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

iCARUS 52, 320--334 (1982)

Thermal Structure of the Atmosphere of Venus from Pioneer Venus Radio Occultations 1 ARVYDAS J. KLIORE AND INDU R. PATEL Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109 Received January 15, 1982; revised August 30, 1982 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 - 6 8 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.

INTRODUCTION

The Pioneer Venus Orbiter spacecraft was inserted into a 24-hr orbit about Venus on December 4, 1978. At that time the orbital geometry permitted the spacecraft to be occulted by Venus near periapsis of the orbit. These occultations continued until mid-February of 1979, and they constitute the first occultation season, encompassing orbits 1 through 70. A second occultation season began on October 6, 1979, and continued through December 19, 1979, including orbits 305 through 379. The third occultation season, which began on September 22, 1980, and ended on October 26, 1980, included orbits 657 through 691. The fourth occultation season occurred between SepPaper presented at "An International Conference on the Venus Environment," Palo Alto, California, November 1-6, 1981.

tember 4 and October 30, 1981, orbits 1003 to 1059. Each occultation provides an opportunity to perform two radio occultation measurements, one during the entry into occultation, and one during the exit. During the first occultation season, radio occultation data were taken during every orbit, and to date 53 of the successfully acquired entry and exit measurements have been processed and analyzed. During the second occultation season, tracking conflicts reduced the number of orbits during which data were taken, and only 14 measurements have been analyzed. Similarly, during the third occultation season only eight measurements of the atmosphere structure were made. During the fourth season only 12 measurements were successfully acquired. The locations of the radio occultation measurements reported in this paper are 320

0019-1035/82/110320-15 $02.00/0 Copyidght© 1982by AcademicPress, Inc. All fightsof reproductionin any formreserved.

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shown in Fig. 1. The entry measurements of the first season occurred at high northern latitudes near the terminator on the nightside. The exit measurements were made at latitudes from about 60° South to the equator, also on the nightside. The second, third, and fourth season measurements were all made on the dayside at various latitudes (see Fig. 1). The location of the first season exit measurements made them suitable for the study of the nightside ionosphere of Venus (Kliore et al., 1979) and the other first, second, third, and fourth season measurements provide fairly good coverage in both latitude and solar zenith angle, with the exception of missing coverage of northern hemisphere near-equatorial latitudes, and measurements taken at low solar zenith angles. It is expected that measurements in these regions will be obtained from forthcoming occultation seasons. The processing and analysis of the 87 radio occultation measurements that are described in this paper have provided a suffi-

cient base of data for the investigation of changes of the thermal structure with latitude, solar zenith angle, and time (longterm temporal changes). DATA PROCESSING

The data used to obtain atmospheric structure information were derived from Sband (2293 MHz) and X-band (8407 MHz) signals recorded by the Occultation Data Assembly (ODA) at the Deep Space Network (DSN) stations at Goldstone, California, and Tidbinbilla, Australia (Berman and Ramos, 1980). The detailed procedures of data acquisition and processing have been described previously (Kliore and Patel, 1980) and they will not be reiterated here. The major part of the processing and analysis of data was carried out at the JPL RODAN (Radio Occultation Data Analysis) Facility. The magnetic tapes containing the ODA data, consisting of digital recordings of sampled data from the S-band and Xband open-loop receivers, were first pro-

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cessed at the RODAN facility to extract the frequency and amplitude of the recorded signals as functions of time. The Programmed Oscillator Control Assembly (POCA) steering function is then added back in to produce the actual received S- and X-band frequencies. The frequency changes in the signals caused by orbital motion of the spacecraft, rotation of the Earth, and other predictable sources are removed by subtracting predictions based on the precisely determined orbit of the spacecraft. The resulting frequency residuals represent only the effects of propagation through the ionosphere and the neutral atmosphere of Venus. The S-band residuals are then used to produce the desired atmospheric results after the effects of the ionosphere are removed using the combined S- and X-band residuals. Remaining drifts and biases in the corrected S-band residuals are then removed by fitting a least-squares straight line or quadratic function to that portion of the data lying outside of the region affected by the atmosphere of Venus. These processed residuals, along with a trajectory of the spacecraft relative to Venus and the Earth, are then used to compute the refractive bending angle as a function of the ray asymptote (Kliore, 1972). The Abel integral transform (Fjeldbo and Eshleman, 1968; Kliore, 1972) is then used to invert these data to obtain a vertical profile of the index of refraction as a function of radius. The index of refraction is then transformed to density by assuming a composition of the neutral atmosphere of Venus which consists of gases for which the indices of refraction are known (Essen and Froome, 1951). In accordance with Pioneer Venus mass spectrometer measurements, the Venus atmosphere is assumed to consist of 96% CO2 and 4% N2 (Hoffman et al., 1979). The barometric equation is then integrated downwards to obtain pressure as a function of radial distance, and the perfect gas law is used to obtain temperature. Initial temperatures of 150, 200, and 250°K were assumed

