Data on dynamics of the subcloud Venus atmosphere from Venera Spaceprobe measurements

I C T U S , 17, 6 5 9 - 6 7 4 (1972)

Data on Dynamics of the Subcloud Venus Atmosphere from Venera Spaceprobe Measurements V. V. KERZHANOVICH, M.Ya. MAROV, AND M. K. ROZHDESTVENSKY Institute for Cosmic Research and Institute of Applied Mathematics, USSR Academy of Sciences, Moscow, USSR. R e c e i v e d J u n e 7, 1972 This paper presents the principal results of wind velocity and turbulence measure m e n t s i n t h e V e n u s a t m o s p h e r e d u r i n g t h e V e n e r a flights. B a s e d o n o n e - w a y D o p p l e r m e a s u r e m e n t s w i n d e s t i m a t i o n s were o b t a i n e d as a difference of t h e m e a s u r e d a n d c o m p u t e d d e s c e n t v e l o c i t y values. T h e c o m p u t a t i o n of free p a r a c h u t e d e s c e n t v e l o c i t y w a s p e r f o r m e d b y a n i n d e p e n d e n t m e t h o d t h a t u t i l i z e d a e r o d y n a m i c s of t h e s p a c e p r o b e s a n d t h e p r e s s u r e - t e m p e r a t u r e m e a s u r e m e n t s of t h e V e n u s a t m o s p h e r e . E n t r y p o i n t location, d y n a m i c s o f t h e p a r a c h u t e - s p a c e p r o b e s y s t e m as well as f r e q u e n c y i n s t a b i l i t y of t h e o n - b o a r d c r y s t a l oscillators are basic f a c t o r s w h i c h influenced t h e a c c u r a c y of w i n d a n d t u r bulence estimations. V e n e r a 4 m e a s u r e d a s t r o n g w i n d (up to 4 0 - 5 0 m s e c -1) a n d t u r b u l e n c e a t 0.7-4 b a r levels ( 4 0 - 5 0 k m a l t i t u d e ) ; w i t h i n t h e m e a s u r e m e n t errors n e i t h e r w i n d n o r t u r b u l e n c e were f o u n d a t a l t i t u d e s lower t h a n 4 0 k m . V e n e r a 5 a n d V e n e r a 6 Doppler data indicated very smooth velocity changes during the whole descent w i t h o u t n o t i c e a b l e signs of t u r b u l e n c e . V e n e r a 7 m e a s u r e d a z o n a l w i n d comp o n e n t ; v a l u e s 5 - 1 4 m sec-1 were o b t a i n e d a t 38-53 k m a l t i t u d e s ; b e l o w 38 k m t h e w i n d v e l o c i t y was zero. E s t i m a t i o n of w i n d v e l o c i t y n e a r t h e p l a n e t surface (0-3.Skin) leads t o v a l u e of 0-2.5 m sec -1. B a s e d o n a n a n a l y s i s of t h e a p p a r a t u s c o n s t r u c t i o n c h a r a c t e r i s t i c s a n d t h e r a d i o signal v a r i a t i o n d u r i n g i m p a c t u p o n t h e V e n u s surface, v a l u e s of 2 80 k g c m -2 were o b t a i n e d for t h e b e a r i n g s t r e n g t h of t h e soil.

I. INTRODUCTION Investigation of atmospheric dynamics is extremely important for understanding the physical nature of Venus. Considerable atmospheric depth, slow rotation, and a small angle of inclination of the equator to the orbit make the motion and heat transfer within the Venus atmosphere unlike those on the Earth. There have been many theoretical works devoted to the circulation of the Venus atmosphere. Early work includes the "aeolosphere" hypothesis (0pik, 1961) and several circulation schemes qualitatively analyzed b y Mintz (1961). More recently, a model for the "deep" circulation was suggested b y Goody and Robinson (1966), and investigated numerically by Hess (1968). In this model heat transfer is similar to that in an ocean, with radiation Copyright © 1972by Academic Press, Inc. All rights o f reproduction in any form reserved.

absorption restricted to the upper layers. The motion in the upper atmosphere is from the subsolar to the antisolar point, and the characteristic velocity is about 30msec -1 [5msee -I according to the estimation of Stone (1968)]. The motion in the lower atmosphere is slower and from the antisolar to subsolar point. A new approach to the study of general circulation in planetary atmospheres was developed b y Golitsyn (1968, 1970a, b), in a dimensional analysis similarity theory. Taking such global parameters as albedo, solar constant, mass of the atmosphere, etc., a value of about 1 m sec-' was obtained for the characteristic velocity of atmospheric motion. However, the pattern of the circulation was not given b y the theory. Numerical experiments simulating the circulation in a two-level atmospheric model (Zilitinkevieh et al., 1971; Turikov 659

