Atmosphere Of Venus Research Articles

Periodic variations of oxygen EUV dayglow in the upper atmosphere of Venus: Hisaki/EXCEED observations

AbstractUsing the Extreme Ultraviolet Spectroscope for Exospheric Dynamics (EXCEED) aboard Hisaki and the Solar Extreme Ultraviolet Monitor on the Solar and Heliospheric Observatory, we investigate variations of the extreme ultraviolet (EUV) dayglow brightness for OII 83.4 nm, OI 130.4 nm, and OI 135.6 nm in the Venusian upper atmosphere observed in March–April (period 1), April–May (period 2), and June–July (period 3) in 2014. The result shows that characteristic periodicities exist in the dayglow variations other than the ~27 day solar rotational effect of the solar EUV flux: 1.8, 2.8, 3.1, 4.5, and 9.9 day in period 1; 1.1 day in period 2; and 1.0 and 11 day in period 3. Many of these periodicities are consistent with previous observations and theory. We suggest these periodicities are related to density oscillations of oxygen atoms or photoelectrons in the thermosphere. The cause of these periodicities is still uncertain, but planetary‐scale waves and/or gravity waves propagating from the middle atmosphere, and/or minor periodic variations of the solar EUV radiation flux may play a role. Effects of the solar wind parameters (velocity, dynamic pressure, and interplanetary magnetic field's intensity) on the dayglow variations are also investigated using the Analyser of Space Plasma and Energetic Atoms (ASPERA‐4) and magnetometer aboard Venus Express. Although clear correlation with the dayglow variations is not found, their minor periodicities are similar to the dayglow periodicities. Contribution of the solar wind to the dayglow remains still unknown, but the solar wind parameters might affect the dayglow variations.

Contribution from SOIR/VEX to the updated Venus International Reference Atmosphere (VIRA)

The SOIR instrument on-board Venus Express is an infrared spectrometer covering the 2.2–4.3μm spectral region. This instrument allows the detection of several key species of the Venus atmosphere, including CO2, CO, H2O/HDO, HCl, HF and SO2. From the CO2 density measurements, temperature is inferred giving information on the thermal structure of the atmosphere. Here we described the kind of data (profiles, latitudinal average, etc.) that will be provided to the updated VIRA compilation.

Theoretical Kinetics Study of the Reactions Forming the ClCO Radical Cycle in the Middle Atmosphere of Venus

Abstract A quantum-chemistry and kinetics study of key chemical reactions involved in the ClCO radical cycle of the Venus atmosphere: Cl + CO + M → ClCO + M (1); ClCO + O2 + M → ClC(O)OO + M (2) and ClC(O)OO + Cl → CO2 + ClO + Cl (3) (M = CO2), has been performed at 150–300 K. Unimolecular reaction rate theories on potential energy features derived at the G4//B3LYP/6-311+G(3df) ab initio composite level were employed. Limiting low pressure rate coefficients calculated for Reaction (1) are in good agreement with recommended experimental values. The present results validate rate coefficient values measured for Reaction (2) over relevant strato-mesosphere Venusian conditions. Rate coefficients calculated by the SACM/CT for Reaction (3) are given by k 3 = 2.5 × 10−11 (T/300)0.5 cm3 molecule –1 s –1. In the absence of experimental data, these values provide the first reliable prediction for k 3.

Radiative heating and cooling in the middle and lower atmosphere of Venus and responses to atmospheric and spectroscopic parameter variations

