Thermal periodicities in the Venus atmosphere

A search was made for periodic fluctuations in the thermal brightness temperatures recorded by the Pioneer Venus orbiter's infrared radiometer. Data were averaged in 10 × 10° latitude-longitude bins for each of the 72 days the instrument was in operation. This time series of thermal brightness temperatures was then analyzed to determine the amplitude of fluctuations at periods from 2 to 64 days at four levels in the atmosphere (at the cloud tops and at approximately 70, 80, and 90 km). The amplitude of such fluctuations is small at equatorial latitudes and increases to a maximum at 60–70° latitude at most altitudes. The period of the highest amplitude fluctuation is 5.3±0.4 days (at all altitudes) except at 70–80°, where a 2.9-day period which appears to correspond to the polar dipole dominates the cloud-top channel. The amplitude of the periodic fluctuations is a maximum at the cloud tops, decreasing to a minimum at the 80-km channel, and increasing again at the 90-km channel.     

Structure of the Venus atmosphere

The structure of the Venus atmosphere is discussed. The data obtained in the 1980s by the last Soviet missions to Venus: orbiters Venera 15, 16 and the entry probes and balloons of Vega 1 and 2 are compared with the Venus International Reference Atmosphere (VIRA) model. VIRA is based on the data of the extensive space investigations of Venus in the 1960s and 1970s. The results of the IR Fourier Spectrometry experiment on Venera 15 are reviewed in detail. This instrument is considered as a precursor of the long wavelength channel of the Planetary Fourier Spectrometer on Venus Express.     

Small-scale turbulence in the atmosphere of Venus

Previous studies based on radio scintillation measurements of the atmosphere of Venus have identified two regions of small-scale temperature fluctuations located in the vicinity of 45 and 60 km. A global study of the fluctuations near 60 km, which are consistent with wind-shear-generated turbulence, was conducted using the Pioneer Venus measurements. The structure constants of refractive index fluctuations c_n² and temperature fluctuations c_T² increase poleward, peak near 70° latitude, and decrease over the pole; c_n² varies from 2 × 10^?15 to 1.5 × 10^?14m^{$\text{2}\text{3}$} and c_T² from 4 × 10^?3 to 7 × 10^?2°K²m^{$\text{?2}\text{3}$}. These results indicate greater turbulent activity at the higher latitudes. In the region near 45 km the refractive index fluctuations and the corresponding temperature fluctuations are substantially lower. Based on the analysis of one representative occultation measurement, $\text{c}{}_{\mathrm{<mtext>n</mtext>}}{}^2\text{= 2 × 10}{}^{\mathrm{?16}}\text{m}{}^{\mathrm{<mtext>?2</mtext><mtext>3</mtext>}}\text{and}\text{c}{}_{\mathrm{<mtext>T</mtext>}}{}^2\text{= 7.3 × 10}{}^{\mathrm{?4}}\text{°}\text{K}{}^2\text{m}{}^{\mathrm{<mtext>?2</mtext><mtext>3</mtext>}}$ in the 45-km region. The fluctuations in this region also appear to be consistent with wind-shear-generated turbulence. The turbulence level is considerably weaker than that at 60 km; the energy dissipation rate ε is 4.9 × 10^?5m²sec^?3 and the small-scale eddy diffusion coefficient K is 2 × 10³ cm² sec^?1.     

