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The dark side of Venus
ICARUS 69, 221--229 (1987)
The Dark Side of Venus DAVID A. A L L E N Anglo-Australian Observatory, P.O. Box 296, Epping, New South Wales, 2121, Australia Received July 7, 1986; revised October 9, 1986 Ground-based infrared observations have been made of the night hemisphere of the planet Venus around 1.7 and 2.3 p,m, confirming the continued presence of dark and light patterns at these wavelengths. The data are inconsistent with two published hypotheses for their origin, but allow a third explanation invoking a broken layer of partially opaque clouds seen projected against the thermal background below. It is shown that around 2.3/.~m the major cloud layer at an altitude of about 48 km provides that background, but the intensity of radiation at 1.74 /zm exceeds that expected and is unexplained. © 1987AcademicPress, Inc.
1. INTRODUCTION
In 1983 the dark hemisphere of the planet Venus was found to be unexpectedly bright at wavelengths around 1.7 and 2.3 /zm. Moreover, the radiation from the dark side showed strong spatial intensity modulation in the form of cloud patterns that rotated with a period somewhat longer than that of the low-contrast markings seen at ultraviolet wavelengths. In discussing their discovery, Allen and Crawford (1984; hereinafter paper I) allowed two interpretations of the observations. These two alternatives reflected the two possible sources of the bright background against which the darker patterns were clearly silhouetted. In one interpretation the radiation was scattered sunlight, which would have had to leak in from the bright side along a zone of altitude that offered little continuum absorption. In the other interpretation the radiation was thermal and arose at great depth in the atmosphere. Crucial to the interpretation were the spectral data presented in paper I. The radiation was seen at only selected wavelengths within the terrestrial atmospheric windows, namely in a narrow band centered at 1.74 /.~m and in a broader band between 2.2 and 2.5 ttm. It was not recorded by the 1978/
1979 Pioneer Venus Orbiter, which included a channel 0.02/zm wide centered at 2.03 /xm (Taylor et al., 1980). Figure 1 shows new data which essentially reproduce those of paper I. Within the 2-/.tm band a deep absorption feature identifiable as CO is seen. The spectrum can be understood if the radiation has passed through a significant amount of Venus' atmosphere, as described in paper I. All absorption lines of CO2 listed by Young (1974) must be optically thick so that radiation escapes only between them. This situation would arise, for instance, if the radiation originated very deep in the planet's atmosphere. The depth of the CO absorption confirms that a considerable amount of the atmosphere has been traversed: the mixing ratio of CO is 50 × 10-6 (Connes et al., 1968; Young, 1972). A path of some 200 km atm is implied, assuming that the CO mixing ratio remains similar throughout the atmosphere. It is known that the mixing ratio actually falls, the value at an altitude of 24 km being less than 1 ppm (Oyama et al., 1979). However, for the portions of the atmosphere with which we shall be concerned, and to the accuracy required, this fact can be neglected. Such a path can be achieved between the cloud tops and a height of about 30 km the surface, according to the data retrieved from satellites and
221 0019-1035/87 $3.00 Copyright© 1987by AcademicPress, Inc. All rightsof reproductionin any formreserved.
222
DAVID A. ALLEN stricted. When the phase of the planet exceeds about 0.35, the diffusion of light from the sunlit hemisphere (mostly due to the diffraction spikes of a reflecting telescope) overwhelms the radiation intrinsic to the dark side. At phases below about 0.25, observation of the planet is precluded for many telescopes because of the danger of sunlight striking the primary mirror. The appropriate range of phases occurs twice per synodic period, on either side of the inferior conjunction. Since the observations described in paper I, only two opportunities have presented themselves. Observations designed to test the various interpretations of the cloud patterns were made on the occasion which was more favorable for southern hemisphere observing and are described in this paper.
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WAVELENGTH #m FIG. 1. Spectrum of the dark side of Venus between 1.6 and 2.5 p.m. The broken lines indicate regions where the measured flux originates entirely in sunlight diffused from the illuminated crescent; no dark-side emission can be detected in those spectral regions.
