hood is transparent in the near IR. |
| Publication date: 13 Aug 2009 |
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| Venus express: Highlights of the nominal mission |
| Venus Express is the first European (ESA) mission to the planet Venus. Its main science goal is to carry out a global survey of the atmosphere, the plasma environment, and the surface of Venus from orbit. The payload consists of seven experiments. It includes a powerful suite of remote sensing imagers and spectrometers, instruments for in-situ investigation of the circumplanetary plasma and magnetic field, and a radio science experiment. The spacecraft, based on the Mars Express bus modified for the conditions at Venus, provides a versatile platform for nadir and limb observations as well as solar, stellar, and radio occultations. In April 2006 Venus Express was inserted in an elliptical polar orbit around Venus, with a pericentre height of ~250 km and apocentre distance of ~66000 km and an orbital period of 24 hours. The nominal mission lasted from June 4, 2006 till October 2, 2007, which corresponds to about two Venus sidereal days. Here we present an overview of the main results of the nominal mission, based on a set of papers recently published in Nature, Icarus, Planetary and Space Science, and Geophysical Research Letters. |
| Publication date: 22 Jun 2009 |
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| Climate evolution of Venus |
| The processes in the atmosphere, interior, surface, and near-space environment that together maintain the climate on Venus are examined from the specific point of view of the advances that are possible with new data from Venus Express and improved evolutionary climate models. Particular difficulties, opportunities, and prospects for the next generation of missions to Venus are also discussed. |
| Publication date: 20 May 2009 |
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| Structure of the Venus neutral atmosphere as observed by the Radio Science experiment VeRa on Venus Express |
| The European Space Agency Venus Express Radio Science experiment (VeRa) obtained 118 radio occultation measurements of the Venusian atmosphere between July 2006 and June 2007. Southern latitudes are uniformly sampled; measurements in the northern hemisphere are concentrated near the pole. Radial profiles of neutral number density derived from the occultations cover the altitude range 40-90 km, which are converted to profiles of temperature (T) and pressure (p) versus height (h). Profiles of static stability are found to be latitude-dependent and nearly adiabatic in the middle cloud region. Below the clouds the stability decreases at high latitudes. At an altitude of 65 km, the VeRa T[p(h)] profiles generally lie between the Venus International Reference Atmosphere (VIRA) and VIRA-2 models; the retrieved temperatures at any given pressure level typically are within 5 K of those derived from the Pioneer Venus Orbiter Radio Occultation experiments. A large equator-to-pole temperature contrast of ~30 K is found at the 1-bar (1000 hPa) level. The VeRa observations reveal a distinct cold collar region in the southern hemisphere, complementing that in the north. At the latitudes of the cold collars, the tropopause altitude increases relative to higher and lower latitudes by ~7 km while the temperature drops roughly 60 K. The observations indicate the existence of a wave number 2 structure poleward of ±75° latitude at altitudes of 62 km. |
| Publication date: 23 Apr 2009 |
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| Venus Express Mission |
| Venus Express is well and healthy and has now been providing exciting new data from Venus, our nearby twin planet, for over 2 years. Many of the new results are presented and discussed in the subsequent papers in this special section. The overall scientific objective of Venus Express is to carry out a detailed study of the atmosphere of Venus, including the interaction of the upper atmosphere with the solar wind and the interaction of the lowest part of the atmosphere with the surface of the planet. In addition, the plasma environment and magnetic fields as well as some aspects of the surface of the planet are addressed. For the first time, investigations make systematic use of the transparent infrared spectral windows in order to probe the atmosphere in four dimensions: three spatial dimensions plus time. The spacecraft design is taken from Mars Express with some modifications necessary owing to the specific environment around Venus. The payload is composed of three spectrometers, a camera, a magnetometer, an instrument for detecting energetic particles, and a radio science package. The orbit is polar and highly elliptic, with a pericenter altitude of about 200 km over the northern polar region and an apocenter altitude of 66,000 km. Presently, the coverage of the southern hemisphere is very good, but important gaps still do exist. The coverage of the northern hemisphere is much less dense. Venus Express is a part of the European Space Agency's program for the exploration of the inner solar system, which includes missions to study the Sun, Mercury, Venus, the Moon, Mars, and comets and asteroids. |
