Correlation of simultaneous ultraviolet (0.36 μm) and infrared (8 to 14 μm) images of Venus

XCARVS38, 81-89 (1979)

Correlation of Simultaneous Ultraviolet (0.36 ~m) and Infrared (8 to 14 ~.m) Images of Venus 1 D A V I D J. D I N E R 2 AND J A M E S A. W E S T P H A L Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125

Received February 8, 1978, revised September 5, 1978 We present evidence for a correlation between features observed in simultaneous infrared (8 to 14 ~,m) and ultraviolet (0.36 ~,m) images obtained during Venus' 1975 and 1977 apparitions. The sense of the observed correlation is such that bright uv markings correspond to bright (warm) ir features, and similarly, dark uv markings correspond to dark (cool) ir features. I. INTRODUCTION

Hansen, 1974) have not led to a consistent view of the relative polarization of light and dark regions, perhaps due in p a r t to the low levels of polarization a n d difficulty of the measurements. Ultraviolet (0.35 ~m) a n d infrared (8 to 13 plus 18 to 2 8 ~ m ) scans obtained b y the Venera 9 orbiter (Ksanfomality, 1976; K s a n f o m a l i t y et al., 1976a) show m a n y small scale features in which a correlation of relatively low uv albedo and low ir brightness t e m p e r a t u r e is exhibited (i.e., d a r k uv = d a r k ir and similarly, bright uv = bright ir). However, there are also cases, particularly in Venera 10 data, in which no obvious correlation is seen and one scan pair which suggests a correlation in the opposite sense. We imaged Venus in the uv and ir during the 1975 and 1977 apparitions. T h e data presented below suggest a correlation of dark uv features with relatively cool ir regions, in the same sense exhibited by some of the Venera data. However, given the diversity of results reported b y the Venera experimenters and low contrast of the features involved, we shall refrain from generalizations based upon this limited data set.

One of the m o s t puzzling aspects of the Venus u p p e r a t m o s p h e r e is the presence of ultraviolet markings. These light a n d dark features have been p h o t o g r a p h e d since the 1920s (Wright, 1927; Ross, 1928). Several hypotheses for their origin have been proposed, and the reader is referred to T r a v i s (1975) for a review of this question. I n a t t e m p t s to differentiate Venus' light and d a r k regions observationally, various experimenters have sought correlations of these markings with other observable phenomena. Young (1974) reported on the inability to correlate CO2 line strengths with isolated light a n d dark features, although he presented marginal evidence for somewhat lower overall CO2 a b u n d a n c e when dark markings cover most of the area of the disk. Polarization data, from Mariner 10 (Hapke, 1974) and from ground-based experiments (Fountain, 1974; Coffeen and Contribution No. 3027 of the Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, Calif. 91125. 2Present address: Earth and Space Sciences Division, Jet Propulsion Laboratory, 4800 Oak Drive, Pasadena, Calif. 91103. 81

0019-1035/79/040081-09502.00/0 Copyright O 1979 by Academic Press, Inc. All rights of reproduction in a n y form reserved.

82

DINER AND WESTPHAL II. OBSERVATIONS

The observations were obtained at the e a s t - a r m focus of the Hale 200-in. telescope on P a l o m a r Mountain. The telescope was equipped with a "wobbling" Gregorian secondary mirror positioned a b o u t 1 m above the prime focus. The f/72 b e a m from the secondary was split into a reflected infrared component and a t r a n s m i t t e d ultraviolet/visible c o m p o n e n t using a dichroic containing a Liberty Mirror I R 81-E conductive gold reflection coating and placed at a 45 ° angle to the light path. The infrared radiation from Venus was sensed with a gallium doped g e r m a n i u m bolometer housed in a Low dewar and cooled to 2°K b y p u m p i n g on liquid helium. The dewar contained an internal filter to limit the ir bandpass to 8 to 14~m. An internal 2 - m m focal plane aperture provided a resolution of 1.1 arcsec on the sky. For the uv measurements, the central portion of the f/72 b e a m t r a n s m i t t e d by the dichroic was diverted b y a series of small (--~2-cm-diameter) fiats onto the S-20 photocathode of an I T T FW-130 photomultiplier. The photomultiplier was housed in a cold box a n d cooled with dry ice to reduce the t h e r m a l d a r k current. As in the case of the infrared measurements, a 2-ram focal plane aperture was used. A 3600-A interference filter with a full width at halfm a x i m u m bandpass of 200/~ isolated the region of the s p e c t r u m used for the ultraviolet observations. The portion of the b e a m not removed b y the small central fiat was viewed with a 12-cm-diameter field

