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Heat Transfer in the Venus Atmosphere
Journal of the Atmospheric Sciences
Abstract
The successful flights of the Venera 4, 5 and 6 probes resulted in
direct, in situ measurements of the chemical composition, pressure,
temperature and density of the Venus lower atmosphere, and gave rise to
a tentative atmospheric model for the planet. These measurements permit
a more definite discussion of the mechanisms responsible for the
observed characteristics of the thermal conditions on Venus.In this
paper, an approximate analysis of heat transfer processes in the Venus
atmosphere is made, representing the continuing development of the
preliminary study described by Avduevsky et al. Radiative fluxes have
been calculated, a model of the convective motions in the lower
atmosphere has been evaluated, and the importance of radiative and
convective energy transfer in the planet's total heat balance has been
estimated.
... Regarding the stability of the atmosphere, early temperature and pressure measurements made by Venera mission and Pioneer Venus, pointed towards a highly stable atmosphere with a vertical temperature gradient of the order of 7.7 K/km, lower than the adiabatic rate (Avduevsky et al., 1970). However, more recent analyses found instability regions (at 20-30 km and 25-33 km) that might be associated with convection and turbulence (Seiff, 1983). ...
The main purpose of this thesis is to contribute to a better understanding of the dynamics of the Venusian atmosphere, constraining its characterization at cloud top level, complementing the observations of the Venus Express spacecraft with ground-based observations. In order to achieve this objective of studying the atmospheric super-rotation, I measured the zonal wind velocity (crosswind the planet from East to West) and its spatial and temporal variability, by means of high precision spectroscopy and Doppler velocimetry. The observations were made with the high-resolution spectrograph UVES with ESO’s Very Large Telescope (VLT) and with the high-resolution ESPaDOnS spectropolarimeter at the Canada-France-Hawaii telescope (CFHT) at Mauna Kea observatory. I use a method of direct measurement of the planetary winds based on high resolution spectroscopy in the visible range. The atmosphere of Venus contains highly scattering aerosols in suspension, mostly at cloud top layer. This cloud deck covers entirely the planet, imposing an unusually high albedo. These particles are carried by the wind, carrying with them the information on atmospheric dynamics. The upper cloud layer is located about 70 km altitude. This altitude corresponds to the maximum velocity of zonal superrotation. The method used, in the case of long-slit observations obtained with VLT/UVES, allowed to characterize the spatial variations of zonal wind as a function of latitude (since the slit is thin compared to the size of the disk image of Venus), and a comparison with VEx/VIRTIS cloud tracking was also performed using CFHT/ESPaDOnS.
... major part of the heating flux is thus deposited at high altitudes. The absorbed flux near the planet surface at the subsolar point F abs is estimated at F abs " 100?200100?100?200 W.m ?2 (Avduevsky et al. 1970; Lacis 1975; Dobrovolskis & Ingersoll 1980 ), which implies J 2 " 10?410?4 W.kg?1kg?1 . Finally, layers below 60 km are characterized by a strongly negative temperature gradient (Seiff et al. 1980 ) and are, therefore, weakly stratified, or convective. ...
Context. Atmospheric tides can strongly affect the rotational dynamics of planets. In the family of Earth-like planets, which includes Venus, this physical mechanism coupled with solid tides makes the angular velocity evolve over long timescales and determines the equilibrium configurations of their spin.
Aims. Unlike the solid core, the atmosphere of a planet is subject to both tidal gravitational potential and insolation flux coming from the star. The complex response of the gas is intrinsically linked to its physical properties. This dependence has to be characterized and quantified for application to the wide variety of extrasolar planetary systems.
Methods. We develop a theoretical global model where radiative losses, which are predominant in slowly rotating atmospheres, are taken into account. We analytically compute the perturbation of pressure, density, temperature, and velocity field caused by a thermogravitational tidal perturbation. From these quantities, we deduce the expressions of atmospheric Love numbers and tidal torque exerted on the fluid shell by the star. The equations are written for the general case of a thick envelope and the simplified one of a thin isothermal atmosphere.
Results. The dynamics of atmospheric tides depends on the frequency regime of the tidal perturbation: the thermal regime near synchronization and the dynamical regime characterizing fast-rotating planets. Gravitational and thermal perturbations imply different responses of the fluid, i.e. gravitational tides and thermal tides, which are clearly identified. The dependence of the torque on the tidal frequency is quantified using the analytic expressions of the model for Earth-like and Venus-like exoplanets and is in good agreement with the results given by global climate models (GCM) simulations.Introducing dissipative processes such as radiation regularizes the tidal response of the atmosphere, otherwise it is singular at synchronization.