660

¥ . v . K E R Z H A N O V I C H , M . Y a . MAROV, A N D M. K. R O Z H D E S T V E N S K Y

and Chalikov, 1971 ; Chalikov et al., 1971) led to a rather simple two-cell scheme with the center of lower pressure on the day side shifted toward the night terminator; the center of high pressure is on the night side near the sunrise terminator. The characteristic velocity for the whole atmosphere is about 5 m/sec. Numerical modeling of penetrating convection (Marov et al., 1970; Polezhaev and Vlasjuk, 1971) can be directly applied to heat transfer in the Venus atmosphere. In spite of its closeness to the Earth experimental data about the Venus circulation are scantier than, for example, for circulation in the atmosphere of Saturn. The 4-day circulation (Boyer and Camichel, 1965) discovered from UV photographs was thought by m a n y authors until recently to represent the planetary rotation. We know little about this circulation. Its height and velocity distribution are unknown. Kuiper's (1969) estimate of the Venus radius in the UV is 6145 km, suggesting t h a t this circulation occurs in the upper atmosphere. Other data [for example, those concerning shifts of the terminator, caused in Goody's opinion (1965) by convective motion] are not numerous and have an indirect character. Before the Venera flights there were no experimental data about the dynamics of the subcloud Venus atmosphere. First estimates of upper limits upon vertical motion magnitudes were obtained from direct measurements of thermodynamic parameters by Venera 4, 5, 6 (Avduevsky et al., 1969, 1970a, b). Results described earlier (Kerzhanovich et al., 1969, 1970, 1971; Kerzhanovich, 1972) and discussed in the present paper are based on velocity measurements and experimental data on the dynamics of the descent, apparatus which provide direct information about wind velocity in the Venus atmosphere. II. M~THOD OF MEASURING W I N D VELOCITY Our determination of wind velocity is based on the fact that as a consequenceof parachute drag the radial component

of wind velocity is additively included in the apparatus radial velocity (component in the Earth direction) measured by the Doppler method. To separate it out, all components connected with the mutual motion of Venus and the Earth, their rotations, and also the velocity of parachute descent in a quiet atmosphere are subtracted from measured values of full velocity. The geometry of the descent apparatus is shown in Fig. 1. Here V, U, VR, UR, Vv, Uv, Vh, Uh are, respectively, the velocity of the descent apparatus relative to the Venus surface, the wind velocity, and their radial, vertical and horizontal components. Note t h a t radial refers to Earthcentered coordinates, and vertical to Venus centered. The radial component includes simultaneously vertical and horizontal components of wind velocity with cos ~ and sin ~ as factors, where ~ is a local zenith angle of the Earth : U R = -U~ cos ~ ÷ U h sin ~ cos(flu - A), (1) where flu and A are azimuths of the E a r t h and the wind velocity. The angle ~ is equal to the planetocentric angle between the point of descent and a subterrestrial point. Hence, it is clear t h a t the horizontal component contributes more and is measured the more precisely the more the angle ~ is, i.e., the farther from the subterrestrial point the descent of apparatus occurs at. The direction of a measured component also depends on the location of the area of descent relative to the subterrestrial point: if both lie on the same meridian, then the meridional component of wind velocity is determined; if descent is near the Venus equator then the zonal component is primarily defined. The estimation of vertical velocity is more precise the less the angle ~ is and the closer the area of apparatus descent is located to the subterrestrial point. The ambiguity of separating contributions of average horizontal and vertical wind velocity is resolved by calculating an average vertical velocity of the descent apparatus according to measurements of temperature and atmospheric pressure (Avduevsky et al., 1969, 1970a, b; Oboukhov and Golitsyn, 1969 ; Golitsyn

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/ V I~'IG. 1. Geometry of descent apparatus. and Kerzhanovich, 1971). Applying the hydrostatic equation, the equation of state and the formula for velocity o£ quasistationary descent,

Based on temperature and pressure measurements, the velocity of descent is defined with the relation

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(2)

where M, S and CR are the mass, characteristic area, and aerodynamic drag coefficient of the parachute apparatus system ; K is the volume of the descent apparatus, and Yo is the angle of planing, several relations for determining velocity can be obtained. 22

d H _ 1 dp R o T d lnP dt pg dt - t~g dt ' (3)

where H is altitude, and p, P, T, and/~ are density, pressure, temperature, and molecular weight, respectively. Using only temperature measurements the velocity is determined as : V~

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662

v . v . KERZHANOVICtt~ M.Ya. MAROV~ AND M. K. ROZHDESTVENSKY

The error in defining this velocity is equal to a fraction of one meter/see. Such independent determinations of horizontal and vertical components cannot be realized for turbulent fluctuations of wind velocity. Here, if it is necessary, hypotheses of turbulent structure, for example, isotropy, can be applied. III. DYNAMICS OF THE PARACHUTEDESCENT APPARATUS SYSTEM

Two factors should be analyzed more extensively. They are important to the accuracy and interpretation of the measurements. Until now it was assumed t h a t the parachute-descent apparatus system is fully dragged by wind and reproduces the wind changes without errors. However, the inertia of the system and the complex aerodynamic character of descent in a turbulent atmosphere can lead to considerable difference between changes of the parachute velocity and variations of wind velocity. To evaluate this discrepancy equations of motion were obtained and mathematical simulation of parachute descent in a disturbed Venus atmosphere was performed (Kerzhanovieh, 1972). The velocity of the descent apparatus in a quiet atmosphere coincides almost exactly with the velocity of quasi-stationary descent. Nonstationarity can manifest itself slightly only at a high velocity (i.e., at high altitudes) and its influence decreases proportionally to the cube of velocity. In general, the descent of the apparatus with parachute is not vertical but has a plane angle close to the balance angle of the parachute, which was small for Venera's parachutes. The inertia of the system is most important at high altitudes where the density is small. One of the important parameters-the drag time of the parachute-descent apparatus s y s t e m - - r d was obtained. This time was defined as the time after which the velocity change of the descent apparatus will differ from a change of wind velocity by not more than 10%. Therefore, for velocity fluctuations with a charac-

teristic time exceeding the drag time and a space scale exceeding the magnitude 1,nj. = 7d" Vs, the parachute descent apparatus system reproduces practically without distortions fluctuations of wind velocity. For fluctuations of wind velocity with smaller scales the measurements of the behavior of the parachute-descent apparatus system can lead only to a qualitative estimation of turbulence. The drag time decreased during descent from 5 to 2sec for Venera 4, from 8 to 4sec for Venera 5, 6; and from 12-15 to 4-5 see for Venera 7. Differences in the magnitude ~ are connected with differences in descent velocity; about 10m/sec for Venera 4 after parachute opening and 60m/see for Venera 7. Correspondingly the minimum resolved turbulence scale--lmin--is equal to 50-10m for Venera 4, 200 to 10m for Venera 5, 6, and from 900-1000 to 80 100m for Venera 7. I V . DOPPLER ~V[EASUREMENTS OF DESCENT APPARATUS VELOCITY

