Originally published in:
Venus and Mars: Atmospheres, Ionospheres and Solar Wind Interactions. (edited by J.G. Luhmann, M. Tatrallyay and R.O. Pepin) 225-236, American Geophysical Union, Washington D.C., 1992.
Despite these differences, Venus is Earth-like in many ways. It is not much closer to the sun than is the Earth, 0.72 Astronomical Units (AU) versus the Earth's 1.0 AU. Moreover, the high albedo of the Venus clouds, 0.8, reflects much more of the sunlight than does the average albedo of the Earth, 0.3. The net result is that less solar energy enters the lower atmosphere of Venus than the terrestrial atmosphere. The paradoxical high surface temperature of Venus occurs because unlike the Earth, the surface of Venus cannot radiate into space. The radiating layer, which emits the infrared radiation to balance the solar flux, occurs high in the atmosphere. Since temperature rises with decreasing altitude (and increasing pressure) in a planetary atmosphere, the surface becomes much, much warmer than on Earth. This is the so-called greenhouse effect.
The efficiency of this effect on Venus, in which just a fraction of the solar energy received by the Earth can cause such high temperature, is of great interest to terrestrial scientists who find the Earth to be slowly warming and "greenhouse" gases to be increasing too.
Venus is similar to the Earth in size and mass and therefore in density. Its similar density suggests similar composition. The radius of Venus is 6052 km versus the terrestrial 6371 km, 95% of the Earth's radius. The volume of Venus is 86% of that of the Earth and its mass 80% of the terrestrial mass. While this may reflect similar internal composition, such internal similarities are not reflected in the atmosphere which is 96.5% CO2 and 3.5% N2 compared with the 78% N2 and 21% O2 composition of the atmosphere of the Earth. However, as mentioned above, this difference may have much to do with the lack of water on Venus.
While Venus may not be the evil twin sister of the Earth, it certainly is a paradoxical twin. These paradoxes have attracted the interest of scientists since the beginning of the space age when Venus became a target of both the US and USSR space programs. The Soviet program consisted of over a dozen successful Venera missions from 1965 to 1983, followed by the VEGA balloon missions. The American program consisted of Mariners 2, 5, and 10 which flew by Venus in 1962, 1967 and 1974, and the Pioneer Venus spacecraft which reached Venus in December 1978. It is the purpose of this paper to discuss the Pioneer Venus mission, principally the orbiter mission and some of its scientific highlights. In particular, we stress those results which pertain to the comparison of the atmospheres, ionospheres and solar wind interactions of Venus and Mars. In the sections below we discuss first the missions themselves and then the science return.
The orbiter instruments were selected in 1974. Table 3 lists these instruments and the responsible investigators. Table 4 lists the mechanical, electrical and telemetry specifications for the orbiter. The structure of the orbiter and the bus were kept identical to save costs. The orbiter was launched first on May 20, 1978 on a "type II" trajectory which first went beyond 1 AU and then fell into 0.72 AU. The probe mission followed on August 8 on a "type I" trajectory which was more direct. The orbiter and probe missions arrived at Venus, on December 5 and 9 respectively. All six spacecraft (including the probes) operated successfully. The orbiter in fact continues to transmit data at this writing over 12 years later.
The orbiter mission was designed around the 243 Earth day sidereal period of Venus, for in that length of time, the slowly rotating (in a retrograde sense) planet would completely pass under the spacecraft and be mapped by its radar altimeter. In order to map the planet and obtain in situ atmospheric data, the altitude of periapsis was to be maintained in the range of about 150 to 170 km. This required the expenditure of hydrazine propellant. Sufficient hydrazine was available for the prime mission but after almost 600 days, the propellant supply was nearly exhausted and solar gravitational effects began to raise periapsis. Periapsis rose until about 1986 after which it began to fall as shown in Figure 1. In 1992, the spacecraft will enter the atmosphere and be lost after the slight amount of remaining fuel is exhausted. One important unfortunate consequence of this long term rise and fall of the periapsis altitude is that no in situ atmospheric data were obtained from Pioneer Venus at solar minimum. As illustrated in Figure 2, the initial data were obtained during a period of high solar activity. The entry phase is expected to occur at a more moderate level of solar activity, but unfortunately no low altitude in situ data will have been obtained under solar minimum conditions.
The orbit selected was nearly polar, with an inclination of 1050 and a periapsis latitude of close to 170 N. The near polar orbit facilitated radar mapping. The orbit period was selected to be 24 hours for operational purposes. This orbit period maintained the same daily schedule for spacecraft operations. The principal change in the orbit over the mission, other than the change in the altitude of periapsis, was a change in the latitude of periapsis from north to south. This is illustrated in Figure 3.
