Mariner 10 observations of hydrogen Lyman alpha emission from the venus exosphere: Evidence of complex structure

Planet. Space Sci., Vol. 28, pp. 687-701

0032M)633]80/0701-0687502.00/0

© Pergamon Press Ltd., 1980. Printed in Northern Ireland

M A R I N E R 10 O B S E R V A T I O N S O F H Y D R O G E N L Y M A N ALPHA EMISSION FROM THE VENUS EXOSPHERE: EVIDENCE OF COMPLEX STRUCTURE P. Z. TAKACS,* A. L BROADFOOT, G. R. sMrrH Kitt Peak National Observatory, Tucson, A Z 85726, U.S.A. and S. KUMAR

Jet Propulsion Laboratory, Pasadena, CA 91103, U.S.A. (Received 12 December 1979)

Abstract--The Ultraviolet Spectrometer Experiment on the MARINER 10 spacecraft measured the hydrogen Lyman a emission resonantly scattered in the Venus exosphere at several viewing aspects during the encounter period. Venus encounter occurred at 1 7 : 0 1 G M T on 5 February 1974. Exospheric emissions above the planet's limb were measured and were analyzed with a spherically symmetric, single scattering, two-temperature model. On the sunlit hemisphere the emission profile was represented by an exospheric hydrogen atmosphere with T c = 2 7 5 + 5 0 K and n c = 1.5×105cm -3, and a non-thermal contribution represented by T H = 1 2 5 0 ± 1 0 0 K with n i l = 500 + 100 cm -3. The observations of the dark limb showed that the spherically symmetric model used for the sunlit hemisphere was inappropriate for the analysis of the antisolar hemisphere. The density of the non-thermal component had increased at low altitudes, <12,000km, and decreased at high altitudes, >20,000 km, by comparison. We conclude that the non-thermal source is on the sunward side of the planet. Analysis of the dark limb crossing suggests that the exospheric temperature on the dark side is <125 K if the exospheric density remains constant over the planet; upper limits are discussed. An additional source of Lyman a emission, 70+ 15 R, was detected on the dark side of the planet and is believed to be a planetary albedo in contrast to multiple scattering from the sunlit side. Our analysis of the MARINER 10 data is consistent when applied to the MARINER 5 data. 1. INTRODUCTION T h e o b s e r v a t i o n of t h e r e s o n a n c e s c a t t e r e d emission f r o m H y d r o g e n in t h e V e n u s a t m o s p h e r e , which i n d i c a t e d a d o u b l e scale h e i g h t ( B a r t h et al., 1967), h a s b e e n with us since t h e flight of MARINER 5, T h e d a t a was e x a m i n e d i n d e p e n d e n t l y b y S t e w a r t (1968) a n d W a l l a c e (1969) b o t h of w h o m also identified two scale heights in t h e h y d r o g e n intensity profile. T h e largest scale h e i g h t was usually associated with H at t h e e x o s p h e r i c t e m p e r a t u r e with several possible ways to p r o d u c e t h e l o w e r scale h e i g h t by a mass or t e m p e r a t u r e f a c t o r of two. N o n e of t h e p o s s i b l e e x p l a n a t i o n s were w i t h o u t difficulties. K u m a r a n d H u n t e n (1974) r e v i e w e d t h e V e n u s h y d r o g e n p r o b l e m a n d suggested t h a t the e x o s p h e r i c t e m p e r a t u r e of 350 K was c o n s i s t e n t with t h e i r i o n o s p h e r i c model, b u t leaving a " h o t " or n o n - t h e r m a l V e n u s h y d r o g e n corona. T h e M A R I N E R 5 e x p e r i m e n t was p e r f o r m e d * Present address: Brookhaven National Laboratory, Upton, NY 11973, U.S.A. 687

again f r o m M A R I N E R 10. T h e p r e l i m i n a r y analysis of t h e H y d r o g e n L y m a n - a d a t a ( B r o a d f o o t et al., 1974) did n o t clarify the situation; a t e m p e r a t u r e of a b o u t 400 K was r e p o r t e d with n o clear e v i d e n c e of a s e c o n d c o m p o n e n t . S u b s e q u e n t l y t h e analysis of t h e H e l i u m 5 8 4 ~ emission profile ( K u m a r a n d B r o a d f o o t , 1975) gave i n d e p e n d e n t confirmation t h a t t h e e x o s p h e r i c t e m p e r a t u r e was n e a r 400 K. This result e l i m i n a t e d t h e speculations a n d left a " h o t " H y d r o g e n c o m p o n e n t as t h e anomaly, as suggested by K u m a r a n d H u n t e n (1974). A re-analysis of t h e M A R I N E R 5 d a t a ( A n d e r son, 1976) with a r a d i a t i v e t r a n s f e r m o d e l s h o w e d conclusively t h a t the intensity profile o b s e r v e d f r o m M A R I N E R 5 was d u e to h y d r o g e n at a low exospheric t e m p e r a t u r e a n d an additional s o u r c e of " h o t " h y d r o g e n . In r e t r o s p e c t t h e implied t h e r m a l c o n d i t i o n s in t h e two c o m p o n e n t h y d r o g e n exosp h e r e were t h e s a m e as t h o s e f o u n d b u t not u n d e r s t o o d by S t e w a r t (1968) a n d by Wallace (1969, his case 4). N o w a m o r e d e t a i l e d analysis of t h e M A R I N E R 10 h y d r o g e n 1 2 1 6 ~ d a t a shows t h a t t h e " h o t " c o m p o n e n t was r e c o r d e d by t h e M A R I N E R 10 e x p e r i m e n t also.

P. Z. TAKACS, A. L. BROADFOOT, G. R. SMITH and S. KUMAR

688

T h e source of the hot hydrogen c o m p o n e n t is not understood. K u m a r et al. (1978) have reviewed the p r o p o s e d mechanisms for ionospheric production of hot hydrogen and the global circulation models invoked to explain the low total hydrogen density. R e c e n t m e a s u r e m e n t s by Bertaux et al. (1978), with a resonance absorption cell p h o t o m e t e r aboard V E N E R A 9 and 10, indicate a source of non-thermal hydrogen atoms in the region of the shock p r o d u c e d by the direct interaction of the solar wind with the upper atmosphere. T h e y are able to fit their bright limb data, which extends only out to about Rp = 10,000 km, with a single temperature exosphere at about 500 K with an exobase density of 1,5 × 1'04 c m - 3 . This is an order of magnitude below the densities r e q u i r e d for the cold c o m p o n e n t at the exobase by two t e m p e r a t u r e models. In our approach to the analysis of the data we lean heavily on A n d e r s o n ' s radiative transfer m o d eling of the M A R I N E R 5 data as a confirmation that the approximate approach and assumptions, which h a v e been used o v e r the years, do give good

results. W e use a spherically symmetric exospheric m o d e l and confine our examinations to optically thin regions of the atmosphere. W e have used a thermal characterization of the cold and hot hydrogen components as a mechanism of separation in the modeling process. H o w e v e r , the nature of the " h o t " c o m p o n e n t is still not understood and there is no evidence to suggest that it has a thermal distribution in the classical sense. W e try to emphasize this point by reference to the " h o t " c o m p o nent as a non-thermal c o m p o n e n t since it does not relate to the thermal structure of the atmosphere. 2. E N C O U N T E R