The second factor determining the accuracy (and the possibility) of measuring wind velocity with one-way Doppler information is the frequency stability of on-board oscillators. The error is introduced through inaccurate knowledge of the value of the frequency or its unpredictable drift, which can be interpreted as a velocity variation. It is known t h a t variations of external temperature is the main factor influencing the frequency stability of the oscillator. In the course of descent, when the temperature variation outside of the apparatus is about 450°C, the temperature inside the apparatus can vary considerably. To decrease temperature-frequency drift thermostabilized crystal oscillators in D e w a r vessels were used in the descent apparatus. However, temperature drifts in such oscillators during descent can lead to a slowly varying error of measured velocity up to 5-7m/see. For subsequent correction of the error, a method of calculating temperature-frequency drift was developed which used oscillator characteristics measured on the Earth and

663

SUBCLOUD VENUS ATMOSPHERE

telemetry measurements of temperature inside the apparatus during descent. The remaining error has a smooth slowly increasing character and at the end of descent does not exceed 1-1.5m/sec. Systematic error of velocity measurements is reduced by means of frequency calibrations carried out during the interplanetary flight, and prediction of the oscillator frequency at the moment of e n tr y into the Venus atmosphere. An error is caused by individual aging of crystal resonators. The extent of correction depends on the number of calibrations made and the time from the last measurement of frequency until ent r y into the atmosphere. The accuracy of measuring velocity fluctuations is defined by short-time frequency instability and did not exceed 0.30.5 m/sec for these oscillators.

V.

~:~ESULTS OF M E A S U R E M E N T S

line with strikes shows the location of the sunrise terminator; a and fl are areas of high radar scattering; the zero meridian crosses the direction to the E a r t h at inferior conjunction. As seen from Fig. 2 all apparatus descended on the night side of the planet. If, by E a r t h analogy, we define the time between solar passages at a given point as a Venus solar day and the time for change of solar longitude by 15 ° as a Venus solar "hour, " then Vencra 4, 5, 6, 7 descended at about 4-5AM. The Venera 4 descent was located at a distance about 1600kin to the north of the subterrestrial point; Venera 7, l l 0 0 k m to the west of it ; and Venera 5 and 6, very close to it. From the foregoing it follows t h a t velocity measurements of Vcncra 4 and Venera 7 can determine both vertical and horizontal components of wind velocity, and Venera 5 and 6 measurements only the vertical component.

Venera 4, 5, 6

Let us turn now to the results of measurement. The schematic map of descent points for Venera 4, 5, 6, 7 is shown in Fig. 2 (stars) (Avduevsky et al., 1971b). Crossed circles show the location of the subterrestrial point during descent, and a

In the course of data processing, measured Doppler frequencies were corrected for temperature frequency drift of the oscillators and also for refractive frequency change of the signal while descending through the atmosphere; the

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F r o . 2. S c h e m a t i c m a p o f l o c a t i o n o f t h e a r e a s o f d e s c e n t o f V e n e r a 4, 5, 6, 7. T h e s u b t e r r e s t r i a l p o i n t ; a r e a o f t h e d e s c e n t ; a n d t h e s u n r i s e t e r m i n a t o r a r e s h o w n f o r e a c h f l i g h t ; a, f~ a r e t h e a r e a s of high radar scattering.

664

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m a g n i t u d e of the latter correction is small and does not exceed 0.3m/see for Venera 7. Radial velocity components of b o t h the Venus surface at the area of descent and the receiving a n t e n n a on the E a r t h were s u b t r a c t e d from the values of the full radial velocity. The period of Venus rotation was assumed to be equal to t h a t of resonance rotation. The t o t a l error introduced in such calculations does not exceed 0.3m/see. Vertical velocities for Venera 4, 5, and 6 were calculated from the formula for quasi-stationary descent (2), employing the a e r o d y n a m i c characteristics of the p a r a c h u t e descent a p p a r a t u s system and direct measurements of t e m p e r a t u r e and pressure, b u t w i t h o u t using models of the atmosphere. After subtracting this c o m p o n e n t the difference of velocity from zero can be associated only with wind and measurem e n t errors. The m a x i m u m systematic UR

error associated with i n a c c u r a c y in extrapolating the frequency of the on-board oscillator to the e n t r y m o m e n t is equal to

(+_ (+

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m/sec for Venera 4, 5, 6, and 7, respectively. (Systematic error for Venera 7 decrease to ±1.5 m/see, after applying as base point the f r e q u e n c y m e a s u r e m e n t s after landing on the planet's surface). Figures 3-5 show the Doppler differences received. These differences include the t e m p e r a t u r e drift of oscillators in order to see clearly the errors introduced. The t e m p e r a t u r e drift c o m p u t e d is shown b y dashed lines (curve 1 corresponds to the most probable drift, curves 2 and 3 to the e x t r e m e ones). Zero frequency, with the m a x i m u m systematic errors m e n t i o n e d above, corresponds to zero velocity of the wind.

