;01.J'R�AL OF GEOPHYSICAL RESEARCH VOL. 68, No. 16 AUGUST 15. 1963

Infrared Photometric Mapping of Venus through the 8- to 14-Micron Atmospheric Window1

BRUCE C. MURRAY, ROBERT L. WILDEY, AND JAMES A. WESTPHAL Division of Geological Sciences CaliforniaInstitute of Technology, Pasadena

Abstract. The 200-inch Hale telescope has been used to obtain high-resolution maps, on four mornings juxtaposed about the Mariner 2 encounter, of the brightness temperature of Venus in the 8- to 14-micron wavelength interval. The resolution was about 1/30 of the disk of Venus. The signal-to-noise ratio was in excess of 100. The maps reveal (1) a general limb darkening, (2) a bilateral symmetry about the planet's orbital plane, (3) a very slight wedging of the contours as the only day-to-night effect, and (4) a transient temperature anomaly in the southern hemisphere .

Introduction. The thermal emISSIOn from reduction and calibration of observations for Venus was first measured by Pettit and Nichol­ extended sources are described by Murray and son [1924]. The measurements have been re­ Wildey [1963b], together with a discussion of viewed by Pettit [1961]. Characteristic bright­ the possible systematic errors. The atmospheric ne8S temperatures were found, and the interest­ extinction correction used in the present study ing discovery was made that the dark part of is that adopted previously by Wildey and Mw'­ the disk of Venus was emitting about as ray [1963] and is due to Westphal (private strongly as the sunlit part. These observations communication) . It may be an important source were made on the 100-inch telescope at Mount of systematic uncertainty. The reduced data Wilson with a vacuum thermocouple. Sinton and are presented in the form of a map of brightness Strong [1960] have made observations with a temperature (the temperature for which the spectrophotometer incorporating a Golay cell measured specific intensity equals a Planck on the 200-inch telescope. They have recorded function weighted precisely the same over wave­ the average center-to-limb brightness profile and length) . real departures from blackbody spectral dis­ The telescope's hour-angle drive was adjusted tribution. to slightly exceed the earth's rotation rate. Be­ In the present study high-resolution bright­ ginning at the north limb of the Venus disk a ness-temperature contour maps have been con­ series of right-ascension scans, separated by structed on four successive nights juxtaposed small southerly decrements in declination, was about the Venus encounter of the Mariner 2 thus made. The scans were reproducible, and the spacecraft. Preliminary results have been re­ signal-to-noise ratio near the center of the disk ported briefly [Murray, Wildey, and Westphal, was about 100 to 1. This is seen in scans 1 and 1963] . 2 of Figure 2, which correspond to tracks 1 and The observations. The observations were 2 of Figure 1. The scans were reduced at in­ collected at the east-arm Cassegrain focus of the tervals commensurate with the photometer reso­ 200-inch telescope during the morning twilights lution (about 1.5 seconds of arc, 1/30 of the disk of December 14, 15, 16, and 17, 1962. The diameter, or 400 km at the Venus subearth photometer and detector (mercury-doped ger­ point). The resulting brightness temperatures manium photoconductor) used have been de­ were fixed geographically by translating the scan scribed by Westphal et al. [1963] and more parallel to right ascension until its length be­ briefly by Murray and Wildey [1963a]. The came the chord of a circle whose diameter was preassigned as the length of the scan that bi­ 1172 sected the disk. This procedure was necessitated 1 Contribution of the Division of Geological by the lack of precision with which the tele­ Sciences of the California Institute of Technology, Pasadena, CaUfomia. scope's declination circles can be read, even 4813 4814 MURRAY, WILDEY, AND WESTPHAL

N 4 per dil1erentiully. Approximately 20 cent of the scans were thrown out as a result. The posi. tional ncerta n y produced a u i t north-soutt o crenulation in the contours pl tted on the re. suIting brightness-temperature maps. tempera The mean brightness ture for the ce t di on fou n er of the sk the r nights wa, 2° (s.d.), 2soK low 20soK ± about er than the 1,2-j----+-----+------t- commonly accepted value. This includes night. to-night calibration errors and possible f1uctua. tions in atmospheric extinction. No correctiot has been made for unknown transmission [oiOe, within the telescope, and this may be a seriolii cause of systematic error. The obscuration 0: the prime-focus cage (13 per cent) combine,: with the laboratory reflectivity at 10 micron; I (0.9S) of each of the four mirrors in S the tele­ scope optics give an over-all telescope traIll'l • PHOTOMETER RESOLUTION mission of O.S1. The brightness temperatutt' 1. of Fig. Index map scans reproduced in Fig­ would be uniformly increased by 7°K as are· of ure 2. The an-owhead denotes the direction each scan. suIt. However, if the reflectivity of each of tht Scans 1 and 2 are exactly coincident in order four were only 0.85, perhaps to test reproducibility. mirrors owing to

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f-ONE MIN. OF TIME4 chart c Scans 1 and Fig. 2. Tracings of selected strip re ords of output voltage versus time. from of a 2 December 13, 1962, illustrate reproducibility; 3 and 4 show closure 'hot spot' as ob ve on ser d December 15, 1962. Scan 3 is a right-ascension scan; 4, a declination scan. T indi­ cates terminator crossing. INFRARED PHOTOMETRIC MAPPING OF VENUS 4815