Net thermal radiation in the atmosphere of Venus

The four entry probes of the Pioneer Venus mission measured the radiative net flux in the atmosphere of Venus at latitudes of 60°N, 31°S, 27°S, and 4°N. The three higher latitude probes carried instruments (small probe net flux radiometers; SNFR) with external sensors. The measured SNFR net fluxes are too large below the clouds, but an error source and correction scheme have been found (H. E. Revercomb, L. A. Sromovsky, and V. E. Suomi, 1982, Icarus52, 279–300). The near-equatorial probe carried an infrared radiometer (LIR) which viewed the atmosphere through a window in the probe. The LIR measurements are reasonable in the clouds, but increase to physically unreasonable levels shortly below the clouds. The probable error source and a correction procedure are identified. Three main conclusions can be drawn from comparisons of the four corrected flux profiles with radiative transfer calculations: (1) thermal net fluxes for the sounder probe do not require a reduction in the Mode 3 number density as has been suggested by O. B. Toon, B. Ragent, D. Colburn, J. Blamont, and C. Cot (1984, Icarus57, 143–160), but the probe measurements as a whole are most consistent with a significantly reduced mode 3 contribution to the cloud opacity; (2) at all probe sites, the fluxes imply that the upper cloud contains a yet undetected source of IR opacity; and (3) beneath the clouds the fluxes at a given altitude increase with latitude, suggesting greater IR cooling below the clouds at high latitudes and water vapor mixing ratios of about 2–5 × 10^?5 near 60°, 2–5 × 10^?4 near 30°, and 5 × 10^?4 near the equator. The suggested latitudinal variation of IR cooling is consistent with descending motions at high latitudes, and it is speculated that it could provide an important additional drive for the general circulation.     

Sulfur chemistry in the middle atmosphere of Venus

Venus Express measurements of the vertical profiles of SO and SO₂ in the middle atmosphere of Venus provide an opportunity to revisit the sulfur chemistry above the middle cloud tops (～58 km). A one dimensional photochemistry-diffusion model is used to simulate the behavior of the whole chemical system including oxygen-, hydrogen-, chlorine-, sulfur-, and nitrogen-bearing species. A sulfur source is required to explain the SO₂ inversion layer above 80 km. The evaporation of the aerosols composed of sulfuric acid (model A) or polysulfur (model B) above 90 km could provide the sulfur source. Measurements of SO₃ and SO (a¹Δ → X³Σ^-) emission at 1.7 μm may be the key to distinguish between the two models.     

Acoustic-gravity waves in the thermosphere of Venus

Properties of acoustic-gravity waves in the upper atmosphere of Venus are studied using a two-fluid model which includes the effects of wave-induced diffusion in a diffusively separated atmosphere. In conjunction with neutral mass spectrometer data from the Pioneer Venus orbiter, the theory should provide information on the distribution of wave sources in the Venus upper atmosphere. Observed wave structure in species density measurements should generally have periods ?30–35 min, small N₂, CO, and O amplitudes, and highly variable phase shifts relative to CO₂. A near resonance may exist between downward phase-propagating internal gravity and diffusion waves near the 165-km level at periods near 29 min. As a result, if very large He wave amplitudes are observed near this level, it will indicate that the wave source is below the 150- to 175-km level and that the exospheric temperature is close to 350°K. Wave energy dissipation may be an important mechanism for heating of the nightside Venus thermosphere. Large-density oscillations in stratospheric cloud layer constituents are also possible and may be detectable by the Pioneer Venus large probe neutral mass spectrometer.     

Physical parameters of the atmosphere of Venus

The following parameters have been computed for the Cytherean atmosphere: pressure, density, speed of sound, collisional frequency, column mass, density scale, mean-free-path, viscosity, pressure scale, mean particle velocity, number density at an altitude from 0 to 170 km.The chemical composition, the temperature distribution function of the altitude, the surface pressure and the surface temperature measured by Pioneer Venus have been used as input data for these computations.     

Infrared limb darkening of the Venus atmosphere

An analysis of the limb darkening component obtained by Ingersoll and Orton [Icarus21 (1974), 121–128] from the thermal infrared maps of Venus published by Murray, Wildey, and Westphal [J. Geophys. Res.68 (1963), 4813–4818] and Westphal, Wildey, and Murray [Astrophys. J.142 (1965), 799–802] shows that the Cytherean cloud tops were close to radiative equilibrium in 1962. A method for obtaining the optical depth, the extinction coefficient, and the extinction scale height from such data is derived and values are extracted from Marov's [Icarus16 (1972), 415–461] standard model of the Venus atmosphere.     

