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Studies of the atmosphere of Venus by means of spacecraft: Solved and unsolved problems
Adv. Space Rex Vol. 29, No. 2, pp. 215-225,2002 B 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved.
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STUDIES OF THE ATMOSPHERE OF VENUS BY MEANS OF SPACECRAFT: SOLVED ANIl UNSOLVED PROBLEMS V. I. Moroz Institute for Space Research of Russian Academy of Sciences, Profsojuznaja 84/32, A4oscow, I I7810, Russia ABSTRACT
Many spacecrafl were used for exploration of the atmosphere of Venus. Their list consists of 25 items, including fly-by missions, orbiters, descent and landing probes and even balloons. VENERA-4 (1967) was near the beginning of this list, providing the first time in situ experiments on other planet. It started a long sequence of successful Soviet Venera missions. However after the year 1985 there were no missions to Venus in Russia. It probably was a strategic error. Now several groups of scientists in other countries work on proposals for new missions to Venus. The goal of this paper is to present a brief review of already solved and still unsolved problems in the studies of the Venus’ atmosphere and to possible future aims in this field. 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved. INTRODUCTION There are two bodies in the Solar System which are most similar to our own: Venus and Mars. From technical point of view they are simplest targets for planetary spacecraft except the Moon. The first attempt at robotic flights to these two planets were made in the early 6Os, a few years after the launch of the first Sputnik. Venera 4 (1967) was the first spacecraft that visited another planet and transmitted to the Earth telemetric signals with results of in situ measurements taken there. Later, for of almost 20 years Venus had the high priority in the Soviet Space program. We had 15 successful venusian spacecraft, and obtained really interesting results, described in a set of professional books and comprehensive reviews (Kuzmin and Marov, 1974; Moroz et al., 1981; Krasnopolsky, 1982; Hunten etal.,1983; Kliore et a1.,1985; Ksanfomality, 1985; Bougher et al.,1997). The most important elements were landing probes, they were unique, having no analogs in NASA missions. It is very pity that this line of exploration was interrupted after 1985. I think that it was a serious strategic error.Now we may see that new proposals for flight to Venus are being made in other countries. Russian experts hardly will be able to participate technically in such new projects in the next few years, but our experience may be still useful. Here I will try to summarize my current understanding of scientific goals for future missions to Venus. Missions of different kinds may be under discussion: a “classic” descent/landing probe (like Venera, C below), a multiple atmospheric probe mission (like Pioneer Venus, M below), balloons (like in the Vega mission or advanced, B below), a single atmospheric probe (like earlier Venera, S below), orbiter (0 below), aircraft or glider(A below), lander or rover with a large lifetime(L), and a sample return mission (SRM). The three last options have no examples in previous programs. Below we will use these designations to say which type of mission may be more suitable for a given sort of study.Discovery of the near infrared emission of the night side of Venus (Allen and Crawford, 1984) showed that there is a possibility of the effective sounding of its lower atmosphere even from the Earth. However there is now renewed interest in the study of Venus by means of spacecraft. This interest is reflected in talks of C3.1 Symposium of 33d COSPAR Scientific Assembly presented by
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Chassefiere (2000), Lavoiser, an example of combined VB+SA types of mission, by Oyama et al. (2000), another example of B, Huntress et al. (2OOO),VAMP,an example of the S type of mission, and Formisano (2000), ISHTAR, an example of 0 type. WHAT IS KNOWN (KEY FACTS)
Main characteristics of the atmosphere and climate of Venus are: a) CO2 and N2 are main constituents of the atmosphere (- 97 and 3 % correspondingly). b) The sharp anomaly in the water abundance: the Ml stock about 10’ times less than on the Earth. c) The overall mass of the atmosphere is - 100 times more than on the Earth and is - 0.0001 from the full mass of the planet; surface pressure is P, - 90 bars . d) The very hot surface, T, - 735K. e) The low effective temperature (T, - 229K), even lower than for the Earth (249K). The very high ratio Ts/Te -3. t) The total cloud cover with very high albedo (- 0,76). g) Super-rotation: the motion of the atmosphere with speed - lOOm/s relative to the solid body of the planet. h) The specific outgoing IR flux near the poles is more than near equator. i) The very cold (-100 K) night upper atmosphere (the cryosphere instead of the thermosphere). So there are large differences between atmospheres of the Venus and Earth in spite of similarity in size, mass and even the input of solar energy. We know these facts from previous studies but still are not able to explaine them. So a general main goal of future studies is to understand why Venus is what it is. It seems that just analysis of the already known facts can not give the answer. New measurements have to be made, and they will be listed and discussed below. In addition some data of the old missions still wait re-assessment, comparisons etc. KEY PROBLEMS
I would select four key problems (or more precisely, groups of problems) for concentration of efforts in future missions to Venus, namely: minor constituents, including the atmosphere/surface exchange, and noble gases, (I) heat transfer, greenhouse effect, (II) (111) dynamics, especially super-rotation, (IV) aerosols, clouds and hazes, their chemical composition at different heights. Possibly upper atmosphere, ionosphere and solar wind/planet interaction could be added but it seems that these subjects were studied more deeply in previous missions, especially due to 14 years measurements by the Pioneer Orbiter. However it I doesn’t mean that there are no more unanswered questions in this field. An important practical question for future missions is the visibility of the surface from a descent probe or balloon. The atmosphere puts significant restrictions on this, so it will also be discussed here. MINOR CONSTITUENTS
Many gases beside CO2 and NZ have been identified in the atmosphere of Venus including water vapor H20, carbon monoxide CO, sulfur dioxide SO2, vapors of muriatic and fluoric acids (HCI and HF), noble gases (Ne, Ar, Kr, Xe). All of them are so called minor constituents with mixing ratio no more than hundredths of a percent. Nevertheless, some of minor constituents play the major role in the atmospheric processes. For example Hz0 and SO2 are strong absorbers of IR radiation, giving substantial inputs to greenhouse effect. Minor constituents participate in chemical conversions, creation of aerosol particles, chemical interaction with surface materials. The most recent data about the chemical composition of the atmosphere of Venus were reviewed by Esposito et al. (1997), Moroz and Zasova (1997), and deBergh et al.( 1998).
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Water vapor Hz0 total abundance and vertical profile attract special attention by several reasons:
(a) the scarcity of water on this planet compared to Earth; (b) the large role in the thermal regime of the planet; Venus would be colder without any water at all; (c)participation in cloud formation; (d) the enrichment of D, by two order of magnitude in contrast with the Earth; (e) a possible interaction with surface material. There were many measurements, by telescopes from the Earth and on the descent probes, Soviet and American. Results were summarized by Ignatiev et al. (1997,1999), Moroz and Zasova (1997), de Bergh et al. (1998). A lot of strong discrepancies are seen in their lists. It seems that the most reliable profile of Hz0 mixing ratio between the surface and main cloud deck (Figure 1) was found by: (1) observations of spectra of night glow emission of Venus (Pollack et al.,l993, Taylor et al.,l997), and (2) Venera 11, 13 and 14 data reassessment (Ignatiev et al.,l997). There is still a significant ambiguity concerning the mixing ratio Venera 11 0 profile below 10 km. Earlier d 60 ‘I-:: interpretation of in situ spectroscopic 4 i 0 ; ;I;:;: ;; measurements indicated a decrease in , .i 50 . .. . ... . . Model A I : the water mixing ratio between the I :’ ----Model B k 0 clouds and surface (Moroz et al., 1979). A 40 a’ This conclusion has been discarded 1 b__ -+ E later by the more refined analysis of the same spectra by Ignatiev et z 30 cl ai(1997). The possibility for an jP 0.1 increase up to 50-70 ppm below 5 km 0 ..= 20 _..* exists (Ignatiev et al.,l997, model A) . i but a constant mixing ratio (model B, 4 ,‘I see Figure 1) also is admitted. lo- j_ D ,. +;...r& . ‘T MC However the total Hz0 content in the I &-J . . . . . . . . _ atmosphere of Venus and in the layer 0 100 80 60 40 20 0 between the surface and height 10 km fHoolPPm depends not very strongly on the choice of mixing ratio profile fitted to the results of in situ spectroscopic is 1.6 Fig. 1. Examples of vertical profiles of the Hz0 mixing ratio and 1.3 g/cm’ for models A and B cor- in the Venus atmosphere. Dotted line is the polynomial respondingly, the content below 10 km approximation of all Venera 12, 13, and 14 data according 1.O and 0.6 g/cm2 for same cases. These the analysis by Ignatiev et al.( 1997), model A. Dashed line: values are not very different from the approximation by a constant mixing ratio of data between 4 of our Earth. Even earliest interpretation and 58 km (same paper, model B). P (solid line) is mixing of of Venera 11 data (Moroz et al., 1979 ratio profile according the analysis of Earth based observati1980) led to approximately the same es- on observations of spectra of NIR night emission of Venus timates of total Hz0 content on Venus . Pollack et al., 1993). MC is obtained by Meadows and It seems that optical methods like those Crisp (1996) from night emission within the 1 pm band. D those applied earlier (analysis of absrp- is profile inferred by Donahue et al. (1997) from Pioneer tion or emission bands are inefficient in Venus mass spectrometer measurements solving the problem of water profile determination below 10 km. The reason is that both methods are characterized by an integration over a long path within the atmosphere. Therefore some kind of local measurements are necessary like mass spectrometry, gas chromatography or active optical spectrometry (like those described by Bertaux et aLl996, for another spectral range). Also there is still an open question why so many local