| Publication date: 19 Mar 2009 |
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| Venus cloud top winds from tracking UV features in Venus Monitoring Camera images |
| To date dynamical observations of the Venus clouds have delivered mainly either only short-term or long-term averaged results. With the Venus Monitoring Camera (VMC) it finally became possible to investigate the global dynamics with a relatively high resolution in space and time on a long-term basis. Our findings from manual cloud feature wind tracking in VMC UV image sequences so far show that the details of the mesospheric dynamics of Venus appear to be highly variable. Although the general rotation of the atmosphere remained relatively stable since Mariner 10, more than 30 years ago, by now, there are indications of short-term variations in the general circulation pattern of the Venus atmosphere at cloud top level. In some cases, significant variations in the zonal wind properties occur on a timescale of days. In other cases, we see rather stable conditions over one atmospheric revolution, or longer, at cloud top level. It remains an interesting question whether the irregularly observed midlatitude jets are indeed variable or simply become shielded from view by higher H2SO4 haze layers for varying time intervals. Winds at latitudes higher than 60°S are still difficult to obtain track because of low contrast and scarcity of features but increasing data is being collected. Over all, it was possible to extend latitudinal coverage of the cloud top winds with VMC observations. Thermal tides seem to be present in the data, but final confirmation still depends on synthesis of Visible and Infrared Thermal Imaging Spectrometer and VMC observations on night and dayside. Although poorly resolved, meridional wind speed measurements agree mainly with previous observations and with the presence of a Hadley cell spanning between equatorial region and about 45°S latitude. |
| Publication date: 17 Mar 2009 |
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| Vortex circulation on Venus: Dynamical similarities with terrestrial hurricanes |
| Some dynamical and morphological similarities exist between the vortex organization of the atmosphere in the northern and southern hemispheres of Venus and the tropical cyclones/hurricanes on Earth. An S-shape feature detected in the center of the vortices on Venus from Pioneer Venus Orbiter and Venus Express observations has also been seen in tropical cyclones. This feature can be simulated with an idealized nonlinear and non-divergent barotropic model and, like in the vortices on Venus and in tropical cyclones, it is found to be transient. Given the challenges in measuring the deep, atmospheric circulation of Venus, the morphological similarities provide clues toward understanding the processes involved in the maintenance of Venus' atmospheric super rotation. |
| Publication date: 25 Feb 2009 |
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| Introduction to the special section on Venus Express: Results of the Nominal Mission |
More than 25 spacecraft from the United States and the Soviet Union visited Venus in the 20th century, but in spite of the many successful measurements they made, a great number of fundamental problems in the physics of the planet remained unsolved [Taylor, 2006; Titov et al., 2006]. In particular, a systematic and long-term survey of the atmosphere was missing, and most aspects of atmospheric behavior remained puzzling. After the Magellan radar mapping mission ended in 1994, there followed a hiatus of more than a decade in Venus research, until the European Space Agency took up the challenge and sent its own spacecraft to our planetary neighbor. The goal of this mission, Venus Express, is to carry out a global, long-term remote and in situ investigation of the atmosphere, the plasma environment, and some aspects of the surface of Venus from orbit [Titov et al., 2001; Svedhem et al., 2007].
Venus Express continues and extends the investigations of earlier missions by providing detailed monitoring of processes and phenomena in the atmosphere and near-space environment of Venus. Radio, solar, and stellar occultation, together with thermal emission spectroscopy, sound the atmospheric structure in the altitude range from 150 to 40 km with vertical resolution of few hundred meters, revealing strong temperature variations driven by radiation and dynamical processes.