balloons that have entered the Venus atmosphere (e.g., Pioneer data, Seiff et al., 1979; Vega data, Linkin et al., 1986). If sunlight directly provides the radiation, then the long path could arise as it courses along a zone between the uppermost cloud decks. Again using the data of Seiff et al., paths of a few thousand kilometers would be needed. Since this is of the same order as the observed physical distance from the terminator, multiple scattering would have to be an insignificant effect, and most of the light would have to percolate around the cloud top with a minimum number of scatterings until the light finally scattered upward and outward. A typical intensity is a thousandth of that reflected directly from the sunlit hemisphere. For an Earth-based observer, access to the dark side of Venus is severely re-
2. THE OBSERVATIONS The observations were made in 1985 May using the same infrared equipment (the IRPS) at the 3.9-m Anglo-Australian Telescope as employed for those of paper I. Imaging, spectroscopy, and polarimetry were undertaken, the latter with the Hatfield polarimeter (Bailey and Hough, 1982), which also uses the IRPS as the detector. Polarimetry of the dark side has not previously been attempted. In contrast to the 1983 observations, spectroscopy was attempted at selected positions on the dark side in order to explore the dependence of the absorption on position and pattern intensity. Clouds and bad seeing plagued the imaging, so that none of the data attains the quality of those of paper I. Table I lists the dates and times of the images that were secured. Only a loose confirmation of the rotation period derived in paper I was possible. The image taken on May 4 appeared to be of the same face as that seen on April 28 slightly more than one rotation later (Figs. 2 and 3), but many of the features had changed appreciably, so that it is only possible to deduce a rotation period slightly un-
THE DARK SIDE OF VENUS TABLE I 1985 IMAGES OF VENUS
Date (1985) April April April April May May May
28 30 30 30 02 04 04
Time (UT) h
m
00 54 00 22 20 37 21 30: 20 46 20 21 21 00:
Wavelength (p.m) 2.41 2.41 2.41 1.2 2.41
narrowband narrowband narrowband broadband narrowband
1.2 broadband
Comments
Fig. 2
Fig. 3 Fig. 4
der 6 days. The previous value (paper I) was 5.4 - 0.1 days. An image was also made in the J window, 1.1-1.4/xm. Reproduced in Fig. 4, it shows none of the structure revealed at longer wavelengths, but nonetheless has detectable dark-side emission. The origin of the
223
light is almost certainly the 02 airglow band at a wavelength of 1.27/zm. The fact that no dark patterns are seen at this wavelength clearly indicates that the majority of the airglow emission arises at a greater altitude than the absorbing clouds lie. The spectra confirmed those given in paper I. A typical spectrum, covering the two relevant telluric atmospheric windows, is shown in Fig. I. Once again, radiation from the dark side was seen only at wavelengths between the CO2 absorption bands, in other words within the Venusian atmospheric windows. As in paper I, deep absorption is seen in the CO overtone rotation bands around 2.35 ~m. On May 2 conditions permitted spectroscopy at several positions on the dark side of the planet, particularly regions having a great difference in their relative brightness. These spectra covered the wavelength re-
FIG. 2. Image of Venus taken on UT 1985 April 28, Oh 54m at a wavelength of 2.41/xm. The spatial resolution was 2 arcsec. Fluctuations in the seeing and minor irregularities in the telescope scanning result in the streaks extending from the sunlit crescent, which is grossly saturated on this gray scale display.
224
DAVID A. ALLEN
FIG. 3. Image of Venus taken on UT 1985 May 4, 20h, 21m. The observing details were identical to those of Fig. 2, and it is thought that approximately the same face of the planet is in view, the bright bar seen in Fig. 2 having been carried part way into the terminator. The image is smaller because of the greater distance of the planet.
gions 2.1-2.4 /zm and therefore included three distinct spectral r e g i o n s - - a portion dominated by light diffused f r o m the bright crescent (mostly b y the diffraction spikes of the telescope), a region in which the darkside c o n t i n u u m is dominant, and a region in which the CO absorption bands are strong. A corresponding spectrum of the sunlit crescent was also taken. The contribution o f scattered light from the sunlit crescent was subtracted by fitting to the first of these spectral regions, so that the resulting spectra represent true darkside emission. In Fig. 5 two such spectra are displayed, on a logarithmic scale, together with their difference. The two refer to regions differing in intensity by about a factor of 3; they show no perceptible difference through the CO bands. N o r can any difference be detected b e t w e e n spectra
taken at a range of distances from the terminator. The polarimetry was p e r f o r m e d at the n a r r o w b a n d wavelength of 2.3 /zm, corresponding to the p e a k of dark-side emission on the short-wavelength side of the CO absorption. The B e c k l i n - N e u g e b a u e r source was conveniently placed to provide a check on the calibration, and the instrumental polarization was, as usual for the Hatfield polarimeter, found to be negligible. A m e a s u r e m e n t of the sunlit crescent yielded a detectable polarization near 2%, with the position angle closely parallel to the line of cusps, as expected. Polarization of the dark-side radiation was seen at some places, but the intensity of the polarized emission was close to that expected for radiation diffracted from the sunlit crescent. W h e n correction for this was made, no un-
THE DARK SIDE OF VENUS derlying polarization could be detected from the dark side. A limit of 0.5% is estimated. 3. INTERPRETATION
The new data are used to explore the nature of the patterns. This issue breaks naturally into two components: the origin of the radiation at wavelengths in the range 1.72.5/.tm and the nature of the darker regions apparently silhouetted against it. (a) Origin of the radiation. Since no airglow emission is expected to match the known wavebands of the emission, and since the 02 airglow lacks the structure, only two sources of radiation can be considered. Either sunlight is scattered out, having been initially scattered around the planet at relatively high altitude, or the radiation is purely thermal, originating deep in the atmosphere. There are several reasons for discounting
225
the first of these interpretations. As noted above, the radiation could not have experienced significant multiple scattering or else the CO absorption would be deeper. That being so, the depth of the CO band should be considerably greater near the dark limb than toward the terminator. No such effect is evident in the spectra. Also, the radiation would be expected to be quite highly polarized by the scattering that directed it outward from the cloud or haze layer. Finally, the intensity would be expected to fall off from the terminator, and there is no indication of this happening (although the presence of scattered radiation from the sunlit crescent weakens this statement). It must be concluded that the radiation is thermal and of deep origin. By terrestrial analogy, we are viewing Venus through a net curtain. The uppermost clouds form that curtain and by day reflect sunlight back to dazzle us. By night, however, we become voyeurs able to peep into the backlit
FIG. 4. Image of Venus taken as Figs. 2 and 3, but using the broadband J filter. The source of emission on the dark side is expected to be the 1.27-/zm 02 airglow line.