Thermal structure of the atmosphere of Venus from Pioneer Venus radio occultations

Eighty-seven measurements of the thermal structure in the atmosphere of Venus between the altitudes of about 40 and 85 km were derived from Pioneer Venus Orbiter radio occultation data taken during four occultation seasons from December 1978 to October 1981. These measurements cover latitudes from ?68 to 88° and solar zenith angles of 8 to 166°. The results indicate that the characteristics of the thermal structure in both the troposphere and stratosphere regions are dependent predominantly on the latitude and only weakly on solar illumination conditions. In particular, the circumpolar collar cloud region in the northern hemisphere (latitude 55 to 77°) displays the most dramatic changes in structure, including the appearance of a large inversion, having an average magnitude of about 18°K and a maximum of about 33°K. Also in this region, the tropopause altitude rises by about 4.8 km above its value at low latitudes, the tropopause temperature drops by about 60°K, and the pressure at the tropopause decreases by an average of about 240 mbar. These changes in the collar region are correlated with observations of increased turbulence and greater amplitude of thermal waves in the region, which is located where the persistent circulation pattern in the Venus atmosphere changes from zonally symmetric retrograde rotation to a hemispherical circumpolar vortex. It was shown that the large zonal winds associated with this circulation pattern are not likely to produce distortions in the atmosphere of a magnitude that could lead to temperature errors of the order of the mesosphere inversions observed in the collar region, but under certain circumstances zonal wind distortion could cause errors of 3–4°K.     

Deducing the age of the dense Venus atmosphere

We show how crater size-density counts may be used to help constrain the history of the Venus atmosphere, based on the predictions of simple but reasonable models for crater production, surface erosion, and the effects of atmospheric drag and breakup on incident meteors in the Venus atmosphere. If the atmosphere is old, we may also be able to determine the importance of breakup as a mechanism for destroying incident meteors in a dense fluid. In particular, if the atmosphere is young, the old (uneroded) surfaces will have crater densities upward of 10^?4 km^?2 and a ratio of small (4 km) craters to large (128 km) craters near 10³. If the atmosphere is old and the breakup mechanism is dominant, absolute crater densities on Venus surfaces will be diminished by several orders of magnitude relative to the young atmosphere case. If atmospheric drag is dominant and the atmosphere is old, the absolute crater density will be lowered by perhaps an order of magnitude relative to the young atmosphere case, and the ratio of small to large craters will be reduced to a value near 10^1.5 according to the models. The comparison of crater populations on young, as well as old, surfaces on Venus can help in distinguishing the young and old atmosphere scenarios, especially since the situation may be complicated by currently undetermined erosional and tectonic processes. Once a large fraction of Venus surface has been imaged at kilometer resolution, as the VOIR project promises to do, it could be possible to make an early determination of the age of the Venus atmosphere.     

Oxidation state of the atmosphere and crust of Venus from pioneer Venus results

Recent spacecraft observations of Venus permit a detailed model of sulfur chemistry in the atmosphere-lithosphere system. Pioneer Venus experiments confirm that, as predicted, COS and H₂S are dominant over SO₂ in the lower atmosphere, and that the equilibrium concentrations of S₂ and S₃ are significant. Many criteria serve to bracket the oxidation state of the crust: it is nearly certain that the S₂^2?/SO₄^2? buffer regulates the oxygen fucagity, and that FeO is at least as abundant as Fe₂O₃ in crustal silicates. A highly oxidized crust (as, for example, would result from O₂ absorption complementary to escape of vast amounts of H₂) is incompatible with the gas-phase sulfur chemistry. If the Pioneer Venus mass spectrometer estimates of the abundance of sulfur gases are correct, Earth-like models for the bulk composition of Venus are seriously in error, and a far lower FeO content is required for Venus.     