lens, which converted the f/72 b e a m to f/lO and provided a convenient viewing and guiding field roughly 1 arcmin across. The infrared m e a s u r e m e n t s were performed in a chopping mode, whereas the ultraviolet observations were made in a d c mode, without chopping. Chop was accomplished by driving the wobbling secondary with a 12.5-Hz square wave with a 50% d u t y cycle. The wobble displaces the image in the n o r t h - s o u t h direction, and resulted in alternately integrating on planet and sky for 40 msec each, yielding a total step time of 80 msec per picture element (pixel). I n order to procure u v and ir images with similar timing and step size parameters, each pixel in a uv image received an 80msec integration as well. The signals from the bolometer and photomultiplier were amplified (phase-locked to the chopper in the case of the ir observations) and digitized to 8 bits with an A / D converter. The d a t a numbers (0-255) were written on I B M compatible digital magnetic tape using a Cipher D a t a Products 800 bpi 9-track tape recorder. I m a g e s of Venus were obtained by feeding a staircase voltage r a m p to the wobbling secondary in 64 80-msec steps to sweep the focal plane image of Venus past the entrance apertures. The amplitude of the r a m p was adjusted to yield a mirror throw t h a t encompassed the whole disk and ensured some sky signal beyond each of the limbs. I n those instances in which the chopping mode was employed, the r a m p was m o d u l a t e d b y the chopping square wave.

TABLE I SUMMARY OF OBSERVATIONS

Date (UT)

Times of observation (UT)

Venus phase angle

Venus

semidiameter

(arcsec) 26 September 1975 13 February 1977 14 February 1977

1400-1600 0000-0300 0000-0300

125 ° 102 ° 102 °

21.6 15.4 15.6

uv A N D ir I M A G E S OF V E N U S

83

O

O

O

t,

O O

v~ r5

84

D I N E R AND WESTPHAL ¢'-,

~J

O

¢9 °~ ¢9

v.Q

¢~

la~ O

¢-q

¢9

uv AND ir IMAGES OF VENUS

VENUS 5600~, N

VENUS B-14/.Lm

E

W

85

N

UT 15 FEB 1977

E

Ba

W

Ba

o.

UT 15 FEB 1977 FIG. 3. Contour maps of the images shown in Fig. 1. The contour intervals are arbitrary. Bright features are indicated by a "B" followed by a Greek letter to distinguish individual features; dark features are similarly designated by a " D " and a Greek letter. Features in the ultraviolet and infrared maps denoted by the same symbol are in the same spatial location on the Venus disk. The bright uv regions correlate with bright areas in the 8- to 14-pro image and the dark uv region corresponds to a relatively dark region at 8- to 14-#m. Flyback of the secondary mirror in preparation for the next scan required 640 msec per scan. Since the mirror scanned in declination, the planet was permitted to drift in right ascension by introducing an offset into the telescope sidereal tracking rate. This combination of mirror scan and flyback plus orthogonal telescope drive produced a raster scan pattern b y which twodimensional images of Venus were built up at a rate of 368.6 sec per picture. Incoming scans were monitored in real time with an oscilloscope synchronized with the mirror sweep. The images could also be viewed in real time with a 6-bit digital scan converter and video monitor, or at leisure in the laboratory with a picture playback system (Westphal, 1973). This system also permits individual scans in an image to be accessed and viewed on an oscilloscope and can be used to produce hard copy Polaroid pictures of the two-dimensional data.

I n light of the data handling requirements, uv and ir data were not recorded concomitantly. I n addition, differential refraction in the E a r t h ' s atmosphere would ensure that the uv and ir detectors would not sample identical spots on the Venus disk. Therefore, the uv data were collected when the sky was dark, and the remainder of the twilight time (either afternoon or morning, depending on Venus' elongation) was used for the ir measurements. This resulted in only a short time interval between collection of the uv and ir images. Given the spatial resolution of the data and the apparent Venus atmosphere rotation period of four terrestrial days, nearsimultaneous imaging is adequate. No absolute calibrations of either the infrared or ultraviolet data were made during these observations. Instead, we concentrated on obtaining good spatially resolved measurements of relative brightness varia-

86

DINER AND WESTPHAL

VENUS 5 6 0 0 4

VENUS 8-14prn N

E

UT 14 FEB 1977 N

,

-B7 W

E

D# B8

S UT 14 FEB 1977

S

FIG. 4. Same as Fig. 3 for the ultraviolet and unenhanced 8- to 14-#m images of UT 14 February 1977. tions across the planetary disk, including high spatial frequency transient features with low contrast. Table I presents a summ a r y of the times of observation corresponding to the images discussed in this paper. Also shown for each date are Venus phase angles and semidiameters.

between 3 and 7 separate images. The resulting signal-to-noise ratios are on the order of 100 to 200, which in the case of