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measurements (with gas chromatographs and specific chemical sensors) gave mixing ratios 100 times more than presented in Figure 1. Only the Pioneer Venus mass spectrometer provided values not very different from “opticals”@onanue et al., 1997). However, this experiment was not performed properly in the atmosphere of Venus due to contamination of the inlet system by an aerosol droplet. An advanced mass spectrometer is the key element of the Discovery VAMP proposal (Huntress et al., 2OOO),so it would provide possibly new more precise determination of the Hz0 profile. The water vapor abundance within and above the upper clouds has been determined by various remote sensing methods, including high resolution spectra of solar reflected radiation (the 0.82 pm band and some others), outgoing thermal radiation (pure rotational band, ti25 pm) and microwave radiation. Mixing ratios obtained by the first method varied from 1 to 100 ppm. In the near infrared the Venus cloud deck is a multiple scattering medium, almost conservative in the continuum and with a large optical thickness. In this case it is practically impossible to associate any estimates of the mixing ratio with a definite altitude level. So the most promising is the pure rotational band. There are two sets of measurements, one with a filter radiometer (OIR) aboard Pioneer Venus Orbiter , the second with the Fourier spectrometer aboard Venera 15 orbiter . Re-analysis of these measurements was made recently, by Irwin (1997) for OIR, and Ignatiev et aL(l999) for Venera 15. Their estimates of mixing ratios have a systematic difference in order of value: 100-200 ppm (for heights 60-70 km) according Irwin, and about 10 ppm (on heights 55-62 km) according Ignatiev et al. Hardly such a difference could be explained only by natural variations. Joint analysis of both sets of data is desirable. For measurements on a titure orbiter, spectral measurements would be preferable for the qualification of Hz0 content within and above clouds. Other molecular constituents
Oxygen (02) was identified in the lower atmosphere of Venus in two experiments with gas chromatographs (Pioneer Venus, Oyama et a1.,1980, and Venera 13,14, Mukhin et al.,1983). A confirmation is desirable with application of some other method (for example optical spectrometry in the vicinity of the 7600 A band). Molecular hydrogen (HZ) was identified once in the lowest atmosphere, also by means of gas chromatography (Mukhin et al. 1983). It should be checked in the new experiments. Carbon monoxide CO was detected by several methods, its height profile looks well established to a first approximation (Oyama et al.,1980, Mukhin et al.,1983, Pollack et a1.,1993). However latitudinal variations were found later by flyby observations with NIMS-GALILEO (Taylor et al., 1997). More measurements are necessary for CO monitoring, missions of MZ and/or 0 type would be suitable. SulfUr dioxide is the most abundant among the venusian sulf%rcompounds. The vertical profile is approximately known, latitudinal variations near upper boundary of the main cloud deck were carefully studied (Zasova et al., 1993). However there are some discrepancies in quantitative estimates (Bertaux et al.,1996), so new measurements are required. Other sulfbr compounds like SZ, S3, SO, H2S, COS were sought. The only reliable identification was published for COS (Pollack et a1.,1993). Only upper limits are known for nitrogen compounds. There is reliable identification for HCI and HF, and also the HCI vertical profile. There are no direct data at all, not even upper limits, for phosphorus compounds. However there is a suspicion that condensed phosphorus compounds are chemical components (maybe even dominating) of lower clouds (Andreichikov et al.,1987). Summarizing we may say that detection of sulfur, nitrogen, chlorin, and phosphorus compounds should be included in the list of tasks for future probes of different types. Monitoring of the spectra of night glow emissions of HZO, SOZ, COS, CO from an orbiter would be an important source of information on their horizontal variations. Even the single Galileo Venus flyby said something new about this, showing substantive latitudinal CO variations. Possible pecularities
near the surface
Lead, tin, bismuth, zinc, cadmium, sulfur, all elements of the first group of the Mendeleev table are melted on the hot surface of Venus. They are evaporated, combined chemically one with another
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and with other elements. As a result the list of small constituents in the lower layer of atmosphere (below lo-20km) may have some pecularities, for example to include halides of metals (ZnClz, PbC12, SnCl2, . ..) and halcogenids (CuS, PbS, PbO, ...). The possibility of the presence of such compounds has been discussed by Bracket et al. (1994). Condensates of such “high-temperature” minor constituents may appear on the surface in relatively high (and correspondingly more cold) regions. Maybe such condensation layers are responsible for the very low radar reflectivity in such places like Alpha. Also a near-surface haze can be created by condensation of such substances, and part of them can be transferred up to the main cloud deck. The mixing ratio profile of water vapor, sulfur compounds etc in the lower 10 km is also important. Missions of C and L types are most suitable for studies of this region of the atmosphere, but S and B could be used also. Noble gases The abundance and isotopic ratios of noble gases are important because they can be linked with the earlier evolution of the planet. It is known from measurements in old missions that absolute abundance (g/cm2 of non-radio enic noble gases is 30-100 times more than on the Earth. It is % interesting that J 6/Kr84and Ne2 /NeZ2ratios is nearer to solar than terrestrial one, but NeZo/A? ratio is like on the Earth. There are no data on isotopic ratios for krypton and xenon. So measurement of these ratios could be an important task for fbture missions of all types (beside 0, of course). An advanced mass spectrometer (like in the VAMP proposal, Huntress et al., 2000) is the only suitable instrument. Of course, a Venus SRM mission could bring the most conclusive results in this field, but its feasibility is a big question for the forseeable future. HEAT TRANSFER GREENHOUSE EFFECT The temperature of the venusian atmosphere rises from the clouds down to the surface with an average gradient about 8 K/km that is near to adiabatic. However the troposphere of Venus can not be treated as a single huge convective zone. In reality there are two or even three such zones (Seiff et al., 1985; Linkin et al. 1987, see Figure 2). The static stability S<=O for heights 5OO) layer. Its upper boundary is at 30 km. A part of the data shows that S>O on heights lO
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Fig.2. Static stability of the Venus lower atmosphere. (a) Pioneer Venus Sounder and Venera IO,11,12; (b) Pioneer Venus North Probe (Seiff et al., 1985); (c) Vega 2 (1, solid and dotts; Linkin et al., 1987 ) in comparioson with Pioneer Sounder (2; Seiff et al., 1985). DYNAMICS
Substantial part of the atmosphere of Venus (layers between 10 and 85 km) flows constantly westward , in the same retrograde direction as the rotation of the solid body of planet but with much higher speed. As a result, the visible period of the planet rotation (defined Corn motions of UV cloud features) is about 4 terrestrial days, although the real one is 243 days. The most spectacular observations were provided by the flight of Vega 1 and Vega 2 balloons (Sagdeev et al, 1996). This super-rotation is one of the most intriguing attributes of the venusian atmosphere and still did not find a clear explanation. The vertical profile of zonal speed was measured by the Doppler method using radio signals Corn different entry probes beginning with Venera 4. Additional estimates of the retrograde zonal circulation from the cloud top level to 80 km were provided by calculations of the balanced flow from the measured thermal field and the assumption of cyclostrophic balance. A detail information can be found in reviews by Schubert (1983), Kerzhanovich and Limaye (1985), and Gierash et al. (1997). It is interesting that zonal winds of opposite directions were never observed. Such motions must exist (below 10 km?) because the fill time averaged angular momentum of the atmosphere should be equal to zero. Meridional winds were measured only once (by Pioneer probes tracking). Gierash et aL(1997) suggested the precise measurements of the thermal structure and wind below lo-15 km as most important for understanding of the nature of super-rotation. Possibly more observations of meridional winds could be also useful. Measurements of both types should be provided in several places, so mainly M and B type missions would be suitable. The thermal field above the clouds and motions of their W features should be monitored as previously by orbiters to provide more information about the properties of zonal winds at high levels. AEROSOLS The overall vertical structure of the particulate medium in the atmosphere of Venus is shown in Figure 3. A detail summary of data about the aerosol structure and their discussion were given by Esposito et al. (1983, 1997) Ragent et al. (1985), and Moroz et al. (1998). Aerosols play two
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important roles: (a) participation in radiation transfer processes, (b) exchange with gases through sublimation/condensation processes and chemical conversions. Below we will list the points where a serious qualification is desirable. Height, km
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Fig.3. A scheme of the vertical structure of the aerosol medium in the atmosphere of Venus.