GEOMI~IfRY AND DATA

T h e g e o m e t r y of the Venus encounter observations is illustrated in Figs. 1 and 2. In Fig. l a , we have transformed the spacecraft trajectory into a planet-fixed cylindrical coordinate system in which the azimuth, or clock, angle about the planet-Sun axis has been suppressed. T h e two parameters presented in Fig. l(a) are the Sun-spacecraft-planet elevation angle (cone angle) and the spacecraftplanet range. Figure l(b) is a plot of the planet

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(a) The solid line marks the planet-spacecraft range as a function of planet celestial cone angle in the sun-planet-spacecraft plane. Tick marks along the trajectory are Flight Data System (FDS) frame counts: 1 frame = 42 s. Arrows and letters show boresight vector directions and approximate spatial volume sampled during each data block. " A " = dark limb drift; " B " = bright limb drift; " C " = step sequence; " D " = post-encounter television mosaics. The solar zenith angle, X, of the boresight closest approach point to the planet center, R o, is also shown. (b) The celestial clock angle of the planet relative to the spacecraft-Sun-Canopus reference plane, as a function of FDS count marked on the trajectory above.

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(a) dark limb drift, range to planet from 35,000 km to 20,000 km; (b) bright limb drift, range =13,000km; (c) post-encounter slews at range =40,000kin; (d) far-encounter slews at range 100,000 km. clock angle as a function of time along the trajectory corresponding to Fig. l(a). The motion of the spacecraft caused the pole of the platform coordinate system to cross the planet after encounter, so that the UVS field-of-view limb tangent point moved from the east limb, over the north pole to the west limb. The spatial regions delineated by the letters in Fig. 1 refer to specific time periods into which the data are grouped owing to variations in observation angle and spacecraft range over the encounter period. Marks along the trajectory curve indicate spacecraft flight data system (FDS) elapsed time from launch in units of 42 s. Figure 2 shows the projection of the instrument field of view on the Venus disk for the various data periods; the slit size is constant in each view. The preliminary analysis of the exospheric hydrogen distribution was based on data from the fixed-platform drift in " B " of Fig. l(a). This sequence did not provide sufficient integration time to allow separation of the low-intensity highaltitude component from the interplanetary background emission. Additional measurements of the atmosphere above the limb were made during later television imaging sequences. The spectrometer boresight was offset 4.8 ° from the camera's so that as the camera examined the planet's limb, the spectrometer probed at varying heights in the exosphere. A complete description of the airglow spectrometer experiment appears in Broadfoot et al. (1977) and will not be discussed here.

The nature of the imaging sequence motion made the airglow data retrieval difficult. The parameter for describing the ultraviolet spectrometer observations is the tangent height, Th, the perpendicular distance of the center of the planet from the plane defined by the long axis of the spectrometer slit and the boresight vector. When the boresight vector passes through the point on this plane nearest the planet, we have the condition Th = Rp where Rp is the conventional distance from the planet center to the boresight closest approach point. Data were selected when we could confirm that Th = Rp. This selection process eliminated data obtained when the altitude range covered by the slit length significantly exceeded the projected slit width. The interval marked " A " in Fig. 1, the "dark limb drift," includes all of the pre-encounter data when the scan platform was held at a fixed cone position 54 ° from the spacecraft-Sun axis. The scan platform position was fixed several days before the encounter, resulting in an uninterrupted observation of the interplanetary background. The platform position allowed the boresight vector of the slit to cross the dark limb of the planet at the tangency point as shown in Fig. 2(a). The distance Rp changed as a result of the spacecraft-planet relative drift motion. Data from the " A " period is shown in Fig. 3. Scan platform motion toward the pole of the planet began shortly after the dark limb crossing, at Rp = 3000 km. It should be noted that

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The time sequence starts at frame 54870 approximately 11 hours before encounter, and ends on the disk of the planet at frame 57770. Open circles = 1000 km altitude bins; solid circles = 500 km and 250 km altitude bins, the highest resolution occurring near the limb and on the disk. The exosphere emission appears above the interplanetary background at altitudes below 30,000 kin. during the approach the spacecraft was outside of the geometric shadow of Venus. The observations were made looking through the shadow. The second block of data, " B " , consists of data from another drift experiment off the sunlit limb followed by data taken during a television imaging sequence. The data to 7000 km in Fig. 4 were obtained with the scan platform fixed at a position to have the projection of the spectrometer slit tangent to the limb at the crossing. This geometry is shown in Fig. 2(b). The imaging sequence following the drift viewed the planet limb which placed the boresight at altitudes greater than 7000 kin. During this imaging sequence, the Rp point moved from 18 ° solar zenith angle to approximately 60 ° and rotated over the north pole of the planet as shown in Fig. 2(c). The spacecraft range to the planet center increased from 12,800 to 37,000 km during acquisition of this data block. The statistical error is smaller than the size of the points in Fig. 4. We attribute the irregularity of the emission profile, which is larger than the statistical error, to the randomness of the data acquisition and summation technique rather than to real emission structure. At the conclusion of the imaging sequence, starting just after the end of " B " at frame count 57890, a series of 4 slews was excecuted with the aspect shown in Fig. 2(c). Due to a pointing error, these slews did not achieve their primary objective, a measurement of the helium limb profile, but they are useful in mapping the Lyman ~ hydrogen profile. The spacecraft range during this sequence

was about 38,000 km. Data from these slews appear in Fig. 5. The third block of data, indicated by " C " in Fig. 1, consists of a sequence of 4 slews, extending from Rv = 0 km to Rp = 28,000 km. This sequence began at frame count 58070 and ended at frame count 58090 at a spacecraft range of about 100,000 kin. The planet aspect is shown in Fig. 2(d); the instrument F O V relative to planet latitude remained constant for the remainder of the post encounter period. Data from this slew sequence are shown in Fig. 6. Additional data from the end of this block, obtained during a discrete stepping sequence, are combined with data from block " D " . Block " D " starts at frame count 58160 at a spacecraft range of 135,000 kin, and extends well past the encounter period to frame count 62030 at a range of 1.5× 10 6 km. The observing sequence during this period consisted of television mosaics of the planet which, because of our instrument offset angle, resulted in a range of Rp from 10,000 to 130,000 km. Data from the early part of this period are shown in Fig. 7. The slope of the 'hot" component becomes lost in the interplanetary background at about 50,000 km. Long integration times at each altitude result in high statistical accuracy in the data. Unfortunately, spacecraft pointing information is less accurate and data points near the limb are not useful and have been suppressed. Two corrections have been made to the data; the first to remove contributions due to internal scattering noise and dark current, the second to remove