A sophisticated radiative transfer model that considers absorption, emission, and multiple scattering by gaseous and particulate constituents over the broad spectral range 0.125–1000µm is applied to calculate radiative fluxes and temperature change rates in the middle and lower atmosphere of Venus (0–100km). Responses of these quantities to spectroscopic and atmospheric parameter variations are examined in great detail. Spectroscopic parameter studies include the definition of an optimum spectral grid for monochromatic calculations as well as comparisons for different input data with respect to spectral line databases, continuum absorption, line shape factors, and solar irradiance spectra. Atmospheric parameter studies are based on distinct variations of an initial model data set. Analyses of actual variations of the radiative energy budget using atmospheric features that have been recently retrieved from Venus Express data will be subject of a subsequent paper.The calculated cooling (heating) rates are very reliable at altitudes below 95 (85)km with maximum uncertainties of about 0.25K/day. Heating uncertainties may reach 3–5K/day at 100km. Using equivalent Planck radiation as solar insolation source in place of measured spectra is not recommended. Cooling rates strongly respond to variations of atmospheric thermal structure, while heating rates are less sensitive. The influence of mesospheric minor gas variations is small, but may become more important near the cloud base and in case of episodic SO2 boosts. Responses to cloud mode 1 particle abundance changes are weak, but variations of other mode parameters (abundances, cloud top and base altitudes) may significantly alter radiative temperature change rates up to 50% in Venus' lower mesosphere and upper troposphere.A new model for the unknown UV absorber for two altitude domains is proposed. It is not directly linked to cloud particle modes and permits an investigation of radiative effects regardless of the absorbers’s chemical composition. A globally averaged Bond albedo of Venus of 0.763 is inferred in accordance with previous results. Considering the gaseous UV absorbers SO2 and CO2 shortward of 0.32µm, the globally averaged deposited solar net flux at the top of atmosphere (TOA) and the outgoing thermal net flux differ by only 1.5Wm−2 around the mean value of 159Wm−2 for the selected initial atmospheric model. Global radiative equilibrium can be achieved by moderate adjustments of cloud mode and UV absorber abundances. Half of the TOA solar net flux is absorbed by atmospheric constituents at altitudes above 63km. Consideration of the unknown UV absorber provides about 50% more heating at 68km compared with a neglect of this opacity source. Less than 5% of the incident flux reaches the surface. There is a broad net cooling region between 70 and 80km with a strong increase of cooling toward the poles. A net radiative temperature change rate gradient is also observed at 65km where heating occurs at low latitudes. At altitudes above 80km, net heating dominates the low and mid latitudes, while net cooling prevails at high latitudes leading to a dominant global average net heating that has to be balanced by dynamical processes to maintain the observed thermal structure. The results of energy balance response analyses will serve as reference for ongoing investigations and provide a profound data base to improve the understanding of radiative forcing of atmospheric dynamical processes.

Analysis of the radiative budget of the Venusian atmosphere based on infrared Net Exchange Rate formalism

AbstractA detailed one‐dimensional analysis of the energy balance in Venus atmosphere is proposed in this work, based on the Net Exchange Rate formalism that allows the identification in each altitude region of the dominant energy exchanges controlling the temperature. Well‐known parameters that control the temperature profile are the solar flux deposition and the cloud particle distribution. Balance between solar heating and infrared energy exchanges is analyzed for each region: upper atmosphere (from cloud top to 100 km), upper cloud, middle cloud, cloud base, and deep atmosphere (cloud base to surface). The energy accumulated below the clouds is transferred to the cloud base through infrared windows, mostly at 3–4 μm and 5–7 μm. The continuum opacity in these spectral regions is not well known for the hot temperatures and large pressures of Venus's deep atmosphere but strongly affects the temperature profile from cloud base to surface. From cloud base, upward transport of energy goes through convection and short‐range radiative exchanges up to the middle cloud where the atmosphere is thin enough in the 20–30 μm window to cool directly to space. Total opacity in this spectral window between the 15 μm CO2 band and the CO2 collision‐induced absorption has a strong impact on the temperature in the cloud convective layer. Improving our knowledge of the gas opacities in these different windows through new laboratory measurements or ab initio computations, as well as improving the constraints on cloud opacities would help to separate gas and cloud contributions and secure a better understanding of Venus's atmosphere energy balance.

Vertical Wavenumber Spectra of Gravity Waves in the Venus Atmosphere Obtained from Venus Express Radio Occultation Data: Evidence for Saturation