to produce the temperature profiles presented in this paper. The differences in the temperature profiles introduced by the choice of initial temperatures disappear in most cases above the level of about 1 mbar pressure, which occurs in an altitude of about 80-90 km. DEPENDENCE OF ATMOSPHERIC TEMPERATURE STRUCTURE ON LATITUDE

Nine temperature profiles, derived from data taken at locations separated by about I0° in latitude, are shown in Fig. 2. The profiles are displayed with altitude on the vertical axis (relative to a 6052-km surface radius), and the horizontal temperature scale extends from 150 to 400°K for each profile. The orbit number and latitude of measurement identify each profile at the top. The most striking feature of Fig. 2 is the dramatic change in the temperature structure as one moves from low latitudes (left) to high latitudes (right). At low latitudes, the temperature lapse rate changes very gradually above the tropopause (marked on each profile by a horizontal line), decreasing somewhat but still retaining a rather large temperature lapse rate in the stratosphere. 2 At mid-latitudes a more distinct tropopause begins to be evident, a growing inversion appears above the tropopause, and the stratospheric temperature lapse rate becomes much lower. At latitudes from about 55 to 75° North [the collar region described by Taylor et al. (1979, 1980)] both the height of the tropopause and the magnitude of the inversion reach maximum dimensions. Finally, in the polar regions from 80 to 90°, the tropopause again descends to a lower altitude, and the stratosphere becomes nearly isothermal. Profiles from low-latitude, mid-latitude, collar, and polar regions are shown superimposed in Fig. 3. In addition to the infor2 In this paper the word stratosphere is used to describe that part of the middle a t m o s p h e r e lying above the tropopause, which is here defined as the altitude at which the nearly adiabatic tropospheric temperature lapse rate begins to show a decrease.

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the temperature at the 1-bar pressure level was obtained by interpolation from each profile. In Fig. 4 the results are plotted against latitude. In this and all other data plotted against latitude different symbols are used for m e a s u r e m e n t s made in the northern and southern hemispheres. A practically constant t e m p e r a t u r e of about 346°K is o b s e r v e d from the equator to about 55 °, at which point it begins to decrease almost linearly with increasing latitude and reaches a value of about 323°K at polar latitudes b e t w e e n 80 and 90 ° . These temperatures agree very well with those reported from probe data at 1 bar (Seiff et al., 1980) which are also plotted in Fig. 4. It is difficult to estimate the realistic uncertainty in the computation of temperature. Estimates of errors based on the rand o m phase jitter noise in the original Doppler frequency data are always unrealistically low. Systematic errors, such as those arising from nonlinear free space baselines produced by imperfect orbit determination, contribute more to the actual error and are difficult to estimate. Therefore, an empirical method was used to esti-

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Fro. 4. The variation of temperature with latitude at the 1-bar pressure level. Points taken from Pioneer Venus probe measurements (Seiff et al., 1980) are also shown. mate the uncertainty in temperature based on the assumption that in the data displayed in Fig. 4 the 1-bar temperature from about 20 to 50 ° latitude is actually constant, and that the scatter observed in the data represents typical errors in the computation of the temperature profiles. The standard deviation of all data points in this range of latitudes was found to be 1.5°K. The use of this figure to represent the uncertainty of temperature measurements is perhaps unduly conservative because of the likelihood that the scatter in the data points represents not only errors in temperature measurement, but also real temperature changes due to dynamical processes in the atmosphere. H o w e v e r , it does serve as a useful error estimate in the context of this study. Therefore, the 3or error in temperature measurements presented in this paper will be adopted as _4.5°K. This uncertainty could undoubtedly be reduced if unlimited amounts of time and resources were available for the analysis of each set of data.