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FRAME57810, END AT 57890. Data out to Rp = 7000 km are from fixed-platform drift at X = 18°; data beyond 7000 km are from a television imaging sequence, X increasing to 63 ° as Rp increases monotonically. Statistical error is smaller than the size of the data points. The fit from the two-temperature model is shown for two cold temperatures and densities: T c = 275 K with Nc = 1.5 × 105 c m -3, and Tc = 300 K with N¢ = 3 × 105 cm -3. The hot component in both cases is T¢ = 1250 K, N¢ = 5 × 102 cm -3. The two curves show the relative insensitivity of the bright limb data to the cold model parameters. The model curves lie below the data in the region where multiple scattering effects become important, at R e < 7500 kin.

The interplanetary background emission has been removed. The statistical error is smaller than the size of the data points. A sequence design error prevented the slews from reaching the planet disk. The absolute altitude scale was determined by data from the He1584/1~ and O I 1 3 0 4 ~ channels just off the planet's limb, which fixed each slew relative to the steep slope in each of these emissions. The model of Fig. 4, our standard front side model, was used to generate the curve.

the c o n t r i b u t i o n of r e s o n a n c e - s c a t t e r e d h y d r o g e n emission f r o m t h e i n t e r p l a n e t a r y m e d i u m . T h e consideration of the first correction was d e s c r i b e d by B r o a d f o o t and K u m a r (1978). In all cases, the s c a t t e r e d light correction is a small fraction of the o b s e r v e d L y m a n a signal. T h e L y m a n a sky m a p of B r o a d f o o t and K u m a r (1978) was used to estim a t e the s e c o n d correction. T h e track of the observing s e q u e n c e was p l o t t e d on the L y m a n a sky m a p which was taken 8 days p r e v i o u s to e n c o u n t e r ; the i n t e r p l a n e t a r y b a c k g r o u n d signal varied f r o m 80 to 250 R and r e q u i r e d p o i n t - b y - p o i n t correction of the data in Figs. 4 - 7 . A g o o d m e a s u r e of the p o s t - e n c o u n t e r b a c k g r o u n d level is given in Fig. 7. O b s e r v a t i o n s g r e a t e r than 5 0 , 0 0 0 k i n f r o m the p l a n e t show a c o n s t a n t level of 250 R n e a r R . A . 180 °, decl. + 2 2 °.

D a t a f r o m the p r e - e n c o u n t e r dark limb drift of Fig. 3 are p r e s e n t e d in a modified fashion in Fig. 8. A g a i n the signal has b e e n c o r r e c t e d for internal scattering, noise, and dark c u r r e n t as n o t e d above. T h e interplanetary emission signal was m e a s u r e d as we a p p r o a c h e d the planet a n d was f o u n d to be 209±1R in t h e direction R . A . 24 °, d e c l . - 1 9 . 5 ° (cf. B r o a d f o o t and K u m a r , 1978). 3. T H E M O D E L

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T h e radiative transfer m o d e l that we have dev e l o p e d to d e s c r i b e the M A R I N E R 10 h y d r o g e n L y m a n c~ observations deals mainly with the optically thin regions of the V e n u s e x o s p h e r e . W e d o not include the effects of multiple scattering and c o n s e q u e n t l y restrict our c o m p a r i s o n on the day side to emissions sufficiently far f r o m the sunlit surface that only a single scattering source n e e d be considered. T h e following c o n s i d e r a t i o n s w e r e included in the calculations. W e use a solar flux g-value of

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Upper (solid) curve uses the same model, but with no solar attenuation above 6300 km beyond the terminator; i.e., with a sharp cutoff in solar flux below that altitude and complete transmission above it. The dashed curve is the approximation to the screening function using a sharp cutoff at 7800 km above the terminator.

2 . 2 × 1 0 - a s - 1 ; this value was calculated by the method of Vidal-Madjar (1975) from a 10.7 cm solar flux of 7 5 x 10-22Wm-2I-Iz -1, which gave a line center solar flux of 4.0 = 1011 ph c m - 2 S -1 A - 1 at Venus. The model calculates the column density in the field of view of the instrument by dividing the slit into a 7 x 31 element array, performing a column integration over each element and weighting each array element by the response function of the instrument. We have constrained our modeling to use a spherically symmetric hydrogen distribution since we do not have a set of observations which could guide the use of a detailed asymmetric atmospheric model, and second because departures from the spherical case are the observables we wish to investigate and identify. Sphericity is a reasonable approximation for a single line of sight through the atmosphere on the day side, regardless of the fact that the real atmosphere may have substantial differences between the day side and night side (Dickinson and Ridley, 1977). Both Wallace (1969) and Anderson (1976) appear to have shown that the atmosphere is asymmetric although there are some problems with their interpretations which we will discuss later. The asymmetry described by

Kumar et al. (1978) resulted from the work of Dickinson and Ridley and Wallace. We use a two-temperature, non-rotating hydrogen exospheric model, calculated according to the formulation of Hartle (1971). The hot component, which dominates at high altitudes, is characterized by a thermal distribution in our model, although its origin and nature are unknown. Departures from the thermal distribution model would represent the signature we wish to examine. However, we find that the altitude profile of the hot component emission can be fit with a thermal distribution at a single temperature. All of the analysis assumes that the line profile of the hot emission is Gaussian with a width characteristic of the apparent temperature. If the velocity distribution of the hot hydrogen atoms deviates significantly from Maxwell-Boltzmann, the transmission of photons from this component by the cold component will be altered somewhat. Until we know more about the nature of the hot component, we will assume a Gaussian line profile. The cold component has a well-behaved barometric distribution characterized by the temperature and density at the exobase reference altitude, which is 6305 km in this model (Anderson, 1976).