~

VENF UE SB IRR-U UA VR1Y977

III. DATA PROCESSING Due to the different modes of sampling in declination and right ascension, images were obtained with unequal pixel spacings in the two orthogonal directions. Furthermore, the images were acquired in a 64 X 64 format, whereas the picture playback system requires 256 X 256 arrays. Both of these problems were corrected in an image reformatting procedure, which employed a bilinear interpolation scheme to expand the arrays while simultaneously compressing one axis in order to circularize the Venus images. Following this reformatting step, images were registered and added together to improve the signal-to-noise ratio of the data. The images presented in this paper are composites produced from the addition of

NORTH

SOUTH

FIG. 5. Ultraviolet and 8- to 14-#m scans through the same region of the Venus disk on UT 14 February 1977. The bright and dark regions defined in Fig. 4 are identified. The error associated with the measurements may be estimated from the magnitude of the noise in the data beyond the planetary limbs, i.e., in the sky. This uncertainty is roughly 2 to 3 times the thickness of the scan lines plotted in this figure.

uv AND ir I M A G E S OF V E N U S

87

O

88

DINER AND WESTPHAL

VENUS 5 6 0 0 ~

VENUS 8-14Fro

N

E

W

N

UT 26 SEPT 1975

E

S UT 26 SEPT 1975

W

S

FIo. 7. Contour maps of the images shown in Fig. 6. The contour intervals are arbitrary. The contours in the northern hemisphere of the uv map are narrower and more pointed than the contours in the south. The presence of the infrared polar anomaly is apparent in the corresponding contour map and it may be seen that the anomaly (depression of the flux levels in the north) extends over much of the northern hemisphere. the ir d a t a represents an u n c e r t a i n t y of 1 lO z-~ K in relative m e a s u r e m e n t s of 8 to 14 #m brightness t e m p e r a t u r e . I m a g e averaging to reduce noise was followed by a final processing step which involved the scan-by-scan removal of a n y r e m n a n t low-frequency drift in the sky background. Points on opposite sides of the planet served as endpoints for a linearly sloping background level t h a t was subtracted from each scan. At this point, a "finished" composite image represents the best estimate of the actual brightness distribution over the disk of Venus, and the data numbers, minus a uniform sky level, are directly proportional to the p l a n e t a r y flux. IV. RESULTS AND CONCLUSIONS Figures 1 and 2 show two sets of u v - i r image pairs obtained on U T 13 and 14 F e b r u a r y 1977. One of the most prominent features in the ir images is the d a y - n i g h t

contrast; these images were obtained at eastern elongation and the evening terminator is in view. This contrast between the illuminated and dark limbs has been previously observed in the infrared m a p s of M u r r a y et al. (1963) and Venera 9 and 10 orbiter scans ( K s a n f o m a l i t y et al., 1976b). The infrared images show a n u m b e r of smaller-scale bright and d a r k tilted bandlike features with contrasts on the order of 1 to 5%. I t is a p p a r e n t t h a t these features change in appearance from one day t o the next. As m a y be seen in Figs. 1 and 2, the ultraviolet images also display light and dark markings. Several other methods of displaying the d a t a are given in the following figures. Figures 3 and 4 present contour m a p s of the uv and ir images in Figs. 1 and 2; the contour intervals are a r b i t r a r y and were selected to give the best representation of the data. Figure 5 shows ultraviolet and infrared scans t a k e n from the images of 14 F e b r u a r y which slice through the same