226
DAVID A. ALLEN I O
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FIG. 5. Detailed spectra of two portions of the dark side after subtraction of the diffused light from the sunlit crescent. The error bars are shown only where larger than the symbols. The crosses indicate the difference on the same logarithmic scale. Errors bars have been omitted from the difference spectrum for clarity, but can be estimated from the individual spectra above. room behind. We know the temperature profile of the atmosphere from Seiff et al. and Linkin et al. and can determine the brightness temperature of the radiation at various wavelengths from the present data. The radiation at 1.74 /zm is considerably hotter than that in the 2.3- to 2.4-t~m window. The two temperature regimes are 470 and 360°K, respectively, and these correspond in the Pioneer and Vega data to altitudes of 30 and 48 km. The 2.3-~m radiation can immediately be identified as arising from deep within the densest cloud deck (region B of Ragent and Blamont (1979)) at an altitude of 48 km, but
the 1.74-/zm radiation would appear to be of deeper origin. H o w e v e r , below this cloud bank lies only a tenuous layer of clouds at a temperature still much cooler than 470°K (Ragent and Blamont, 1979; T o m a s k o et al., 1979). Must we believe that the major cloud deck is transparent at 1.74 /zm and that some u n k n o w n material at an altitude of about 30 km is opaque at that wavelength? These unlikely requirements encourage instead the belief that an extra radiation mechanism is operating at 1.74 /zm, though no obvious candidate mechanism exists. The resolution of the present spectra do not preclude a single emission line pro-
THE DARK SIDE OF VENUS ducing the radiation, or most of it, at 1.74 /zm.
Because the absorption patterns appear identical in morphology and intensity at 1.74/zm and in the 2-/xm window, whatever causes the excess emission must lie below the absorbing material and therefore below the altitude at which the airglow arises. The peak surface brightness at 1.74, 2.30, and 2.41/zm defines an energy distribution which closely follows the energy distribution of the sunlit crescent. This fact might be taken as evidence that the radiation is indeed sunlight scattered by a neutral material in the atmosphere. More likely, however, this is a curious coincidence. 4. NATURE OF THE DARK MARKINGS
Accepting that the radiation is thermal and originates deep within the atmosphere, what possibilities exist to explain the dark regions, the patterns on the net curtains? The temperature of the dark regions is about 30°K lower than that of the bright, and in so well mixed an atmosphere there is little likelihood that clouds of so large a temperature difference could be maintained at the same altitude. Indeed, the Pioneer probes entering the atmosphere did not find more than 2°K temperature differences at any altitude between points well dispersed across the planet, and the largest recorded difference is 6.5°K from the two Vega balloons (Linkin et al., 1986). It might then be thought that the dark clouds are opaque and lie in a broken layer above the zone providing the background radiation. This view was espoused in paper I. The dark clouds would then radiate as blackbodies at the lower temperature appropriate to their elevation. This, however, cannot be the case. If the dark clouds were opaque and elevated relative to the brighter regions, then the path length of CO absorption would be significantly less to the dark regions than to the bright. The difference, using the data of Seiff et al., and again neglecting the fall in CO abundance at the depths involved, amounts to a factor of
227
about 2, and would be clearly visible in Fig. 5. No such difference is seen. It must be concluded that the dark markings are optically thin so that they attenuate the radiation from deeper in the atmosphere and do not themselves emit appreciably at these wavelengths. What can be said about the ,altitude of the dark clouds? The only definite~upper limit is that set by the airglow, which is estimated to arise in the range 60-100 km altitude (Parisot and Moreels, 1980). The lower limit is determined as follows. At the location of one of the spectra the clouds transmitted only about one-third of the underlying radiation; the optical thickness of those clouds was therefore at least 0.67, the lower bound being appropriate if they contribute no thermal continuum whatsoever. This is sufficiently close to unity that for the present purposes we can regard them as having unit emissivity. Following the arguments above, they must have been cool enough not to contribute much of their own radiation at 2.4/zm; a limit of 10% is somewhat arbitrarily estimated. For unit emissivity the corresponding temperature is 315°K and the lower bound to the altitude is 50 km. Within this range of altitude lies the relatively tenuous but deep cloud layer referred to by Ragent and Blamont as region C. Ragent and Blamont found this region to be reasonably uniform at the four sites they sampled. If the dark markings are to be identified with region C, then the clouds cannot be uniform across the entire planet. On the other hand, it is not uncommon for the dark markings to be quite extensive; Fig. 6 shows such a case. The limited data available from the Pioneer probes do not necessarily contradict an assignment of the dark markings to region C. 5. CONCLUSIONS
By the use of polarimetry and spectroscopy at selected locations on the dark side of Venus, it has been shown that two cloud zones on the planet can be studied remotely
228
DAVID A. ALLEN
FIG. 6. An example of the relatively uniform dark cover typical of about 25% of the planet. The gray scale display in this image is almost identical to that used in Figs. 2 and 3.