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Unlike previous radiative transfer analyses of Venus hydrogen emissions (Wallace, 1969; Anderson, 1976), we consider the effect of two temperatures on the propagation of photons through the Cytherean exosphere. The temperature effect is not significant in modeling the bright limb data, but is very important in describing the dark limb data. In an optically thin atmosphere, the observed hydrogen emission can be written as 47ri = g0N

(1)

where go is the emission rate factor in number of solar photons scattered/sec-atom (Chamberlain, 1961), and N is the column density of hydrogen atoms along the line of sight. In order to describe regions of moderate optical depth, we can preserve the formulation in (1) by considering an effective g-factor as a function of r. The vertical optical depth at line center,

"ro = croN where tro is the temperature-dependent line center absorption cross-section. We write (1) as 47ri = { g(r')n(~-') ds

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where the integral is over the line of sight, n(~-') is the volume density at T' along the line of sight. We then write g(T') as a product g(r') = goSo('r')T('r', r, a )

dominates at low altitudes. The density of the hot component is such that it is always optically thin, even along a tangent column at the exobase. As a result, the transmission function differs significantly from unity only in the region where the cold component dominates. In this region, the hot source is only a small fraction of the total source, and on the day side we may use a single transmission function appropriate for a cold source propagating through a cold absorber. The transmission function in this case (c~ = 1) has the form (Holstein, 1974) T(~',z)=~Ij~

e - ~ 2 e x p ( - I ' r ' - ' r l e - ~ 2 ) dx

(5)

where x = V - V o / A v c is the frequency from line center expressed in units of the cold component Doppler width. The source function in (4) on the bright limb is calculated by considering the probability that a photon incident at the top of the atmosphere will travel down to a depth r' and be scattered. Again, we neglect the difference in temperature between the two components and treat the density distribution as if it were at the cold temperature. With these approximations, the source function on the bright limb is So (~") = ~

e -~2 exp ( - 7 ' e -~2) dx.

(6)

(3)

where So('r') is the single-scattering source function at "r', T('r', "r, o~) is a function that describes the transmission of source photons from T' to r, and = (TdTH) ~/2 is the ratio of the line widths of the two temperature components in the model assuming both have Gaussian line shapes. Using (3) in (2) gives 4wI(r) = go [ dsSo('r')T('r', % ot)n('r'). (4) The source is a function of both the line shape of the attenuated solar flux reaching ~" and of the line shape of the scattering component at T'. The solar flux shape depends on the temperature and density of the absorbing gas down to the level of ~-'; the line shape of the scatters depends on the local temperature at ~-'. Likewise, the transmission function depends on the temperature of the scattering source and the temperature of the gas between source and observer. The observations off the day side of the planet are best described, as shown in the next section, by a two-temperature atmosphere--a hot temperature on the order of 1 2 5 0 K that dominates at high altitude, and a cold temperature of 275 K that

Model--dark

limb

The dark limb model can be considered as two separate problems. First, sunlight is scattered from the hydrogen exosphere behind the terminator. Second, this resonance scattered emission becomes the radiation source which we use to probe the cold hydrogen exosphere on the dark side in an extinction experiment. The geometry of the M A R I N E R I0 observations is such that this emission source region is always at sufficiently high altitude that only the hot component is illuminated by sunlight. We may again consider only the effect of the cold component on the attenuation of incident solar flux for the same reasons as discussed in the bright limb case. Since scattering in the source region behind the terminator is done only by the hot component and the screening above the terminator is dominated by a cold line profile, detailed consideration of the difference in line shapes between the incident flux and the scattering gas is necessary. In the next step, the hot source photons are transmitted through the cold gas near the dark limb, requiring the same detailed line shape considerations. The source term

Hydrogen Lyman alpha emission from the Venus exosphere in (4) for the dark limb observations is

So(~',a)=-~w

e . . . . . exp (-l-e-X2) dx

(7)

where ~- is the line center optical depth of the cold c o m p o n e n t from the top of the atmosphere down to the source region along the line to the Sun, and the frequency p a r a m e t e r is again measured in units of the cold c o m p o n e n t D o p p l e r width. The second term in the integral in (7) is the probability that a photon at frequency x will reach an optical depth ~-; the first term is the probability that a photon at frequency x will scatter from the hot component. A l t h o u g h we do not know the nature of the hot component, we m a k e the assumption that the source region on the far side of the shadow scatters isotropically with a D o p p l e r width characteristic of the hot apparent temperature. T h e explicit form for the transmission function near the dark limb is O£ ~"

T(-r, a) --- ~--~ J e

~2x2

exp (-~-e

x2

)dx,

(8)

where the first term in the integral is the frequency distribution of source photons, and the second term is the probability of reaching the observer through a cold c o m p o n e n t of optical depth ~-. The hot c o m p o n e n t is sufficiently thin that it can be neglected as a secondary absorber. Pure absorption by the CO2 atmosphere is negligible at altitudes we are concerned with in our data. In both the bright and dark limb cases, the thermal profile of the interplanetary hydrogen Lyman a line is so much broader than the cold gas absorber that we assume the emission is not significantly attenuated and is cut off sharply at the exobase. T h e interplanetary background signal has been subtracted from all data before comparison with the m o d e l calculations. 4. R E S U L T S A N D D I S C U S S I O N

Bright limb The data in the four sets of observations in Figs. 4 - 7 are used to d e t e r m i n e the m o d e l parameters above the sunlit hemisphere. T h e data in Fig. 4 are d o m i n a t e d by the emission profile of the cold component while the data in Fig. 7 are primarily the hot c o m p o n e n t emission profile. T h e data sets in Figs. 5 and 6 are primarily of the transition region. A good m o d e l fit to the hot hydrogen emission c o m p o n e n t is shown in Fig. 7. T h e m o d e l p a r a m e ters were TH = 12504- 100 K and nH = 5.04-1.0 x102 cm 3. T h e excess emission feature

695

around 24,000 km is difficult to explain, since additional data points (not shown) at larger Rp follow the extension of the model curve very closely. This may be the signature of the solar wind shock wave observed by Bertaux et al. (1978) from measurements taken aboard V E N E R A 9 and 10. The g e o m e t r y of the observations in Fig. 7 at large Rp would place the instrument's F O V along the asymptote of the shock wave trailing off behind the planet, thus enhancing any perturbation in the " n o r m a l " exospheric hydrogen distribution along the shock fromt. This type of signature was observed along the bow shock at Mercury as well (cf. Shemansky and Broadfoot, 1977). T h e data from the limb drift in Fig. 4 are dominated by the cold component. Two curves are shown to demonstrate the insensitivity of the model to m o d e r a t e changes in temperature and density. The two sets of parameters are Tc = 275 K with n c = 1 . 5 x 1 0 5 c m -3, and T c = 2 5 0 K with no= 3.0 x l 0 s cm 3. W e have chosen the first set as our standard bright limb cold c o m p o n e n t model, although both sets appear to fit the data at the higher altitudes, away from the region where multiple scattering is important. W e have calculated that below 8 0 0 k i n the effects of multiple scattering b e c o m e significant and the data points are expected to be above the model curve. A second measure of the hot and cold transition region is shown in Fig. 5; a good fit is achieved with the standard model. Figure 6 shows additional data from the disk through the limb to high altitude; the range, pointing uncertainties, and statistics prevent a good match, but the hot and cold characteristics are again demonstrated. O u r model results are compared in Table 1 with similar work done with the M A R I N E R 5 data (Wallace, 1969; Anderson, 1976). Wallace's results are scaled to a reference level of 6305 km to compare with the other two results. An error in Wallace's calculation, an inverted temperature dependence in the scattering cross-section, was pointed out by A n d e r s o n (1976). H o w e v e r , both A n d e r son's and Wallace's methods give the same results in the optically thin region of the atmosphere. An additional m e a s u r e m e n t of the exospheric temperature of Venus was provided by the helium 5 8 4 ~ airglow m e a s u r e m e n t made at the same time as the limb drift in Fig. 4. K u m a r and Broadfoot (1975) reported a temperature of 375 ± 105 K; rework of that data recently shows that the lower limit is preferable. The M A R I N E R 5 and 10 observations had somewhat different geometric characteristics off the