Abstract By using the vertical temperature profiles obtained by the radio occultation measurements on the European Space Agency (ESA)’s Venus Express, the vertical wavenumber spectra of small-scale temperature fluctuations that are thought to be manifestations of gravity waves are studied. Wavenumber spectra covering wavelengths of 1.4–7.5 km were obtained for two altitude regions (65–80 and 75–90 km) and seven latitude bands. The spectra show a power-law dependence on the high-wavenumber side with the logarithmic spectral slope ranging from −3 to −4, which is similar to the features seen in Earth’s and Martian atmospheres. The power-law portion of the spectrum tends to follow the semiempirical spectrum of saturated gravity waves, suggesting that the gravity waves are dissipated by saturation as well as radiative damping. The spectral power is larger at 75–90 km than at 65–80 km at low wavenumbers, suggesting amplitude growth with height of unsaturated waves. It was also found that the wave amplitude is larger at higher latitudes and that the amplitude is maximized in the northern high latitudes. On the assumption that gravity waves are saturated in the Venusian atmosphere, the turbulent diffusion coefficient was estimated. The diffusion coefficient in the Venusian atmosphere is larger than those in Earth’s atmosphere because of the longer characteristic vertical wavelength of the saturated waves.

Constraints on a potential aerial biosphere on Venus: I. Cosmic rays

While the present-day surface of Venus is certainly incompatible with terrestrial biology, the planet may have possessed oceans in the past and provided conditions suitable for the origin of life. Venusian life may persist today high in the atmosphere where the temperature and pH regime is tolerable to terrestrial extremophile microbes: an aerial habitable zone. Here we argue that on the basis of the combined biological hazard of high temperature and high acidity this habitable zone lies between 51km (65°C) and 62km (−20°C) altitude. Compared to Earth, this potential venusian biosphere may be exposed to substantially more comic ionising radiation: Venus has no protective magnetic field, orbits closer to the Sun, and the entire habitable region lies high in the atmosphere – if this narrow band is sterilised there is no reservoir of deeper life that can recolonise afterwards. Here we model the propagation of particle radiation through the venusian atmosphere, considering both the background flux of high-energy galactic cosmic rays and the transient but exceptionally high-fluence bursts of extreme solar particle events (SPE), such as the Carrington Event of 1859 and that inferred for AD 775. We calculate the altitude profiles of both energy deposition into the atmosphere and the absorbed radiation dose to assess this astrophysical threat to the potential high-altitude venusian biosphere. We find that at the top of the habitable zone (62km altitude; 190g/cm2 shielding depth) the radiation dose from the modelled Carrington Event with a hard spectrum (matched to the February 1956 SPE) is over 18,000 times higher than the background from GCR, and 50,000 times higher for the modelled 775 AD event. However, even though the flux of ionising radiation can be sterilizing high in the atmosphere, the total dose delivered at the top of the habitable zone by a worst-case SPE like the 775 AD event is 0.09Gy, which is not likely to present a significant survival challenge. Nonetheless, the extreme ionisation could force atmospheric chemistry that may perturb a venusian biosphere in other ways. The energy deposition profiles presented here are also applicable to modelling efforts to understand how fundamental planetary atmospheric processes such as atmospheric chemistry, cloud microphysics and atmospheric electrical systems are affected by extreme solar particle events. The companion paper to this study, Constraints on a potential aerial biosphere on Venus: II. Solar ultraviolet radiation (Patel et al., in preparation), considers the threat posed by penetration of solar UV radiation. The results of these twin studies are based on Venus but are also applicable to extrasolar terrestrial planets near the inner edge of the circumstellar habitable zone.

Hinode SOT Plate Scale Reinvestigated by G-Band Images on the 2012 Transit of Venus

The Hinode Solar Optical Telescope (SOT) successfully observed the transit of Venus with an unprecedented high spatial resolution on 5 – 6 June 2012, providing images of the aureole refracted by the atmosphere of Venus and the dark Venus disk against the bright solar surface. The transit of Venus provided a unique opportunity for calibrating the plate scale of SOT images. With the examination of the radius of the dark Venus disk, we determined the plate scale of G-band 430.5 nm images with high accuracy: 0.05369±0.00005 arcsec pixel−1. The radius was defined at the intensity level of the 0.5 transmittance and compared with the angular radius of Venus including the thickness of the atmosphere determined with the measurements of SPICAV onboard Venus Express. Thanks to the high spatial resolution, SOT images show that the dark Venus can be well represented by an ellipse. We observed 7.6 km difference in altitude between the equator and the polar regions.