by imaging at wavelengths between 2.2 and 2.5/xm. These zones are tentatively identified with B and C, two of the known cloud decks on the planet, lying deeper into the atmosphere than both the clouds which reflect sunlight back so efficiently and the high-level haze layer responsible for the patterns seen at ultraviolet wavelengths. Ground-based observers can satisfactorily study these clouds for at most a 2-week interval of time twice every synodic period. For this reason observations from an orbiting spacecraft are desirable. It is recommended that a near-infrared imager be considered for inclusion in the instrument
package of subsequent Venus Orbiter missions. REFERENCES ALLEN, D. A., AND J. W. CRAWFORD (1984). Cloud structure on the dark side of Venus. Nature 307, 222-224. BAILEY, J., AND J. H. HOUGH (1982). A simultaneous infrared/optical polarimeter. Publ. Astron. Soe. Pacif. 94, 618-623. CONNES, P., J. CONNES, L. D. KAPLAN, AND W. S. BENEDICT (1968). Carbon monoxide in the Venus atmosphere. Astrophys. J. 152, 731-743. LINKIN, V. M., V. V. KERZHANOVICH, A. N. L1PATOV, A. A. SHURUPOV,A. SEIFF,B. RAGENT,R. E. YOUNG, A. P. INGERSOLL, D. CRISP, L. S. ELSON, R. A. PRESTON, AND J. E. BLAMONT (1986). Ther-
THE DARK SIDE OF VENUS mal structure of the Venus atmosphere in the middle cloud layer. Science 231, 1420-1422. OYAMA, V. I., G. C. CARLE, F. WOELLER, AND J. B. POLLACK (1979). Venus lower atmospheric composition: Analysis by gas chromatography. Science 203, 802-805. PARISOT,J. P., AND G. MOREELS (1980). Oxygen 1.27 # m emission from the atmosphere of Venus. Icarus 42, 46-53. RAGENT, B., A N D J. B L A M O N T (1979). Preliminary results of the Pioneer Venus nephelometer experiment. Science 203, 790-792. SEIFF, A., D. B. KIRK, S. C. SUMMER,R. E. YOUNG, R. C. BLANCHARD,D. W. JUERGENS, J. E. LEPETICH, P. F. INTRIERI, J. T. FINDLAY, AND J. S. DERR (1979). Structure of the atmosphere of Venus up to 110 kilometres: Preliminary results from the four Pioneer Venus entry probes. Science 203, 787790. TAYLOR, F. W., R. BEER, M. T. CHAHINE, D. J. DINER, L. S. ELSON, R. D. HASKINS,D. J. McCLEESE, J. V. MARTONCHIK, P. E. REICHLEY, S. P.
229
BRADLEY, J. DELDERFIELD, J. T. SCHOFIELD, C. B. FARMER, L. FROIDEVAUX, J. LEUNG, M. T. COFFEY, AND J. C. GILLE (1980). Structure and meteorology of the middle atmosphere of Venus: Infrared remote sensing from the Pioneer Orbiter. J. Geophys. Res. 85, 7963-8006. TAYLOR,F. W., D. J. DINER, L. S. ELSON,D. J. MCCLEESE, J. V. MARTONCHIK, J. DELDERFIELD, S. P. BRADLEY, J. T. SCHOFIELD, J. C. GILLE, AND M. T. COFFEY (1979). Temperature, cloud structure, and dynamics of Venus middle atmosphere by remote sensing from Pioneer Orbiter. Science 205, 6567. TOMASKO, M. G., L. R. DOUSE, J. PALMER, A. HOLMES, W. WOLFE, N. D. CASTILLO, AND P. n . SMITH (1979). Preliminary results of the solar flux radiometer experiment aboard the Pioneer Venus multiprobe mission. Science 203, 795-797. YOUNG, L. D. G. (1972). High resolution spectra of V e n u s - - A review. Icarus 17, 632-658. YOUNG, L. D. G. (1974). Infrared spectrum of Venus. Int. Astron. Union Syrup. 65, 77-160.