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| Publication date: 17 Dec 2008 |
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| Atmospheric structure and dynamics as the cause of ultraviolet markings in the clouds of Venus |
| When seen in ultraviolet light, Venus has contrast features that arise from the non-uniform distribution of unknown absorbers within the sulphuric acid clouds and seem to trace dynamical activity in the middle atmosphere. It has long been unclear whether the global pattern arises from differences in cloud top altitude (which was earlier estimated to be 66-72 km), compositional variations or temperature contrasts. Here we report multi-wavelength imaging that reveals that the dark low latitudes are dominated by convective mixing which brings the ultraviolet absorbers up from depth. The bright and uniform mid-latitude clouds reside in the 'cold collar', an annulus of cold air characterized by 30 K lower temperatures with a positive lapse rate, which suppresses vertical mixing and cuts off the supply of ultraviolet absorbers from below. In low and middle latitudes, the visible cloud top is located at a remarkably constant altitude of 72±1 km in both the ultraviolet dark and bright regions, indicating that the brightness variations result from compositional differences caused by the colder environment rather than by elevation changes. The cloud top descends to 64 km in the eye of the hemispheric vortex, which appears as a depression in the upper cloud deck. The ultraviolet dark circular streaks enclose the vortex eye and are dynamically connected to it. |
| Publication date: 04 Dec 2008 |
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| Ground-based and Venus Express Coordinated Campaign |
Beyond their intrinsic interest, ground-based observations have proven their usefulness in supporting spacecraft observations of Solar System bodies. Probably the most spectacular illustration ever was provided during the descent of the Huygens Probe on Titan, when the radio astronomy segment detected the "channel A" carrier signal from Huygens and allowed the recovery of the Doppler Wind Experiment that had been compromised by the failure of the corresponding Cassini channel (Lebreton et al., 2005). Furthermore, ground-based science observations performed during or around the Huygens mission provided new, complementary information on Titan's atmosphere and surface, helping to put the Huygens observations into context (Witasse et al., 2006). Another example of a successful ground-based campaign is the Deep Impact event, when numerous Earth-based and Earth-orbiting observatories monitored comet 9P/Tempel 1 when it was hit by the impactor (Meech et al., 2005).
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| Publication date: 15 Aug 2008 |
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| Variable winds on Venus mapped in three dimensions |
| We present zonal and meridional wind measurements at three altitude levels within the cloud layers of Venus from cloud tracking using images taken with the VIRTIS instrument on board Venus Express. At low latitudes, zonal winds in the Southern hemisphere are nearly constant with latitude with westward velocities of 105 ms-1 at cloud-tops (altitude ~ 66 km) and 60-70 ms-1 at the cloud-base (altitude ~ 47 km). At high latitudes, zonal wind speeds decrease linearly with latitude with no detectable vertical wind shear (values lower than 15 ms-1), indicating the possibility of a vertically coherent vortex structure. Meridional winds at the cloud-tops are poleward with peak speed of 10 ms-1 at 55° S but below the cloud tops and averaged over the South hemisphere are found to be smaller than 5 ms-1. We also report the detection at subpolar latitudes of wind variability due to the solar tide. |
| Publication date: 10 Jul 2008 |
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| First detection of hydroxyl in the atmosphere of Venus |
| We report measurements of night-side airglow emission in the atmosphere of Venus in the OH (2-0), OH (1-0), O2(a-X) (0-1), and O2(a-X) (0-0) bands. This is the first detection of the first three of these airglow emissions on another planet. These observations provide the most direct observational constraints to date on H, OH, and O3, key species in the chemistry of Venus' upper atmosphere.
(This is an abbreviated abstract from the paper "First detection of hydroxyl in the atmosphere of Venus", by Piccioni et al, published in Astronomy & Astrophysics, A&A 483, L29-L33 (2008)) |
| Publication date: 19 May 2008 |
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| Mars Express/Venus Express |
The European space Agency (ESA) undertook a bold experiment with the Mars Express mission: to develop rapidly a low cost platform for planetary exploration. The myriad scientific achievements of this mission prove the success of the experiment. ESA took a second bold step by adapting the Mars platform for the Venus Express mission, and doing so rapidly and most cost-effectively. While the differences in Venus and Mars necessitated several changes in instrumentation, there are many objectives that remain the same at the two planets. When we issued a call to the MEX and VEX communities for a volume of brief articles covering the latest results from these two missions, the response from those examining the interaction of the solar wind and energetic particles with the planets was most swift. The authors were asked to keep their presentations to four published pages. The guest editor in turn attempted to shepherd these papers through the reviewing process quickly. In those instances where the editor had a conflict of interest, R. J. Strangeway assumed the duties of the editor.
The articles that passed review before the press deadline are included therein. They include discussions of the various plasma boundaries at Venus and Mars, the nature of their plasma environments, the discovery of energetic neutral particles, the configuration of the magnetic field near the planets, space weather and the loss of atmosphere. Papers included contain both modeling and observational work and are written by some of the newest members of the community as well as many of the veteran research scientists. We especially thank the referees of these papers who responded promptly to help speed these early results to the readers of Planetary and Space Science.