Comparative pick-up ion distributions at Mars and Venus: Consequences for atmospheric deposition and escape

Without the shielding of a substantial intrinsic dipole magnetic field, the atmospheres of Mars and Venus are particularly susceptible to similar atmospheric ion energization and scavenging processes. However, each planet has different attributes and external conditions controlling its high altitude planetary ion spatial and energy distributions. This paper describes analogous test particle simulations in background MHD fields that allow us to compare the properties and fates, precipitation or escape, of the mainly O+ atmospheric pick-up ions at Mars and Venus. The goal is to illustrate how atmospheric and planetary scales affect the upper atmospheres and space environments of our terrestrial planet neighbors. The results show the expected convection electric field-related hemispheric asymmetries in both precipitation and escape, where the degree of asymmetry at each planet is determined by the planetary scale and local interplanetary field strength. At Venus, the kinetic treatment of O+ reveals a strong nightside source of precipitation while Mars' crustal fields complicate the simple asymmetry in ion precipitation and drive a dayside source of precipitation. The pickup O+ escape pattern at both Venus and Mars exhibits low energy tailward escape, but Mars exhibits a prominent, high energy ‘polar plume’ feature in the hemisphere of the upward convection electric field while the Venus ion wake shows only a modest poleward concentration. The overall escape is larger at Venus than Mars (2.1×1025 and 4.3×1024 at solar maximum, respectively), but the efficiency (likelihood) of O+ escaping is 2–3 times higher at Mars. The consequences of these comparisons for pickup ion related atmospheric energy deposition, loss rates, and detection on spacecraft including PVO, VEX, MEX and MAVEN are considered. In particular, both O+ precipitation and escape show electric field controlled asymmetries that grow with energy, while the O+ fluxes and energy spectra at selected spatial locations show characteristic signatures of the pickup related acceleration and precipitation.

Photochemical control of the distribution of Venusian water

We use the JPL/Caltech 1-D photochemical model to solve continuity diffusion equation for atmospheric constituent abundances and total number density as a function of radial distance from the planet Venus. Photochemistry of the Venus atmosphere from 58 to 112km is modeled using an updated and expanded chemical scheme (Zhang et al., 2010, 2012), guided by the results of recent observations and we mainly follow these references in our choice of boundary conditions for 40 species. We model water between 10 and 35ppm at our 58km lower boundary using an SO2 mixing ratio of 25ppm as our nominal reference value. We then vary the SO2 mixing ratio at the lower boundary between 5 and 75ppm holding water mixing ratio of 18ppm at the lower boundary and finding that it can control the water distribution at higher altitudes. SO2 and H2O can regulate each other via formation of H2SO4. In regions of high mixing ratios of SO2 there exists a “runaway effect” such that SO2 gets oxidized to SO3, which quickly soaks up H2O causing a major depletion of water between 70 and 100km. Eddy diffusion sensitivity studies performed characterizing variability due to mixing that show less of an effect than varying the lower boundary mixing ratio value. However, calculations using our nominal eddy diffusion profile multiplied and divided by a factor of four can give an order of magnitude maximum difference in the SO2 mixing ratio and a factor of a few difference in the H2O mixing ratio when compared with the respective nominal mixing ratio for these two species. In addition to explaining some of the observed variability in SO2 and H2O on Venus, our work also sheds light on the observations of dark and bright contrasts at the Venus cloud tops observed in an ultraviolet spectrum. Our calculations produce results in agreement with the SOIR Venus Express results of 1ppm at 70–90km (Bertaux et al., 2007) by using an SO2 mixing ratio of 25ppm SO2 and 18ppm water as our nominal reference values. Timescales for a chemical bifurcation causing a collapse of water concentrations above the cloud tops (>64km) are relatively short and on the order of a less than a few months, decreasing with altitude to less than a few days.

The millimeter-wavelength sulfur dioxide absorption spectra measured under simulated Venus conditions

Over 130 laboratory measurements of the 2–4mm wavelength opacity of sulfur dioxide in a carbon dioxide atmosphere under simulated conditions for the upper Venus troposphere (temperatures between 308 and 343K and pressures between 0.03 and 2bar) have been made. These measurements along with the centimeter wavelength measurements by Steffes et al. (Steffes, P.G. et al. [2015]. Icarus 245, 153–161) have been used to empirically assess existing formalisms for sulfur dioxide opacity in a carbon dioxide atmosphere (Fahd, A.K., Steffes, P.G. [1992]. Icarus 97(2), 200–210; Suleiman, S.H. et al. [1996]. J. Geophys. Res.: Planets 101(E2), 4623–4635). The Van Vleck and Weisskopf Model (VVW) used by Fahd and Steffes with the JPL rotational line catalog (Pickett, H. et al. [1998]. J. Quant. Spectrosc. Radiat. Transfer 60(5), 499–890) was found to fit 85.88% of all 500 measurements within the 2-sigma uncertainty. This work will improve the confidence in retrievals of the atmospheric abundance of sulfur dioxide from millimeter-wavelength observations of the Venus atmosphere.