Latitudinal thermal structure changes are to a great extent characterized by changes in the properties of the tropopause and the mesosphere inversion. It is likely that these phenomena are related to the persistent atmospheric circulation pattern of Venus (Schubert et al., 1980; Schubert, 1982), which also manifests itself in the persistent cloud patterns observed in the ultraviolet images of Venus (Travis et al., 1979). Four quantities were measured in connection with the tropopause, as shown in Fig. 5. They are the tropopause altitude, pressure, temperature, and the depth of the mesosphere inversion above the tropopause. The tropopause altitudes are shown plotted against latitude in Fig. 6. The error bars in Fig. 6 were obtained as follows: 8hv = 3(8ho 2 + Ah2) I/2, where

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The quantities trx, cry, and orz are, respectively, the formal uncertainties in the x, y , and z components of the orbital state vector, and Ah is the altitude interval between adjacent data points. In effect, the error bars represent a 3tr orbit determination error. As one would expect from the appearance of Figs. 2 and 3, the tropopause altitude, which has a value of about 56.3 km at low latitudes, begins to rise at about 40° latitude and reaches a maximum of about 61.1 km in the collar region between approximately 55 and 77° , decreasing again to about 57.7 km in the polar region. Also plotted in Fig. 6 are the altitudes of the 300°K temperature level and of the 1-bar pressure level. In contrast to the tropopause altitude, the 300°K level remains constant from the equator to about 65 ° latitude, at which point it begins to decrease as one moves poleward. The level of the 1-bar pressure surface remains fairly constant, decreasing an average of about 0.7 km from equator to pole. This altitude difference is consistent with that produced under the assumption of cyclostrophic balance by zonal winds of 60-80 msec -~ magnitude. The scatter of the data points in the I-bar pressure temperature level, especially at lati-

325

tudes between 20 and 50°, should be representative of the empirically determined altitude uncertainty of the measurements, and the computed standard deviation of the data points in this region is 0.19 km, leading to a 3tr uncertainty of +--0.57 km. One obvious feature of Fig. 6 is that the scatter of the points representing the tropopause altitude is much greater than the scatter in the 300°K temperature level, and especially the l-bar level, suggesting that the changing tropopause altitude reflects actual changes in the dynamics of the atmosphere. However, measurements taken during different occultation seasons, spaced some 300 days apart in time, tend to be similar at approximately the same latitudes, although significant differences between measurements from the same season and also between measurements from different seasons are observable, especially in the collar region. This would seem to be a result of the altitude of the tropopause in the collar region being dependent upon the structure of the border of the collar cloud features, which is variable with longitude and time. The atmospheric pressure, in millibars, at the tropopause level, plotted on a logarithmic scale, is shown as a function of latitude in Fig. 7. The behavior of pressure with latitude is consistent with the changes in the tropopause altitude. At equatorial and low latitudes, where the tropopause altitude is lower, the pressure is high, having an average value of about 385 mbar. As the tropopause altitude increases, the pressure falls, reaching a minimum value of about 147 mbar in the collar region. Near the pole, where the tropopause altitude is again lower, the pressure increases to an average value of about 253 mbar. Again, there is little difference between measurements taken during different occultation seasons, except in the collar region between about 70 and 80° latitude, in which some of the measurements were apparently made on the collar and some were already in the polar region, depending on the longitude of measurement.