(1)Variations of the vertical profile: latitudinal, longitudinal, daily, random. The position of the lower boundary of the clouds and their profile near this boundary that was measured on Venera 8 (which landed near terminator), Vega 1 and Vega 2 ( landed at night), differs substantively from that observed by other probes (all of which landed by day except the Pioneer Venus night probe). The vertical profile above 55 km changes drastically with the latitude, from a very smooth decrease of aerosols density on lower latitudes to the sharply abrupt on higher latitudes and even the formation of a double shallow structure (the polar dipole). Missions of C, S, M types equipped by nephelometers and/or particle size spectrometers are necessary for future measurements. M could be most suitable. An excellent tool for future studies of upper clouds and haze above them would be an 0 type mission equipped with a thermal IR radiometer (like OIR on Pioneer orbiter), thermal IR spectrometer (like FS on Venera 15) and a UV imaging system.
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(2)The chemical comnosition of particles. It looks well established that the venusian upper clouds particles consist of a water solution of sulfuric acid. However, in situ analysis of the particles elemental composition on the Vega 1 and Vega 2 probes showed the presence of chlorine, phosphorus end even iron in addition to sulfur (Andreichikov et al., 1987). Moreover, not sulfur, but phosphorus dominated below 55 km. It would be desirable to measure in future experiments not only the elemental but also the molecular composition of the cloud material. First attempts were made by two other experiments on Vega 1 and Vega 2 , but their results were not conclusive. The next generation of such experiments requires complicated instrumentation like a combination of gas chromatograph and mass spectrometer, with a unit for pyrolytic and/or chemical processing. (3) Size distribution. It depends on height, and even its general shape is not well established. Three modes were found by Pioneer Sounder (Knollenberg and Hunten, 1980), but this was not confirmed by Vega 1 and Vega 2. There is a suspicion that large particles were simply missed in these latest experiments due to their low number density. However even these rare large particles may significantly influence the extinction and mass density. So a check is necessary in future missions, with an instrument providing a real ability to detect large particles at low concentration. The possibility to spend a long enough time in clouds is very important for such studies. (4) The UV absorber. The reflectivity of the Venus clouds declines in the spectral range below about nearly 5000 A. In the near UV the local albedo shows substantial variations in contrast with the visible range (where Venus appears through the telescope as a featureless object). Sulfuric acid’ water solution is absolutely transparent in this range. Consequently some absorbing admixture to sulfuric acid must exist or some of upper clouds’ particles must consist of not sulfuric acid but of some other compound. The nature of UV absorber is a subject of longstanding discussions, but there is still no prevalent solution. Meanwhile, this mysterious substance is very important for the thermal regime and probably for dynamics, providing a large supply of solar energy above 55 km. A spectrophotometer for the 3000-4000 A range together with a chemical analyzer on a C, M, S, A or B type probe could be helpful for identification of the W absorber. The most difficult point by in situ studies of clouds was that previous missions (of C and M types) passed the main cloud deck in a few minutes. So A and B missions would be in principle more effective, but they may encounter more difficulties with a mass restrictions for scientific instruments than conventional C probe. In any case some simple type aerosol experiments (like nephelometer) are desirable for every S or M entry probe. THE ATMOSPHERE AS AN OBSTACLE FOR IMAGING FROM A DESCENT PROBES AND BALLOONS Only four optical images of the surface of Venus are available at present. They were obtained by Venera 9, 10 (1978) and Venera 13,14( 1982) descent probes, all of them after landing. Now we will discuss briefly the possibilities of imaging during the descent before the landing from different heights or from balloon. Let designate the intensity of the radiation received by an individual pixel of a CCD camera oriented to nadir as Bi(z, A). This quantity is the sum of two parts. The first is the surface radiation from its small element (corresponding to given pixel), attenuated by the atmosphere, and the second is the radiation of the atmosphere itself. Only the first part contents the useful information about the surface features. This part is B,=B I (0,4exp(f
- h)lcos(z,)),
(1)
where Bi (0,1) is intensity (=brightness) on the height z=O, to is the total optical thickness of the undercloud atmosphere, t is its optical depth between the lower boundary of clouds and the surface. The atmospheric column between the surface element and camera collects the scattering radiation from all directions. The corresponding part of the intensity is a background that spoils the image. So we may introduce a visibility factor V(z, 12)as the ratio
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V(z,4 = W BI (z,A),
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(2)
and use it as a criterion of the quality of the image obtained through the scattering atmosphere. Here B,(z, A) is the full brightness of radiation coming from nadir direction, including the scattered atmospheric radiation. It depends from t and from the surface albedo averaged on a whole area visible from the height z. The ability to see contrast varations requires even higher visibility ratios. We estimated the visibility factor for different heights at three wavelengths: 0.65, 0,85 and 1.02 urn. All three are outside of CO2 and Hz0 bands. The results are disappointing: if to accept V(z, 1) = 0.1 as a minimal admittable value then surface features imaging would be possible only below - 2 km 1.02um. at 0.65 pm, -4kmat0.85pm,and-10kmat Approximately 10% of the brightness in the 1 pm window is the thermal radiation due to the high (- 735K) temperature of the surface. So observations of the surface not only on the day side but also on the night side of the Venus are possible if the sensitivity of camera is high enough for this. It is interesting that in this case the estimated visibility factor V(z) is much better than on the dayside. Observations will be possible from all heights below -45 km. This possibility looks very attractive. Radiation from the surface of Venus in 1 pm window is detectable even by observations from an orbiter. However it is reflected partly by clouds, and it would be not easy to distinguish surface brightness and clouds optical depth variations. Possibly imaging of the night side in several (two as minimum) windows could solve the problem and give simultaneously information about horizontal variations of the surface brightness temperature and cloud structure. CONCLUSION It seems that a return to study of Venus could be well justified scientifically. Most of the already existing data (although still not all), obtained in old missions has been carefully studied, and a set of goals for new missions is identified. The valuable technical experience obtained earlier is still available. New approaches are forseen, like high temperature electronics. The main problem is a balanced selection between solar system bodies as targets for future scientific missions. At present Mars and small bodies look the most popular candidates. However we think that Venus should be included in the long term plane of space research. REFERENCES Allen, D.A., and J.W. Crawford, Cloud structure on the dark side of Venus, Nature 307, 222 - 224, 1984. Andreichikov, B.M., O.K. Akhmetshin, B.N. Korchuganov, L.M.Mukhin, V.I. Ogorodnikov, I.V.Petrianov, and V.I. Skitovich, X-Ray analysis of the aerosols composition in venusian clouds on Vega 1 and Vega 2 probes, Kosmich. Med., 25,721_736,1987(in Russian). Bertaux J.L., Th. Wiedemann, A.Hauchecorn, V.I.Moroz, and A.P.Ekonomov, Vega-l and Vega-2 entry probes: an investigation of local UV absorption in the atmosphere of Venus, J. Geophys. Res., 101, 12709, 1996. Bougher, S.W., D.M.Hunten, and Phillips, R.J., eds., Venus II, The University of Arizona Press, Tucson, Arizona., 1997. Bracket, R.A., B.Fegley,Jr., and R.E.Arvidson, Volatile transport on Venus and implications for surface geochemstry and geology, preprint, 1994. Chassefiere, E., The Lavoisier mission, a system of descent probe and balloon flotilia for geochemical investigation of the deep atmosphere of Venus, abstract, 33d COSPAR Scientific Assembly, Warsaw, 2000. Crisp, D., and Titov, D.V., The thermal balance of the lower atmosphere of Venus, in Venus II ,eds.S.W.Bougher, D.M.Hunten, and R.J.Phillips, The Univerisity of Arizona Press, Tucson, pp 353-384,1997.