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The time of averaging is equal to 6sec for Vencra 4 and l0 scc for Vcncra 5 and 6. In Fig. 3 the time scale after 8h 02m is contracted. From the comparison of Figs. 3-5 it is seen that variations of signal frequency received for the first 20-23 min from Venera 4, contrary to Venera 5 and 6, have a clear nonmonotonic character. According to the oscillator tests made after the Vencra 4 flight and also during the following flights of automatic stations, it is

improbable that such change was associated with intrinsic variations of oscillator frequency; it cannot be explained either by any reasonable factors which can influence conditions of radiowave propagation. The most likely cause of the frequency variation in this interval was velocity change of the descent apparatus by wind action. The right curve in Fig. 6 shows the altitude profile of wind velocity obtained by graphically averaging the measurement

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FIG. 5. F l u c t u a t i o n s o f the vertical c o m p o n e n t o f the w i n d v e l o c i t y during descent o f V e n e r a 6. 1 is t h e m o s t probable; 2, 3, the e x t r e m e t e m p e r a t u r e drift o f t h e oscillator frequency.

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F~G. 6. A l t i t u d e profile of w i n d v e l o c i t y a c c o r d i n g to V e n e r a 4 m e a s u r e m e n t s . T h e r i g h t profile is o b t a i n e d if t h e D o p p l e r difference is c a u s e d b y h o r i z o n t a l w i n d o n l y ; t h e left profile o b t a i n e d w h e n t h e m e a n v a l u e of v e r t i c a l flow is s u b t r a c t e d .

data in Fig. 3, with the assumption t h a t all the frequency changes are associated with a horizontal wind velocity. Fitting to the altitude scale follows Marov's model (1971) of the temperature measurements. I t is believed t h a t all frequency variations after 08.02.00 are associated only with temperature drift of frequency. The left curve shows the profile of wind velocity which results if a limiting average velocity of vertical flow, based on temperature and pressure measurements (Avduevsky et al., 1969), is subtracted from the radial wind component. The horizontal bar shows the systematic error. The location of the Venera 4 descent point relative to the subterrestrial point shows t h a t the measured wind component is approximately a meridional one, and the direction of wind is from the pole to the

equator. From Fig. 6 it is seen that the wind velocity distribution has a maximum of 4050m/see at a height of about 51 km. At lower altitudes, the wind velocity became smaller and below 40km it is equal to zero (0 ± 10.5m/see with regard to systematic error). Before analyzing velocity fluctuations we shall make some comments. The different dynamical conditions and degree of thermal stratification imply t h a t the inertial range in which turbulence is close to isotropy (Monin and Yaglom, 1967), occupies a greater range of space scales in the Venus atmosphere than in the Earth's atmosphere. The coefficient of kinematic viscosity, whose magnitude determines the microsca]e of turbulence, is 1.5-40 times less in the Venus atmosphere (depending on height) than in the Earth's atmosphere, even near the surface. On the other hand, since the tropopause altitude in the Venus atmosphere (60km, 5.Iarov, 1971) is 3.5-5 times higher than in the Earth's atmosphere (ll-17km), and the scale height exceeds t h a t on the Earth (16.8-6km vs 8.5-6.5km), the largest vertical scale in the inertial interval in the Venus atmosphere must not be less than t h a t in the Earth's. Also, the nearly adiabatic temperature gradient in the Venus atmosphere favors isotropic turbulence (Monin and Yaglom, 1967; Vinnichenko et al., 1968). I f turbulence is considered to be isotropic, then fluctuation values of the vertical and horizontal wind components are similar (more exactly, differ by the factor (~)1/2). As seen below, the characteristic scale of turbulence has a magnitude of 100-200m, larger than the magnitude 1,1i, ; in this case fluctuations of the horizontal wind component essentially do not influence the vertical velocity of the automatic station, and owing to the smallness of the angle ~ the fluctuations observed are associated basically with the vertical wind component. The fluctuations are obtained by subtracting from the wind velocity its averaged values. Uncertainty in separating the fluctuations from the averaged velocity leads to some ambiguity. During process-

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ing, small corrections due to t h e aerod y n a m i c s of the p a r a c h u t e - a p p a r a t u s syst e m were introduced. The r o o t m e a n square v a l u e of wind v e l o c i t y fluctuations in t h e i n t e r v a l till 8 h 02rain is equal to 0.75m/sec, which corresponds to a relative i n t e n s i t y of t u r b u l e n c e a u / u of 0.04 a t t h e m e a n wind v e l o c i t y in this i n t e r v a l (equal to a b o u t 20m/sec). Figure 7 shows the t i m e correlation function of fluctuations of the vertical wind velocity c o m p o n e n t o b t a i n e d b y 20 min of averaging. The correlation function decreases quickly a t r ~ 25sec, which corresponds to a t u r b u l e n c e m e a s u r e a b o u t 1002 0 0 m (if t u r b u l e n c e is considered to be homogeneous) w h e n the m e a n descent velocity is equal to 8 m/sec. I t should be noticed t h a t t h e presence of t u r b u l e n c e a t 40 to 55-km a l t i t u d e is consistent w i t h t h e large gradients of wind v e l o c i t y o b s e r v e d a t these heights. I f the R i c h a r d s o n n u m b e r R i is used as a criterion of t u r b u l e n c e d e v e l o p m e n t , where R i = g y - ~,a/Tfl z ( T is the m e a n t e m p e r a t u r e of the l a y e r ; ~, ~a is the real a n d a d i a b a t i c t e m p e r a t u r e gradients, a n d fl = d U / d H the g r a d i e n t of wind velocity), t h e n t u r b u l e n c e m u s t occur w i t h g r e a t e s t p r o b a b i l i t y at heights where R i < Ri¢r, a n d Ric~ ~ 0.5 - 1 (Vinnichenko et al., 1968). F i g u r e 8 shows values of t h e R i n u m b e r calculated according to t h e m e a s u r e d wind velocity. Values of y were o b t a i n e d f r o m t e m p e r a t u r e a n d pressure m e a s u r e m e n t s ( A v d u e v s k y et al., 1969). A t 45 to 53-kin altitudes R i