The Dark Side of Venus DAVID A. A L L E N Anglo-Australian Observatory, P.O. Box 296, Epping, New South Wales, 2121, Australia Received July 7, 1986; revised October 9, 1986 Ground-based infrared observations have been made of the night hemisphere of the planet Venus around 1.7 and 2.3 p,m, confirming the continued presence of dark and light patterns at these wavelengths. The data are inconsistent with two published hypotheses for their origin, but allow a third explanation invoking a broken layer of partially opaque clouds seen projected against the thermal background below. It is shown that around 2.3/.~m the major cloud layer at an altitude of about 48 km provides that background, but the intensity of radiation at 1.74 /zm exceeds that expected and is unexplained. © 1987AcademicPress, Inc.
1. INTRODUCTION
In 1983 the dark hemisphere of the planet Venus was found to be unexpectedly bright at wavelengths around 1.7 and 2.3 /zm. Moreover, the radiation from the dark side showed strong spatial intensity modulation in the form of cloud patterns that rotated with a period somewhat longer than that of the low-contrast markings seen at ultraviolet wavelengths. In discussing their discovery, Allen and Crawford (1984; hereinafter paper I) allowed two interpretations of the observations. These two alternatives reflected the two possible sources of the bright background against which the darker patterns were clearly silhouetted. In one interpretation the radiation was scattered sunlight, which would have had to leak in from the bright side along a zone of altitude that offered little continuum absorption. In the other interpretation the radiation was thermal and arose at great depth in the atmosphere. Crucial to the interpretation were the spectral data presented in paper I. The radiation was seen at only selected wavelengths within the terrestrial atmospheric windows, namely in a narrow band centered at 1.74 /.~m and in a broader band between 2.2 and 2.5 ttm. It was not recorded by the 1978/
1979 Pioneer Venus Orbiter, which included a channel 0.02/zm wide centered at 2.03 /xm (Taylor et al., 1980). Figure 1 shows new data which essentially reproduce those of paper I. Within the 2-/.tm band a deep absorption feature identifiable as CO is seen. The spectrum can be understood if the radiation has passed through a significant amount of Venus' atmosphere, as described in paper I. All absorption lines of CO2 listed by Young (1974) must be optically thick so that radiation escapes only between them. This situation would arise, for instance, if the radiation originated very deep in the planet's atmosphere. The depth of the CO absorption confirms that a considerable amount of the atmosphere has been traversed: the mixing ratio of CO is 50 × 10-6 (Connes et al., 1968; Young, 1972). A path of some 200 km atm is implied, assuming that the CO mixing ratio remains similar throughout the atmosphere. It is known that the mixing ratio actually falls, the value at an altitude of 24 km being less than 1 ppm (Oyama et al., 1979). However, for the portions of the atmosphere with which we shall be concerned, and to the accuracy required, this fact can be neglected. Such a path can be achieved between the cloud tops and a height of about 30 km the surface, according to the data retrieved from satellites and
221 0019-1035/87 $3.00 Copyright© 1987by AcademicPress, Inc. All rightsof reproductionin any formreserved.
222
DAVID A. ALLEN stricted. When the phase of the planet exceeds about 0.35, the diffusion of light from the sunlit hemisphere (mostly due to the diffraction spikes of a reflecting telescope) overwhelms the radiation intrinsic to the dark side. At phases below about 0.25, observation of the planet is precluded for many telescopes because of the danger of sunlight striking the primary mirror. The appropriate range of phases occurs twice per synodic period, on either side of the inferior conjunction. Since the observations described in paper I, only two opportunities have presented themselves. Observations designed to test the various interpretations of the cloud patterns were made on the occasion which was more favorable for southern hemisphere observing and are described in this paper.
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WAVELENGTH #m FIG. 1. Spectrum of the dark side of Venus between 1.6 and 2.5 p.m. The broken lines indicate regions where the measured flux originates entirely in sunlight diffused from the illuminated crescent; no dark-side emission can be detected in those spectral regions.