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Figure 8 shows the variation of the tropopause temperature with latitude. The temperature drops sharply from an average of about 284°K at latitudes below 40 ° to the collar region, in which it reaches a minimum of about 224°K. In the polar region the average temperature rises to about 241°K. The most striking behavior is exhibited by the magnitude of the stratosphere inversion, which is shown plotted against latitude in Fig. 9. This feature shows the greatest variability with latitude, with little or no inversion from the equator to about 50°, and very large inversions, reaching magnitudes up to about 33°K, and an average value of about 20°K, in the collar region. The deep inversions characteristic of the collar region are observed during the first and second occultation seasons, during which the high-latitude measurements were taken in the northern hemisphere, and in

one measurement of the fourth season, taken at 68° South. The inversion is practically absent in the three higher-latitude measurements of the third season, which were taken in the southern hemisphere. Thus the absence of significant inversions in the temperature profiles of the third season can be interpreted either as a hemispheric or temporal effect, although the latitudes of measurement were not as high in the third season as they were in the first and second seasons. The dramatic changes in the thermal structure of the atmosphere in the collar region, as reflected in the properties of the tropopause and the stratospheric inversion, are very well correlated with the increased intensity in scintillations due to turbulence (Woo et al., 1982) and observations of thermal waves in the latitude region of the collar (Apt and Leung, 1982). It is therefore

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suggested that various phenomena are connected with the persistent large-scale atmosphere circulation pattern of Venus, which appears to change at about a latitude of 50 ° from a zonally symmetric pattern to a circumpolar vortex. It is important to point out that the observations indicate that the most obvious features of the thermal structure of the Venus atmosphere, such as the tropopause height and stratosphere inversion, are determined principally by the latitude of measurement, and are not obviously affected by solar illumination conditions (dayside or nightside). In their publications, the Venera 9 and 10 radio science investigators have often segregated their measurements into dayside and night-

side categories (Kolosov et al., 1980; Yakovlev et al., 1979), inferring diurnal effects in temperature structure. In reality, the Venera nightside measurements were all made at low and moderate latitudes, showing a rather uniform stratosphere structure, and the dayside measurements were made at various latitudes from moderate to high (72 ° lat.) which naturally displayed the inversions characteristic of the collar region. It is important to avoid the impression that these features are a consequence of the dayside locations of the measurements rather than their latitude. The changes in thermal structure with latitude were, however, exploited by the Venera investigators to estimate zonal wind veloci-

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ties in the region of the tropopause (Chub and Yakovlev, 1980). A similar, but more complex, investigation of the wind field, in the Venus atmosphere using the data described in this paper is currently in progress. (Newman, Schubert, Kliore, and Patel, in preparation) In Table I, averages and standard deviations of the tropopause and inversion parameters are listed for each of the four regions of the cloud surface that appear to

have diverse properties. The four regions are the equatorial and mid-latitude region, which covers latitudes from the equator to 40 ° , corresponding to 64.3% of the cloud surface; the transition region, from 40 to 55 ° latitude, accounting for 17.6%; the collar region, from 55 to 77 ° latitude, or 15.5% of the cloud surface; and finally, the polar region, from 77 to 90 ° latitude, occupying only 2.6% of the surface.

TABLE

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Equatorial and mid-latitude, 0 - 4 0 ° lat. Transition, 4 0 - 5 5 ° lat. Collar, 5 5 - 7 7 ° lat. Polar, 7 7 - 9 0 ° lat.

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TEMPORAL

VARIABILITY

OF THERMAL

STRUCTURE

B e c a u s e m e a s u r e m e n t s w e r e made at similar latitudes during each o f the three occultation s e a s o n s , it is possible to compare the thermal structure at certain latitudes at points in time separated by s o m e 300 days. The results o f several such c o m parisons are s h o w n in Figs. 10 through 14. In each of these figures, the vertical error bars reflect the altitude uncertainty due to orbital position errors. Measurements in the polar regions are available from the first, second, and fourth seasons. The c o m p a r i s o n o f Fig. 10 inv o l v e s three m e a s u r e m e n t s , o n e made at a latitude o f 85.4 ° North in the first season, one at a latitude o f 78.7 ° in the s e c o n d sea-

son, and a m e a s u r e m e n t at latitude 84.2 ° in the fourth season. The nearly isothermal character o f the stratosphere is evident in all three profiles, as are the small inversions b e t w e e n the tropopause and about 75 km altitude, at w h i c h point all three profiles develop a high temperature lapse rate. The troposphere appears to be warmer in the second season, but this could be the result of an altitude error. The error bars attached to each profile denote the 3tr uncertainty. A comparison o f first- and s e c o n d - s e a s o n measurements in the collar region at latitudes o f 75.5 ° and 77.1 ° and a fourth-season measurement at - 6 7 . 8 ° latitude is s h o w n in Fig. 11. For the first and s e c o n d s e a s o n s the general shape o f the stratosphere structure is similar, although the first-season (night-