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de Bergh C., Bezard B., Crisp D., Maillard J.-P., Gwen T.,Pollack J. and Grinspoon D, Water in the deep atmosphere of Venus from high-resolution spectra of the night side, Adv. Space Res. 15, (4)79 - (4)88, 1995. de Bergh, C., V. Moroz, F.W. Taylor, D. Crisp, B. Bezard, L.V. Zasova, Venus Interanational Atmosphere: the composition of the atmosphere of Venus below 100 km altitude, absfract, 32d COSPAR Scientific Assembly, Nagoya, 1998. Donahue, T.M., D.H. Greenspoon D.H., R.E. Hatle, and R.R.Hodges,Jr., Ion/neutral escape of hydrogen and deuterium: evolution of water, in Venus II, eds. Bougher, S.W., D.M.Hunten, and Phillips, R.J., Venus II, pp. 385-414, The University of Arizona Press, Tucson, Arizona., 1997. Drossard, P., B. Bezard, Th. Encrenaz, E. Lellouch, M. Roos, F.W. Taylor,A.D. Collard, S.B. Calcutt, J. Pollack, D. Grinspoon, R.W. Carlson, K.H. Baines, and L.W. Kamp, Search for spatial variations of the I-I20 abundance in the lower atmosphere of Venus from NIMS-Galileo, Planet. Space Sci., 41,495 - 504,1993. Esposito, L.W., R.G. Knollenberg, M.Ya. Marov, O.B. Toon, and R.P.Turco, The clouds and hazes of Venus, in Venus, eds. D.M.Hunten, L.Colin, T.M. Donahue, and V.I.Moroz, pp 484-564, The Univerisity of Arizona Press, Tucson, Arizona, 1983. Esposito, L.W., J.L. Bertaux, V. Krasnopolsky, V.I. Moroz, and L.W. Zasova, Chemistry of lower atmosphere and clouds, in Venus II, eds. Bougher, S.W., D.M.Hunten, and Phillips, R.J., Venus II, pp. 4 15-458, The University of Arizona Press, Tucson, Arizona., 1997. Formisano, V., ISHTAR Venus orbiter, abstract, 33d COSPAR Scientific Assembly, Warsaw, 2000. Gierash, P.J., R.M.Goody, R.E.Young, A. Del Genio, R.Greeley, C.B. Leovy, and M. Newman, The general circulation of the atmosphere of Venus: an assessment, in Venus II, eds. Bougher, S.W., D.M.Hunten, and Phillips, R.J., Venus II, pp. 459-500, The University of Arizona Press, Tucson, Arizona., 1997. Hunten, D.M., L. Colin, T.M. Donahue, T.M.,and V.I. Moroz, eds., Venus, The University of Arizona Press, Tucson, Arizona., 1983. Huntress, W.T.,Jr., B.Clark, and the VAMP team, Venus Atmosphere Measurements Probe (VAMP):a Discovery Program proposal, abstract, 33d COSPAR ScienriJc Assembly, Warsaw, 2000. Ignatiev N.I., V.I. Moroz, B.E. Moshkin. A.P. Ekonomov, V.I. Gnedykh,A.V. Grigoriev and I.V. Khatuntsev, Water vapor in the lower atmosphere of Venus: a new analysis of optical spectra measured by entry probes, Planet. Space Sci., 45, 427 ,1997. Ignatiev N.I., V.I. Moroz, L.V. Zasova and I.V. Khatuntsev, Water vapor in the middle atmosphere of Venus: an improved treatment of the Venera 15 IR spectra, Planet. Space Sci., 47, 1061-1075, 1999. Irwin P.G., Temporal and spatial variations in the Venus mesosphere retrieved from Pioneer Venus OIR, Adv. Space Res.,l9 (8) 1169, 1997. Kerzhanovich, V.V., and S.S. Limaye, Circulation of the atmosphere of Venus from the surface to 100 km, Adv. Space Res., 5, 59-83, 1985. Kliore, A.J., V.I.Moroz V.I., and G.M.Keating, eds., The Venus International Reference Atmosphere, Adv. Space Rex, 5,1985. Knollenberg, R.G., and D.M. Hunten, Microphysics of the clouds of Venus: Results of the Pioneer Venus particle size spectrometer experiment, JGeophys. Res., 85, 8039-8058, 1980. Krasnopolsky, V.A. Photochemistry of the atmosphere of the Mars and Venus, Springer Verlag, Berlin/Heidelberg/New York/Tokyo. 1982. Ksanfomality, L.V., The planet Venus, Nauka, Moscow ( in Russian),1985. Kuzmin, A.D. and Marov, M.Ya. Phvsics of the planet Venus, Nauka, Moscow ,1974( in Russian). Linkin, V.M., J. Blamont, S.I. Deviatkin, S.P. Ignatova, V.V. Kerzhanovich, A.N. Lipatov, K. Malik, B.I. Stadnyk, Ya.V. Sanotsky, P.G. Stoliarchuk, A.V. Terterashvili, G.A. Frank, and L.I. Khliustova, Thermal structure of the atmosphere of Venus according results of measurements on the “Vega-Tprobe, Kosmich. issled., 25, 659-672,1987 (in Russian).
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Meadows,V.S., and D.Crisp, Ground-based near-infrared observations of the Venus nightside:the thermal structure and water abundance near the surface, J. Geophys.Res., lOl(#E2),4595-4622, 1996. Moroz, V.I., The atmosphere of Venus, Space Sci. Rev.,29,3-127, 1981. Moroz ,V.I., N.A Parfentiev, and N.F. Sanko, Spectrophotometric experiment aboard the Venera 11 and Venera 12 probes.2.Analysis of spectral data of of the Venera 11 by adding method, Kosmich. issled., 17, 727-7451979 (in Russian). Moroz ,V.I., Yu.M. Golovin., A.P.Ekonomov, B.E. Moshkin., N.A Parfent’ev. and N.F. Sanko, Spectrum of the Venus day sky, Nature 284,242 - 244, 1980. Moroz, V. I., and L. V. Zasova, VIRA-2: A review of inputs for updating the Venus International Atmosphere, AdvS’ce Res.,19,#8, 1191-1201, 1997. Moroz, V.I., B. M. Andreichikov, J. Blamont, L.W. Esposito, V. M. Linkin ‘, B. E. Moshkin, B. Ragent, F. W.Taylor and L.V. Zasova, Particulate matter in the atmosphere of Venus, abstract, 32d COSPAR Scient@c Assembly, Nagoya.,1998. Mukhin, L.M., B.G. Gelman, N.I. Lamonov, V.V. Melnikov, D.F. Nenarokov, B.P. Okhotnikov, V.A. Rotin, V.N. Khokhlov, Gas chromatographic analysis of the chemical composition of the atmosphere of Venus on Venera 13 and Venera 14 descent probes, Kosmich. issled.,21, 225230, 1983 (in Russian). Oyama, V.I., G.L. Carle, F. Woeller, J.B. Pollack, R.T. Reynolds, and R. A. Craig, Pioneer Venus gas chromatography of the lower atmosphere of Venus, J. Geophys. Res., 85, 7881-7903, 1980. Oyama, K.-I., T.Imamura, and T.Abe, Feasibility study for the Venus atmosphere mission, abstract, 33d COSPAR Scientific Assembly, Warsaw, 2000. Pollack J.B., J.B. Dalton, D. Grinspoon, R.B. Wattson, R. Freedman, D. Crisp, D.A. Allen, B. Bezard, C. de Bergh, L. Giver, Q. Ma and R. Tipping, Near-infrared light from Venus’ nightside : a spectroscopic analysis Icarus, 103, 1 , 1993. Ragent, B., L. W. Esposito, M. G. Tomasko, M. Ya. Marov, V. P. Shari, and V. N. Lebedev , Particulate Matter in the Venus Atmosphere, Adv.S’ce Res. 5,#11, 85-116 (1985). Sagdeev, R Z, V.M. Linkin, V.V. Kerzhanovich, A.N. Lipatov, A.A. Shurupov A A, J.E. Blamont ,D. Crisp, A.P. Ingersoll, L.S. Elson, R.A. Preston, C.E. Hildebrand, B.A. Ragent, A. Seif A, R.G. Young, L. Bologh, Yu.N. Alexandrov, N.A. Armand, P.V., Bakitko R V, A.S. Selivanov, Overview of VEGA balloon in situ meteorological measurements, Science 321, 1411-1414, 1986. Seiff A., Schofield J.T., Kliore A.J., Taylor F.W., Limaye S.S., Revercomb H.E., Sromovsky L.A., Kerzhanovich V.V., Moroz V.I. and Marov M.Ya., Models of the structure of the atmosphere of Venus from the surface to 100 kilometers altitude, Adv. Space Res 5, 3 - 58, 1985. Schubert G., General circulation and the dynamical stae of the Venus atmosphere, in Venus. eds. Hunten, D.M., L. Colin, T.M. Donahue, T.M.,and V.I. Moroz, pp. 681-765, University of Arizona Press, Tucson, Arizona., 1983. Taylor, F.W., D.Crisp, and B. Bezard, Near infrared sounding of the lower atmoshere of Venus, in Venus II, eds. Bougher, S.W., D.M.Hunten, and Phillips, R.J., pp. 459-500, The University of Arizona Press, Tucson, Arizona., 1997. Tomasko, M.G. ,The thermal balance of the lower atmosphere of Venus, in Venus, eds. D.M.Hunten, L.Colin, T.M. Donahue, and V.I.Moroz, pp 604-63 l,The Univerisity of Arizona Press, Tucson, Arizona, 1983. Zasova, L. V., V. I. Moroz, L. W. Esposito, and C. Y. Na , SO2 in the middle atmosphere of Venus: IR measurements from VENERA-15 and comparison to UV data, Icarus 105,92-109 (1993).
