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Chapter 3 UPPERATMOSPHERE PLASMA AND INTERACTIONS INTRODUCTION Observational data on the upper atmosphere of Venus are lim- ited but have already produced promise of exciting results in future missions. Mariner 5 obtained a profile of electron density on the days ide provided evidence for a very interest- , ing interaction between the solar wind and the planetary iono- sphere, and identified an extensive region of ionization on the nights ide of the planet (the plasma tail). An ultraviolet airglow experiment on Mariner 5 detected a halo of Lyman-a emission around Venus. The emission showed an unexpected pro- file, suggesting that two distinct neutral species are involved in scattering the solar radiation. The simplest model of the ultraviolet dayglow requires large concentrations of deuterium, as well as hydrogen, in the outer atmosphere and stimulated a number of theoretical dis- cussions of :planetary evolution. A question of great impor- tance concerns the fate of water on VenUs: Was Venus at some distant epoch covered by an. ocean, and if so, what happened to the water? Alternatively, was Venus formed \vith much less water than was the earth? The isotopic ratio of hydrogen to deuterium should clarify these questions, and only upper- atmospheric measurements, where the fractional abundance of light elements is high, seem likely to provide the answers. An isotopic analysis of hydrogen compounds in the lower atmo- sphere is more difficult and does not appear feasible with mass spectroscopy in the present state of the art. The interaction of the planet with the solar wind is an intriguing phenomenon in own right. The 1968 report (PZaneta:r>y ExpZo:t'ation 1968-1975) identified studies relevant to the understanding of the origin and evolution of the solar system as a prime goal in the planetary program. It should be emphasized that the upper atmospheres of Venus and Mars and their modes of interaction with the solar \vind are similar, but they are very differen.t from those of the earth. The solar wind can modify atmospheric composition in a variety of 17
18 ways: it provides a source of hydrogen comparable to the pres- ent evaporation r~te, gases ionized near the limb of the plan- et will be carried away by the solar wind, and the apparent absence of nitrogen on Mars may be a tonsequence of solar-wind scavenging. The stability of carbon dioxide atmospheres on both Venus and Mars is poorly understood. Upper-atmospheric measurements are essential if the recombination process is to be established. Theoretical models for the primitive terrestrial atmosphere can be refined onZy if present conditions on Mars and Venus are understood. THERMAL STRUCTURE OF NEUTRAL ATMOSPHERE At present, observational data relating to the thermal struc- ture of the upper atmosphere of Venus are extremely limited. They consist of a single profile of Lyman-a emission and a single dayside electron density profile provided by Mariner 5. The temperature information in these data is indirect. The airglow measurements suggest an exospheric temperature of ei- ther 350 K or 700 K, with the larger value appropriate if thermospheric concentrations of deuterium are comparable with those of hydrogen. The smaller value could be achieved if the upper atmosphere of Venus contained large amounts of H2. De- duction of temperatures from the ionospheric data requires certain assumptions with regard to the composition of the neu- tral atmosphere and involves a detailed ionospheric theory which, although plausible, remains relatively untested. The simplest interpretation of the ionospheric data favors the higher value for exospherictemperature mentioned above. The lower value. cannot, however, be categorically excluded. Detailed radiative equilibrium studies of the upper at- mosphere of Venus give exospheric temperatures in the range 500-700 K and are consistent therefore with the observational data. However, analogous studies of radiative equilibrium have been carried out for Mars, and in this case the agree- ment with observation is less convincing. The simplest inter- pretation of Mariner 4 data suggests that radiative calcula- tions overestimate temperatures by perhaps a factor of 2. The apparent discrepancy for Mars has led some workers to question the agreement obtained for Venus. On the other hand, the dif- ficulties encountered for Mars may simply reflect the impor-