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FIG. 10. Comparison of measurements taken in the polar region during the first, second, and fourth seasons.

side) measurement shows more small-scale structure. The fourth-season measurement displays a similar inversion magnitude, but the corresponding temperatures are some 15-20°K warmer. This could be due to hemispherical asymmetry or temporal variability in the collar structure. The differences in altitude in the troposphere could be due to orbit uncertainties or could reflect a real difference in temperature caused by temporal changes in the dynamics of the collar region. A comparison of the structure at about 62 ° North latitude in measurements taken during the first and second seasons, and a measurement at - 6 1 . 5 ° from the third season is shown in Fig. 12. The structure above the tropopause is quite different, with the third-season measurement not displaying the prominent inversion that is so characteristic of collar latitude measurements in the first, second, and fourth seasons. However, the third-season measurement was taken in the opposite hemisphere, thus the observed differences could again be caused by n o r t h - s o u t h asymmetry. A comparison of mid-latitude profiles taken during the first, third, and fourth seasons at latitudes of - 4 6 . 8 , 44.8 and 47.7 ° is shown in Fig. 13. The general trends in the

I 300

I 350

I 400

450

TEMPERATURE, K

F I G . 1 1. Comparison of first-, second-, and fourthseason measurements taken in the circumpolar collar region. The difference in altitude is most probably due to orbit uncertainties, although day-night effects may play a role,

stratosphere are quite similar, although the detailed structures are different, with the third season dayside measurement showing a substantially cooler stratosphere. The same can be said about the comparison of low latitude measurements taken during the first, second, and fourth seasons 90

[

80

i

i

~.~,,

i

i

HIGH LATITUDES

~

FIRST SEASON, N I G H T , 62N I.AT = 6 2 . 0 °, SZA = 119.0 °

70

I

------

SECOND SEASON, DAY, 354N LAT = 61.5 ° SZA = 7 4 . 8 °

----

THIRD SEASON, DAY, 679X = . o, :63.0 o

I

60

. ~

4C

I

150

200

I

250

I

300

I

350

I

400

450

TEMPERATURE, K

F I G . 1 2 . Comparison of first, second, and third season measurements taken at high latitudes. It is notable that the inversion features above the tropopause appear to have persisted for nearly 300 days in the case of the first and second season northern hemisphere observations, but no such feature exists in the southern hemisphere observation.

VENUS ATMOSPHERE THERMAL STRUCTURE 90

I

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\\.~x

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I

L?~-~ \~--b~x

----

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--

FIRST SEASON, NIGHT, 27X LAT~ --46.8 °, SZA : 128.1 °

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t

MID-LATITUDE

X~ 70~

I

.

THIRD SEASON, DAY, 679N tAT = 44.8 °, SZA : 52.5 °

-,

FOURTH SEASON, DAY, 1055N

~

~ . o

= 46.7°

--

< 50

4O

3O 150

I 200

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I 300

~ 350

t

400

450

TEMPERATURE, K

FIG. 13. Comparison of mid-latitude measurements

taken during the first, third, and fourth seasons. Although the measurements were taken in opposite hemispheres, the general characteristics of the thermal structure above the tropopause are quite similar.

at latitudes of - 2 7 . 1 , - 2 1 . 9 , and 33.2 ° . Here the agreement in the troposphere is quite good, and the mesosphere structure differs in detail, but not in general characteristics (Fig. 14). It should be kept in mind that the preceding comparisons were based on similarities in latitude only, and that the measurements being compared were different in longitude, solar illumination conditions, and, of course, time of measurement. In the case of comparisons with third season observations, the measurements are from similar latitudes but different n o r t h - s o u t h hemispheres. Thus there is insufficient data to generalize the observations to the entire planet; however, some salient observations can be made. a. In the collar region (55 to 78 ° lat.) the structure of the stratosphere inversion region appears to have changed very little between the first and second occultation season, an interval of about 300 days. This seems to indicate that the circumpolar vortex structure at the collar latitudes is a fairly persistent feature. In contrast, there is very little similarity in structure between measurements taken at similar latitudes in