The Pioneer Venus Orbiter has lasted now well past its prime mission, in fact, over 18 times its prime mission at this writing. This period has not passed without some anxious moments but the operations personnel of the NASA Ames Research Center, its contractors, and the Deep Space Network personnel have always prevailed. Nevertheless, the story of the Pioneer Venus Orbiter has been one of continually declining reserves. Above we mentioned that the gas supply for altitude maneuvering is now nearly depleted. In Figure 4 we show the declining current supplied by the solar cells. Presently the available power is not sufficient to keep all needed equipment working all of the time, and power sharing and battery usage have become necessary. The objective of the operations personnel has become to husband resources in such a way that they all run out simultaneously in late 1992 as the orbiter enters the atmosphere.
Surface Processes. The Pioneer Venus Orbiter was the first spacecraft to use radar to obtain a global image of the Venus surface. Figure 5 shows the surface topography resulting from these data. There are small continental areas, large plains, and mountains which appear to be volcanic in origin. The appearance of apparent volcanos together with the inferences based on the expected lifetime of the sulfuric acid clouds leads one to the paradigm of an active, young surface for Venus in contrast to the older surfaces seen on the Moon, Mercury, and Mars. The altimetry data obtained from the radar provide a more accurate figure of the planet, and more accurate absolute heights of mountain ranges than are available from either the Moon, Mercury or Mars, whose surfaces have been imaged stereoscopically. Global altimetry is the most accurate way to obtain this important planetary information.
The ultraviolet spectrometer on the orbiter also provided some important evidence for an active surface. The Fegley et al.  inferences indicate volcanism on the million year time scale, but the UVS data indicate that volcanism is occurring now. Figure 6 shows the decay of the SO2 haze layer above the clouds from the injection of PVO into orbit to the present [L. W. Esposito, personal communication, 1990]. This is not a solar cycle effect because these data cover more than a full solar cycle and are not seen to return to earlier levels. There was clearly a large injection of SO2 high into the upper atmosphere of Venus in about 1978, and a subsequent decay of the gas. A large volcanic eruption is the only known terrestrial analog of such a phenomenon.
Atmospheric Processes. The Pioneer Venus Orbiter provided many constraints on the circulation and structure of the lower atmosphere through its infrared investigation [Taylor et al., 1980]. However two of the perhaps most surprising results were obtained from the ultraviolet spectrometer and the plasma wave instrument. As illustrated in Figure 7, atomic oxygen emissions at 1304 Angstrom (130.4 nm) are seen periodically [Phillips et al., 1986]. These emissions appear to be the Venus analog of terrestrial aurora despite the fact that Venus has no detectable intrinsic magnetic field [Russell et al., 1980]. The source of the particles which cause these variable aurora is still uncertain.
The second result that was surprising, at least to some, was the discovery of evidence for lightning. The Soviet landers, Veneras 11-14, all carried electromagnetic antennas to search for lightning associated Very Low Frequency (VLF) signals. Such signals were detected on all four missions [Ksanfomaliti, 1979; 1983]. Initial data from Pioneer Venus at low altitudes on the nightside of Venus also showed such signals [Taylor et al., 1979]. Figure 8 shows a sample of such signals detected by PVO [Russell et al., 1988]. The local time distribution of these signals suggests that they arise in the lower atmosphere rather than the upper atmosphere or ionosphere. Their occurrence maximizes from 2000 to 2200 LT. This could arise if the sources are at low altitudes because of the retrograde circulation of the atmosphere (combined with attenuation by the ionosphere). The local time distribution is shown in Figure 9 [Russell et al., 1989a]. Other characteristics of the signals which support their lightning origin are the fall off in occurrence of the high frequency signals with increasing altitude [Russell et al., 1989a] the near constancy with altitude of the Poynting flux at low frequencies [Russell et al., 1989b], the polarization of the low frequency signals, and the apparent lack of Doppler shift of the signals [Scarf and Russell, 1988]: Figure A (23k) illustrates schematically our present understanding of Venus lightning. Most recently Sonwalker et al  and Ho et al  have examined the wave propagation consequences of the hypothesis that the 100Hz waves arise in the atmosphere. If they do, the steep increase of index of refraction with increasing altitude will refract the waves vertically. These authors find that if they assume vertical propagation and look for waves consistent and inconsistent with whistler mode propagation, they get two classes of quite distinct waves one of which has all the properties expected for upward propagating whistler mode waves and one which very much resembles the higher frequency non-whistler mode waves. This is very strong evidence for a source of these waves below the ionosphere.