19 tance of heat-transfer m~chanisms such as eddy transport and dissipation of tidal waves, at present poorly understood and inadequately treated by theoretical models. Resolution of the conflict is clearly desirable. The in- ference of large deuterium concentrations for Venus, and de- ductions relating to the evolutionary history of the planet, assumes that the exospheric temperature is known and large (~700 K). The exospheric temperature plays an important role in the determinatipn of the evaporation rate .of atmospheric gases. It also determines the extent of the upper atmosphere and influences the details of the interaction of the planet with the solar wind. Reliable observational data on thermal structure would clarify matters not only for Venus but also for Mars. These data could be obtained from vertical distri- butions of neutral species, measured by mass spectroscopy in the appro~imate height range 150-300 km. COMPOSITION OF NEUTRAL ATMOSPHERE All measurements related to the composition of the atmospheres of Venus and'Mars indicate one important and astonishing fact: the atmospheres are practically pure CO2. On the other hand, CO2 is only a minor constituent in the present atmosphere of the earth in which oxygen is present in the form of Â°2. These gross differences can be understood, at least quali- tatively, if one postulates the absence of liquid water for Venus and Mars. Carbon on earth is locked in the crust mostly as CaC03, but in the absence of water on Mars and Venus and at the high temperature of Venus carbon freely enters the atmo- sphere as CO2' Another important question is raised by the persistence of molecular CO2 on Venus (and Mats) into the upper atmosphere. Although solar ultraviolet radiation dissociates CO2 into CO and 0, above the cloud tops o.f Venus the concentration of CO is only about 2 parts in 105 and that of Â°2 is less than about 10 parts in 105. The ultraviolet .airglow e~periments aboard the Venera probes indicate that the atomic oxygen density at 300 kIn in the upper atmosphere late in Venus's night is only 10-5 times the density at the same altitude on earth. For the atmosphere of Mars, similar conclusions can be drawn from the ionospheric chemistry, and resonance fluorescence measurements carried out on Mariners 6 and 7 give very low values for the
20 o density. The CO2 atmospheres of Venus and Mars are aston- ishingly stable despite the low probability of recombination of ground state o and CO. To account for the absence of 02 and CO in the lower at- mosphere, several possibilities exist. For example, in the presence of water photochemistry, CO and Â°2 are efficiently recombined by OH and H in a catalytic cycle; but the process does not account for the low concentration of atomic oxygen above 100 km. The only scheme proposed to date in this alti- tude regime is speculative and invokes formation. of an unstable radical C03by a reaction in which the metastable oxygen atom is produced by the primary dissociation event. Recombination is achieved in a subsequent reaction of C03 and CO. The C03 scheme may also be important in. the lower atmosphere, particu- larlyif the concentrations of hydrogen compounds are as low as current models suggest. The primary measurements needed to clarify CO2 photochem- istry appear to be determinations of the composition of the atmosphere, particularly below 200 km. Most interesting would be an observation of C03' A further interesting compositional question concerns the interpretation of Lyman-a observations performed by Mariner 5. The deuterium hypothesis suggests an over-all enhancement of the D/H ratio in the atmosphere of Venus as compared with earth. As mentioned earlier, if this enhancement were con- firmed, it would have profound implications for models of planetary evolution. By analogy with earth we should expect that Venus over geologic time should have released an amount of water equivalent to a surface pressure of approximately 270 atm. Such a large concentration of H2O is not observed at the present epoch. ~an we explain the loss of a whole ocean of water on Venus? A model of a "runaway greenhouse" has been proposed in which an ocean might evaporate rapidly when the solar radia- tion is as strong as it is at Venus. Water vapor would be dissociated by sunlight, and H atoms would escape thermally from the upper atmosphere. Oxygen released in this manner could react with crustal materials. In a primitive atmosphere these processes could be very rapid, and it appears that they would indeed lead to a large depletion of water. After most of the water was gone, the atmosphere would begin to resemble the present one, with CO2 as the major constituent and a simi- lar exospheric temperature. At this point in the argument we see the possibility of a relevant observable parameter, because in the present Venus
