High-dispersion spectroscopic observations of Venus near superior conjunction

~CAI~US34, 46--51 (1978)

High-Dispersion Spectroscopic Observations of Venus near Superior Conjunction II. The Carbon Dioxide Band at 7883 ~, A. T. Y O U N G , L. D. G. Y O U N G , AND R. A. J. S C H O R N Department of Physics, Texas A (~ M University, College Station, Texas 77843 Received April 21, 1977; revised August 12, 1977 Nine plates of the 7883-~ CO2 band were taken between phase angles 7.2 and 10.7° in 1971. A curve-of-growth analysis of 28 rotational lines in the band indicates an average rotational temperature of 236 :t: 8°K; the average slope of the curve of growth was 0.63 :t: 0.06. The results for this band are compared to those for the 7820-.~ band. INTRODUCTION

Venus phase angles are representative values. Perhaps these few observations were made on days when an unusually small a m o u n t of CO2 was above the clouds. Calculations made for a realistic Venus model atmosphere (Young and Kattawar, 1977) show that a single well-mixed cloud will produce equivalent widths that increase between i = 0 ° and i = ~ 60 ° for a cloud of Rayleigh scatterers; the corresponding curve for isotropic scatterers decreases monotonically with increasing phase angle from i = 0 ° to i = 180 ° . Similar results have been found b y Regas et al. (1973) and for isothermal, isobaric slabs (Whitehill and Hansen, 1973). Measurements of the polarization of Venus at different wavelengths show that her cloud particles actually behave as Mie scatterers. I n this case, one would again expect something similar to an "inverse phase effect," due to the back-scattering lobe, for small (i < 60 °) Venus phase angles. 2 Ultimately, the "best" scattering model

A series of high-dispersion measurements of the near-infrared C02 bands in the spect r u m of Venus has shown t h a t the phase curve of equivalent width, for a particular line, tends to increase slightly from small Venus phase angles up to phase angles of ~ 6 0 °. These data have been reviewed b y Young (1972). As the 7883-• band is the weakest CO~ band I that can be routinely observed, it is important for " t y i n g - d o w n " theoretical scattering models. For this band, only four observations have been published (Young, et al., 1971) for Venus phase angles smaller than i = 40 °, and none for i > 130 °, so more observations are needed near inferior (i = 180 °) and superior (i = 0 °) conjunction. Since the CO2 abundance above the clouds of Venus (or, alternatively, the equivalent width for a particular line) is known to v a r y from day to day (Kuiper, 1952; Young, 1972), it is important to determine whether the "low" equivalent widths which have been reported at small 1It has about half the total band strength of the COs band at 7820 ~_ (Young, 1971).

See, for example, Figs. 2 and 3 of Whitehill and Hansen (1973). 46

0019-1035/78/0341-0046502.00/0 Copyright • 1978 by Academic Press, Inc. All rights of reproduction in any form reserved.

VENUS C02 BAND AT 7883 A for the cloud particles has to be one which gives the best fit to the Venus phase curve.

47

T 505 • LDGY 0 RAJS

20 -

OBSERVATIONS

15

The present observations were made in September 1971. Spectra were taken only on days when there was no aureole around the Sun. The 7883 A bands were taken on the same plates as the 7820 A CO~ bands; the circumstances of these observations are given by Young et al. (1977), hereafter referred to as Paper I. We used more solar lines for our intensity calibration for the 7883-A band than for the 7820-A band, and more than were used in previous work on the 7883-A band (Gray et al., 1969; Young et al., 1971). The 16

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FIG. 1. Equivalent widths of carbon dioxide lines in the 7883-.~ band. Two independentmeasurements of each spectrum are shown. Data for plates T297, T298, and T302 are displayed.

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FIG. 2. Equivalent widths of carbon dioxide lines in the 7883-&band. Data for plates T303, T308, and T309 are shown. solar lines, and their equivalent widths as reported by Moore et al. (1966), are: X 7797.588 (79 mA), 7800.000 (61 mA), 7807.916 (64 mA), 7835.317 (42 mA), 7836.130 (64 mh), 7838.15 (5 mh), 7849.984 (66 mA), 7855.405 (25 mA), 7855.822 (3 m/~), 7861.045 (12 mA), 7863.799 (15 mA), 7869.635 (26 mA), 7869.94 (10 mA), 7912.384 (15 m/~), 7912.870 (40 mA), 7913.438 (17 mA). Because we used more weak solar lines in the intensity calibration for the weak COs band at 7883 A, the conversion from square millimeters of chart paper to milliangstroms differed slightly from that used for the 7820 A band on the same plate. This difference, for a given measurer (RAJS or LDGY) was always much smaller than the