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| Publication date: 15 May 2008 |
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| A dynamic upper atmosphere of Venus as revealed by VIRTIS on Venus Express |
| The upper atmosphere of a planet is a transition region in which energy is transferred between the deeper atmosphere and outer space. Molecular emissions from the upper atmosphere (90-120 km altitude) of Venus can be used to investigate the energetics and to trace the circulation of this hitherto little-studied region. Previous spacecraft and ground-based observations of infrared emission from CO2, O2 and NO have established that photochemical and dynamic activity controls the structure of the upper atmosphere of Venus. These data, however, have left unresolved the precise altitude of the emission owing to a lack of data and of an adequate observing geometry. Here we report measurements of day-side CO2 non-local thermodynamic equilibrium emission at 4.3 m, extending from 90 to 120 km altitude, and of night-side O2 emission extending from 95 to 100 km. The CO2 emission peak occurs at 115 km and varies with solar zenith angle over a range of 10 km. This confirms previous modelling, and permits the beginning of a systematic study of the variability of the emission. The O2 peak emission happens at 96 km +- 1 km, which is consistent with three-body recombination of oxygen atoms transported from the day side by a global thermospheric sub-solar to anti-solar circulation, as previously predicted. |
| Publication date: 29 Nov 2007 |
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| A warm layer in Venus' cryosphere and high-altitude measurements of HF, HCl, H2O and HDO |
| Venus has thick clouds of H2SO4 aerosol particles extending from altitudes of 40 to 60 km. The 60-100 km region (the mesosphere) is a transition region between the 4 day retrograde superrotation at the top of the thick clouds and the solar-antisolar circulation in the thermosphere (above 100 km), which has upwelling over the subsolar point and transport to the nightside. The mesosphere has a light haze of variable optical thickness, with CO, SO2, HCl, HF, H2O and HDO as the most important minor gaseous constituents, but the vertical distribution of the haze and molecules is poorly known because previous descent probes began their measurements at or below 60 km. Here we report the detection of an extensive layer of warm air at altitudes 90-120 km on the night side that we interpret as the result of adiabatic heating during air subsidence. Such a strong temperature inversion was not expected, because the night side of Venus was otherwise so cold that it was named the 'cryosphere' above 100 km. We also measured the mesospheric distributions of HF, HCl, H2O and HDO. HCl is less abundant than reported 40 years ago. HDO/H2O is enhanced by a factor of 2.5 with respect to the lower atmosphere, and there is a general depletion of H2O around 80-90 km for which we have no explanation. |
| Publication date: 29 Nov 2007 |
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| Lightning on Venus inferred from whistler-mode waves in the ionosphere |
| The occurrence of lightning in a planetary atmosphere enables chemical processes to take place that would not occur under standard temperatures and pressures. Although much evidence has been reported for lightning on Venus, some searches have been negative and the existence of lightning has remained controversial. A definitive detection would be the confirmation of electromagnetic, whistler-mode waves propagating from the atmosphere to the ionosphere. Here we report observations of Venus' ionosphere that reveal strong, circularly polarized, electromagnetic waves with frequencies near 100 Hz. The waves appear as bursts of radiation lasting 0.25 to 0.5 s, and have the expected properties of whistler-mode signals generated by lightning discharges in Venus' clouds. |
| Publication date: 29 Nov 2007 |
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| Little or no solar wind enters Venus' atmosphere at solar minimum |
| Venus has no significant internal magnetic field, which allows the solar wind to interact directly with its atmosphere. A field is induced in this interaction, which partially shields the atmosphere, but we have no knowledge of how effective that shield is at solar minimum. (Our current knowledge of the solar wind interaction with Venus is derived from measurements at solar maximum.) The bow shock is close to the planet, meaning that it is possible that some solar wind could be absorbed by the atmosphere and contribute to the evolution of the atmosphere. Here we report magnetic field measurements from the Venus Express spacecraft in the plasma environment surrounding Venus. The bow shock under low solar activity conditions seems to be in the position that would be expected from a complete deflection by a magnetized ionosphere. Therefore little solar wind enters the Venus ionosphere even at solar minimum. |
| Publication date: 29 Nov 2007 |
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