Water vapor near Venus cloud tops from VIRTIS-H/Venus express observations 2006–2011

This work aims to give a summary of the water vapor at the cloud top of Venus atmosphere using the complete set of observations made using high spectral resolution channel (-H) of Visible and Infrared Thermal Imaging Spectrometer (VIRTIS), on board the ESA Venus Express orbiter, to measure the cloud top altitude and the water vapor abundance near this level. An initial analysis of these measurements by Cottini et al. (2012) was limited to data in 140 orbits in the period 2007–2008. These observations were limited to the Northern hemisphere due to observational geometry in this early part of the mission. In the present paper, the analysis is extended to a larger dataset covering the years 2006–2011, significantly improving the latitudinal coverage. Altitude of the cloud tops, corresponding to unit optical depth at a wavelength of 2.5µm, is equal to 69±1km at low latitudes, and decreases toward the pole to 62–64km. The water vapor abundance is equal to 3±1ppm in low latitudes and it increases reaching a maximum of 5±2ppm at 70–80° of latitude in both hemispheres, with a sharp drop in the polar regions. This can be explained by the specific dynamics of the atmosphere of Venus affecting the distribution of water vapor such as the transfer of water vapor in the Hadley cell and the dynamic in the polar vortex. The average height of the cloud tops and the H2O near this level are symmetric with respect to the equator. As a function of local solar time, the water vapor shows no particular dependence, and the cloud tops exhibit just a weak maximum around noon. Over 5 years of observations the average values of the cloud top altitude and the water vapor were quite stable in low and middle latitudes, while in high latitudes both quantities in 2009–2011 years are systematically higher than in 2006–2008. Short period variations increasing with latitude are observed, from approximately less than ±1km for cloud tops and ±1ppm for water vapor in low latitudes to, respectively, ±2km and ±2ppm in high latitudes. As a rule there is no correlation between variations of the cloud top altitude, the water vapor content, and the UV brightness. However, numerous examples can be found when UV dark features, with a characteristic size of a few degrees of latitude (several hundred kilometers), coincide with regions of higher cloud tops.

Thermal structure of Venus׳ nightside mesosphere as observed by infrared heterodyne spectroscopy at 10 μm

Ground-based heterodyne spectroscopy is used to observe the night side of Venus by probing single pressure broadened CO2 absorption lines. From the pressure induced line broadening, the predominant temperature at different altitude layers can be deduced. It is found, that heterodyne spectroscopy is sensitive to probe the mesosphere between ~60km and 90km with an altitude resolution of ~4.5km.During two observing campaigns in March and May 2012, four different locations on the planet were investigated. Herein, we report on the retrieval of vertical temperature profiles in the nightside atmosphere of Venus. Retrieval of atmospheric parameters is based on a Levenberg–Marquard χ2 optimization that iteratively compares observed data to telluric transmittance corrected Venus׳ top-of-atmosphere spectra calculated using a radiative transfer algorithm. The deduced profiles are compared to the Venus International Reference Atmosphere and some found to be in satisfactory agreement. Sub-Doppler resolution Infrared heterodyne observations can provide temperature measurements that complement existing sub-mm and space based observations.

Gamma rays and cosmic rays at Venus: The Pioneer Venus gamma ray detector and considerations for future measurements

We draw attention to, and present a summary archive of the data from, the Pioneer Venus Orbiter Gamma-ray Burst Detector (OGBD), an instrument not originally conceived with Venus science in mind. We consider the possibility of gamma-ray flashes generated by lightning and model the propagation of gamma rays in the Venusian atmosphere, finding that if gamma rays originate at the upper range of reported cloud top altitudes (75km altitude), they may be attenuated by factors of only a few, whereas from 60km altitude they are attenuated by over two orders of magnitude. The present archive is too heavily averaged to reliably detect such a source (and we appeal to investigators who may have retained a higher-resolution archive), but the data do provide a useful and unique record of the cosmic ray flux at Venus 1978–1993. We consider other applications of future orbital gamma ray data, such as atmospheric occultations and the detection of volcanic materials injected high in the atmosphere.

