Meteorological variations in the middle clouds of Venus from VEGA balloon in situ measurements

Adv. Space Res. Vol. 7, No. 12, pp. (12)337—(12)342, 1987 Printed in Great Britain. All rights reserved.

0273—1177/87$O.O0 + .50 Copyright © 1987 COSPAR

METEOROLOGICAL VARIATIONS IN THE MIDDLE CLOUDS OF VENUS FROM VEGA BALLOON IN SITU MEASUREMENTS Lee S. Elson* and the VEGA Balloon Science Team *M/S 183—301, Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Dr., Pasadena, CA 91109, U.S.A.

ABSTRACT

Instruments aboard the gondolas of the two VEGA balloons obtained in situ measurements of pressure, temperature, vertical velocity relative to the balloon, cloud particle backscatter, lightning and the ambient light level. Atmospheric motions at the balloon float altitudes were also determined from Earth—based tracking results. To illustrate the history of the balloon flights and to facilitate comparisons between some of the different observed quantities, measurements of pressure, temperature and backscatter are presented as time series for the entire lifetime of each balloon. Both long and short period variations have been detected. In addition, the environmental entropy encountered by each balloon will be discussed. INTRODUCTION The VEGA balloons were the first platforms to explore the horizontal domain of the middle cloud tegion of the Venus atmosphere by sampling the atmosphere at 75 second intervals for a period of about 46 hours. Because of planned periods of transmitter power conservation, approximately 22.5 hours of data were collected from each balloon. An overview of the balloon experiment has been given by /1/. An overview of the in situ measurements has been published /2/. RESULTS AND DISCUSSION Figures 1 and 2 show several quantities measured by VEGA 1 and 2 respectively as functions of time. VEGA 1 experienced the greatest altitude changes during the beginning and end of it’s flight whereas VEGA 2 encountered relatively calm conditions after deployment but very active conditions after about 35 hours of flight. During this active period, VEGA 2 Doppler results /3/ indicated a rather large change in line of sight velocity. The large vertical excursions of the balloons were caused by vertical winds in the atmosphere which occasionally exceeded 3 rn/s. As can be seen from Figures la and 2a, the majority of the excursions from the balloons’ equilibrium positions occurred in the downward direction. Both balloons slowly lost altitude as helium diffused through their skin. It is easily seen that pressure and temperature are strongly correlated for both balloons, indicating that variations in these quantities were primarily caused by vertical motions of the balloons in a region with a large (10.2 K/km ) vertical temperature gradient. Towards the ends of the flights, it is believed that solar heating of the temperature sensors had a significant effect on the measurements of temperature /4/. Backscatter measurements made by the nephelometer on VEGA 1 are shown in Figures Ic and ld. No useful measurements were returned by VEGA 2. As discussed elsewhere /2/, the main uncertainty In the backscatter measurements is related to the choice of constant offset. The relative variations are more certain. The solid line in Figure Ic represents a quadratic fit of backscatter to pressure. Figure Id shows the fitted residual, i.e. the difference between each data value and its quadratic fit. From these two figures, it is seen that although pressure and backscatter are related, presumably as a result of vertical stratification of the clouds, some variability on both large and small scales must be attributed to other mechanisms. Figures lb and 2b show no obvious evidence of large—scale (solar or planet fixed) temperature variations. This is not true for the velocity, as measured by Doppler residuals /3/, where a planetary scale feature is suggested by the preliminary analysis of the data. A planetary scale feature also seems to be present in the backscatter data. Although this can be seen in the data itself (Figure lc), It is most evident in the residual plot shown in Figure id. Based on an estimate /3/ that the balloon traveled about 1/4 of the way around the planet during it’s first 40 hours, this feature is suggestIve of a vavenumber 1 phenomenon, so that if this is a solar fixed effect, it would be diurnal in nature. This result is not

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dependent on the polynomial order chosen for the fit: a similar result occurs for a sixth order fit to pressure. As has been pointed out /2/, there are substantial variations in backscatter on shorter time scales. A notably coherent feature seems to be present near 30 hours in Figure ld.

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Fig. 2. As in Fig. 1 but for VEGA 2. Time is measured relative to 0 hours U.T. on 15 June, 1985. Cloud backscatter values are not displayed.

Pressure and temperature data can be used to study the characteristics of the ambient atmosphere. One approach to doing this is to plot temperature as a function of pressure /4/. An alternate approach is to examine the variations in the entropy of the gas surrounding the balloon as a function of time. Figures 3 and 4 show the quantity S/R where S is the entropy at a given pressure and temperature, and R is the gas constant. The data displayed do not include a small number of observations for which data decommutation /5/ ambiguities existed, nor do they include day side observations. The change of S with pressure or height can be related to dT’/dz, the departure of the lapse rate from the adiabatic lapse rate, giving a measure of atmospheric stability. As can be seen in Figure 3, VEGA 1 measurements seem to be fairly tightly clustered around a line representative of a lapse rate which is stable by about 1 K/km. VEGA 2, in contrast, shows more scatter which may be indicative of varying conditions during Its flight. The VEGA 2 data seem to lie closest to a lapse rate which is less stable than VEGA 1. The solid line in Figure 4 represents a lapse rate which is stable by .21 K/km. The large differences between values of S/R for VEGA 1 and 2 are real and are a reflection of the 6.5 K difference in temperature reported by /4/.

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Fig. 3. Entropy as a function of pressure for VEGA 1 from 5 U.T. to 35 U.T. Data points are given by + symbols, and the solid line shows an approximate fit of the data to dT’/dz’l.02 K/km.

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Fig. 4. Same as Fig. 3 but for VEGA 2 from 5 U.T. to 35 U.T. The solid line shows dT’/dz.C 0.21 K/km.

AQ(NOWLEDGEMENTS The work described in this paper was funded partially by the National Aeronautics and Space AdmInistration through the Ames Research Center and the Jet Propulsion Laboratory, California Institute of Technology.

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