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STUDIES OF THE ATMOSPHERE OF VENUS BY MEANS OF SPACECRAFT: SOLVED ANIl UNSOLVED PROBLEMS V. I. Moroz Institute for Space Research of Russian Academy of Sciences, Profsojuznaja 84/32, A4oscow, I I7810, Russia ABSTRACT
Many spacecrafl were used for exploration of the atmosphere of Venus. Their list consists of 25 items, including fly-by missions, orbiters, descent and landing probes and even balloons. VENERA-4 (1967) was near the beginning of this list, providing the first time in situ experiments on other planet. It started a long sequence of successful Soviet Venera missions. However after the year 1985 there were no missions to Venus in Russia. It probably was a strategic error. Now several groups of scientists in other countries work on proposals for new missions to Venus. The goal of this paper is to present a brief review of already solved and still unsolved problems in the studies of the Venus’ atmosphere and to possible future aims in this field. 0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved. INTRODUCTION There are two bodies in the Solar System which are most similar to our own: Venus and Mars. From technical point of view they are simplest targets for planetary spacecraft except the Moon. The first attempt at robotic flights to these two planets were made in the early 6Os, a few years after the launch of the first Sputnik. Venera 4 (1967) was the first spacecraft that visited another planet and transmitted to the Earth telemetric signals with results of in situ measurements taken there. Later, for of almost 20 years Venus had the high priority in the Soviet Space program. We had 15 successful venusian spacecraft, and obtained really interesting results, described in a set of professional books and comprehensive reviews (Kuzmin and Marov, 1974; Moroz et al., 1981; Krasnopolsky, 1982; Hunten etal.,1983; Kliore et a1.,1985; Ksanfomality, 1985; Bougher et al.,1997). The most important elements were landing probes, they were unique, having no analogs in NASA missions. It is very pity that this line of exploration was interrupted after 1985. I think that it was a serious strategic error.Now we may see that new proposals for flight to Venus are being made in other countries. Russian experts hardly will be able to participate technically in such new projects in the next few years, but our experience may be still useful. Here I will try to summarize my current understanding of scientific goals for future missions to Venus. Missions of different kinds may be under discussion: a “classic” descent/landing probe (like Venera, C below), a multiple atmospheric probe mission (like Pioneer Venus, M below), balloons (like in the Vega mission or advanced, B below), a single atmospheric probe (like earlier Venera, S below), orbiter (0 below), aircraft or glider(A below), lander or rover with a large lifetime(L), and a sample return mission (SRM). The three last options have no examples in previous programs. Below we will use these designations to say which type of mission may be more suitable for a given sort of study.Discovery of the near infrared emission of the night side of Venus (Allen and Crawford, 1984) showed that there is a possibility of the effective sounding of its lower atmosphere even from the Earth. However there is now renewed interest in the study of Venus by means of spacecraft. This interest is reflected in talks of C3.1 Symposium of 33d COSPAR Scientific Assembly presented by
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Chassefiere (2000), Lavoiser, an example of combined VB+SA types of mission, by Oyama et al. (2000), another example of B, Huntress et al. (2OOO),VAMP,an example of the S type of mission, and Formisano (2000), ISHTAR, an example of 0 type. WHAT IS KNOWN (KEY FACTS)
Main characteristics of the atmosphere and climate of Venus are: a) CO2 and N2 are main constituents of the atmosphere (- 97 and 3 % correspondingly). b) The sharp anomaly in the water abundance: the Ml stock about 10’ times less than on the Earth. c) The overall mass of the atmosphere is - 100 times more than on the Earth and is - 0.0001 from the full mass of the planet; surface pressure is P, - 90 bars . d) The very hot surface, T, - 735K. e) The low effective temperature (T, - 229K), even lower than for the Earth (249K). The very high ratio Ts/Te -3. t) The total cloud cover with very high albedo (- 0,76). g) Super-rotation: the motion of the atmosphere with speed - lOOm/s relative to the solid body of the planet. h) The specific outgoing IR flux near the poles is more than near equator. i) The very cold (-100 K) night upper atmosphere (the cryosphere instead of the thermosphere). So there are large differences between atmospheres of the Venus and Earth in spite of similarity in size, mass and even the input of solar energy. We know these facts from previous studies but still are not able to explaine them. So a general main goal of future studies is to understand why Venus is what it is. It seems that just analysis of the already known facts can not give the answer. New measurements have to be made, and they will be listed and discussed below. In addition some data of the old missions still wait re-assessment, comparisons etc. KEY PROBLEMS
I would select four key problems (or more precisely, groups of problems) for concentration of efforts in future missions to Venus, namely: minor constituents, including the atmosphere/surface exchange, and noble gases, (I) heat transfer, greenhouse effect, (II) (111) dynamics, especially super-rotation, (IV) aerosols, clouds and hazes, their chemical composition at different heights. Possibly upper atmosphere, ionosphere and solar wind/planet interaction could be added but it seems that these subjects were studied more deeply in previous missions, especially due to 14 years measurements by the Pioneer Orbiter. However it I doesn’t mean that there are no more unanswered questions in this field. An important practical question for future missions is the visibility of the surface from a descent probe or balloon. The atmosphere puts significant restrictions on this, so it will also be discussed here. MINOR CONSTITUENTS
Many gases beside CO2 and NZ have been identified in the atmosphere of Venus including water vapor H20, carbon monoxide CO, sulfur dioxide SO2, vapors of muriatic and fluoric acids (HCI and HF), noble gases (Ne, Ar, Kr, Xe). All of them are so called minor constituents with mixing ratio no more than hundredths of a percent. Nevertheless, some of minor constituents play the major role in the atmospheric processes. For example Hz0 and SO2 are strong absorbers of IR radiation, giving substantial inputs to greenhouse effect. Minor constituents participate in chemical conversions, creation of aerosol particles, chemical interaction with surface materials. The most recent data about the chemical composition of the atmosphere of Venus were reviewed by Esposito et al. (1997), Moroz and Zasova (1997), and deBergh et al.( 1998).
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Water vapor Hz0 total abundance and vertical profile attract special attention by several reasons:
(a) the scarcity of water on this planet compared to Earth; (b) the large role in the thermal regime of the planet; Venus would be colder without any water at all; (c)participation in cloud formation; (d) the enrichment of D, by two order of magnitude in contrast with the Earth; (e) a possible interaction with surface material. There were many measurements, by telescopes from the Earth and on the descent probes, Soviet and American. Results were summarized by Ignatiev et al. (1997,1999), Moroz and Zasova (1997), de Bergh et al. (1998). A lot of strong discrepancies are seen in their lists. It seems that the most reliable profile of Hz0 mixing ratio between the surface and main cloud deck (Figure 1) was found by: (1) observations of spectra of night glow emission of Venus (Pollack et al.,l993, Taylor et al.,l997), and (2) Venera 11, 13 and 14 data reassessment (Ignatiev et al.,l997). There is still a significant ambiguity concerning the mixing ratio Venera 11 0 profile below 10 km. Earlier d 60 ‘I-:: interpretation of in situ spectroscopic 4 i 0 ; ;I;:;: ;; measurements indicated a decrease in , .i 50 . .. . ... . . Model A I : the water mixing ratio between the I :’ ----Model B k 0 clouds and surface (Moroz et al., 1979). A 40 a’ This conclusion has been discarded 1 b__ -+ E later by the more refined analysis of the same spectra by Ignatiev et z 30 cl ai(1997). The possibility for an jP 0.1 increase up to 50-70 ppm below 5 km 0 ..= 20 _..* exists (Ignatiev et al.,l997, model A) . i but a constant mixing ratio (model B, 4 ,‘I see Figure 1) also is admitted. lo- j_ D ,. +;...r& . ‘T MC However the total Hz0 content in the I &-J . . . . . . . . _ atmosphere of Venus and in the layer 0 100 80 60 40 20 0 between the surface and height 10 km fHoolPPm depends not very strongly on the choice of mixing ratio profile fitted to the results of in situ spectroscopic is 1.6 Fig. 1. Examples of vertical profiles of the Hz0 mixing ratio and 1.3 g/cm’ for models A and B cor- in the Venus atmosphere. Dotted line is the polynomial respondingly, the content below 10 km approximation of all Venera 12, 13, and 14 data according 1.O and 0.6 g/cm2 for same cases. These the analysis by Ignatiev et al.( 1997), model A. Dashed line: values are not very different from the approximation by a constant mixing ratio of data between 4 of our Earth. Even earliest interpretation and 58 km (same paper, model B). P (solid line) is mixing of of Venera 11 data (Moroz et al., 1979 ratio profile according the analysis of Earth based observati1980) led to approximately the same es- on observations of spectra of NIR night emission of Venus timates of total Hz0 content on Venus . Pollack et al., 1993). MC is obtained by Meadows and It seems that optical methods like those Crisp (1996) from night emission within the 1 pm band. D those applied earlier (analysis of absrp- is profile inferred by Donahue et al. (1997) from Pioneer tion or emission bands are inefficient in Venus mass spectrometer measurements solving the problem of water profile determination below 10 km. The reason is that both methods are characterized by an integration over a long path within the atmosphere. Therefore some kind of local measurements are necessary like mass spectrometry, gas chromatography or active optical spectrometry (like those described by Bertaux et aLl996, for another spectral range). Also there is still an open question why so many local