331

the north and south hemispheres with a time interval of about 600 days, suggesting that the structure of the circumpolar vortex is not symmetrical between the north and south hemispheres, and is variable with time. The fourth season observations also support this conclusion. b. In the mid- and low latitudes (55 ° to the equator) there is little difference in the thermal structure of the stratosphere between measurements taken during different occultation seasons or in opposite hemispheres. c. The temperature profiles determined from radio occultation data agree very well with in situ measurements by the Pioneer probes (Seiff et al., 1980) and with remote sensing data obtained by the infrared radiometer instrument (Taylor et al., 1980) at similar latitudes (Kliore and Patel, 1980). E F F E C T OF DISTORTIONS D U E TO A T M O S P H E R I C ROTATION

Because spherical symmetry is assumed in the inversion of data using the Abel integral transform, any distortions introduced in the atmosphere by high zonal winds could produce erroneous temperature structure profiles when the spherical symmetry assumption is invoked (Hubbard et al., 1975; Eshleman, 1975). This effect has 90

I

I

[

t

I

LOW LATITUDES 80

E

x.'~,,~%. \N~-

FIRST SEASON, NIGHT, 47X tAT = -27.1 °, SZA = 148.5 °

q~,.. \k'~,

7O

- - - - - - SECOND SEASON, DAY, 350X LAT -- -21.9 ° SZA = 62. I ° -

""k~',- . . . . "~,

FOURTH SEASON, TERM., 1041X LAT = -33.2 °, SZA = 98.8 °

< 50

40 30 150

i

200

I

250

I

300

I

350

L

400

450

TEMPERATURE, K

FIG. 14. Comparison of low-latitude measurements during the first, second, and fourth seasons. The stratosphere structures are similar in the three profiles, although significant temperature differences exist.

332

KLIORE AND PATEL

already been proposed as a technique to study zonal winds in the giant planets (Kliore et al., 1976). Thus the question arose of whether the high zonal winds observed in the atmosphere of Venus, at least up to the level of the clouds (Counselman et al., 1980), could be effective in introducing a sufficient degree of distortion to the stratosphere in the collar region to produce the ubiquitous stratosphere inversion. In order to investigate this possibility, an approximate analysis was undertaken, as described below. The angle ot between the true vertical direction in the absence of a zonal wind motion and the actual direction of the gravity gradient in the presence of a zonal wind of velocity v can be described by U2

a -- tan 0 - Rg'

(2)

in which 0 is the latitude, R is the distance from the planet center, v is the zonal velocity, and g is the acceleration of gravity. Combining this with Eq. (6) of Eshleman (1975), it is possible to derive the following approximation for the gravitational perturbation in the derived temperature structure when neglecting the effects of zonal winds: 2TeDv 2 AT - - tan 0 tan ~, HRg

(3)

where T is the temperature in Kelvin, • is the refractive bending angle, D is the distance of the spacecraft from the occulting limb, H is the scale height, ~ is the angle between the projection of the spacecraft velocity vector and the density gradient direction, and the other quantities are defined in Eq. (2). Using Eq. (3) with d~ = 45 ° and a model of the zonal wind velocity with altitude abstracted from Counselman et al. (1980) the estimated gravitational perturbation in the derived temperature was computed for the high-latitude (collar) measurement of orbit 53 entry. The results are shown in Fig. 15, which also shows the zonal wind model. F r o m Fig. 15 it is obvi-

90

80

~

<

ZONAL WIND SPEEDms-1 20 40 60 I I I

0 I

~

~

80 I

100

ZONALWIND

6o

ZONAL WIND ~ GRAVITATIONAL ~'~ DISTORTION ORB53N, LAT = 71.5 ~ ~.. 40 AS DERIVED J - - - - EFFECTOF DISTORTrON 30 I I L ] 150 200 250 300 350 TEMPERATURE, K 50