48

YOUNG, YOUNG, AND SCHORN TABLE I R E S U L T S FOR M E A S U R E M E N T S

OF T H E C A R B O N ] ) I O X I D E B A N D AT 7 8 8 3 J~

NEAR S U P E R I O R C O N J U N C T I O N Plate number

Square-root absorption law Rotational temperature (°K)

W0 (0.5) (m.~)

Absolute calibration error ( % )

Curve of growth absorption law Slope of curve of growth

Rotational temperature (°K)

Reference temperature (°K)

W0 (b) (m£)

T297Y

240 230220

5.2

6.3

0.652 4- 0.012

,47250 2 244

240

3.94 4- 0 . 0 3

T2978

, 215 205196

5.(}

5.5

0.749 4- 0.019

, 237 234233

241/

3 . 1 7 4- 0.04

224 17210

5. l

--

0.690 4- 0.015

239 236233

240

3.[13 4- 0 . 0 5

T298Y

247 2 4 2237

5.1

11.7

0.632 4- 0.0111

.249 24;)241

240

4 . 8 0 :l- 0.04

T2988

228 218209

5.5

6.7

0.666 -4- 0.016

, 240 237233

2411

4.03 4- 0 . 0 4

T298

2 36230 .241

4.8

--

0.663 4- 0.018

, 244 239235

240

4 . 3 2 4- 0.04

T302Y

22 "234o217

4.8

7.2

0.559 4- 0.015

226~.9~

23(}

4 . 4 0 :l: 0.03

T3028

,230 223216

4.6

8.3

0.595 4- 0.021

, 2;3(i 231226

230

:3.90 :t_ 0.04

T302

231 2 2 4218

4.7

--

0.577 -4-0.018

,, 234 231228

230

4.15 4 - 0 . 0 4

T303Y

2 31224 239

4.0

6.11

0.602 4- 0,022

. 2;36 '231226

230

3.34 :t: 0 . 0 4

T303S

209199' 219

3.4

8.2

0.644 4 - 0 . 0 2 1

226222~

230

2.64 :k 0 . 0 4

T303

,233 226219

3.1

--

0.626 4 - 0 . 0 2 2

, , 234 229224

23(}

2.99 4- 0 . 0 4

T308Y

, 265 260255

5.2

6.7

0.559 4 - 0 , 0 1 2

, . 266 261256

250

4.81 :t: 0 . 0 3

T297

2

"absolute calibration error" (given in column 4 of Table I) estimated by that measurer, based on measurements of solar line equivalent widths. As discussed previously (Young et al., 1975), the equivalent widths we need for our calibration should be for integrated sunlight, while those of Moore et al. refer to the center of the solar disk. Each measurer used an individual intensity calibration to convert squares to equivalent widths of the CO2 lines in the Venus spectrum. The results are shown in Figs. 1 to 3. Comparison of these figures with the corresponding figures in Paper I, shows that the same measurer was usually "high" for both bands on each plate.

DETERMINATION OF ROTATIONAL TEMPERATURE AND OTHEI~ PARAMETERS We used the same technique as in Paper I; the results of these calculations, based on 28 rotational lines in the 7883-/~ C02 band, are given in Table I. Here the letter following the plate number, given in column 1, identifies the measurer: Y = L D G Y and S = RAJS. Below these two results is the "average" result obtained by fitting the combined data simultaneously. The second 3 and third columns give the rotational temperature, T(0.5), and the intercept, W010.5), found from a linear leastsquares fit to the logarithm of the rotational energy distribution as a function of

49

V E N U S COs B A N D AT 7883 T A B L E I--Continued Plate number

Square-root absorption law Rotational temperature (°K)

Wc (0.5) (m.~)

Absolute calibration error (%)

Curve of growth absorption law

Slope of curve of growth

Rotational temperature

(°K)

Reference temperature (°K)

W0 (b) (m.~)