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measurements (with gas chromatographs and specific chemical sensors) gave mixing ratios 100 times more than presented in Figure 1. Only the Pioneer Venus mass spectrometer provided values not very different from “opticals”@onanue et al., 1997). However, this experiment was not performed properly in the atmosphere of Venus due to contamination of the inlet system by an aerosol droplet. An advanced mass spectrometer is the key element of the Discovery VAMP proposal (Huntress et al., 2OOO),so it would provide possibly new more precise determination of the Hz0 profile. The water vapor abundance within and above the upper clouds has been determined by various remote sensing methods, including high resolution spectra of solar reflected radiation (the 0.82 pm band and some others), outgoing thermal radiation (pure rotational band, ti25 pm) and microwave radiation. Mixing ratios obtained by the first method varied from 1 to 100 ppm. In the near infrared the Venus cloud deck is a multiple scattering medium, almost conservative in the continuum and with a large optical thickness. In this case it is practically impossible to associate any estimates of the mixing ratio with a definite altitude level. So the most promising is the pure rotational band. There are two sets of measurements, one with a filter radiometer (OIR) aboard Pioneer Venus Orbiter , the second with the Fourier spectrometer aboard Venera 15 orbiter . Re-analysis of these measurements was made recently, by Irwin (1997) for OIR, and Ignatiev et aL(l999) for Venera 15. Their estimates of mixing ratios have a systematic difference in order of value: 100-200 ppm (for heights 60-70 km) according Irwin, and about 10 ppm (on heights 55-62 km) according Ignatiev et al. Hardly such a difference could be explained only by natural variations. Joint analysis of both sets of data is desirable. For measurements on a titure orbiter, spectral measurements would be preferable for the qualification of Hz0 content within and above clouds. Other molecular constituents
Oxygen (02) was identified in the lower atmosphere of Venus in two experiments with gas chromatographs (Pioneer Venus, Oyama et a1.,1980, and Venera 13,14, Mukhin et al.,1983). A confirmation is desirable with application of some other method (for example optical spectrometry in the vicinity of the 7600 A band). Molecular hydrogen (HZ) was identified once in the lowest atmosphere, also by means of gas chromatography (Mukhin et al. 1983). It should be checked in the new experiments. Carbon monoxide CO was detected by several methods, its height profile looks well established to a first approximation (Oyama et al.,1980, Mukhin et al.,1983, Pollack et a1.,1993). However latitudinal variations were found later by flyby observations with NIMS-GALILEO (Taylor et al., 1997). More measurements are necessary for CO monitoring, missions of MZ and/or 0 type would be suitable. SulfUr dioxide is the most abundant among the venusian sulf%rcompounds. The vertical profile is approximately known, latitudinal variations near upper boundary of the main cloud deck were carefully studied (Zasova et al., 1993). However there are some discrepancies in quantitative estimates (Bertaux et al.,1996), so new measurements are required. Other sulfbr compounds like SZ, S3, SO, H2S, COS were sought. The only reliable identification was published for COS (Pollack et a1.,1993). Only upper limits are known for nitrogen compounds. There is reliable identification for HCI and HF, and also the HCI vertical profile. There are no direct data at all, not even upper limits, for phosphorus compounds. However there is a suspicion that condensed phosphorus compounds are chemical components (maybe even dominating) of lower clouds (Andreichikov et al.,1987). Summarizing we may say that detection of sulfur, nitrogen, chlorin, and phosphorus compounds should be included in the list of tasks for future probes of different types. Monitoring of the spectra of night glow emissions of HZO, SOZ, COS, CO from an orbiter would be an important source of information on their horizontal variations. Even the single Galileo Venus flyby said something new about this, showing substantive latitudinal CO variations. Possible pecularities
near the surface
Lead, tin, bismuth, zinc, cadmium, sulfur, all elements of the first group of the Mendeleev table are melted on the hot surface of Venus. They are evaporated, combined chemically one with another
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and with other elements. As a result the list of small constituents in the lower layer of atmosphere (below lo-20km) may have some pecularities, for example to include halides of metals (ZnClz, PbC12, SnCl2, . ..) and halcogenids (CuS, PbS, PbO, ...). The possibility of the presence of such compounds has been discussed by Bracket et al. (1994). Condensates of such “high-temperature” minor constituents may appear on the surface in relatively high (and correspondingly more cold) regions. Maybe such condensation layers are responsible for the very low radar reflectivity in such places like Alpha. Also a near-surface haze can be created by condensation of such substances, and part of them can be transferred up to the main cloud deck. The mixing ratio profile of water vapor, sulfur compounds etc in the lower 10 km is also important. Missions of C and L types are most suitable for studies of this region of the atmosphere, but S and B could be used also. Noble gases The abundance and isotopic ratios of noble gases are important because they can be linked with the earlier evolution of the planet. It is known from measurements in old missions that absolute abundance (g/cm2 of non-radio enic noble gases is 30-100 times more than on the Earth. It is % interesting that J 6/Kr84and Ne2 /NeZ2ratios is nearer to solar than terrestrial one, but NeZo/A? ratio is like on the Earth. There are no data on isotopic ratios for krypton and xenon. So measurement of these ratios could be an important task for fbture missions of all types (beside 0, of course). An advanced mass spectrometer (like in the VAMP proposal, Huntress et al., 2000) is the only suitable instrument. Of course, a Venus SRM mission could bring the most conclusive results in this field, but its feasibility is a big question for the forseeable future. HEAT TRANSFER GREENHOUSE EFFECT The temperature of the venusian atmosphere rises from the clouds down to the surface with an average gradient about 8 K/km that is near to adiabatic. However the troposphere of Venus can not be treated as a single huge convective zone. In reality there are two or even three such zones (Seiff et al., 1985; Linkin et al. 1987, see Figure 2). The static stability S<=O for heights 5O
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NORTtI
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I@)
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_/VENERA
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6 7 8 9 10 dT/dr - I‘, K/km
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Fig.2. Static stability of the Venus lower atmosphere. (a) Pioneer Venus Sounder and Venera IO,11,12; (b) Pioneer Venus North Probe (Seiff et al., 1985); (c) Vega 2 (1, solid and dotts; Linkin et al., 1987 ) in comparioson with Pioneer Sounder (2; Seiff et al., 1985). DYNAMICS
Substantial part of the atmosphere of Venus (layers between 10 and 85 km) flows constantly westward , in the same retrograde direction as the rotation of the solid body of planet but with much higher speed. As a result, the visible period of the planet rotation (defined Corn motions of UV cloud features) is about 4 terrestrial days, although the real one is 243 days. The most spectacular observations were provided by the flight of Vega 1 and Vega 2 balloons (Sagdeev et al, 1996). This super-rotation is one of the most intriguing attributes of the venusian atmosphere and still did not find a clear explanation. The vertical profile of zonal speed was measured by the Doppler method using radio signals Corn different entry probes beginning with Venera 4. Additional estimates of the retrograde zonal circulation from the cloud top level to 80 km were provided by calculations of the balanced flow from the measured thermal field and the assumption of cyclostrophic balance. A detail information can be found in reviews by Schubert (1983), Kerzhanovich and Limaye (1985), and Gierash et al. (1997). It is interesting that zonal winds of opposite directions were never observed. Such motions must exist (below 10 km?) because the fill time averaged angular momentum of the atmosphere should be equal to zero. Meridional winds were measured only once (by Pioneer probes tracking). Gierash et aL(1997) suggested the precise measurements of the thermal structure and wind below lo-15 km as most important for understanding of the nature of super-rotation. Possibly more observations of meridional winds could be also useful. Measurements of both types should be provided in several places, so mainly M and B type missions would be suitable. The thermal field above the clouds and motions of their W features should be monitored as previously by orbiters to provide more information about the properties of zonal winds at high levels. AEROSOLS The overall vertical structure of the particulate medium in the atmosphere of Venus is shown in Figure 3. A detail summary of data about the aerosol structure and their discussion were given by Esposito et al. (1983, 1997) Ragent et al. (1985), and Moroz et al. (1998). Aerosols play two
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important roles: (a) participation in radiation transfer processes, (b) exchange with gases through sublimation/condensation processes and chemical conversions. Below we will list the points where a serious qualification is desirable. Height, km
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3vm
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20
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0.01
1 0.1 Extinction coefficient, u, km-’
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Fig.3. A scheme of the vertical structure of the aerosol medium in the atmosphere of Venus.