21 atmosphere deuterium escapes slowly and light hydrogen perhaps a thousand times more readily. Continued escape should lead to a great enrichment of deuterium in the remaining water vapor. Some confirming evidence already exists from the Lyman-a mea- surements made by Mariner 5. The anomalous distribution of this radiation around Venus is most readily explained by a large relative abundance of deuterium. It is important to obtain an unambiguous confirmation of this inference and as much further information as possible on the vertical distributions. The latter can be compared with computed effects of a r.apid upward flow to give a check on the escape rates. Measurements of the D/H ratio could in principle be made from a probe in the lower atmosphere) where hydrogen is com- bined in molecular forms. But measurement of atoms in the upper atmosphere may be considerably easier) especially because several independent methods exist. Dayglow of Lyman-a gives the sum of both isotopes, but resolution can be obtained by use of absorption cells containing one or the other isotope. An undeveloped but promising method is to induce resonance scat- tering by a hydrogen or deuterium lamp aboard the spacecraft. A neutral mass spectrometer has the capability to make the measurement) and it should therefore be designed with this in mind. The abundance of helium is also an interesting parameter. If the rate of emission of radiogenic helium (4He) from the crust of Venus is similar to that of the earth, the flow should be of the order of 2 x 106 cm-2 see-I. This is probably more than is accreted from the solar wind if, as suggested by the Mariner 5 observations, the solar wind is diverted around the planet. Other sources of helium appear to be negligible. The escape of helium from the top of the atmosphere by thermal evaporation is probably negligible. Thus the total atmospheric content should be essentially the total production since the planet was formed 4 x 109 years ago, namely, about 2 x 1023 atoms cm-2. This is about 1000 times greater than the helium content of the terrestrial atmosphere, and it is sufficient to permitHe+ ions to play a.n important role in the formation of the topside ionosphere. Because helium is likely to playa major role in the formation of the topside iono- sphere and plasma tail, and because the total abundance might contain information concerning the formation of the planet itself, observations of neutral and ionized helium are of prime importance. Minor neutral constituents should be measured throughout the upper atmosphere with a neutral mass spectrometer. If
22 the earth's atmosphere 1%... valid guide, the atmosphere of Venus should contain about 1% N2, 0.01% Ar, and even less Ne. The inert gases are especially interesting because their dis- tributionsprovide a good measure of structure of the at- mosphere, und;i.storted chemical . Measurements of the relative abundance a function altitude of any two chemically inert gases yie1ciimportant information on the de- gree of turbulent mixing the :Lower atmosphere and on the location of the turbopause. It is necessary to measure at least the relative concentrations of two inert constituents at two altitudes, one in lower atmosphere below the turbo- pause and one in the upper atmosphere where molecular diffu- sion is dominant. Then, because it can be assumed that the relative abundance of two constituents is uniform below the turbopause, it is possible to estimate the altitude of the turbopause from considerations of diffusive separation in the upper atmosphere. 1t1s advantageous to perform the measure- ment.s on gases of widely differing mass (e.g., He and Ar), and if the measurements can be made at more than two altitudes, more detailed information on the nature of the turbulence can be obtained. Isotopic abundances can provide important clues concern- ing the origin of the atmosphere and the possibility of the extent to which there .is a primeval component. Isotopes of Ne, Ar, Kr, and Xe are of inte.rest in this respect. Argon-40, which is of radiogenic origin, is likely to be the most abun- dant of these. isotopes and thus relatively easy to dete.ct; its abundan.ce should yield information on the .constitution of the outer crust of the planet. THE IONO~PHERE Mariner 5 dete.cted ele.ctron of Venus. A peak a well-developed ionosphere .concentrat.ion on the da~side of about 5 x 10 cm-3 was observed .at 140 km, and the dayside ionosphere was sur- prisingly extensive; the electron density is large (~104 cm-3) and approximately constant in the altitude range 300-500 km. The density decreases rapidly above 500 kID, and this sharp .cutoffin the dayside ionosphere presumably represents a tran- sition from planetary to interplanetary plasma. The transition has been variously termed the ionopause or anemopause (wind pause).