Venus high temperature atmospheric dropsonde and extreme-environment seismometer (HADES)

The atmospheric composition and geologic structure of Venus have been identified by the US National Research Council׳s Decadal Survey for Planetary Science as priority targets for scientific exploration; however, the high temperature and pressure at the surface, along with the highly corrosive chemistry of the Venus atmosphere, present significant obstacles to spacecraft design that have severely limited past and proposed landed missions. Following the methodology of the NASA Innovative Advanced Concepts (NIAC) proposal regime and the Collaborative Modeling and Parametric Assessment of Space Systems (COMPASS) design protocol, this paper presents a conceptual study and initial feasibility analysis for a Discovery-class Venus lander capable of an extended-duration mission at ambient temperature and pressure, incorporating emerging technologies within the field of high temperature electronics in combination with novel configurations of proven, high Technology Readiness Level (TRL) systems. Radioisotope Thermal Power (RTG) systems and silicon carbide (SiC) communications and data handling are examined in detail, and various high-temperature instruments are proposed, including a seismometer and an advanced photodiode imager. The study combines this technological analysis with proposals for a descent instrument package and a relay orbiter to demonstrate the viability of an integrated atmospheric and in-situ geologic exploratory mission that differs from previous proposals by greatly reducing the mass, power requirements, and cost, while achieving important scientific goals.

Update of the Venus density and temperature profiles at high altitude measured by SOIR on board Venus Express

The SOIR instrument on board Venus Express regularly sounds the Venus atmosphere using the solar occultation technique. The density and temperature profiles are inferred from SOIR spectra recorded in the infrared. The method has been described in a previous publication (Mahieux et al., 2012. J. Geophys. Res. 117. doi:10.1029/2012JE004058.). This paper is devoted to the update of the VAST (Venus Atmosphere from SOIR measurements at the Terminator) compilation that was initiated in the above cited work, which gives the mean CO2 number density and temperature profiles for different latitude bins. The method has been improved and has been applied to more data. The new compilation which is given on the same latitudinal grid now distinguishes between the two sides of the terminator. The compilation also confirms the main thermal layering characteristics that were identified in the earlier version: the succession of a warm layer (230±30K, 1−σ standard deviation) at a pressure level of 3.2×10−7mbar (~140km), a very cold layer (125±32K) at 2.5×10−5mbar (~123km), a warm layer (204±17K) at 0.01mbar (~102km) and finally a colder layer at 0.4mbar (171±34K, ~87km). The layering of all the temperature profiles is explained by radiative rather than dynamical processes. The temporal temperature variation is larger than the mean latitudinal temperature variation. VAST is compared with temperature profiles obtained from other Venus Express instruments, VeRa and SPICAV–UV, and ground based measurements.

Thermodynamic properties of supercritical carbon dioxide: Widom and Frenkel lines.

Supercritical fluids are widely used in a number of important technological applications, yet the theoretical progress in the field has been rather moderate. Fairly recently, a new understanding of the liquidlike and gaslike properties of supercritical fluids has come to the fore, particularly with the advent of the Widom and Frenkel lines that aim to demarcate different physical properties on the phase diagram. Here, we report the results of a computational study of supercritical carbon dioxide, one of the most important fluids in the chemical industry. We study the response functions of CO_{2} in the supercritical state and calculate the locations of their maxima (Widom lines). We also report the preliminary calculations of the Frenkel line, the line of crossover of microscopic dynamics of particles. Our insights are relevant to physical processes in the atmosphere of Venus and its evolution.