(1)Variations of the vertical profile: latitudinal, longitudinal, daily, random. The position of the lower boundary of the clouds and their profile near this boundary that was measured on Venera 8 (which landed near terminator), Vega 1 and Vega 2 ( landed at night), differs substantively from that observed by other probes (all of which landed by day except the Pioneer Venus night probe). The vertical profile above 55 km changes drastically with the latitude, from a very smooth decrease of aerosols density on lower latitudes to the sharply abrupt on higher latitudes and even the formation of a double shallow structure (the polar dipole). Missions of C, S, M types equipped by nephelometers and/or particle size spectrometers are necessary for future measurements. M could be most suitable. An excellent tool for future studies of upper clouds and haze above them would be an 0 type mission equipped with a thermal IR radiometer (like OIR on Pioneer orbiter), thermal IR spectrometer (like FS on Venera 15) and a UV imaging system.
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(2)The chemical comnosition of particles. It looks well established that the venusian upper clouds particles consist of a water solution of sulfuric acid. However, in situ analysis of the particles elemental composition on the Vega 1 and Vega 2 probes showed the presence of chlorine, phosphorus end even iron in addition to sulfur (Andreichikov et al., 1987). Moreover, not sulfur, but phosphorus dominated below 55 km. It would be desirable to measure in future experiments not only the elemental but also the molecular composition of the cloud material. First attempts were made by two other experiments on Vega 1 and Vega 2 , but their results were not conclusive. The next generation of such experiments requires complicated instrumentation like a combination of gas chromatograph and mass spectrometer, with a unit for pyrolytic and/or chemical processing. (3) Size distribution. It depends on height, and even its general shape is not well established. Three modes were found by Pioneer Sounder (Knollenberg and Hunten, 1980), but this was not confirmed by Vega 1 and Vega 2. There is a suspicion that large particles were simply missed in these latest experiments due to their low number density. However even these rare large particles may significantly influence the extinction and mass density. So a check is necessary in future missions, with an instrument providing a real ability to detect large particles at low concentration. The possibility to spend a long enough time in clouds is very important for such studies. (4) The UV absorber. The reflectivity of the Venus clouds declines in the spectral range below about nearly 5000 A. In the near UV the local albedo shows substantial variations in contrast with the visible range (where Venus appears through the telescope as a featureless object). Sulfuric acid’ water solution is absolutely transparent in this range. Consequently some absorbing admixture to sulfuric acid must exist or some of upper clouds’ particles must consist of not sulfuric acid but of some other compound. The nature of UV absorber is a subject of longstanding discussions, but there is still no prevalent solution. Meanwhile, this mysterious substance is very important for the thermal regime and probably for dynamics, providing a large supply of solar energy above 55 km. A spectrophotometer for the 3000-4000 A range together with a chemical analyzer on a C, M, S, A or B type probe could be helpful for identification of the W absorber. The most difficult point by in situ studies of clouds was that previous missions (of C and M types) passed the main cloud deck in a few minutes. So A and B missions would be in principle more effective, but they may encounter more difficulties with a mass restrictions for scientific instruments than conventional C probe. In any case some simple type aerosol experiments (like nephelometer) are desirable for every S or M entry probe. THE ATMOSPHERE AS AN OBSTACLE FOR IMAGING FROM A DESCENT PROBES AND BALLOONS Only four optical images of the surface of Venus are available at present. They were obtained by Venera 9, 10 (1978) and Venera 13,14( 1982) descent probes, all of them after landing. Now we will discuss briefly the possibilities of imaging during the descent before the landing from different heights or from balloon. Let designate the intensity of the radiation received by an individual pixel of a CCD camera oriented to nadir as Bi(z, A). This quantity is the sum of two parts. The first is the surface radiation from its small element (corresponding to given pixel), attenuated by the atmosphere, and the second is the radiation of the atmosphere itself. Only the first part contents the useful information about the surface features. This part is B,=B I (0,4exp(f
- h)lcos(z,)),
(1)
where Bi (0,1) is intensity (=brightness) on the height z=O, to is the total optical thickness of the undercloud atmosphere, t is its optical depth between the lower boundary of clouds and the surface. The atmospheric column between the surface element and camera collects the scattering radiation from all directions. The corresponding part of the intensity is a background that spoils the image. So we may introduce a visibility factor V(z, 12)as the ratio
Atmosphere of Venus
V(z,4 = W BI (z,A),
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(2)
and use it as a criterion of the quality of the image obtained through the scattering atmosphere. Here B,(z, A) is the full brightness of radiation coming from nadir direction, including the scattered atmospheric radiation. It depends from t and from the surface albedo averaged on a whole area visible from the height z. The ability to see contrast varations requires even higher visibility ratios. We estimated the visibility factor for different heights at three wavelengths: 0.65, 0,85 and 1.02 urn. All three are outside of CO2 and Hz0 bands. The results are disappointing: if to accept V(z, 1) = 0.1 as a minimal admittable value then surface features imaging would be possible only below - 2 km 1.02um. at 0.65 pm, -4kmat0.85pm,and-10kmat Approximately 10% of the brightness in the 1 pm window is the thermal radiation due to the high (- 735K) temperature of the surface. So observations of the surface not only on the day side but also on the night side of the Venus are possible if the sensitivity of camera is high enough for this. It is interesting that in this case the estimated visibility factor V(z) is much better than on the dayside. Observations will be possible from all heights below -45 km. This possibility looks very attractive. Radiation from the surface of Venus in 1 pm window is detectable even by observations from an orbiter. However it is reflected partly by clouds, and it would be not easy to distinguish surface brightness and clouds optical depth variations. Possibly imaging of the night side in several (two as minimum) windows could solve the problem and give simultaneously information about horizontal variations of the surface brightness temperature and cloud structure. CONCLUSION It seems that a return to study of Venus could be well justified scientifically. Most of the already existing data (although still not all), obtained in old missions has been carefully studied, and a set of goals for new missions is identified. The valuable technical experience obtained earlier is still available. New approaches are forseen, like high temperature electronics. The main problem is a balanced selection between solar system bodies as targets for future scientific missions. At present Mars and small bodies look the most popular candidates. However we think that Venus should be included in the long term plane of space research. REFERENCES Allen, D.A., and J.W. Crawford, Cloud structure on the dark side of Venus, Nature 307, 222 - 224, 1984. Andreichikov, B.M., O.K. Akhmetshin, B.N. Korchuganov, L.M.Mukhin, V.I. Ogorodnikov, I.V.Petrianov, and V.I. Skitovich, X-Ray analysis of the aerosols composition in venusian clouds on Vega 1 and Vega 2 probes, Kosmich. Med., 25,721_736,1987(in Russian). Bertaux J.L., Th. Wiedemann, A.Hauchecorn, V.I.Moroz, and A.P.Ekonomov, Vega-l and Vega-2 entry probes: an investigation of local UV absorption in the atmosphere of Venus, J. Geophys. Res., 101, 12709, 1996. Bougher, S.W., D.M.Hunten, and Phillips, R.J., eds., Venus II, The University of Arizona Press, Tucson, Arizona., 1997. Bracket, R.A., B.Fegley,Jr., and R.E.Arvidson, Volatile transport on Venus and implications for surface geochemstry and geology, preprint, 1994. Chassefiere, E., The Lavoisier mission, a system of descent probe and balloon flotilia for geochemical investigation of the deep atmosphere of Venus, abstract, 33d COSPAR Scientific Assembly, Warsaw, 2000. Crisp, D., and Titov, D.V., The thermal balance of the lower atmosphere of Venus, in Venus II ,eds.S.W.Bougher, D.M.Hunten, and R.J.Phillips, The Univerisity of Arizona Press, Tucson, pp 353-384,1997.