N infrared radiation of Venus. The differential accuracy of these brightness-temperature meas­ urements is about YacK. The brightness-temperature contour maps. Figures 3 to 6 display the brightness-tempera­ ture contour maps for the four nights of ob-

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Fig. 3. Eight- to 14-micron brightness-tempera­ ture map of Venus for the morning of December 13, 1962. Lower chart indicates location of data points used to construct map. The terminator and the projected direction to the sun are indicated, respectively, by a heavy line and by the symbol for the sun. The brightness temperatures shown are systematically 7° to 28°(?)K too low because of uncertain telescope transmission losses. Fig. 4. Eight- to 14-micron brightness-tempera­ ture map of Venus for the morning of December 14, 1962. Lower chart indicates location of data thinness of the coatings , the entire 28GK dis­ points used to construct map. The terminator and the projected dir ction to the sun are indicated, parity would disappear. The importance of the e respectively, by a heavy line and by the symbol. observations de c bed in this is in s ri paper not The brightness temperatures shown are systemati­ absolute value of he central temperature 7· the t cally to 28° (?)K too low because of uncertain but rather in the two-dimensional picture of the telescope tranBIIlission losses. 4816 MURRAY, WILDEY, AND WESTPHAL

N celestial north and east and the line of tte terminator. Features common to all the maps include (11 a general decrease in brightness from center tc no night-to-day limb; (2) effect except aVer" slight wedging of the contours, diverging toward

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DEC. 16. 1962 1315-1431 u. T.

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Fig. 5. Eight- to 14-micron brightness-tempera­ ture map of Venus for the morning of December ··· · 1962. 15, Lower chart indicates location of data · · points used to construct map. The terminator and \S············i i ···.·.ii·· H···· / the projected direction to the sun are indicated, '--" · respectively, by a heavy line and by the symbol. The brightness temperatures shown are systemati­ 7° Fig. 6. Eight- 14-micron brightness-tempera. cally to 28°(?)K too low because of uncertain to telescope transmission losses. ture map of Venus for the morning of December 16, 1962. Lower chart indicates location of data points used to construct map. The terminator and servation. Immediately beneath each map is the projected direction to the sun are indicated, respectively, by a heavy line and by the symbol. shown a chart of the corresponding data points. The brightness temperatures shown are systemati· The projection of the radius vector from Venus 7° 28°(?)K cally to too low because of uncertlLinI to the sun is shown on each map, together with telescope transmission losses. INFRARED PHOTOMETRIC MAPPING OF VENUS 4817

rk side; and (3) a bilateral symmetry of that the planet is allowed to rotate with a period the da tour pattern about the diameter in small compared with the thermal relaxation the con Venus's orbital plane. In addition, the map for time of its atmosphere and surface materials. December 15 shows a well-defined anomalous Another global distribution of the planet's whose existence was also evident on a 'hot spot' atmospheric effective temperature which, when oss Venus the same g declination scan acr mornin combined with intense limb darkening, can also (see scans 3 and 4 of Figures 1 and 2). The produce the observed bilateral symmetry is one �aps for the mornings of December 14 and 16 wherein the emitted flux is at a maximum at the reveal this hot spot to be but one phase of subsolar point , decreases as a function of the transient phenomena taking place over an ex­ great-circle distance away from that point to a tended region in the vicinity of the southern minimum value just on the nighttime side of cusp of the planet. Because of the temporal, the terminator, and then increases to a second ly geog aphic, variation of this and presumab r maximum at the antisolar point. This very spe­ anomalous feature it fulfills the usual definition cial distribution is a priori possible for a syn­ of a storm. chronously rotating planet upon which advec­ Discu.ssion. The general decrease in bright­ tive energy transport may be important. ness from center to limb is most obviously ex­ However, the two interpretations differ in re­ plained in terms of the dependence on angle gard to their prediction of the integral bright­ with local vertical of the specific-intensity vec­ ness, in the 8- to 14-micron region, viewed as a tor characterizing the emergent radiation field. function of phase angle (sun-Venus-earth This phenomenon is analogous to limb darken­ angle). The latitudinally dependent flux ('rota­ ing as observed in the visible region in the sun, tional') interpretation should show no particular if we assume that the total Venus atmosphere is phase effect, whereas the case of dependence optically thick from 8 to 14 microns. One is thus on geodetic distance from the subsolar point, or led from a consideration, in fairly general terms, 'synchronous' interpretation, should display a of the equation of radiative transfer [Munch, phase effect of perhaps 5cK. In fact, no phase 1960; and othersJ toward the conclusion that effect has yet been observed [Sinton, 1963; Sin­ the temperature on Venus increases with depth ton and Strong, 1960J. But the uncertainties in down through the range in the atmosphere from the present data do not permit an unqualified which most of the 8- to 14-micron radiation rejection of the possibility of some phase effect emerges. (Sinton, private communication) . More ob­ The absence of a strong night-to-day effect in servations of integral brightness and the ap­ the infrared emission suggests that the tempera­ parent distribution of radiation at different ture of the emitting atmospheric layers is nearly phases will be required before the three-dimen­ independent of the planetary diurnal insolation sional distribution of effective temperature will variation. Furthermore, the energy loss from be completely established on observational the planet in this wavelength band balances grounds alone; however, the latitudinally de­ about one-third of the total solar energy ab­ pendent distribution does seem to be the more sorbed on the illuminated hemisphere. There­ likely at present. Thus, for a truly synchron­ fore, energy redistribution mechanisms are im­ ously rotating planet, if the infrared emission plied that render a major fraction of the local is indeed latitudinally dependent, the solar energy loss on the planet similarly insensitive to energy must be redistributed within the plane­ diurnal insolation variation. tary atmosphere not only with such efficiency A second constraint upon the planetary en­ that no day-night effect is present but also ergy redistribution mechanism is strongly im­ with such sensitivity to Coriolis forces (as the plied by the fact that the contours show only latitudinally dependent parameter left) bilateral rather than radial symmetry. This dis­ that approximately 20 per cent less energy is tribution is suggestive of real lateral tempera­ radiated away at the poles as at a point on the ture variations, most simply a latitudinal cool­ equator of equal distance from the subsolar ing toward the 'poles' away from Venus's orbital point. plane. Such a condition is easily visualized as a Recent radar observations [Goldstein and latitude effect due to solar insolation provided Carpenter, 1963J are interpreted to indicate a 4818 MURRAY, WILDEY, AND WESTPHAL