T308S

217~

4.7

4.9

0.833 4 - 0 . 0 2 5

249 245241

250

2.604-0.05

T308

236~

4.9

--

0.690-¢-0.018

258 253248

250

3.71 4 - 0 . 0 4

T309Y

219~

4.1

8.8

0.519 4 - 0 , 0 1 3

227 224221

230

3.964-0.03

T309S

239 231223

4.4

8.6

0.571 4 - 0 . 0 2 3

241 235230

230

3.87 -4-0.05

T309

229 223217

4.2

--

0.5454-0.018

235 230225

230

3.924-0.04

T311Y

225~

5.8

7.4

0.563 i 0 . 0 1 9

234 229224

240

5,184-0.04

T311S

243 226211

4.2

7.9

0.744 4 - 0 . 0 2 5

260 255249

240

2.654-0.05

T311

236 222209

4.9

--

0.681 4 - 0 . 0 4 2

247 238229

240

3.584-0.09

T316Y

250 238228

5.8

8.2

0.617 4 - 0 . 0 2 7

259 252245

240

4.71 4 - 0 . 0 6

T316S

225216

5.7

5.1

0.6454-0.014

245 241238

240

4.344-0.03

T316

239 231225

5.7

--

0.648 4 - 0 . 0 1 5

248 244240

240

4.384-0.04

T317Y

223 218213

4.5

7.9

0.609 4 - 0 . 0 1 5

233 230228

230

3.72 2=0.03

T317S

222 218214

5.6

5.2

0.543 4 - 0 . 0 1 6

228 225221

230

5.124-0.04

T317

226 218210

5.0

--

0.576 4- 0.016

230 227224

230

4.424-0.04

226 4 - 7

4.8 4-0.7

7.0 4 - 1 . 3

0.6334-0.055

236±8

2364-7

3.87 4 - 0 . 5 0

Average

234

squared angular momentum J " ( J " + assuming

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1),

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square-root law. The fourth column gives the intensity calibration error based on the solar calibration lines. The fifth column gives the slope of the curve of growth (equivalent width versus line intensity) for the reference temperature given in column 7. Columns 6 3 and 8 give the rotational 3 The two temperatures presented as superscripts and subscripts following each value of T, correspond to the least-squares slope plus and minus its standard deviation.

temperature and intercept, found using the computed slope of the curve of growth given in column 5. At the bottom of Table I, the mean values and the standard deviations of the individual values are given for the 9 plates taken near superior conjunction. Since there are 9 plates, the standard deviations of the means are just of the tabulated (individual) standard deviations. 4 4 In Paper I, the tabulated standard deviations also refer to the individual values and not the means, contrary to what was stated there.

50

YOUNG, YOUNG, AND SCHORN

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FIG. 3. Equivalent widths of carbon dioxide lines in the 7883-.~ band. Data for plates T311, T316, and T317 are shown. DISCUSSION The average rotational t e m p e r a t u r e found f r o m the curve-of-growth analysis, assuming a constant C02 line halfwidth, is seen to be 236 4 - 8 ° K . The external s t a n d a r d deviation, 8°K, is slightly larger t h a n the internal s t a n d a r d deviation of 5°K, suggesting t h a t the rotational t e m perature was varying during the course of the observations. The fact t h a t the reference t e m p e r a t u r e was different from plate to plate also suggests a t e m p o r a l variation in the t e m p e r a t u r e . On the other hand, the 7820-/~ b a n d on the same plates, was found to be constant with time at 241 -4- 2°K. Since the 7820-A b a n d is the stronger of the two, and has more measurable CO2 lines (46 compared to 28 for the

7883-A band), we believe the results from P a p e r I, which indicate a constant rotational temperature, should be given more weight. 5 A comparison of the results obtained for the two bands on each plate is given in Table II. In the fourth column we show the difference in rotational t e m p e r a t u r e found for the two bands. The m e a n difference is just over twice its s t a n d a r d deviation, which is marginally statistically significant. However, it is opposite in sign to a n y real difference one might expect (the weaker b a n d should be formed deeper in the atmosphere, where it is warmer). On the other hand, both bands indicate the a m o u n t of CO2 above the cloud tops, Wo(b), is varying with time. The value of aext, when corrected for internal error, indicates a real variation between 11 and 12% (RMS) for each band. This is similar to the d a y - t o - d a y variation in these bands studied b y Young et al. (1974, 1975) at phase angles near 80 °, using plates t a k e n with the same instrument. One might argue t h a t the good correlation (r = 0.71) between the Wo(b) values for the two bands might be due to varying contamination of our spectra b y sky light. However, we have tried to observe only under good conditions; if only Rayleigh scattering is involved, it can be shown t h a t the sky contribution to our spectra is less t h a n 2%. Because our daily fluctuations are comparable to those seen at large elongations, where sky effects are surely negligible, we believe our data are free of a n y serious systematic error. Our current values for equivalent widths of these bands confirm the low values reported previously at small phase angles, and we believe this "inverse phase effect" is real. 5This is confirmed by the F test. : ( O ' e x t 2 / O ' i n t 2 ) = 2 . 8 , nl = 8, n2 = 9 gives P ~ 92%, which is only weakly significant, even for the 7883 ~_ data alone.