balloons that have entered the Venus atmosphere (e.g., Pioneer data, Seiff et al., 1979; Vega data, Linkin et al., 1986). If sunlight directly provides the radiation, then the long path could arise as it courses along a zone between the uppermost cloud decks. Again using the data of Seiff et al., paths of a few thousand kilometers would be needed. Since this is of the same order as the observed physical distance from the terminator, multiple scattering would have to be an insignificant effect, and most of the light would have to percolate around the cloud top with a minimum number of scatterings until the light finally scattered upward and outward. A typical intensity is a thousandth of that reflected directly from the sunlit hemisphere. For an Earth-based observer, access to the dark side of Venus is severely re-
2. THE OBSERVATIONS The observations were made in 1985 May using the same infrared equipment (the IRPS) at the 3.9-m Anglo-Australian Telescope as employed for those of paper I. Imaging, spectroscopy, and polarimetry were undertaken, the latter with the Hatfield polarimeter (Bailey and Hough, 1982), which also uses the IRPS as the detector. Polarimetry of the dark side has not previously been attempted. In contrast to the 1983 observations, spectroscopy was attempted at selected positions on the dark side in order to explore the dependence of the absorption on position and pattern intensity. Clouds and bad seeing plagued the imaging, so that none of the data attains the quality of those of paper I. Table I lists the dates and times of the images that were secured. Only a loose confirmation of the rotation period derived in paper I was possible. The image taken on May 4 appeared to be of the same face as that seen on April 28 slightly more than one rotation later (Figs. 2 and 3), but many of the features had changed appreciably, so that it is only possible to deduce a rotation period slightly un-
THE DARK SIDE OF VENUS TABLE I 1985 IMAGES OF VENUS
Date (1985) April April April April May May May
28 30 30 30 02 04 04
Time (UT) h
m
00 54 00 22 20 37 21 30: 20 46 20 21 21 00:
Wavelength (p.m) 2.41 2.41 2.41 1.2 2.41
narrowband narrowband narrowband broadband narrowband
1.2 broadband
Comments
Fig. 2
Fig. 3 Fig. 4
der 6 days. The previous value (paper I) was 5.4 - 0.1 days. An image was also made in the J window, 1.1-1.4/xm. Reproduced in Fig. 4, it shows none of the structure revealed at longer wavelengths, but nonetheless has detectable dark-side emission. The origin of the
223
light is almost certainly the 02 airglow band at a wavelength of 1.27/zm. The fact that no dark patterns are seen at this wavelength clearly indicates that the majority of the airglow emission arises at a greater altitude than the absorbing clouds lie. The spectra confirmed those given in paper I. A typical spectrum, covering the two relevant telluric atmospheric windows, is shown in Fig. I. Once again, radiation from the dark side was seen only at wavelengths between the CO2 absorption bands, in other words within the Venusian atmospheric windows. As in paper I, deep absorption is seen in the CO overtone rotation bands around 2.35 ~m. On May 2 conditions permitted spectroscopy at several positions on the dark side of the planet, particularly regions having a great difference in their relative brightness. These spectra covered the wavelength re-
FIG. 2. Image of Venus taken on UT 1985 April 28, Oh 54m at a wavelength of 2.41/xm. The spatial resolution was 2 arcsec. Fluctuations in the seeing and minor irregularities in the telescope scanning result in the streaks extending from the sunlit crescent, which is grossly saturated on this gray scale display.
224
DAVID A. ALLEN
FIG. 3. Image of Venus taken on UT 1985 May 4, 20h, 21m. The observing details were identical to those of Fig. 2, and it is thought that approximately the same face of the planet is in view, the bright bar seen in Fig. 2 having been carried part way into the terminator. The image is smaller because of the greater distance of the planet.
gions 2.1-2.4 /zm and therefore included three distinct spectral r e g i o n s - - a portion dominated by light diffused f r o m the bright crescent (mostly b y the diffraction spikes of the telescope), a region in which the darkside c o n t i n u u m is dominant, and a region in which the CO absorption bands are strong. A corresponding spectrum of the sunlit crescent was also taken. The contribution o f scattered light from the sunlit crescent was subtracted by fitting to the first of these spectral regions, so that the resulting spectra represent true darkside emission. In Fig. 5 two such spectra are displayed, on a logarithmic scale, together with their difference. The two refer to regions differing in intensity by about a factor of 3; they show no perceptible difference through the CO bands. N o r can any difference be detected b e t w e e n spectra
taken at a range of distances from the terminator. The polarimetry was p e r f o r m e d at the n a r r o w b a n d wavelength of 2.3 /zm, corresponding to the p e a k of dark-side emission on the short-wavelength side of the CO absorption. The B e c k l i n - N e u g e b a u e r source was conveniently placed to provide a check on the calibration, and the instrumental polarization was, as usual for the Hatfield polarimeter, found to be negligible. A m e a s u r e m e n t of the sunlit crescent yielded a detectable polarization near 2%, with the position angle closely parallel to the line of cusps, as expected. Polarization of the dark-side radiation was seen at some places, but the intensity of the polarized emission was close to that expected for radiation diffracted from the sunlit crescent. W h e n correction for this was made, no un-
THE DARK SIDE OF VENUS derlying polarization could be detected from the dark side. A limit of 0.5% is estimated. 3. INTERPRETATION
The new data are used to explore the nature of the patterns. This issue breaks naturally into two components: the origin of the radiation at wavelengths in the range 1.72.5/.tm and the nature of the darker regions apparently silhouetted against it. (a) Origin of the radiation. Since no airglow emission is expected to match the known wavebands of the emission, and since the 02 airglow lacks the structure, only two sources of radiation can be considered. Either sunlight is scattered out, having been initially scattered around the planet at relatively high altitude, or the radiation is purely thermal, originating deep in the atmosphere. There are several reasons for discounting
225
the first of these interpretations. As noted above, the radiation could not have experienced significant multiple scattering or else the CO absorption would be deeper. That being so, the depth of the CO band should be considerably greater near the dark limb than toward the terminator. No such effect is evident in the spectra. Also, the radiation would be expected to be quite highly polarized by the scattering that directed it outward from the cloud or haze layer. Finally, the intensity would be expected to fall off from the terminator, and there is no indication of this happening (although the presence of scattered radiation from the sunlit crescent weakens this statement). It must be concluded that the radiation is thermal and of deep origin. By terrestrial analogy, we are viewing Venus through a net curtain. The uppermost clouds form that curtain and by day reflect sunlight back to dazzle us. By night, however, we become voyeurs able to peep into the backlit
FIG. 4. Image of Venus taken as Figs. 2 and 3, but using the broadband J filter. The source of emission on the dark side is expected to be the 1.27-/zm 02 airglow line.