23 An ionosphere was detected also on the nightside of Venus. Here the peak electron concentration is approximately 104 cm-3 at about 140 km. The nightside ionosphere is also extensive, with densities of order 102-103 out to heights of approxi- mately 3500 km. Theoretical models seem to a satisfactory description for the lower levels of the dayside ionosphere. Both the peak density and the scale height near the peak are satisfactorily described by photochemical equilibrium, if the major neutral gas is CO2. The identity of the major positive ion is, how- ever, unknown. Published models favor COZ+. On the other hand, even trace amounts of 0 can alter this picture, because the reaction C02+ +0 -+ Â°2+ + CO is rapid, and in this case Â°2+ would dominate. Formation of an Â°2+ ionosphere has several important con- sequences. Molecular hydrogen is destroyed by the reaction C02+ +H2 -+ C02~ + H and not affected by collisions with Â°2+' If Â°2+ dominates, the possibility of large concentra- tions of HZ in the outer atmosphere is enhanced, and the evi- dence for D is correspondingly weakened.. A measurement of ion composition is evidently very important. Additional e1ectron- density profiles, measured over a range of zenith angles and solar-activity conditions, would provide valuable checks on contemporary theoretical models. Measurements of the ionic and neutral constituents of the lower ionosphere region can provide information on a pos- sible D-region formed by galactic cosmic radiation. The mea- surementsshould also provide some hints regarding the exis- tenceof attaching species. Present understanding of the dayside ionosphere above 250 km is minimal. Several important questions arise: How does the ionopause remain in equilibrium? Are there temporal variations of the shape and characteristics of the ionopause and topside ionosphere? What are the main characteristics of the plasma in the topside ionosphere (especially the tempera- ture~ composition, and bulk motion), and where is it produced? Two distinct classes of model have been invoked to ex- plain the observations: one assumes that there is a magnetic field in the ionosphere (either of planetary origin or inter- planetary origin), and the other assumes that there is none. In the first case the equilibrium of the ionopause is deter- mined by a balance of the solar-wind pressure on the outside with the combined pressure of the magnetic field and plasma in the topside ionosphere. There is essentially no bulk mo- tion of the ionosphericplasma,which is producedin situ by
24 photoionization of light atmospheric gases (ll,D, He). The plasma temperature is maintained at about 4000 K by suppres- sion of heat conduction caused by the presence of the magnetic field. In the second case, where there is no magnetic field, the plasma is not constrained and can flow around the planet to the nightside. In this way it seems possible to produce the "tail" of plasma having a density of about 103 cm-3, observed on the downstream side of Venus by Mariner 5. Once again the mean temperature of the plasma must be about 4000 K, but it is conceivable that there is a substantial contribution to the pressure from photoelectrons, which can escape into the tail instead of being absorbed locally. Also, if these ions are produced by charge-exchange at low altitudes involving H, D, and He atoms, they can achieve substantial energies as they are ejected upwards by the vertical electric field which must exist in the lower ionosphere. The presence of substantial quantities of 0 atoms in the upper atmosphere could constitute a difficulty for this model as far as n+ and D+ ions are con- cerned, because these are lost rapidly by charge-exchange yielding 0+. He+ ions can be lost by charge-exchange with molecules, and there is also a question concerning any model involving He+ in that the helium abundance is unknown. If these problems are to be resolved, it is evident that we need observations of the bulk motion of the plasma in the topside ionosphere and measurements of its composition, den- sity, and temperature, both on the sunlit hemispheres of the planet and in ,the tail. In addition, we require measurements of the magnetic field in the ionosphere and of the abundances of H, D, and He in the neutral atmosphere. It seems difficult to suggest appropriate sources of thermal energy and ionization in the upper atmosphere on the mightside. The most plausible source seems to be transport from the days ide ionosphere, whose temperature and density at about 500 km elevation are higher than required on the nightside. Alternative suggestions seem unlikely. Capture of the solar wind requires cooling and an increase of density by a factor of the order of 100; it seems more likely that the wake. would diffuse out if transport across the interface is not strongly inhibited. Any possible observations of plasma flow and of magnetic field connectivity with either the denser atmosphere or the interplanetary region (which might be made by simultaneous measurements of magnetic-field direction and the anisotropies of energetic electrons) would help to resolve these questions. Observations of the strength, direction, and