Ricr; below 4 5 - 4 0 k m , R i ~ Ricr, agreeing w i t h t h e o b s e r v e d c h a r a c t e r o f wind velocity fluctuations. As seen f r o m Figs. 4 a n d 5 fluctuations of the vertical c o m p o n e n t of wind velocity in the course of descent of Venera 5 a n d 6 were within m e a s u r e m e n t errors for the r a n g e of t i m e scales f r o m 2-10 sec to 10-20 rain. T h e small difference at the beginning of the Venera 6 descent b e t w e e n m e a s u r e d a n d calculated velocity is m o s t p r o b a b l y connected with errors in defining the velocity of descent Vs. T h e root m e a n square m a g n i t u d e of fluctuations was p r a c t i c a l l y c o n s t a n t a n d was equal to 0.28-0.32m/sec for Venera 5 a n d 0.24-0.28 m/see for Vencra 6. M a x i m u m values of fluctuations do n o t

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exceed 0.Sm/see. Assuming isotropic turbulence the same upper values are obtained for fluctuations of horizontal velocity. The small magnitude of fluctuations indicates weak convective currents, agreeing with temperature and pressure measurements (Avduevsky et al., 1970a, b) and consistent with small gradients of horizontal wind velocity. Indirect estimation of the possible wind velocity can be obtained if a relative intensity of turbulence T = au/u in the Venus atmosphere is assumed to be similar to t h a t in the Earth's atmosphere. According to experimental data (Vinnichenko et al., 1968) the magnitude of T depends on wind velocity and height, and in the Earth's atmosphere is equal to 0.02-0.1 (for Venera 4 T is equal to 0.04). Assuming au ~ 0.3m/see we find t h a t the wind velocity during the descent of Venera 5 and 6 does not exceed 3-15m/sec; however, such estimations are highly uncertain. I t should be remembered t h a t 0.3 m/see is the limit of measure-

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ment errors, and the real magnitude of velocity fluctuations could be less. Mutual agreement of temperature and pressure measurements on Venera 5 and 6 (Avduevsky et al., 1970a, b) and also with velocity measurements (Golitsyn and Kerzhanovich, 1971) also indicates small values of possible regular vertical flows. Venera 7 Figures 9, 10, and 12 show changes of radial velocity during descent of Venera 7. Estimates of wind velocity were obtained for intervals before 08.19.03 and after 08.34.00, where the character of descent was similar to the designed one and was smooth so t h a t the velocity could be calculated on the basis of measurements of the exterior temperature, independent of Doppler measurements. The velocity was computed in three ways. First, the formula for the quasi-stationary descent velocity (2) was used, with the density determined from the atmospheric model as a

A"

~0-

O _A 45

>

~0-

0~00

OBOe~

OB~O0O

o8~-o0

MOSCOW T I N~ ,])ecern6ez'~5~1970

FIG. 9. Calculated and measured velocity of descent for Venera 7 from 08.04.20 08.13.10. Measured velocity ; velocity calculated according to Eq. (2) ; velocity calculated according to Eq. (4) ; and velocity calculated according to Nqs. (6, 7) are all displayed.

669

SUBCLOUD VE:hTUS ATMOSPHERE V,~'.j nl- sec" i

>...

.

.

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MOSCOW TIME,DesernI~e'c~5,1970 i

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10. M e a s u r e d a n d c a l c u l a t e d velocities o f d e s c e n t o f V e n e r a 7 f r o m 08.13.10 till 08.19.03.

function of temperature (Avduevsky et a l . , 1971a, b). Secondly, the vertical velocity was obtained directly from the temperature measurements using Eq. (4), which associates temperature and velocity of quasi-stationary descent. This calculation does not use the atmospheric model nor a value of the aerodynamic drag coefficient of the parachute-apparatus system. Then, thirdly, the velocity of descent as a function of time was obtained from relations determining velocity and time as a function of altitude

/

t(H) =

~'~.

®

®

.I"

H,

dh

(6)

.o V(h)

V(H) = (2g [M - ~p(H)]],n C,,Sp(H)

)

"

(7)

To calculate velocity, the formula of quasistationary descent was used. Density as a function of height was defined from the atmospheric model. The initial altitude for (6) was obtained from the atmospheric model at one measured temperature point only. Corrections for the nonstationary

.,~°/ "'*'x*---Q ---*'~

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,

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FIG. ] 1. A l t i t u d e progdes of w i n d velocity according to Venera 7 measurements, corresponding to three ways of calculating descent velocity.