696

P. Z. TAKACS,A. L. BROADFOOT,G. R. SMITH and S. KUMAR

bright limb. T h e M A R I N E R 5 observations were at right angles to the planet-sun line (cone angle of 90°). This is comparable to our data in Fig. 4 out to 7 8 0 0 k i n where the cone angle was 106 ° . T h e higher altitude data in Fig. 4 were taken at cone angles that monotonically increased from 106 ° to 155 ° as illustrated in Fig. 1. T h e observations were concentrated in the 8000 to 9000 km region during this change in cone angle, but no systematic change in intensity was d e t e c t e d as the cone angle increased. W e chose the 9000 k m altitude to compare our observed intensity with that of M A R I N E R 5: Wallace r e p o r t e d a corrected intensity of 2.18 kR, 4.2 times greater than our measured emission rate of 520 R. T h e difference in intensity is consistent with the drop in solar Lyman ct flux of 1.86 between the two missions and the apparent decrease in the hot c o m p o n e n t density of about a factor of 2 as noted in Table 1.

Bright disk Figures 4, 6 and 7 show data taken while the field of view was on the bright disk of the planet. T h e disk data from Fig. 4 were taken while the line of sight was nearly perpendicular to the planet-Sun line. T h e other figures include data from well after encounter, when the line of sight was nearly antisolar. Thus the range of solar zenith angles and viewing angles for the data points a m o n g the figures is considerable. With either geometry, the r e c o r d e d intensities show a disk brightness of about 16.5 kR, with a p r o n o u n c e d limb brightening to 21.5 k R in the close range view. Modeling of the bright disk intensity is beyond the scope of this work. A n d e r s o n (1976) estimates an equivalent m e a n disk intensity of 40 k R for the M A R I N E R 5 observations in order to c o m p a r e with the Moos and R o t t m a n (1971) m e a s u r e m e n t of 27 kR. A l t h o u g h M A R I N E R 5 did not measure the bright disk intensity directly, we may infer the result from A n d e r s o n ' s limb profile m o d e l that extends onto the bright disk. O u r limb drift m e a s u r e m e n t s across the bright disk and limb were m a d e at nearly the

same aspect as that of M A R I N E R 5, so that we may compare that portion of Fig. 4 directly with his model. O u r m e a s u r e m e n t of 1 7 k R at Ro= 5000 km is to be c o m p a r e d with A n d e r s o n ' s m o d e l intensity of 29 k R at that same point. The ratio of these intensities is 1.7, which is nearly identical to the drop in the solar flux at Lyman a between the two m e a s u r e m e n t s of 1.86. This is perhaps as it should be, since the cold c o m p o n e n t temperature and density deduced from each m o d e l are identical. O u r m e a s u r e m e n t of the subsolar intensity from the post-encounter period, Figs. 6 and 7, indicates an intensity of 16.5 k R averaged over the bright disk. Scaling this value up by the limb intensity ratio factor, 1.7, we estimate that M A R I N E R 5 would have seen an average bright disk intensity of about 28 kR. T h e remainder of the 40 k R equivalent m e a n disk intensity is a product of the hot c o m p o n e n t corona around the planet. T h e hot corona during the M A R I N E R 10 encounter period was not as extensive as during the M A R I N E R 5 period, so that we would estimate an equivalent m e a n disk intensity somewhat less than the 23.5 k R that would result from scaling the 16.5 k R disk value by the M A R I N E R 5 ratio. The three equivalent m e a n disk intensity values---our measurement, M o o s and R o t t m a n ' s measurement, and A n d e r son's m o d e l - - a g r e e to within the uncertainty in the solar flux variation.

Dark limb T h e dark limb observations from Fig. 3 are rep r o d u c e d in Fig. 8 on an expanded linear scale with the interplanetary background contribution subtracted. T h e exospheric signal rises above the background at an R o of about 30,000 km, continues to increase toward the limb, and decreases sharply before the geometric limb is encountered. In order to investigate the symmetry in the atm o s p h e r e we c o m p a r e d the dark side observations with predictions of our standard bright limb density and t e m p e r a t u r e in the spherically symmetric model. Careful consideration of the source and

T A B L E 1. VENUS HYDROGEN EXOSPHERE TEMPERATURES AND DENSITIES (EXOBASE REFERENCE LEVEL AT

Day side Hot Cold Hot Cold Hot Cold

1250-4-100 275+50 1020 325 1020± 100 275+50

5:~ 1 (2) 1.5~ 1 (5) 9.2 (2) 1.5 (5) 1.3 (3) 2 ± 1 (5)

Dark disk, terminator

Dark limb 800d:200 150±25 1130

1500±200 150±50

1.0 (3) 2±1 (5)

2.0±0.5 (3) 1.0:v 0.5 (5) 7.1 (2)

6305 km)

Reference This work. Wallace (1969) (Not scaled by 0.725) Anderson (1976)