226
DAVID A. ALLEN I O
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i
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FIG. 5. Detailed spectra of two portions of the dark side after subtraction of the diffused light from the sunlit crescent. The error bars are shown only where larger than the symbols. The crosses indicate the difference on the same logarithmic scale. Errors bars have been omitted from the difference spectrum for clarity, but can be estimated from the individual spectra above. room behind. We know the temperature profile of the atmosphere from Seiff et al. and Linkin et al. and can determine the brightness temperature of the radiation at various wavelengths from the present data. The radiation at 1.74 /zm is considerably hotter than that in the 2.3- to 2.4-t~m window. The two temperature regimes are 470 and 360°K, respectively, and these correspond in the Pioneer and Vega data to altitudes of 30 and 48 km. The 2.3-~m radiation can immediately be identified as arising from deep within the densest cloud deck (region B of Ragent and Blamont (1979)) at an altitude of 48 km, but
the 1.74-/zm radiation would appear to be of deeper origin. H o w e v e r , below this cloud bank lies only a tenuous layer of clouds at a temperature still much cooler than 470°K (Ragent and Blamont, 1979; T o m a s k o et al., 1979). Must we believe that the major cloud deck is transparent at 1.74 /zm and that some u n k n o w n material at an altitude of about 30 km is opaque at that wavelength? These unlikely requirements encourage instead the belief that an extra radiation mechanism is operating at 1.74 /zm, though no obvious candidate mechanism exists. The resolution of the present spectra do not preclude a single emission line pro-
THE DARK SIDE OF VENUS ducing the radiation, or most of it, at 1.74 /zm.
Because the absorption patterns appear identical in morphology and intensity at 1.74/zm and in the 2-/xm window, whatever causes the excess emission must lie below the absorbing material and therefore below the altitude at which the airglow arises. The peak surface brightness at 1.74, 2.30, and 2.41/zm defines an energy distribution which closely follows the energy distribution of the sunlit crescent. This fact might be taken as evidence that the radiation is indeed sunlight scattered by a neutral material in the atmosphere. More likely, however, this is a curious coincidence. 4. NATURE OF THE DARK MARKINGS
Accepting that the radiation is thermal and originates deep within the atmosphere, what possibilities exist to explain the dark regions, the patterns on the net curtains? The temperature of the dark regions is about 30°K lower than that of the bright, and in so well mixed an atmosphere there is little likelihood that clouds of so large a temperature difference could be maintained at the same altitude. Indeed, the Pioneer probes entering the atmosphere did not find more than 2°K temperature differences at any altitude between points well dispersed across the planet, and the largest recorded difference is 6.5°K from the two Vega balloons (Linkin et al., 1986). It might then be thought that the dark clouds are opaque and lie in a broken layer above the zone providing the background radiation. This view was espoused in paper I. The dark clouds would then radiate as blackbodies at the lower temperature appropriate to their elevation. This, however, cannot be the case. If the dark clouds were opaque and elevated relative to the brighter regions, then the path length of CO absorption would be significantly less to the dark regions than to the bright. The difference, using the data of Seiff et al., and again neglecting the fall in CO abundance at the depths involved, amounts to a factor of
227
about 2, and would be clearly visible in Fig. 5. No such difference is seen. It must be concluded that the dark markings are optically thin so that they attenuate the radiation from deeper in the atmosphere and do not themselves emit appreciably at these wavelengths. What can be said about the ,altitude of the dark clouds? The only definite~upper limit is that set by the airglow, which is estimated to arise in the range 60-100 km altitude (Parisot and Moreels, 1980). The lower limit is determined as follows. At the location of one of the spectra the clouds transmitted only about one-third of the underlying radiation; the optical thickness of those clouds was therefore at least 0.67, the lower bound being appropriate if they contribute no thermal continuum whatsoever. This is sufficiently close to unity that for the present purposes we can regard them as having unit emissivity. Following the arguments above, they must have been cool enough not to contribute much of their own radiation at 2.4/zm; a limit of 10% is somewhat arbitrarily estimated. For unit emissivity the corresponding temperature is 315°K and the lower bound to the altitude is 50 km. Within this range of altitude lies the relatively tenuous but deep cloud layer referred to by Ragent and Blamont as region C. Ragent and Blamont found this region to be reasonably uniform at the four sites they sampled. If the dark markings are to be identified with region C, then the clouds cannot be uniform across the entire planet. On the other hand, it is not uncommon for the dark markings to be quite extensive; Fig. 6 shows such a case. The limited data available from the Pioneer probes do not necessarily contradict an assignment of the dark markings to region C. 5. CONCLUSIONS
By the use of polarimetry and spectroscopy at selected locations on the dark side of Venus, it has been shown that two cloud zones on the planet can be studied remotely
228
DAVID A. ALLEN
FIG. 6. An example of the relatively uniform dark cover typical of about 25% of the planet. The gray scale display in this image is almost identical to that used in Figs. 2 and 3.