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F I o . 12. M e a s u r e d a n d c a l c u l a t e d v e l o c i t y o f V e n e r a 7 d e s c e n t before l a n d i n g on t h e surface. Crosses r e p r e s e n t m e a s u r e m e n t s for t h e p r o b a b l e t e m p e r a t u r e d r i f t o f t h e oscillator f r e q u e n c y ; circles for t h e e x t r e m e t e m p e r a t u r e drift ; a n d s t a r s t h e calculated velocity.

character of motion at high altitudes were introduced in all three cases. Measured and calculated values of radial velocity are given in Figs. 9 and 10. The time of averaging is equal to 10see. I t is seen t h a t as a whole the velocity of descent in the intervals analysed is close to the calculated one. The profile of wind velocity obtained as a difference between the calculated and measured velocity values is shown in Fig. 11. The three curves correspond to the three ways of calculating vertical velocity. On the ordinate the more compressed scale relates to the horizontal component of wind velocity, the other scale to the vertical component. Wind directed from the antisolar point to the sunrise terminator corresponds to positive values of velocity. Profiles are obtained with spatial averaging of measurements equal to 1 km, t ha t is, more than the magnitude of lmin in this interval. A horizontal line shows the maximum constant systematic error in determining the wind velocity, associated with the inaccuracy in defining the frequency of the on-board oscillator. The abscissa scale of the ordinate corresponds to the altitude obtained by integrating the velocity from the planetary surface, or to the altitude defined by fitting the measured temperature to the atmospheric model. The difference of the two

scales of height in the interval under consideration is almost constant and is equal to 3.3km, which corresponds to a Venus radius equal to 6053km compared to 6050 assumed in the atmospheric model. Figure 11 shows t hat the wind velocity has a maximum of about 7-14m/see at 48-km altitude (with H taken from the atmospheric model), where the wind is directed toward the sunrise terminator. At 45-km altitude the direction of the wind velocity changes and a maximum of (5-7) m/sec is reached at a height of 43kin. However, because of systematic errors it is difficult to say definitely whether this is really a variation of direction or simply a decrease in wind velocity, since systematic errors can shift curves 1-3 as a whole (Fig. 11 ). Small wind velocities with a small angle ~ justify neglecting the horizontal motion of the descent apparatus to obtain a height temperature distribution (Avduevsky et al., 1971a, b). I f we assume t h a t only vertical motions occur during descent, then the velocities do not exceed 1.2-3.0m/scc for descending flow and 1-1.4m/see for ascending flow. Obtaining an estimate of wind velocity near the Venus surface is of special interest because, according to the "aeolospherie"

SUBCLOUD

VENUS

hypothesis, strong wind must exist in the lower atmosphere in order to heat the planet's surface. To obtain velocity in the interval of descent after 08.34.00 we note t h a t although an accurate coefficient of aerodynamic drag for the descent apparatus is unknown here, it is probable t h a t variations of CR here were small. This follows because: (1) the average velocity of the descent apparatus changed monotonically in this interval, and (2) the relative increase of the Reynolds number is small (less t h a n 15%) and its absolute magnitude is large (Re ~ 2.7 × 107). At such large Reynolds numbers motion becomes independent of Re and the CR magnitude is independent of the velocity; this is verified by experimental data. Taking CR to be constant we can define the change of velocity relative to the final one by the methods mentioned above. In the calculation, it was assumed t h a t the wind velocity during the last 6 sec of descent (altitude 0-100m) was equal to zero and t h a t the descent apparatus moved vertically. Figure 12 shows measured and calculated (stars) velocities of descent. Points correspond to the measured frequency and most probable drift and circles to the maximum frequency drift of the on-board oscillator. Profiles of wind velocity at altitudes 0-3.5kin can be obtained by comparing measured and calculated values of the velocity given in Fig. 12. The response time of the descent apparatus to the wind in this interval is equal to 4-5 sec and accurate values of wind velocity are determined by averaging over an interval of more than 100m. To exclude the influence of short-time variations of velocity of the descent apparatus which are not produced by wind, the velocity profile was calculated from measurements averaged over an interval of 30sec (about 500m). Wind velocity distributions are shown in Fig. 13. Curve 1 corresponds to the most probable values of descent velocity and curve 2 to the extreme temperature drift on the on-board oscillator frequency. The figure shows t h a t the wind velocity increases gradually to the value 2.5m/sec

67 l

ATMOSPHERE

Hi Km

/ /Z

/

/

l

I

./x l

Liar)m. sec "4

WIND

VELOCIW

~ h~ ~ m mSeC(

FIG. 13. H e i g h t profile of w i n d v e l o c i t y n e a r the surface. 1, p r o b a b l e ; 2, e x t r e m e .

(maximum 5m/sec) at a height of 3.5km. The corresponding vMue of vertical velocity at this height is equal to 0.Sm/see (maximum l m/sec). Motion from the sunrise terminator toward the antisolar point corresponds to negative values of horizontal velocity. We remind the reader t h a t the calculated values were obtained supposing t h a t the wind velocity near the surface is equal to zero. I f we abandon this assumption, then the curves in Fig. 13 show the difference between the wind velocity at a given height and the wind velocity at the surface layer. Considering the wind velocity in the boundary layer to be known we could obtain in general the temperature distribution near the surface from the measured velocity values. However, a simple calculation shows t h a t the difference of velocity in this interval for adiabatic or isothermal atmospheres does not exceed 0.1m/sec, which is beyond the limits of measurement errors.