Hydrogen Lyman alpha emission from the Venus exosphere absorption line profiles was included as states in the previous section. The result is shown in the lower curve in Fig. 8. At high altitude the model prediction is too high; toward the planet it crosses the data and becomes too low. The cutoff at the limb appears to occur too far from the surface in comparison to the sharp cutoff in the data. The quality of the data on the dark side is poor; however, it does have some significant characteristics which cannot be set aside. (1) The observed intensity at 30,000 km is significantly different from the day to night side of the planet. We can be more specific by reference to Fig. 1. When Rp is five Venus radii from the dark side, perpendicular to a solar zenith angle of 144 °, the maximum in the Lyman a source function is well clear of the influence of the planetary shadow. The intensity across that region is compared to the intensity from the front side, perpendicular to a solar zenith angle of about 63 °. The data corrected for interplanetary background emission are shown in Figs. 7 and 8; note the intensity scale change. The intensity on the dark side is lower by a factor of 2 or 3. (2) The second notable feature of the dark side measurements is the two very high data points near the limb. The error bars on these two points are large and the run of the curve represented by the points is quite subjective, but the high intensity is real and the sharp cutoff at the limb is real and has to be satisfied by the model. (3) Finally, the emission rate from the dark side of the planet on the limb is a good statistical measurement although, again, the profile is subject to interpretation. (a) The non-thermal hydrogen component on the dark side. The distribution of source intensity in the dark side measurements could be strongly influenced by the atmospheric screening at the terminator. We note that the maximum in the intensity source function will follow the shadow line in Fig. 1., which is defined by the transmission function at the terminator, until it reaches the intersection with the perpendicular to the line of sight at about 12,000 km. The maximum will follow this perpendicular to greater Rp at 144 ° solar zenith angle. In order to examine the effect of screening on the profile, we replaced the transmission function with a sharp step transition near the exobase, 6300 km, The result is shown by the solid curve in Fig. 8. We find that even without screening we cannot produce enough emission from the standard front side atmosphere to satisfy the observations. We also conclude that the intensity profile is not

697

particularly sensitive to the formulation of the transmission function. We find that the transmission function which generated the lower curve in Fig. 8 could be replaced by a sharp shadow edge at 7800 km without measurable effect on the emission source function. We use this sharp screening function at 7800 km in the remaining model calculations. In order to adjust the model to fit the dark side data at high altitude, we must reduce the apparent temperature. If the high emission rate at the limb is to be provided the density must be elevated. With T~ = 800 K and nH= 2.0 × 103 we can approach the two end conditions. The curve runs lower than one would like in the mid region. However, we have concluded above that the hydrogen distribution is asymmetric and there can be little doubt that the measured profile will be affected by the hydrogen gradient from the front side X ~ 63 ° to the backside position at X = 1440. We believe the single thermal representation and spherical symmetry is no longer valid; the data are not good enough to test an improved model. Another model would be based on the production of non-thermal hydrogen which is beyond the scope of this work. The exospheric characteristics of the non-thermal hydrogen seem to be the following. (1) As a result of the interaction of the nonthermal hydrogen and the atmosphere, the vertical distribution appears to be well represented by a thermal profile of about 1250 K and has reasonable uniformity over the sunward hemisphere at least to zenith angles of 63 °. (2) The exobase density of the non-thermal component increases above the dark hemisphere. (3) The high altitude density of the non-thermal component decreases on the dark side. Although the preparation of an improved model is beyond the scope of this paper, we do note the similarity between the observed characteristics of the non-thermal hydrogen on Venus and those of the exospheres of the moon and Mercury (Shemansky and Broadfoot, 1977). In that work, the exobase is the surface of the planet and the exospheric density distribution appears to be strongly modified by accommodation of the particles with the surface; there is a drop in apparent temperature and an increase in density across the terminator to the dark side. Preparation of a model for Venus where the exobase is gaseous may be more tractable than the lunar and Mercury cases where knowledge of the physics of surface interactions is still lacking. The analogy with the lunar and Mercurian exospheres does allow us to suggest that

698

P . Z . TAKACS, A. L. BROADFOOT, G. R. SMITH and S. KUMAR

t h e s o u r c e of t h e n o n - t h e r m a l c o m p o n e n t is c o n fined to t h e sunlit h e m i s p h e r e . (b) The cold component on the dark side. T h e fit to t h e intensity c u r v e at high altitude does n o t affect o u r i n t e r p r e t a t i o n of t h e d a r k limb o n c e we h a v e p r o v i d e d sufficient intensity in t h e 4 0 0 0 8000 k m region. W e h a v e used a modified TH = 1000 K, n H = 1.1 × 103 c m -3 as t h e far side s o u r c e a n d h a v e used a series of low t e m p e r a t u r e a n d density c o m b i n a t i o n s t o e x a m i n e t h e d a r k limb extinction effect. F o u r curves are s h o w n in Fig. 9. A s a r e f e r e n c e c u r v e no. 1 has n¢ = 0. C u r v e no. 2 h a s t h e f r o n t side t e m p e r a t u r e a n d density, T¢ = 275 K a n d n~ = 1.5 × 105 c m - 3 ; it shows a roll-off w h i c h is n o t s h a r p e n o u g h to satisfy t h e m e a s u r e d intensity m u c h closer to t h e limb. F o r c u r v e no. 3 we d r o p t h e t e m p e r a t u r e to 175 K at t h e s a m e density for c u r v e no. 2. This shows t h e t r e n d of t h e t e m p e r a t u r e a d j u s t m e n t to shift t h e roll-off t o w a r d t h e limb, b u t d o e s n o t c o m e close e n o u g h to t h e a p p a r e n t s t e e p cutoff in t h e data. A f u r t h e r r e d u c tion in t e m p e r a t u r e to 125 K, c u r v e no. 4, shows i m p r o v e m e n t , b u t this t e m p e r a t u r e s e e m s very low in c o m p a r i s o n to t h e p r e d i c t i o n of D i c k i n s o n a n d Ridley (1977).

MARINER 10 VENUS HI 1216 ~. DARK LIMB DRIFT

200 ~Z

I/

150 W

%, t 2~,.\ \

~125 IOO

=E ~_

75

Zhi E n

25 12

I

I

II

IO

FIG, 9. EFFECT O F

I

I

9 8 Rpx 10 ~ I k m )

CHANGING

,

,

MARINER IO VENUS

-r

Ir5

HI ~2~6k

n~ hi

125

¢Y Z

IO0

1~,'. I

~

o

(/) (~

7550 25 0

t

'i ~'x

i

Rp x 103 (kin)

FIG. 10. EFVECT O F

COLD

COMPONENT

DENSITY

ON THE

DARK LIMB MODEL. ALL CURVES USE A HOT COMPONENT OF

T H = 1000 K, n H = 1.1 (3) cm -3. (1) Tc = 125 K, n c = 1.5 (5) cm-3; s a ~ e as in Fig. 9, curve # 4 ; (2) T~ = 175 K, nc = 1.5 (4) cm-3; (3) T c = 125 K, n~= 1.5(6)cm -3. Curves (1) and (2) show range of parameters for reasonable fits to data. Curve (3) shows that even with the lowest temperature, the density cannot be much greater than 1.5 (5) cm -3, otherwise the absorption edge moves away from the limb too far.

/

bJ .-I

tlJ

,

/

~_ 1 7 5

Z 0

200

l

I

7

6

THE TEMPERATURE

5

OF THE

C O L D C O M P O N E N T O N T H E D A R K LIMB.