The aeronomy of the upper atmosphere of Venus

Density profiles for CO, O, and O₂ in the Cytherean atmosphere above 90 km are plotted with eddy diffusion coefficient (K) as a parameter, subject to the constraint that the mixing ratios of CO and O₂ approach their observed value or values under the observed upper limit at the lower boundary. It is then shown that the value of K puts upper limits on the amount of hydrogen (in the form of H₂O, HCl, and H₂) the atmosphere near 90km can contain. This value is a function of the density and temperature of hydrogen at the critical level and the magnitude of the total escape flux, where unspecified flux mechanisms other than thermal are postulated ad hoc. In general these constraints call for large values of K to accomodate the atomic hydrogen produced by measured mixing ratios of HCl and H₂O. Hence they constrain thee amount of O in the upper atmosphere to values well under 1% at 130 km unless there are very large hydrogen escape fluxes, 10⁷ cm^?2sec^?1 or larger. The freedom to assume arbitrary amounts of H₂ in the atmosphere is also restricted. We suggest either very effective escape mechanisms—despite low exospheric hydrogen densities—or novel excitation mechanisms for O(3³S) and O(3⁵S) in the upper atmosphere.     

Reassessment of net radiation measurements in the atmosphere of Venus

Net radiative flux measurements by instruments on the Pioneer Venus Day, North, and Night probes are too large below 30 km to be consistent with present estimates of atmospheric opacity. We evaluate the only known mechanisms which could potentially have caused significant errors in the deep atmosphere, namely, (1) radiation field perturbations behind each probe due to its thermal wake, (2) cloud particle deposition on the sensor windows, and (3) thermal perturbations within the radiation sensor produced by gas flow through the sensor window retainers. Thermal analysis of the wake effect shows that temperature perturbations are not large enough to produce significant flux perturbations when gas opacity and sensor field-of-view characteristics are taken into account. The particle deposition effect is rejected because it requires a signature in the measured radiation profile which is not observed. The absence of such a feature also implies that mode 3 cloud particles are either not sulfuric acid or are far less numerous than previously reported. We find that the third mechanism is the most likely source of the large net flux measurements. However, this error is not sufficiently constrained by laboratory data to allow rigorous corrections to the measured flux profiles. If we use radiative transfer calculations to constrain the fluxes at 14 km and limited laboratory data to estimate the altitude dependence of the error, then we obtain a plausible set of corrected flux profiles which are roughly consistent with reasonable H₂O mixing ratios below the clouds.     

Temperature waves in the solar atmosphere

The properties of five-minute temperature waves in the photosphere are investigated. The phase and amplitude relations of temperature and acoustic waves are deduced. It is expected that the five-minute oscillations represent a mixture of acoustic and temperature waves. The temperature waves are generated due to linear interaction with acoustic waves.It is well known that concurrent with the acoustic waves, temperature or heat waves can appear in the case of nonadiabatic disturbances (Landau and Lifshitz, 1959). The temperature waves are dissipative damped waves. Propagation of nonadiabatic hydrodynamic waves in a stratified medium have been considered by Zhugzdha (1983). If stratification of heat exchange exists, a linear interaction of hydrodynamic and temperature waves arises. The temperature waves must be present in the solar atmosphere.     

Ion-acoustic waves in the solar atmosphere

A model for ion-acoustic waves in the solar atmosphere is presented. In the limit of strongly magnetized plasma this model leads to the Zakharov-Kuznetsov equation which possesses a flat solitary wave solution. An initial-value problem for this equation is solved numerically to show a transition of the flat solitary waves into spherical solitary waves. The paper suggests further developments of an ion-acoustic wave theory that may improve our knowledge of ion-acoustic waves and lead to the possibility of their being detected in the solar atmosphere.     

Cosmic ray ionization of lower Venus atmosphere

Cosmic ray particles passing through dense lower atmosphere of Venus decay giving rise to various charged and neutral particles. The flux and degradation of dominant cascade particles namely neutrinos and pions are computed and ionization contributions at lower altitudes are estimated. Using the height profile of pion flux, the muon flux is computed and used to estimate ionization at lower altitudes. It is shown that cosmic ray produced ionization descends to much lower altitudes intercepting the thickness of Venus cloud deck. The dynamical features of Venus cloud deck are used to allow the likely charging and charge separation processes resulting into cloud-to-cloud lightning discharges.     