672

v . v . KERZHANOVICI-I, M.Ya. MAROV, AND M. K. ROZttDESTVENSKY

VI. DISCUSSION OF RESULTS

The small number of experiments, limited space and "Venus time" distribution (2000-3000km from the sunrise terminator), short descent times, and low accuracy of velocity measurements do not permit definition of the circulation scheme from these data alone. However, because of the slow rotation around its axis, the considerable thermal inertia of the atmosphere, and the absence of seasons, the circulation on Venus probably has a stationary character and is more stable than the Earth's (Golitsyn, 1970b, Zilitinkevich et al., 1971). Therefore, some of its properties can be defined by single experiments. The possibility of meteorological "noise" caused by local conditions should be remembered however. As seen from the data in all the experiments at 20- to 40-km altitudes, and for Venera 7 near the surface, winds and turbulence were not observed. This result confirms modern conceptions about the dynamics of the Venus atmosphere and is an argument against the "aeolospheric' hypothesis. Convective motions at these altitudes were also small in the intervals where measurements were made; their velocities did not exceed 0.3 to 0.5m/sec. These values agree well with theoretical estimates from a model of radiativeconvective heat transfer (Marov et al., 1970). The motion in the upper troposphere is more complex. The absence of turbulence at the beginning of the Venera 5 and Venera 6 descent and the character of variation of wind velocity during the Venera 7 descent when the zonal component was directly measured, show t h a t a 4-day circulation does not occupy heights below at least 50-55km. The possibility of a meridional wind in the direction from pole to equator near the sunrise terminator (Venera 4) agrees with results of numerical experiments (Zilitinkevich et al., 1971; Charikov et al., 1971), although the two-level model does not allow quantitative comparison. Let us touch now upon two results obtained from Venera 7 data which are

not associated with the dyamics of the atmosphere. Since signal frequency and, therefore, absolute velocity of the descent apparatus after landing are known, a rough estimation of the proper rotation of Venus can be obtained because the velocity of the stationary apparatus is determined only by the relative motion of Venus and the receiving antenna on the Earth, and by the rotation of Venus around its axis. As was reported in previous papers (Avduevsky et al., 1971a, b), the signal frequency received from the Venus surface within an accuracy of 3 cps (1 m/see) coincided with the value of the signal frequency emitted from the apparatus, assuming it to be immobile relative to the Venus center. Taking into account the a priori error of extrapolation of signal frequency, this corresponds to a radial component of linear velocity of the Venus rotation less than 2m/see. Hence, a lower bound on the period of Venus rotation can be obtained:

T > 2rrR~AvSin ~ - 44 days, where/1 V = 2m/sec is the systematic error in measuring the velocity, and R~ is the radius of the Venus surface. This estimation of a period is certainly much more rough than t h a t from radar measurements ; however, it is of some interest because it is a direct local measurement of Venus surface velocity. Venera 7 data allow us to obtain an estimate of properties of the ground at the area of landing, by several indirect means (Avduevsky et al., 1971b). From the descent apparatus' static and dynamic strength, we can obtain an upper limit of the soil strength, determined by the velocity of impact with the planet's surface and the apparatus' mass. This estimate indicates t h a t the soil bearing strength did not exceed 80-85kg/cm 2. The analog of such soil on the Earth corresponds, for example, to volcanic tuff. A lower bound obtained from analysis of the signal received on the Earth during landing and also from additional experiments on dynamic models of the descent apparatus shows t h a t soil strength at the

SUBCLOUD VENUS ATMOSPHERE

area of landing exceeds 2kg/cm2; this corresponds to soils like light sands; however, this estimate is not very accurate. The analysis shows also that a liquid surface at the area of landing is essentially excluded. REFERENCES AVDUEVSKY, V. S., MAROV, M.YA., AND ROZI~DESTVENS:KY, M. K. (1969). Results of measurements of parameters of Venus atmosphere obtained by Soviet Automatic Station Venera 4. Kosm. Issled. (Cosmic Res.) 7, 233. AVDUEVS:KY, V. S., MAROV, M.YA., AND ROZHDESTVENS:KY,M. K. (1970a). The results of measurements on the automatic stations Venera 5 and Venera 6 and a model of the atmosphere of Venus. Kosm. Issled. (Cosmic Res.) 8, 871. AVDUEVSKY, V. S., MAI~OV, M.YA., AND ROZHDESTVENSKY, M. K. (1970b). A tentative model of the atmosphere of planet Venus based on the results of measurements of spaceprobes Venera 5 and Venera 6. J . Atmos. Sci., 27,561. AVDUEVSKY, V. S., MAROV, M.YA., AND ROZHDESTVENS:KY, M. K. (1970a). BORODIN, N. F., AND KERZHANOVICH, V. V. (1971a). Soft landing of Venera 7 on the Venus surface and preliminary results of investigations of the Venus atmosphere. J. Atmos. Sci. 28, 263. AVDUEVS:KY, V. S., MAROV, M.YA., ROZI~DESTVENSKY, M. K., KERZItANOVICIt, V. V., BORODI~¢, N. F., AND RYABOV, O. L. (1971b). The results of the Venus atmosphere measurements made by the landing station Venera 7. Paper presented at XIV COSPAI~ Meeting, Seattle, WA. BOYER, CH., AI',ID CAMICHEI~, H., (1965). Le recherche photographique de rotation de Venus. C.R. Acad. Sci. 260, 809. CHALIKOV,D. V., MONIN, A. S., TuRIKOV, V. G., AND ZILITINKEVICtt, S. S. (1971). Numerical experiments of the general circulation of Venus atmosphere. Tellus 23, 488. GOnlTSY~¢, G. S. (1968). Estimates of some characteristics of general circulation of terrestrial planets. Izv. Acad. Sci. U S S R Atmos. Oceanic. Phys. 4, 1131. GOLITSYN, G. S. (1970a). A theory of similarity for large scale motions of planetary atmospheres. Dokl. A]cad. Naulc S S S R 190, 323. GOLITSYN, G. S. (1970b). A similarity approach to the general circulation of planetary atmospheres. Icarus 13, 1. GOLITSYN, G. S., AND KERZHANOVICH, V. V.