All models use a hot component with TH = 1000 K, nH = 1.1 (3)cm -3, and a cold density of n~ = 1.5 (5)cm -3, appropriate for the dayside model. (1) Cold component is absent, T , = 0 K , showing maximum possible unattenuated signal at the limb; (2) T~ =275 K, same as dayside; (3) T~ = 176 K; (4) T~ = 125 K.

In Fig. 10 we d e m o n s t r a t e t h e effect of c h a n g i n g t h e density of t h e low t e m p e r a t u r e c o m p o n e n t . W e h a v e r e p r o d u c e d t h e T~ = 125 K a n d nc = 1.5 × 105 c u r v e no. 4 f r o m Fig. 9 as c u r v e 1 in this figure. W e show a s e c o n d similar curve, no. 2, p r o d u c e d by l o w e r i n g t h e density by a factor of 3, n~ = 0.5 × 104, b u t m a i n t a i n i n g t h e h i g h e r t e m p e r a t u r e 175 K. Finally, we show t h a t t h e density at this z e n i t h angle c o u l d n o t b e very m u c h h i g h e r t h a n t h e f r o n t side density w i t h o u t a drastic r e d u c t i o n in t e m p e r a t u r e ; c u r v e no. 3 was p r o d u c e d by Tc = 125 K with an increase in nc to 1.5 × 10 6. T h e d a t a c a n n o t s u p p o r t a significant increase in density o n t h e d a r k side, e v i d e n c e d by t h e m o v e m e n t of t h e m o d e l a b s o r p tion e d g e away f r o m t h e limb. T h e r e are two effects at t h e limb which s h o u l d b e c o n s i d e r e d briefly b e f o r e finishing o u r inspection of t h e d a r k limb profile to show t h a t n e i t h e r could r e d u c e t h e s h a r p n e s s of t h e limb cutoff in t h e d a t a a n d modify o u r results. First we h a v e a s s u m e d a s h a r p cutoff of t h e interstellar m e d i u m at t h e limb; this c o m p r o m i s e s t h e p o i n t at 6 2 5 0 k m , b u t a n y s h a p e applied to t h e t r a n s m i s s i o n of t h e interp l a n e t a r y emission a n d s u b t r a c t e d f r o m t h e data

Hydrogen Lyman alpha emission from the Venus exosphere would have an inverse effect on the planetary emission causing an increase in the steepness of the measured cutoff. The second effect has to do with estimating the contribution to the measured profile due to multiple scattering from the dark side; we conclude below that the effect would be to steepen the measured profile when a correction is made to the data. Therefore, neither of these effects could reduce the sharpness of the limb cutoff and modify our results. The data quality is not good, but it demonstrates characteristics near the dark limb that allow us to put resonable upper limits on the temperature and density of the low temperature or exospheric hydrogen component on the dark side of the planet. The upper limits would be Tc <: 175 K and nc < 1.5 × 105 cm-3; however these limits cannot both be in effect at the same time. The relationship is Tc = 1 5 0 ± 2 5 K with respect to nc = 1.0~ 0.5 × 105 cm-3; note the reciprocal relationship between changes in temperature and density. (c) Scattering from the dark side atmosphere. We have a measurement of Lyman a scattering from the atmosphere on the dark side of the planet. In Figs. 9 and 10 the calculation of the emission from the hydrogen between the shadow and the spacecraft is shown, for R o < 6 0 0 0 km, the difference between the observation and the model intensity is about 7 0 + 1 5 R ; the difference increases as the sunlit terminator is approached. We expect this multiple scattered or albedo component to decay to zero off the limb of the planet, probably with a scale height similar to the cold hydrogen scale height. An albedo component from the atmosphere would tend to reduce the extinction of the hot component emission source from the far side and would confuse the roll-off caused by the low temperature component, but the sharpness at the limb, which is influenced strongly by the density, would not be changed. The effect of emission being multiply scattered from the day side to the night side of the planet was considered by Anderson (1976) with his radiative transfer model. He found that the multiple scattering emission became very small before reaching the dark limb, and attributed the remaining emission to an elevated temperature of the hot component on the dark side. We believe it is significant that this component of the multiple scattering is very low, and we attribute the excess emission we measure to the albedo of the atmosphere illuminated by the hot component outside of the shadow on the dark side. The albedo of the hydrogen exosphere will be high to the hot component emission as long as unit

699

optical depth is well above the pure absorption region in the lower atmosphere. If we consider the cold component at any point on the dark side as lying at the bottom of a glowing hemisphere whose source is the illuminated hot component, we may estimate the irradiance at that point and consider an effective emission rate due to the atmospheric albedo. A point in the dark side atmosphere is illuminated by an intensity distribution that is brightest toward the terminator and diminishes to near zero along the axis of the geometric shadow. The model curve in Fig. 9 indicates that the horizon brightness which would be seen toward the terminator from the limb tangency point is about 200 R. In order to account for the 70 R emission we measure on the dark side, the mean irradiance over the outward hemisphere would need to be 70 R if the albedo at 1216/~ were 100%. This is a reasonable set of requirements to suggest this as the primary source of the back side emission. The back side is also illuminated by the interplanetary Lyman a emission which is of about the same irradiation level as the hot component. However, the emission line is much broader compared to the cold hydrogen exosphere. The photons would penetrate further down into the cold atmosphere where they have much greater probability of being absorbed by CO2 in the lower atmosphere. The contribution to backscattering due to the interplanetary emission is likely to be much smaller than for the hot component source. A complete model of dark side scattering is beyond the scope of this work. We do suggest that detailed consideration of the line widths of the hydrogen emissions and the scatterers is important in modeling the radiative transport on the dark side of the planet. Although we do not have a detailed analysis, we conclude that the signal seen on the planet dark side is primarily due to the cold lower atmosphere backscattering the hot component emission. The consequences of this mechanism predict that the dark side brightness is directly related to the hot component intensity and does not depend significantly on the cold component density distribution. This conclusion is supported by our interpretation of the M A R I N E R 5 observations discussed in the next section.