51

V E N U S CO2 B A N D AT 7883 T A B L E II COMPARISON OF RESULTS FOR THE 7820- AND 7883-). CO2 BANDS AT THE 1971 SUPERIOR CONJUNCTION OF VENUS

T(b) (°K)

Plate number 7820 T297 T298 T302 T303 T308 T309 T311 T316 T317

241 238 240 244 244 240 240 241 241

Average ~oxt i~t

4± ± 4± 444±

2 4 3 4 4 3 3 2 2

7883 236 239 231 229 253 230 238 244 227

4- 3 ± 5 ± 3 ± 5 ± 5 4- 5 -4- 9 4- 4 ± 3

W(b) (m~.) AT (°K) -5 -}-1 -9 -15 -{-9 -10 -2 -{-3 -14

7820 7.15 6.91 6.54 5.32 7.01 5.82 6.53 7.68 7.47

± 4± ± ± ± ± 44-

0.04 0.07 0.05 0.06 0.06 0.06 0.05 0.04 0.04

7883 3.63 4.32 4.15 2.99 3.71 3.92 3.58 4.38 4.42

4± 4± ± ± ± ± ±

0.05 0.04 0.04 0.04 0.04 0.04 0.09 0.04 0.04

241 4- 0.6

236 4- 2.8

( - 4 . 7 4- 2.8>

6.71 4- 0.25

3.90 4- 0.16

2 3

8 5

8 --

0.76 0.05

0.47 0.05

ACKNOWLEDGMENT This work was supported by NASA under Grant N G R 44-001-117. REFERENCES GRAY, L. D., SCHORN, R. A., AND BARKER, E. (1969). High dispersion spectroscopic observations of Venus. IV. The weak carbon dioxide band at 7883 )`. Appl. Opt. 8, 2087-2093. KUIPER, G. P. (1952). The Atmospheres of the Earth and Planets, p. 370. Univ. Chicago Press, Chicago. MOORE, C. E., MINNAERT, M. G. J., AND HOUTGAST, J. (1966). The Solar Spectrum from 2935 )` to 8770 )`. National Bureau of Standards Monograph 61, Washington, D.C. REGAS, J. L., ]~OESE, R. W., GIVER, L. P., AND MILLER, J. H. (1973). Does spectroscopic evidence require two scattering layers in the Venus atmosphere? J. Quant. Spectrosc. Radiat. Transfer 13, 461-463.

WHITEHILL, L. B., AND HANSEN, J. E. (1973). On the interpretation of the inverse phase effect for CO2 equivalent width on Venus. Icarus 20, 146152. YOUNG, L. D. G. (1971). Measurements of the intensity of the CO2 band at 7883 )` relative to

Ratio 0.51 0.63 0.63 0.56 0.53 0.67 0.55 0.57 0.59

(0.58)

the intensity of the CO2 band at 7820 )`. Appl. Opt. 10, 662-663. YOUNG, L. D. G. (1972). High-resolution spectra of Venus--A review. Icarus 17, 632-658. YOUNG, L. D. G., and KATTAWAR, G. W. (1977). Scattering in the atmosphere of Venus. III. Line profiles and phase curves for Rayleigh scattering. Icarus 30, 360-366. YOUNG, L. D. G., SCHORN, R. A. J., BARKER, E. S., AND WOSZCZKY, A. (1971). High dispersion spectroscopic observations of Venus during 1968 and 1969. I. The carbon dioxide bands at 7820 )` and 7883 )`. Acta Astron. 21, 329-363. YOUNG, L. D. G., SCHORN, R. A. J., AND YOUNG, A. T. (1977). High dispersion spectroscopic observations of Venus near superior conjunction. I. The carbon dioxide band at 7820 )`. Icarus 30, 559-565. YOUNG, A. T., WOSZCZYK,A., AND YOUNG, L. G. (1974). Spectroscopic observations of spatial and temporal variations on Venus. Acta Astron. 24, 55-68. YOUNG, L. D. G., YOUNG, A. T., AND WOSZCZYK,A. (1975). High dispersion observations of Venus during 1972 : The CO2 band at 7820 )`. Icarus 25, 239-267,