25 fluctuations in the magnetic field and of fluctuating electric fields on orbiters that in time survey an extensive region of the wake would be valuable. Interactions across the interface between the wake and the solar wind, whether by viscosity, hy- drodynamic instabilities, or magnetic effects, are likely to be of great interest but seem unlikely to be the source of the nights ide ionosphere. Maintenance of a peak concentration of 104 electrons cm-3 on the nights ide also poses problems, especially in view of the long duration of Venus's night (~lO7 see). It seems un- likely that the observed ionization peak can be maintained by direct nighttime ion production. A more likely source for this ionization may be lateral transport of light (atomic) ions (H+, D+, He+) from the dayside. On the nightside these light ions can charge-exchange with the neutral molecular species (CO2)' The CO2+ ions formed in this manner would diffuse downward. A peak would be formed at an altitude where the divergence of the downward flux is balanced by the chemical loss rate caused by dissociative recombination. This mecha- nism seems to be able to provide a plausible explanation for the observed peak of the nightside ionosphere. There are, however, other possibilities, including scattered ultraviolet radiation from the days ide, photoelectrons from the days ide, leakage of solar-wind plasma into the tail, and cosmic radia- tion. The precise mechanism could be clarified by in situ measurement of ion composition and temperature and by direct measurement of possible ionization sources. An additional phenomenon, of great interest on the night- side, is the mysterious ashen light reported by many visual observers. Its reality has not been established spectroscopi- cally at the telescope, in part because of seeing difficulties, in part because of unavailability of a low-scattered light telescope. The phenomenon should be investigated, probably with airglow instrumentation on an orbiting vehicle. Reports that the intensity of ashen light varies with solar activity raise the interesting suggestion that it is an auroral phenomenon. INTERACTION WITH SOLAR WIND Almost all current ideas on .interplanetary particles and fields in the vicinity of Venus are based on the Mariner 5 and, to
26 some extent, Venera 4 observations. Although it seems likely that the main features described below are basically correct, it would be desirable to check the observations, to study the variability with time, and to fill in more detail. There are hints in these data of surprising features that need to be confirmed by further study. The Mariner 5 magnetometer and plasma observations showed that Venus produced a bow shock in the solar wind at the posi- tion that would be appropriate if the wind flow could not pene- trate a surface in the ionosphere about 500 km above the sur- face on the sunward side. This surface was located much more precisely by the Mariner 5 dual-frequency radio occultation da~a, which 10 cm-3 abrupt drop in electron density from at this 10 cm-3 to gave an over a distance of less than 100 km about elevation over the sunward side, clearly a transition from ionospheric to solar-wind conditions. Except for over-all scale, the observations of Mariner 5 and of Venera 4 show no difference between the bow shocks at earth and Venus and no essential difference in magnetic and plasma conditions out- side and just inside the bow shock. Well inside the shock, not too far from the presumed wake behind Venus, there are indications in the Mariner 5 data that the magnetic and plasma conditions may be different from those in the corresponding position near the earth. The magnetic field is stronger, the field direction more nearly toward the limb of Venus, and the plasma density and velocity lower than expected. A variety of explanations can be advanced. A tran- sient change in solar-wind conditions that came and went at just the right times could explain everything. A lateral ex- pansion into a cavity behind Venus might explain part, but not all, of these observations, but more recent observations near the moon where a similar cavity is known to exist do not par- ticularly support this model. Alternatively, the magnetic field lines of the solar wind may be draped over the front side, moving slowly near the stagnation point but streaming in the wind, parallel to the shadow cylinder in the rear. In the shadow and wake region on the side away from the sun, the observations are even less complete. Venera 4 en- tered this region and experienced substantial magnetic tr~n- sients near the boundary of the optical shadow. It showed that the field inside the shadow is small, probably less than 10 gamma, but its spacecraft field is sufficiently uncertain that there might be essentially no field in this region, there might be a remnant of solar-wind field, or there might be a small field whose source currents are inside Venus. The