Conclusions. We demonstrate the important role played by the physical and dynamical properties of a super-Earth atmosphere (e.g. Coriolis, stratification, basic pressure, density, temperature, radiative emission) in its response to a tidal perturbation. We point out the key parameters defining tidal regimes (e.g. inertia, Brunt-Väisälä, radiative frequencies, tidal frequency) and characterize the behaviour of the fluid shell in the dissipative regime, which cannot be studied without considering the radiative losses.
... A possible explanation of this situation is that the infrared opacity of the gaseous constituents of the Venusian atmosphere is so large that the additional infrared opacity contributed by the cloud has little effect on temperatures. Avduevsky, Marov, Noykina et al. (1970) find, for example, that the radiative fluxes throughout most of the Venusian atmosphere are insensitive to various assumptions on cloud infrared opacity. Several theoretical studies have dealt specifically with the effect of the cloud layer. ...
Recent space probe and Earth-based observations-have provided information on the composition and pressure of the Venusian atmosphere. This information is used to calculate the temperature poflk of the Venusian atmosphere with the use of a thermal equilibrium model. Under conditions of thermal equilibrium, the vertical temperature profile is influenced by vertical convection and radiative heat transfer. Absorption of solar radiation and emission of infrared radiation by carbon dioxide and water vapor are considered to be the dominant radiative transfer processes, and are calculated with a non-grey transmission model. The thermal equilibrium temperatures are calculated with an iterative scheme. Mean temperature profiles are computed for a range of water vapor mixing ratios and surface pressures that represents present uncertainties in these para-meters. Best agreement with the temperature profiles observed by the space probes to Venus is obtained with a model atmosphere that has water vapor mixing ratios of order 10 and surface pressures greater than 60 atm. Undet these conditions the computed surface temperature is about 700' K and the cow.-wive layer extends to a pressure level the order of 0.1 atm. Calculations of the latitudinal variation of the thermal equilibrium temperature profile are also performed with this model atmosphere.
... However, the uncertainty of the amount of the absorption of solar light by the cloud made the study on radiative equilibrium not to be definitive. (Pollack 1969, Avduevsky et al. 1970). ...
The radiative equilibrium and the radiative-convective equilibrium of the Venusian atmosphere corresponding to the averaged insolation are numerically calculated for the layer up to 76km. The non-grey absorption in infra-red region due to carbon dioxide and water vapour and the grey absorption due to the cloud are included in the present study. The amount of solar radiation reaching the ground is assumed to be 1.5% of the solar constant at the orbit of Venus based on the information by Venera-8, and the rest of the absorbed energy is assumed to be distributed uniformly in the cloud layer. The heat trans-port by convection is included by means of eddy mixing of the potential temperature.
The results of the present calculations on the radiative equilibrium and the radiative- convective equilibrium lead to the conclusion that the high surface temperature can be explained in terms of a green house effect provided that the concentration of water vapour assumed in the present study is not far from the correct value. Namely, water vapour is indispensable to the maintenance of the high surface temperature. The cloud also has an important effect concerning the infra-red region on the thermal structure of the atmosphere, especially on that in the upper layer. However the cloud's contribution to green house effect seems not to be indispensable, as indicated from the result that the high surface temperature can be maintained in spite of a small optical thickness of the cloud assumed in the present study. In the calculations of the radiative-convective equilibrium, convection takes place in most part of the atmosphere. The eddy diffusion coefficient is of the order of 106cm²/sec in the lower layer and 107cm²/sec in the upper layer. However a non-convective layer appears below the cloud base under most conditions assumed in the present study. It suggests that a connection between the lower layer and the cloud layer is rather weak. A calculation on the diurnal variation of the Venusian atmosphere was made by the present model in which lateral mixing is neglected. The result indicates that in the night side strong convection occurs to transport heat upward within the cloud layer and prevents cooling of the cloud top. The maximum diurnal temperature range averaged over a layer between the surface and the 1km level turns out to be 0.3°K.