224
V. I. Moroz
de Bergh C., Bezard B., Crisp D., Maillard J.-P., Gwen T.,Pollack J. and Grinspoon D, Water in the deep atmosphere of Venus from high-resolution spectra of the night side, Adv. Space Res. 15, (4)79 - (4)88, 1995. de Bergh, C., V. Moroz, F.W. Taylor, D. Crisp, B. Bezard, L.V. Zasova, Venus Interanational Atmosphere: the composition of the atmosphere of Venus below 100 km altitude, absfract, 32d COSPAR Scientific Assembly, Nagoya, 1998. Donahue, T.M., D.H. Greenspoon D.H., R.E. Hatle, and R.R.Hodges,Jr., Ion/neutral escape of hydrogen and deuterium: evolution of water, in Venus II, eds. Bougher, S.W., D.M.Hunten, and Phillips, R.J., Venus II, pp. 385-414, The University of Arizona Press, Tucson, Arizona., 1997. Drossard, P., B. Bezard, Th. Encrenaz, E. Lellouch, M. Roos, F.W. Taylor,A.D. Collard, S.B. Calcutt, J. Pollack, D. Grinspoon, R.W. Carlson, K.H. Baines, and L.W. Kamp, Search for spatial variations of the I-I20 abundance in the lower atmosphere of Venus from NIMS-Galileo, Planet. Space Sci., 41,495 - 504,1993. Esposito, L.W., R.G. Knollenberg, M.Ya. Marov, O.B. Toon, and R.P.Turco, The clouds and hazes of Venus, in Venus, eds. D.M.Hunten, L.Colin, T.M. Donahue, and V.I.Moroz, pp 484-564, The Univerisity of Arizona Press, Tucson, Arizona, 1983. Esposito, L.W., J.L. Bertaux, V. Krasnopolsky, V.I. Moroz, and L.W. Zasova, Chemistry of lower atmosphere and clouds, in Venus II, eds. Bougher, S.W., D.M.Hunten, and Phillips, R.J., Venus II, pp. 4 15-458, The University of Arizona Press, Tucson, Arizona., 1997. Formisano, V., ISHTAR Venus orbiter, abstract, 33d COSPAR Scientific Assembly, Warsaw, 2000. Gierash, P.J., R.M.Goody, R.E.Young, A. Del Genio, R.Greeley, C.B. Leovy, and M. Newman, The general circulation of the atmosphere of Venus: an assessment, in Venus II, eds. Bougher, S.W., D.M.Hunten, and Phillips, R.J., Venus II, pp. 459-500, The University of Arizona Press, Tucson, Arizona., 1997. Hunten, D.M., L. Colin, T.M. Donahue, T.M.,and V.I. Moroz, eds., Venus, The University of Arizona Press, Tucson, Arizona., 1983. Huntress, W.T.,Jr., B.Clark, and the VAMP team, Venus Atmosphere Measurements Probe (VAMP):a Discovery Program proposal, abstract, 33d COSPAR ScienriJc Assembly, Warsaw, 2000. Ignatiev N.I., V.I. Moroz, B.E. Moshkin. A.P. Ekonomov, V.I. Gnedykh,A.V. Grigoriev and I.V. Khatuntsev, Water vapor in the lower atmosphere of Venus: a new analysis of optical spectra measured by entry probes, Planet. Space Sci., 45, 427 ,1997. Ignatiev N.I., V.I. Moroz, L.V. Zasova and I.V. Khatuntsev, Water vapor in the middle atmosphere of Venus: an improved treatment of the Venera 15 IR spectra, Planet. Space Sci., 47, 1061-1075, 1999. Irwin P.G., Temporal and spatial variations in the Venus mesosphere retrieved from Pioneer Venus OIR, Adv. Space Res.,l9 (8) 1169, 1997. Kerzhanovich, V.V., and S.S. Limaye, Circulation of the atmosphere of Venus from the surface to 100 km, Adv. Space Res., 5, 59-83, 1985. Kliore, A.J., V.I.Moroz V.I., and G.M.Keating, eds., The Venus International Reference Atmosphere, Adv. Space Rex, 5,1985. Knollenberg, R.G., and D.M. Hunten, Microphysics of the clouds of Venus: Results of the Pioneer Venus particle size spectrometer experiment, JGeophys. Res., 85, 8039-8058, 1980. Krasnopolsky, V.A. Photochemistry of the atmosphere of the Mars and Venus, Springer Verlag, Berlin/Heidelberg/New York/Tokyo. 1982. Ksanfomality, L.V., The planet Venus, Nauka, Moscow ( in Russian),1985. Kuzmin, A.D. and Marov, M.Ya. Phvsics of the planet Venus, Nauka, Moscow ,1974( in Russian). Linkin, V.M., J. Blamont, S.I. Deviatkin, S.P. Ignatova, V.V. Kerzhanovich, A.N. Lipatov, K. Malik, B.I. Stadnyk, Ya.V. Sanotsky, P.G. Stoliarchuk, A.V. Terterashvili, G.A. Frank, and L.I. Khliustova, Thermal structure of the atmosphere of Venus according results of measurements on the “Vega-Tprobe, Kosmich. issled., 25, 659-672,1987 (in Russian).
Atmosphere of Venus
225
Meadows,V.S., and D.Crisp, Ground-based near-infrared observations of the Venus nightside:the thermal structure and water abundance near the surface, J. Geophys.Res., lOl(#E2),4595-4622, 1996. Moroz, V.I., The atmosphere of Venus, Space Sci. Rev.,29,3-127, 1981. Moroz ,V.I., N.A Parfentiev, and N.F. Sanko, Spectrophotometric experiment aboard the Venera 11 and Venera 12 probes.2.Analysis of spectral data of of the Venera 11 by adding method, Kosmich. issled., 17, 727-7451979 (in Russian). Moroz ,V.I., Yu.M. Golovin., A.P.Ekonomov, B.E. Moshkin., N.A Parfent’ev. and N.F. Sanko, Spectrum of the Venus day sky, Nature 284,242 - 244, 1980. Moroz, V. I., and L. V. Zasova, VIRA-2: A review of inputs for updating the Venus International Atmosphere, AdvS’ce Res.,19,#8, 1191-1201, 1997. Moroz, V.I., B. M. Andreichikov, J. Blamont, L.W. Esposito, V. M. Linkin ‘, B. E. Moshkin, B. Ragent, F. W.Taylor and L.V. Zasova, Particulate matter in the atmosphere of Venus, abstract, 32d COSPAR Scient@c Assembly, Nagoya.,1998. Mukhin, L.M., B.G. Gelman, N.I. Lamonov, V.V. Melnikov, D.F. Nenarokov, B.P. Okhotnikov, V.A. Rotin, V.N. Khokhlov, Gas chromatographic analysis of the chemical composition of the atmosphere of Venus on Venera 13 and Venera 14 descent probes, Kosmich. issled.,21, 225230, 1983 (in Russian). Oyama, V.I., G.L. Carle, F. Woeller, J.B. Pollack, R.T. Reynolds, and R. A. Craig, Pioneer Venus gas chromatography of the lower atmosphere of Venus, J. Geophys. Res., 85, 7881-7903, 1980. Oyama, K.-I., T.Imamura, and T.Abe, Feasibility study for the Venus atmosphere mission, abstract, 33d COSPAR Scientific Assembly, Warsaw, 2000. Pollack J.B., J.B. Dalton, D. Grinspoon, R.B. Wattson, R. Freedman, D. Crisp, D.A. Allen, B. Bezard, C. de Bergh, L. Giver, Q. Ma and R. Tipping, Near-infrared light from Venus’ nightside : a spectroscopic analysis Icarus, 103, 1 , 1993. Ragent, B., L. W. Esposito, M. G. Tomasko, M. Ya. Marov, V. P. Shari, and V. N. Lebedev , Particulate Matter in the Venus Atmosphere, Adv.S’ce Res. 5,#11, 85-116 (1985). Sagdeev, R Z, V.M. Linkin, V.V. Kerzhanovich, A.N. Lipatov, A.A. Shurupov A A, J.E. Blamont ,D. Crisp, A.P. Ingersoll, L.S. Elson, R.A. Preston, C.E. Hildebrand, B.A. Ragent, A. Seif A, R.G. Young, L. Bologh, Yu.N. Alexandrov, N.A. Armand, P.V., Bakitko R V, A.S. Selivanov, Overview of VEGA balloon in situ meteorological measurements, Science 321, 1411-1414, 1986. Seiff A., Schofield J.T., Kliore A.J., Taylor F.W., Limaye S.S., Revercomb H.E., Sromovsky L.A., Kerzhanovich V.V., Moroz V.I. and Marov M.Ya., Models of the structure of the atmosphere of Venus from the surface to 100 kilometers altitude, Adv. Space Res 5, 3 - 58, 1985. Schubert G., General circulation and the dynamical stae of the Venus atmosphere, in Venus. eds. Hunten, D.M., L. Colin, T.M. Donahue, T.M.,and V.I. Moroz, pp. 681-765, University of Arizona Press, Tucson, Arizona., 1983. Taylor, F.W., D.Crisp, and B. Bezard, Near infrared sounding of the lower atmoshere of Venus, in Venus II, eds. Bougher, S.W., D.M.Hunten, and Phillips, R.J., pp. 459-500, The University of Arizona Press, Tucson, Arizona., 1997. Tomasko, M.G. ,The thermal balance of the lower atmosphere of Venus, in Venus, eds. D.M.Hunten, L.Colin, T.M. Donahue, and V.I.Moroz, pp 604-63 l,The Univerisity of Arizona Press, Tucson, Arizona, 1983. Zasova, L. V., V. I. Moroz, L. W. Esposito, and C. Y. Na , SO2 in the middle atmosphere of Venus: IR measurements from VENERA-15 and comparison to UV data, Icarus 105,92-109 (1993).