Dynamics of polar vortices at cloud top and base on Venus inferred from a general circulation model: Case of a strong diurnal thermal tide

Polar vortices in the presence of a thermal tide are investigated using a Venusian middle atmosphere general circulation model. Around the cloud top, where the warm polar region is maintained by the thermal wind associated with a high latitude jet, the temperature contrast forms the polar vortex pattern. The cold collar and hot oval (monopole) near the pole are enhanced by the polar diurnal tide, and unstable vortices form the hot dipole and tripole. The centroid of the hot oval is displaced from the pole to around 80° by the diurnal tide. The hot dipole appears and breaks up into a tripole when transient vortical and divergent eddies with zonal wavenumbers 2 and higher are predominant within the polar hot oval region. Because the divergence and temperature are a quarter cycle out of phase with the eddy vorticity, the vortical eddies transport heat toward the cold region. Thus, the cloud-top polar vortices are mainly formed by a combination of the diurnal tide and transient baroclinic wave. At the cloud base, isotherms are almost zonally uniform and the eddy temperature structure is not apparent. In contrast, divergence and vorticity have large amplitudes within this region. The vortical eddies have a comma-shaped pattern, which is stably maintained and rotates with a period of about 5 days. The divergence and vorticity might be important in controlling cloud morphology at the cloud base via material transport.

Proton and alpha particle precipitation onto the upper atmosphere of Venus

We study the precipitation of protons and alpha-particles onto the upper atmosphere of Venus, using particle data recorded by the Venus Express spacecraft inside the induced magnetosphere. Our investigations are limited to the dayside close to the terminator. We observe on average a net downward flux of protons, which originate partly from the planetary atmosphere and partly from the solar wind. We present median energy spectra of the precipitating protons divided into two energy ranges, 10–100eV and 100eV–30keV. The total dayside precipitation of solar wind protons is estimated to be 3×1022s−1, assuming only protons with energies above 500eV will reach the exobase. Downgoing protons are frequently observed but only in 3% of the available data records we see He2+. These observations are made close to the induced magnetosphere boundary and we argue that at lower altitude the countrates for alpha-particles fall below detection limits. We estimate the precipitation of He2+ onto the dayside exobase to be 1×1021s−1, which is not enough enough to replace the helium escaping from the planet.

Explosive volcanic activity on Venus: The roles of volatile contribution, degassing, and external environment

We investigate the conditions that will promote explosive volcanic activity on Venus. Conduit processes were simulated using a steady-state, isothermal, homogeneous flow model in tandem with a degassing model. The response of exit pressure, exit velocity, and degree of volatile exsolution was explored over a range of volatile concentrations (H2O and CO2), magma temperatures, vent altitudes, and conduit geometries relevant to the Venusian environment. We find that the addition of CO2 to an H2O-driven eruption increases the final pressure, velocity, and volume fraction gas. Increasing vent elevation leads to a greater degree of magma fragmentation, due to the decrease in the final pressure at the vent, resulting in a greater likelihood of explosive activity. Increasing the magmatic temperature generates higher final pressures, greater velocities, and lower final volume fraction gas values with a correspondingly lower chance of explosive volcanism. Cross-sectionally smaller, and/or deeper, conduits were more conducive to explosive activity. Model runs show that for an explosive eruption to occur at Scathach Fluctus, at Venus’ mean planetary radius (MPR), 4.5% H2O or 3% H2O with 3% CO2 (from a 25m radius conduit) would be required to initiate fragmentation; at Ma’at Mons (~9km above MPR) only ~2% H2O is required. A buoyant plume model was used to investigate plume behaviour. It was found that it was not possible to achieve a buoyant column from a 25m radius conduit at Scathach Fluctus, but a buoyant column reaching up to ~20km above the vent could be generated at Ma’at Mons with an H2O concentration of 4.7% (at 1300K) or a mixed volatile concentration of 3% H2O with 3% CO2 (at 1200K). We also estimate the flux of volcanic gases to the lower atmosphere of Venus, should explosive volcanism occur. Model results suggest explosive activity at Scathach Fluctus would result in an H2O flux of ~107kgs−1. Were Scathach Fluctus emplaced in a single event, our model suggests that it may have been emplaced in a period of ~15 days, supplying 1–2×104Mt H2O to the atmosphere locally. An eruption of this scale might increase local atmospheric H2O abundance by several ppm over an area large enough to be detectable by near-infrared nightside sounding using the 1.18µm spectral window such as that carried out by the Venus Express/VIRTIS spectrometer. Further interrogation of the VIRTIS dataset is recommended to search for ongoing volcanism on Venus.