grant the National c 252-day retrograde rotational period (140-day and G-25210 of Scien e FoUl:. dation. synodic period), and thus the latitudinal de­ pendence may n eed retlect insolation variation. i d REFERENCES However, the thermal relaxation time of the S. C., L. D. Kaplan, and Neugeba atmosphere and surface materials of Venus must Chase, G. UEr. The Mariner 2 infrared radiometer " experimen'L, be long, i.e., measured in years, if both the Science, 139, 909, 1963. infrared and radar observat ons are to be ex­ Goldstein, R. M., and R. L. Carpenter, i Goldstone radar observations of Venus during 1962, Science plained adequately. Possibly, high atmospheric . , pressures and surface temperatures (implying 139,910, 1963. Munch, G., The theory of model stellar at s eric may ccount for such atmos. high mo ph opacity) a pheres, in Stars Stellar Systems, and vol. 6, Stel. a large heat capacity and long relaxation time. la?' edited L. Atmospheres, by J. Greenstein, Pp. Other, completely different explanations of the 1-49, University of Chicago Press, 1960. Murray, and R. L. Wildey, S a phenomenon based on the thermal properties B. C., tell r and planetary observations at 10 microns, .4strollity,'. of the actul'll surface arE', of course, possiblE'. J., 137, 692, 1963a. The 'storm' or 'storm zone' observed in the Murray, and R. Wildey, Surface B. C., L. t€mperu. southern hemisphere does not correlate with any ture variations during the lu ar n nighttime ' visible feature on Venus in photographs taken .4strophys. J., in press, 1963b. Murray, R. Wildey, and A. around December 15 [Smith, 1963J, although B. C., L. J. Westphal. Brightness temperature map of Venus, its effects are apparent on one of the three Science, 140, 391, 1963. infrared scans collected by Mariner 2 on that Pettit, E., Planetary temperature measurements, in date [Chase, Kaplan, and Neugebauer, 1963J. Planets and Satellites, edit€d by G. P. Kuiper, If the storm zone is a result of topographically University of Chicago Press, 1961. Pettit, E., nd S. B. Nicholson, Radiometric induced disturbances in the Venus atmosphere, a 0'0. servation of Venus, Populm Astmn ., 38, 614, the 'mountains' in question must cover an ex­ 1924. tensive area in the southern po ar region. If the l Sinton, \'1. M., Infrared observations of Venus. storm is orographic, it cannot have remained Colloqu0 International d'Astmphysique dr Liege, 300-310, 1963. associated with the mountains of its origin un­ 1962, Sinton, W. and Strong, adiometr c obser· less the rotation period of Venus is approxi­ M., J. R i vations of Venus, Astrophys. J., 131, 470, 1960. mately between 7 and 9 days. Additional, closely Smith, B. A., Photographic appearance of Venus spaced observations such features prove of may a.t the time of Mariner 2 encounter, Technical to be highly illuminating. Rept. 557-63-1, Research Center, New Mexico State University, 1963. We have indeed been fortu­ Acknowledgments. Westphal, J. A., B. C. Murray, and D. E. Martz, , nate to be able to use the 200-inch telescope for An 8-14 fL infra.red astronomical photomet€r, these observations, and we wish to express our .4ppl. Opt., 2, 749-753, 1963. gratitude to the Mount Wilson and P lomar ob­ a Wildey, R. L., and B. C. Murray, Ten-micron servatories. photometry of 25 stars from B8 to M7, Astra· Financial support for this research has been plll/s. J., in press, 1963. made available through grant NsG 56-60 of the National Aeronautics and Space Administration (Manuscrint received Mav 23. 1963.)