Chemical kinetic model for the lower atmosphere of Venus

A self-consistent chemical kinetic model of the Venus atmosphere at 0-47 km has been calculated for the first time. The model involves 82 reactions of 26 species. Chemical processes in the atmosphere below the clouds are initiated by photochemical products from the middle atmosphere (H₂SO₄, CO, S_x), thermochemistry in the lowest 10 km, and photolysis of S₃. The sulfur bonds in OCS and S_x are weaker than the bonds of other elements in the basic atmospheric species on Venus; therefore the chemistry is mostly sulfur-driven. Sulfur chemistry activates some H and Cl atoms and radicals, though their effect on the chemical composition is weak. The lack of kinetic data for many reactions presents a problem that has been solved using some similar reactions and thermodynamic calculations of inverse processes. Column rates of some reactions in the lower atmosphere exceed the highest rates in the middle atmosphere by two orders of magnitude. However, many reactions are balanced by the inverse processes, and their net rates are comparable to those in the middle atmosphere. The calculated profile of CO is in excellent agreement with the Pioneer Venus and Venera 12 gas chromatographic measurements and slightly above the values from the nightside spectroscopy at 2.3 μm. The OCS profile also agrees with the nightside spectroscopy which is the only source of data for this species. The abundance and vertical profile of gaseous H₂SO₄ are similar to those observed by the Mariner 10 and Magellan radio occultations and ground-based microwave telescopes. While the calculated mean S₃ abundance agrees with the Venera 11-14 observations, a steep decrease in S₃ from the surface to 20 km is not expected from the observations. The ClSO₂ and SO₂Cl₂ mixing ratios are ∼10⁻¹¹ in the lowest scale height. The existing concept of the atmospheric sulfur cycles is incompatible with the observations of the OCS profile. A scheme suggested in the current work involves the basic photochemical cycle, that transforms CO₂ and SO₂ into SO₃, CO, and S_x, and a minor photochemical cycle which forms CO and S_x from OCS. The net effect of thermochemistry in the lowest 10 km is formation of OCS from CO and S_x. Chemistry at 30-40 km removes the downward flux of SO₃ and the upward flux of OCS and increases the downward fluxes of CO and S_x. The geological cycle of sulfur remains unchanged.     

Upstream proton cyclotron waves at Venus

Data from the magnetometer MAG aboard the Venus Express S/C are investigated for the occurrence of cyclotron wave phenomena upstream of the Venus bow shock. For an unmagnetized planet such as Venus and Mars the neutral exosphere extends into the on-flowing solar wind and pick-up processes can play an important role in the removal of particles from the atmosphere. At Mars upstream proton cyclotron waves were observed but at Venus they were not yet detected. From the MAG data of the first 4 months in orbit we report the occurrence of proton cyclotron waves well upstream from the planet, both outside and inside of the planetary foreshock region; pick-up protons generate specific cyclotron waves already far from the bow shock. This provides direct evidence that the solar wind is removing hydrogen from the Venus exosphere. Determining the role the solar wind plays in the escape of particles from the total planetary atmosphere is an important step towards understanding the evolution of the environmental conditions on Venus. The continual observations of the Venus Express mission will allow mapping the volume of escape more accurately, and determine better the present rate of hydrogen loss.     

The distribution of carbon monoxide in the lower atmosphere of Venus

We have obtained spatially resolved near-infrared spectroscopy of the Venus nightside on 15 nights over three observing seasons. We use the depth of the CO absorption band at 2.3 μm to map the two-dimensional distribution of CO across both hemispheres. Radiative transfer models are used to relate the measured CO band depth to the volume mixing ratio of CO. The results confirm previous investigations in showing a general trend of increased CO abundances at around 60° latitude north and south as compared with the equatorial regions. Observations taken over a few nights generally show very similar CO distributions, but significant changes are apparent over longer periods. In past studies it has been assumed that the CO latitudinal variation occurs near 35 km altitude, at which K-band sensitivity to CO is greatest. By modeling the detailed spectrum of the excess CO at high latitudes we show that it occurs at altitudes around 45 km, much higher than has previously been assumed, and that there cannot be significant contribution from levels of 36 km or lower. We suggest that this is most likely due to downwelling of CO-rich gas from the upper atmosphere at these latitudes, with the CO being removed by around 40 km through chemical processes such as the reaction with SO₃.