M A R I N E R 5 dark limb The dark limb measurements on M A R I N E R 5 appear to differ substantially from the M A R I N E R 10 observations. However, we will argue that both sets of measurements are in agreement on the bases

700

P . Z . TAKACS, A. L. BROADFOOT, G. R. SMrrn and S. KUMAR

of our M A R I N E R 10 interpretation. Wallace (1969) was unable to discern any evidence for a limb crossing after removing the m e a s u r e d interplanetary background. This implied a high density on the dark side and was the basis for the interpretation of K u m a r et al. (1978). A smooth curve approximation to the M A R I N E R 5 data is shown as curve " A " in Fig. 11. N o t e also that the intensity at the limb, about 500 R, is much higher than the comparable M A R I N E R 10 intensity in Fig. 8 of abour 150 R. Because the observation geometries between the two spacecraft encounters differ significantly, we used the M A R I N E R 5 trajectory parameters with our dark side m o d e l to generate a synthetic intensity profile which we can c o m p a r e directly to the M A R I N E R 5 data; this synthetic m o d e l is curve " B " in Fig. 11. It is noteworthy that both trajectories intersected the limb behind the terminator at about the same zenith angle; the scattering characteristics at the dark side limb have this as a point of commoality. T h e ratio between the M A R I N E R 5 data and our synthetic model, at an altitude free f r o m secondary scattering effects, 9000 km, is 5.1. This compares favorably with the ratio between the M A R I N E R 5 and 10 observations at 9000 k m off

I000

I

|

I

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.//~'

HI 12,~~

500

,J

MARINER 5 TRAdECTORY . / " WITH MARINERI0 ~f IOOOK/175K MODEL o "~

30o

..c

rr,

z z

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IOC

...."" i D .,." / /.

if)

.~

5G

zh i lr

3o

w p.

a.

Ic 21

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18

I 15

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12

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6

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0

Rp X 10 3 Ikm)

FIG. 11. CO~VmAR[SONOF THE PRESENT MODEL TO THE MARINER 5 DATAFROMWALLACE(1969). (A) MARINER 5 data, Wallace (1969); (B) standard model from this work using the MARINER 5 trajectory parameters; (C) MARINER 5 data scaled down by factor of 5.1; (D) difference between "C" and "B" showing magnitude of backscattered albedo component.

the bright limb, 4.2. T h e fact that the M A R I N E R 5 intensities are larger than M A R I N E R 10 intensities in all directions around Venus is due to the g-value being larger by a factor of 1.8, and to the larger hot c o m p o n e n t density, a factor of about 2, during the M A R I N E R 5 encounter period. In order to follow the dark side analysis of the M A R I N E R 10 data in the previous section, we have scaled down the M A R I N E R 5 observations by a factor of 5.1; it is plotted as curve " C " in Fig. 11. T h e slope at altitudes greater than 7000 km follows the slope of the synthetic m o d e l fairly well. Just on the limb, we estimate the emission due to the planetary albedo to be about 84 R by subtracting curve " B " from curve " C ; " this difference curve is plotted as curve " D . " W e note that the magnitude of the dark side c o m p o n e n t for the M A R I N E R 5 data is close to the 70z~15 R which we measure for the backscattered signal in the M A R I N E R 10 results. If we scale the 85 R up by a factor of 5.1, we are suggesting an additional source on the dark limb of the planet of 430 R in the M A R I N E R 5 data. T h e significant result of this comparison is that by use of a single scaling factor, the observations from M A R I N E R 5 and 10 are in good agreement. W e suggest that the accidental lack of a signature at the dark limb in the M A R I N E R 5 observations p r e v e n t e d Wallace (1969) and A n d e r s o n (1976) from recognizing the presence of an additional emission source which is clearly present in the M A R I N E R 10 observations. Finally, the value of the scaling factor 5.1 is significant as a second confirmation that the primary source of the dark side emission is due to backscattering of the hot c o m p o n e n t radiation. W e have m e n t i o n e d before that A n d e r s o n (1976) shows in his radiative transfer calculations that the multiple scattered emission c o m p o n e n t carried to the back limb in the lower atmosphere is very small. If the dark limb source were controlled by multiple scattering in the lower atmosphere, the increase in intensity would be the same as the g-value difference from M A R I N E R 10 to M A R I N E R 5, an increase of 1.8. W e have shown above that the scaling factor required to m a k e the two sets of m e a s u r e m e n t s compatible is 5.1; that ratio was in reasonable a g r e e m e n t with the increase in the hot c o m p o n e n t intensity between the two flights. This allows us to conclude f r o m the M A R I N E R 5 data also that the primary emission f r o m the planet dark side is related to the hot c o m p o n e n t emission backscattered from the cold hydrogen atmosphere.

Hydrogen Lyman alpha emission from the Venus exosphere 5. C O N C L U S I O N S

(1) With m e a s u r e m e n t s of the hydrogen emission profile above the planet on the sunlit hemisphere for comparison, we have prepared a spherically symmetric exospheric m o d e l which will reproduce the intensity profile reasonably well with a two c o m p o n e n t hydrogen exosphere. An exospheric c o m p o n e n t with T c = 2 7 5 + 5 0 K and n~= 1 . 5 w l . 0 x 1 0 ( 5 ) , and a non-thermal c o m p o n e n t which can be simulated by the exospheric p a r a m e ter T ~ - - 1 2 5 0 ± 1 0 0 K and n ~ = 5 T l x l 0 2 c m 3. (2) W e have found that the disk intensity measured by M o o s and R o t t m a n (1971) and M A R I N E R 10 along with the calculation of A n d e r s o n (1976) for M A R I N E R 5 are reasonably consistent with one another. (3) In preparation of the dark side m o d e l we have concluded that it is important to treat the line shape of the exospheric and hot hydrogen explicitly. (4) T h e spherically symmetric m o d e l used on the sunlit side of the planet is not valid for the dark limb measurement. T h e departures from the model suggest increased density at low levels, < 1 2 , 0 0 0 km, and reduced densities at high levels, > 20,000 kin. (5) The characteristics of the non-thermal hydrogen suggest a source on the sunlit hemisphere with some a c c o m m o d a t i o n of the hot exospheric gas with cooler gas of the exobase on the dark side. D e t a i l e d modeling is required. (6) The hydrogen exosphere, cold component, on the dark side has a lower t e m p e r a t u r e than the sunlit side. In o r d e r to have the sunlit side density, the t e m p e r a t u r e must be < 1 2 5 K. Combinations of t e m p e r a t u r e and density covering the range T,: = 150 ± 25 K with nc = (1.0 z~ 0.5) × 105 cm -3 are consistent with the data. (7) T h e r e is a measurable emission from the dark disk of the planet which is due apparently to back scattering of the irradiation of the atmosphere by the sunlit exosphere. D e t a i l e d modeling is required to substantiate this conclusion. (8) W h e n the source of emission from the dark side of the planet [(7) above] is included in the analysis of the M A R I N E R 5 dark limb observations, the two sets of dark limb measurements, M A R I N E R 5 and M A R I N E R 10, are consistent. Acknowledgment--We acknowledge fruitful discussions with D. M. Hunten, D. E. Shemansky, B. R. Sandel, and L. Wallace.

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The Kitt Peak National Observatory is operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation. This work was supported by the Jet Propulsion Laboratory, California Institute of Technology, under NASA contract NAS 7-100. REFERENCES

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