673

( 1971 ). Mutual correspondence of the atmospheric parameters measured on the spaceprobe and its velocity of descent. Kosm. Issled. Cosmic Res. 9, No. 6. GOODY, 1~. (1965). The structure of the Venus cloud veil. J. Geophys. Res. 70, 5471. GooDY, 1~., AND ROBINSON, A. (1966). Discussion of the deep circulation of the Venus atmosphere. Astrophys. J. 146, 339. HEss, S. L. (1968). The hydrodynamics of Mars and Venus. I n "Atmospheres of Mars and Venus." Holt N.Y. KERZHANOVICH, V. V., GOTLIB, V. M. CHETYR:KIN, 1~. V., AND AI~DREEV, B. 1~. (1969). The results of determination of Venus atmosphere dynamics on the Venera 4 radial velocity measurements. Kosm. Issled. (Cosmic Res.) 7, 592. KERZHANOVICH, V. V., A:NI)REEV, B. 1~., GOTLIB, V. M. {1970). Investigation of Venus atmosphere dynamics by the automatic interplanetary stations Venera 5 and Venera 6. Dokl. Akad. N a u k S S S R 194, 288. KERZItANOVICH, V. V., ANDREEV, S . :N., AND GOTLIB, V. M. (1971). Wind velocities and turbulence in the Venus atmosphere obtained from Doppler velocity measurements of automatic interplanetary stations Venera 4-6. Kosm. Issled. (Cosmic Res). 19, No. 2. KUXPER, G. P. (1969). Short communication at X I I COSPAR Meeting, Prague, CSSR. MAROY, M.YA. (1971). The model of the Venus atmosphere. Dokl. Akad. N a u k S S R 196, 67. MAROV, M.YA., AVDUEVS:KY, V. S., NOY:KINA, A. I., POLEZHAEV, V. I., ANI) ZAVELEVICH, F. S. (1970). H e a t transfer in the Venus atmosphere. J. Atmos. Sci. 27,569. MAROV, M.YA. (1972). Venus: A perspective at the beginning of planetary exploration. Icarus 16, 455. MINTZ, Y. (1961). Temperature and circulation of the Venus atmosphere. Planet. ,Space Sci. 5, 141. MONIN, A. S., AND YACLOM, A. M. (1967). "The Statistical Hydrodynamics," p. II, Nauka, Moscow. OBOUKI-IOV, A. M., AND GOLITSYN, G. S. (1969). The dynamics of descending of automatic stations in planetary atmospheres to control measurements data. Kosm. Issled. (Cosmic Res. 7, 150. OPI:K, E. L. (1961). The atmosphere and aeolosphere of Venus. J. Geophys. Res. 66, 2807. POLEZ~AYEY, V. I., AnD VLASIU:K,M. P. (1971). A model of penetrating convection of compressible gas in deep atmosphere. Dold. Alcad. N a u k SSb'R 201,552. STONE, P. H. (1968). Some properties of Hadley

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v.v.

1KERZItANOVICH~ M . Y a . MAROV, A N D M. K. R O Z H D E S T V E N S K Y

Regime on rotating and non-rotating planets. J . A t m o s . Sci. 25, No. 4. TURIKOV, V. G., AND CHALIKOV, D. V. (1971). T h e c o m p u t a t i o n of t h e general c i r c u l a t i o n of t h e V e n u s a t m o s p h e r e . Izv. Acad. Sci. U S S R Atmos. Oceanic P h y s . 7, 705. VINNICHENKO, N. K., PINUS, N. Z., SHMETER,

S. M., AND SHUR, A. N. (1968). T u r b u l e n c e of t h e free a t m o s p h e r e . " H y d r o m e t e o i s d a t " , Leningrad. ZILITINKEVICtI, S. S., MONIN, A. S., TURIKOV, V. G., AND CtIALIKOV, D. V. (1971). N u m e r i c a l m o d e l l i n g of t h e V e n u s i a n a t m o s p h e r e circulation. Dokl. A k a d . Naulc S S S R 197, 1291.

DIscussioN SATAN: I f h o r i z o n t a l velocities are several m / s e e n e a r t h e surface, w i t h a surface p r e s s u r e 100 t i m e s t h a t of t h e e a r t h , w o u l d n o t one e x p e c t large q u a n t i t i e s of d u s t t o b e p i c k e d u p easily.* MAROV: I d o n ' t t h i n k so. O u r d a t a c a n be in e r r o r b y 3 - 4 m / s e c . SAGAN : Are t h e velocities of t e n s of m / s e e a t t h e 50 k m level t i e d t o t h e 100 m / s e e w i n d s o b s e r v e d h i g h e r in t h e atmosphere.* MA~OV : W i t h V e n e r a 4 we were a c t u a l l y o b s e r v i n g a m e r i d i o n a l c o m p o n e n t , b u t p e r h a p s t h e y are in some w a y related. I-IIDE: C a n y o u c o m m e n t on t h e i m p l i c a t i o n s of t h e o b s e r v a t i o n s w i t h r e g a r d to t h e early m o d e l s of tile deep circulation. MA~OV : T h e o b s e r v a t i o n s are c o m p a t i b l e w i t h all early models. SAGAS: I t seems t h a t t h e surface velocities are h i g h e r t h a n t h e few t e n t h s of a m / s e e t h a t is p r e d i c t e d b y G o l i t s y n ' s s i m i l a r i t y t h e o r y . MAROV : W e m u s t r e g a r d t h e s e o b s e r v e d velocities n e a r t h e surface as u p p e r limits.