uv AND ir IMAGES OF VENUS l o c a t i o n s on t h e V e n u s disk. T h i s figure shows t h r e e f e a t u r e s in t h e ir scan, t w o bright areas with an intervening dark region, w h i c h c o r r e l a t e w i t h s i m i l a r f e a t u r e s in t h e u v scan. T h e sense of t h e c o r r e l a t i o n is such t h a t t h e b r i g h t ir areas, t h a t is, t h e w a r m e r regions, a r e r e l a t i v e l y b r i g h t in t h e u l t r a v i o l e t ; s i m i l a r l y , t h e d a r k (cool) ir region correlates with a relatively dark uv area. T h e f e a t u r e s seen in t h e u v - i r s c a n p a i r a r e also i n d i c a t e d in t h e c o r r e s p o n d i n g c o n t o u r m a p s (Fig. 4) a n d m a y be seen in t h e i m a g e s (Fig. 2). O t h e r u v a n d ir f e a t u r e s c o r r e l a t e in t h e i m a g e s a n d c o n t o u r m a p s , w i t h t h e c o r r e l a t i o n in t h e s a m e sense as s h o w n in Fig. 5. A d d i t i o n a l e v i d e n c e for a u v - i r c o r r e l a t i o n is f o u n d in a set of i m a g e s o b t a i n e d on U T 26 S e p t e m b e r 1975 (Fig. 6). I n t h e s e data, a prominent northern polar "collar" w a s o b s e r v e d in t h e i n f r a r e d images. T h e p h a s e a n g l e of V e n u s w a s 125 ° a n d t h e v i s u a l a p p e a r a n c e of t h e p l a n e t in t h e telescope eyepiece indicated a sharper n o r t h e r n c u s p r e l a t i v e t o t h e s o u t h e r n cusp. T h e s e e i n g was g o o d ( ~ 1 arcsec) a n d t h e v i s i b l e n o r t h - s o u t h a s y m m e t r y was v e r i f i e d in r e c o r d e d u v i m a g e s . F i g u r e 7 shows c o n t o u r m a p s of t h e u v - i r i m a g e p a i r f r o m t h i s d a t a . T h e c o n t o u r s in t h e n o r t h e r n p a r t of t h e u l t r a v i o l e t c r e s e n t a r e c l e a r l y n a r r o w e r a n d m o r e p o i n t e d t h a n t h e cont o u r s in t h e s o u t h . T h e n o r t h - s o u t h a s y m m e t r y in t h e 8- t o 14-gin i m a g e is also q u i t e apparent. The correlations suggested by our data m a y b e i n t e r p r e t e d t o m e a n t h a t t h e inf r a r e d p o l a r a n o m a l i e s , as well as o t h e r t r a n s i e n t ir f e a t u r e s , a r e v i s i b l e in r e f l e c t e d sunlight, indicating that they are due to v a r y i n g o p a c i t y c a u s e d b y clouds, a n d n o t thermal variations at constant pressure levels. W e h o p e t h a t t h e s e o b s e r v a t i o n s will be s u p p l e m e n t e d b y t h e m u l t i c h a n n e l infrared radiometer and ultraviolet imaging photopolarimeter experiments aboard the P i o n e e r 1978 V e n u s o r b i t e r .

89

ACKNOWLEDGMENTS We are indebted to Andrew Ingersoll, Glenn Orton, and Fredric Taylor for many helpful suggestions. We would also like to thank Janet Bailey and John Dvorak for assistance with the observations, and are pleased to acknowledge the help of our night assistants, Juan Carrasco, Kevin Jordan, and Gary Tuton. This research was sponsored by NASA under Grant 05-002-003. REFERENCES COFFEEN, D. L., AND ~IANSEN,J E. (1974). Polarization studies of planetary atmospheres. In Planets, Stars and Nebulae Studied with Photopolarirnetry (T. Gehrels, Ed.). Univ. of Arizona Press, Tucson. FOUNTAIN, J. W. (1974). Spatial distribution of polarization over the disks of Venus, Jupiter, Saturn, and the Moon. In Planets, Stars and Nebulae Studied with Photopolarimetry (T. Gehrels, Ed.) Univ. of Arizona Press, Tucson. HAPKE, B. (1974). In The Atmosphere of Venus, pp. 69-76. Proceedings of a conference held at Goddard Institute for Space Studies, New York. KSANFOMALITY,L. V. (1976). Publication 226 of the Space Research Institute of the Academy of Sciences, USSR. KSANFOMALITY,L. V., DEDOVA, E. V., OBUKHOVA, i . F., TEMNAYA~ U. V., AND FIUPPOV, G. F. (1976a). Infrared radiation of the clouds of Venus. Cosmic Res. 14, 670--677. KSANFOMALITY,L. V., DEDOVA, E. V., OBUKHOVA, L. F., PO~RAS, V. M., TEMNAYA, N. V., AND FILIPPOV, G. F. (1976b). Thermal asymmetry of Venus. Soy. Astron. 20, 476-480. MURaAy, B., WILDEY, R. L., AND WESTPHAL, J. A. (1963). Infrared photometric mapping of Venus through the 8- to 14-micron atmospheric window. J. Geophys. Res. 58, 4813-4818. Ross, F. (1928). Photographs of Venus. Astrophys. J. 68, 57-92. TRAVIS, L. D. (1975). On the origin of ultraviolet contrasts on Venus. J. Atmos. Sci. 32, 1190-1200. WESTPHAL, J. A. (1973). Application of the SIT vidicon to astronomical measurements. In Astronomical Observations with Television-Type Sensors (J. W. Glaspey and G. A. H. Walker, Eds.). University of British Columbia, Vancouver. WRIGHT, W. H. (1927). Photographs of Venus made by infrared, and by violet light. Publ. Astron. Soc. Pac. 39, 220-221. YOUNG, A. T. (1974). In The Atmosphere of Venus, pp. 15-16. Proceedings of a conference held at Goddard Institute for Space Studies, New York.