Most models of atmospheric evolution start with the reasonable but
unverified assumption that the original atmospheric inventories of Venus
and Earth were similar. Although the two planets have similar overall
abundances of nitrogen and carbon, the present day water inventory of
Venus is lower than that of Earth by a factor of 105. The
original water abundance of Venus is highly unconstrained. The high D/H
ratio observed, 2.5 ×10- 2 or ≈ 150 times
terrestrial (Donahue et al. 1997) has been cited as evidence of a large
primordial water endowment (Donahue et al. 1982). Yet, given the
likelihood of geologically recent water sources and the large
uncertainty in the modern and past hydrogen and deuterium escape fluxes,
the large D/H may not reflect the primordial water abundance but rather
may result from the history of escape and resupply in the most recent
≈ 109 years of planetary evolution (Donahue et al. 1997,
Grinspoon 1993, 1997). Thus, at present the best arguments for a sizable
early Venusian water endowment remain dependent on models of planet
formation and early volatile delivery. Most models of water delivery to
early Earth involve impact processes that would have also supplied Venus
with abundant water (Grinspoon 1987, Ip et al. 1998, Morbidelli et al.
2000). Stochastic processes could have created large inequities in
original volatile inventory among neighboring planets (Morbidelli et al.
2000). However, given the great similarity in bulk densities and their
close proximity in the Solar System the best assumption at present is
that Venus and Earth started with similar water abundances.
Depending on the value of the Prandtl number, the average velocity imparted to a layer of Boussinesq fluid by traveling thermal waves applied at the upper free surface is found, in the linear case, to be either in the same or opposite direction as that of the moving thermal source. Since the mean flow is in the opposite direction only when the Prandtl number is small, the 4-day retrograde zonal motion of the Venus atmosphere may be evidence that the effective Prandtl number of the upper atmosphere is much less than unity.
The possibility is considered that the cellular convection observed by
Mariner 10 in the subsolar region of the atmosphere of Venus may be
related to laboratory studies of convection with internal heat
generation. It appears probably that the basic dynamics of the Venusian
convection and of the convection observed in the laboratory experiments
are similar. The Venusian cells have highly structured interiors. In the
laboratory, cells with an elongation of greater than about two develop
an internal structure.
It is pointed out that Venus is the planet most similar to the earth in
size, mass, mean density, and the amount of solar energy absorbed. Space
missions conducted by the U.S. and the Soviet Union to explore Venus are
related to Mariner flybys and an employment of Venera landers. Data
concerning uranium, thorium, and potassium abundances and density
measurements provide evidence that Venusian matter differentiated in the
course of evolution during billions of years. Among the terrestrial
planets Venus is distinguished mainly by the massive gaseous envelope
that is responsible for its hot surface. Direct in situ measurements
reliably established temperature and pressure at the Venus surface as
high as 740 K and 90 kg/sq cm, respectively. Attention is given to
planetary figure and surface, the structure and composition of the
atmosphere, the problems of the light and heat budget, the clouds,
dynamics, and upper atmosphere and near-space environment
IN 1956 Kuiper and Chamberlain1 reported the systematic diminution of the equivalent width of weak CO2 bands near 0.8 µm in the spectrum of Venus as the phase angle of the planet increases, especially near inferior conjunction. I found the same effect for strong CO2 bands near 1.6 µm at the time of the inferior conjunction of 1966 (ref. 2). All the spectra were obtained in the region of the intensity equator. I repeated the observations near the inferior conjunction of 1969 for the cusps as well. Fig. 1 shows typical spectra—three of Venus and a comparison spectrum of the Moon. All the spectra were obtained at about the same air mass, those for Venus on March 31 and for the Moon on April 2, 1969, using the diffractional infrared spectrometer at the 125 cm reflector of the Crimean Station of the Sternberg Astronomical Institute.
This paper presents the principal results of wind velocity and turbulence measurements in the Venus atmosphere during the Venera flights.Based on one-way Doppler measurements wind estimations were obtained as a difference of the measured and computed descent velocity values. The computation of free parachute descent velocity was performed by an independent method that utilized aerodynamics of the spaceprobes and the pressure-temperature measurements of the Venus atmosphere. Entry point location, dynamics of the parachute-spaceprobe system as well as frequency instability of the on-board crystal oscillators are basic factors which influenced the accuracy of wind and turbulence estimations.Venera 4 measured a strong wind (up to 40–50 m sec−1) and turbulence at 0.7–4 bar levels (40–50km altitude); within the measurement errors neither wind nor turbulence were found at altitudes lower than 40 km. Venera 5 and Venera 6 Doppler data indicated very smooth velocity changes during the whole descent without noticeable signs of turbulence. Venera 7 measured a zonal wind component; values 5–14 m sec−1 were obtained at 38–53 km altitudes; below 38 km the wind velocity was zero.Estimation of wind velocity near the planet surface (0–3.5 km) leads to value of 0–2.5 m sec−1. Based on an analysis of the apparatus construction characteristics and the radio signal variation during impact upon the Venus surface, values of 2–80 kg cm−2 were obtained for the bearing strength of the soil.
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