I 400

4,50

FIG. 15. The effect on a temperature profile of departures from sphericity introduced by zonal wind gravitational distortion. The zonal wind model is given in the right-hand portion of the figure, which refers to the upper horizontal scale. Although those zonal wind gravitational distortions cannot produce the inversions seen above the troposphere in the collar region, under certain circumstances they can be significant, causing temperature deviations of several °K. ous that the zonal wind motions of the atmosphere are not likely to be responsible for the temperature structure observed in the collar region. H o w e v e r , it also shows that under certain conditions the effect of zonal winds upon the measured temperature could be nonnegligible. In this example, the effect is exaggerated because at the high latitude chosen the zonal wind speed is probably lower than in mid-latitude or equatorial regions. H o w e v e r , it is possible that the effects of an atmosphere gravitationally distorted by zonal winds could amount to 1 or 2°K, especially at lower levels in the atmosphere. The above discussion applies only to distortions caused by centrifugal perturbations of the gravity field. The distortion of the equipotential surfaces by zonal winds in cyclostrophic balance can also produce local altitude and slope changes. If a uniform change of 0.7 km from 40 ° latitude to the pole is assumed at the 1-bar level (see previous section) the resulting slope distortion

VENUS ATMOSPHERE THERMAL STRUCTURE could cause a temperature error of about 3°K. If higher local slope distortions exist, the temperature errors could be higher in those locations. ACKNOWLEDGMENTS The authors are grateful to the Pioneer Project staff at NASA Ames Research Center, especially to R. O. Fimmel, R. Jackson, J. Dyer, J. Cowley, and D. Tristram, and to the Pioneer Navigation Team at JPL, especially to B. G. Williams and S. K. Wong for their help in providing the spacecraft orbits. Thanks are also due to the personnel of the Deep Space Net for their help in data acquisition, especially to A. Bouck, R. Nevarez, W. Hietzke, R. Kursinski, and R. Ryan. Special thanks is due to G. Lindal and D. N. Sweetham for the use of some of their software and to S. Frastaci and H. Hotz for assistance in data processing and analysis. We also wish to acknowledge stimulating discussions with G. Schubert, A. Seiff, R. Woo, D. McCleese, L. Elson, J. B. Cimino, G. F. Lindal, and D. Sweetnam. This paper represents the results of one phase of research carried out at the Jet Propulsion Laboratory, California Institute of Technology, under NASA Contract NAS 7-100. REFERENCES APT, J., AND J. LEUNG (1982). Thermal periodicities in the Venus atmosphere. Icarus 49, 427-437. BERMAN, A., AND R. RAMOS (1980). Pioneer Venus occultation radio science data generation. IEEE Trans. Geosci. Remote Sensing GE-83, 11-14. CHUB, E. V., AND O. I. YAKOVLEV (1980). Temperature and zonal circulation of the atmosphere of Venus based on data of radio probe experiments. Cosmic Res. 18, 331-336. COUNSELMAN, C. C., III, S. A. GOUREVlTCH, R. W. KING, G. B. LORIOT, AND E. S. GINSBERG (1980). Zonal and meridional circulation of the lower atmosphere of Venus determined by radio interferometry. J. Geophys. Res. 85, 8026-8031. ESHLEMAN, V. R. (1975). Jupiter's atmosphere: Problems and potential of radio occultation. Science 189, 876-878. ESSEN, L., AND K. D. FROOME (1951). The refractive indices and dielectric constants of air and its principal constituents at 24,000 Mc/s. Proc. Roy. Soc. B 64, 862-875. FJELDBO, G., AND V. R. ESHLEMAN (1968). The atmosphere of Mars analyzed by integral inversion of Mariner 4 occultation data. Planet. Space Sci. 16, 1035. HOFFMAN, J. H., R. R. HODGES, i . B. McELRoY, AND T. M. DONAHUE, (1979). Venus lower atmospheric composition: Preliminary results from Pioneer Venus, Science, 203, 800.

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Cloud images from the Pioneer Venus Orbiter. Science 205, 74-76. Woo, R., J. A. ARMSTRONG,AND A. J. KLIORE (1982). Small-scale turbulence in the atmosphere of Venus. Icarus 52, 335-345. YAKOVLEV,O. I., A. I. EFIMOV, S. S. MATYUGOV,T. S. TIMOFEEVA, E. V. CHUB, AND G. D. YAKOVLEVA(1978). Radioscopy of the nighttime atmosphere of Venus by probes Venera 9 and Venera 10. Cosmic Res. 16, 88-92.