by imaging at wavelengths between 2.2 and 2.5/xm. These zones are tentatively identified with B and C, two of the known cloud decks on the planet, lying deeper into the atmosphere than both the clouds which reflect sunlight back so efficiently and the high-level haze layer responsible for the patterns seen at ultraviolet wavelengths. Ground-based observers can satisfactorily study these clouds for at most a 2-week interval of time twice every synodic period. For this reason observations from an orbiting spacecraft are desirable. It is recommended that a near-infrared imager be considered for inclusion in the instrument
package of subsequent Venus Orbiter missions. REFERENCES ALLEN, D. A., AND J. W. CRAWFORD (1984). Cloud structure on the dark side of Venus. Nature 307, 222-224. BAILEY, J., AND J. H. HOUGH (1982). A simultaneous infrared/optical polarimeter. Publ. Astron. Soe. Pacif. 94, 618-623. CONNES, P., J. CONNES, L. D. KAPLAN, AND W. S. BENEDICT (1968). Carbon monoxide in the Venus atmosphere. Astrophys. J. 152, 731-743. LINKIN, V. M., V. V. KERZHANOVICH, A. N. L1PATOV, A. A. SHURUPOV,A. SEIFF,B. RAGENT,R. E. YOUNG, A. P. INGERSOLL, D. CRISP, L. S. ELSON, R. A. PRESTON, AND J. E. BLAMONT (1986). Ther-
THE DARK SIDE OF VENUS mal structure of the Venus atmosphere in the middle cloud layer. Science 231, 1420-1422. OYAMA, V. I., G. C. CARLE, F. WOELLER, AND J. B. POLLACK (1979). Venus lower atmospheric composition: Analysis by gas chromatography. Science 203, 802-805. PARISOT,J. P., AND G. MOREELS (1980). Oxygen 1.27 # m emission from the atmosphere of Venus. Icarus 42, 46-53. RAGENT, B., A N D J. B L A M O N T (1979). Preliminary results of the Pioneer Venus nephelometer experiment. Science 203, 790-792. SEIFF, A., D. B. KIRK, S. C. SUMMER,R. E. YOUNG, R. C. BLANCHARD,D. W. JUERGENS, J. E. LEPETICH, P. F. INTRIERI, J. T. FINDLAY, AND J. S. DERR (1979). Structure of the atmosphere of Venus up to 110 kilometres: Preliminary results from the four Pioneer Venus entry probes. Science 203, 787790. TAYLOR, F. W., R. BEER, M. T. CHAHINE, D. J. DINER, L. S. ELSON, R. D. HASKINS,D. J. McCLEESE, J. V. MARTONCHIK, P. E. REICHLEY, S. P.
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BRADLEY, J. DELDERFIELD, J. T. SCHOFIELD, C. B. FARMER, L. FROIDEVAUX, J. LEUNG, M. T. COFFEY, AND J. C. GILLE (1980). Structure and meteorology of the middle atmosphere of Venus: Infrared remote sensing from the Pioneer Orbiter. J. Geophys. Res. 85, 7963-8006. TAYLOR,F. W., D. J. DINER, L. S. ELSON,D. J. MCCLEESE, J. V. MARTONCHIK, J. DELDERFIELD, S. P. BRADLEY, J. T. SCHOFIELD, J. C. GILLE, AND M. T. COFFEY (1979). Temperature, cloud structure, and dynamics of Venus middle atmosphere by remote sensing from Pioneer Orbiter. Science 205, 6567. TOMASKO, M. G., L. R. DOUSE, J. PALMER, A. HOLMES, W. WOLFE, N. D. CASTILLO, AND P. n . SMITH (1979). Preliminary results of the solar flux radiometer experiment aboard the Pioneer Venus multiprobe mission. Science 203, 795-797. YOUNG, L. D. G. (1972). High resolution spectra of V e n u s - - A review. Icarus 17, 632-658. YOUNG, L. D. G. (1974). Infrared spectrum of Venus. Int. Astron. Union Syrup. 65, 77-160.