27 Mariner 5 dual-frequency radio occultation data show a remark- able increase in electron content along the line from Mariner 5 to the earth just when this line passed through the wake re- gion. The most natural explanation is that above the iono- sphere the wake is occupied by plasma with a density of about 103 electrons cm-3 and 103 protons cm-3. If this had a tem- perature of the order of 1000 K, it would have a reasonable scale height, and along the roughly cylindrical boundary be- tween the wake a~d the solar wind there would be an approxi- mate pressure balance between the magnetic pressure of the field observed just outside the wake and kinetic gas pressure of the presumablY stagnant gas inside. It must be emphasized that this model represents a large structure erected on the basis of a very small amount of somewhat uncertain data. A probe measurement of the magnetic and plasma properties over the elevation range from 6000 to 200 km would determine which model was appropriate. One such probe on either the bright or the dark side, but preferably one on each, would provide in- valuable information for the design of all later missions. An orbiter that carried a magnetometer, a plasma probe, a dual- frequency occultation experiment, and a simple 40-keV electron detector would represent a large advance. Observations for 120 days, i.e., for half or more of a Venus year, with a peri- apsis of L 2 'radii of Venus and an apoapsis of 2 to 5 radii of Venus should establish the character of the wake, give con- siderable insight into the mechanism of the anemopause both in the stagnation region and along the wake, and establish the position of the bow. shock and the solar-wind environment of Venus. Such knowledge is essential for a sound understanding of the ionosphere and upper atmosphere of Venus. It is pos- sible that a study of the responses to major changes in the interplanetary field would yield information on the extent to which the interplanetary magnetic field diffuses into the ionosphere, atmosphere, and possibly even the surface of Venus. It would be highly desirable if the plasma probe could make measurements in the high-density, low-temperature wake. It would be interesting to know the extent to which Venus has a tail resembling that of a comet and to see if there is any reason other than the gravitational field that makes the ef- fect of the solar wind on Venus differ from that on a comet. It would be desirable to observe ions, atoms, and molecules heavier than helium both in and just outside of the wake.
28 INTERPLANETARY MEDIUM AND ENERGETIC PARTICLES Plasma probes and magnetometers will be required on many of the Planetary Explorers in order to study the extreme upper atmosphere of Venus and its interaction with the solar wind. These instruments are also capable of making valuable obser- vations in the solar wind both in the immediate vicinity of Venus and during cruise. Probe measurements during the day before encounter and orbiter measurements outside the bow shock are also part of the primary mission because they provide the only available data on the solar-wind environment of Venus at the time planetary observations are made. In addJ..tion,mea- surements made in the solar wind are valuable for their own sake and should be regarded as secondary objectives that sig- nificantly enhance the value of the mission. Comparison of such observations with those made elsewhere in the solar sys- tem will provide much needed information on the large-scale structure of the solar wind. If no other solar-wind observa- tions are being made at this time, it \<lill be even more impor- tant for those made on the Planetary Explorer to be as con- tinuous as possible. Comparison with observations made 11 years earlier whether at 1 A.U. or between 0.7 and 1.4 A.U. will help to indicate the extent to which there is an identi- fiable II-year cycle in solar-wind characteristics. We emphasize the desirability of operating, near earth and during cruise, those instruments that are capable of pro- ducing useful information. They include plasma detectors, magnetometers, and Lyman-a. detectors. There is no evidence for the presence of radiation belts near Venus, and none are expected. However, by analogy with measurements near the earth's bow shock, moderately energetic electrons (10-102 keV) should be produced occasionally at the bow shock of Venus. These particles can be detected \<lith a thin-window Geiger-MUller counter. Electrons in this energy range are also emitted by the sun following solar flares; be- cause absorption will cause the flux in any magnetic field line that enters the atmosphere to become anisotropic, it should be possible to obtain information concerning the struc- ture of the magnetic field around Venus from observations of the streaming direction of these particles. Although observations of galactic and solar cosmic rays near the orbit of Venus are of considerable interest, there is much less interest in observations in the vicinity of the planet (e.g., from an orbiter), because interactions of the
29 cosmic rays with the planet will distort the cosmic-ray dis- tribution. As we noted earlier, cosmic rays (and also solar and galactic x rays) must produce an extensive lower iono- sphere somewhat akin to the terrestrial C and D regions.