Upper Atmosphere and Ionosphere. The comprehensive aeronomy package of the Pioneer Venus Orbiter: the neutral and ion mass spectrometers, the Langmuir probe and the retarding potential analyzer, fulfilled their objectives of determining the chemical and physical properties of the Venus ionosphere [Niemann et al., 1980; Taylor et al., 1980; Brace et al., 1980; Knudsen et al., 1980]. However, the Pioneer Venus data also revealed important temporal variations and dynamical phenomena from the longest periods to the shortest periods studied. At the peak of solar activity, the only time when in-situ data were available, trans- terminator ion flows were observed which reached velocities of several km/s as illustrated in Figure 10 and Figure 11 [Knudsen et al., 1982]. This flow from day to night could easily explain the existence of the nightside ionosphere. This seemed contrary to modeling performed by Gringauz et al.  based on the Venera 9 and 10 data which indicated that electron precipitation maintained the night ionosphere. The resolution of these contradictory results became apparent as the solar cycle waned and the EUV flux from the sun changed [Knudsen et al., 1987; Brace et al., 1988]. The density of the Venus ionosphere is quite sensitive to solar activity. The results of Gringauz et al. were obtained at solar minimum while the initial (and only in-situ) PVO data were at solar maximum. At the other end of the spectrum of time variations, gravity waves have been detected by the neutral mass spectrometer [Kasprzak et al., 1988]. These waves appear to be generated near the terminator in conjunction with antisolar flow of the neutral atmosphere. They also are seen in conjunction with the phenomenon known as disappearing ionospheres [e.g. Luhmann, 1986].
The magnetization of the ionosphere also had some surprises. Since the planet has no detectable planetary magnetic field, it might be expected to have an unmagnetized ionosphere because the diffusion time for magnetic fields into the ionosphere is much longer than the convection time scale. However, as illustrated in Figure 12, the ionosphere exhibits 2 states [Russell and Vaisberg, 1983]. When the solar wind dynamic pressure is low relative to the peak thermal pressure of the ionosphere, the average magnetic field strength in the ionosphere is low and small filaments or ropes of magnetic flux appear. When the solar wind dynamic pressure is high, the boundary between the magnetic layer (magnetic barrier) in the magnetosheath and the thermal plasma in the ionosphere decreases in altitude. The region of transition between predominantly thermal pressure and predominantly thermal plasma pressure thickens and the plasma at low altitudes becomes magnetized. The reason for this dichotomy in behavior for low and high solar wind pressure can be found in the relative roles of convection and diffusion in the ionosphere [Luhmann et al., 1981]. The velocity of the plasma at low altitudes in the subsolar region is downward and varies as illustrated in Figure 13 [Cravens et al., 1984]. When the ionopause is pushed to low altitudes where the plasma becomes collisional, the diffusion rate increases so that a significant amount of magnetic flux can enter the ionosphere. Once in the ionosphere, it is redistributed by the varying velocity of the plasma which acts as a conveyor belt to deposit it at low altitudes. The source of the magnetic flux ropes, however, is not so easy to determine. An instability may be acting on the lower edge of the magnetic barrier in the subsolar region [Russell et al., 1987].
In contrast to the dayside ionosphere, the night ionosphere appears to be quite complex. The night ionosphere contains density holes with strong magnetic fields [Brace et al., 1980; Luhmann et al., 1981]. There are also irregular ionospheric density structures at the top of the ionosphere which may be streamers, clouds or rays [Brace et al., 1982]. There are also strong non- linear waves apparent in the plasma [Brace et al., 1983]. Thus, night ionospheric behavior is still somewhat of a mystery.
The Solar Wind Interaction. The interaction of the solar wind with Venus in many aspects resembles the interaction of the solar wind with a magnetized planet, but in other aspects it resembles the interaction with a comet as illustrated in: Figure B (10k). The magnetized solar wind is stood off by the unmagnetized ionosphere whose maximum thermal pressure, during solar maximum, generally exceeds the dynamic pressure of the solar wind [Luhmann et al., 1987]. As described above when the solar wind dynamic pressure approaches that of the thermal pressure of the ionosphere, the ionosphere becomes magnetized through the downward convection of magnetic field from the magnetosheath. At this time the neutral atmosphere participates in the stress balance through collisions with the downflowing ions. At the top of the ionosphere the magnetic barrier acts both as a lid to the ionosphere and an obstacle to the solar wind flow. The solar wind flow is deflected by this barrier, but since the solar wind is supersonic, this deflection has to be accompanied by the formation of a detached bow shock as sketched in Figure 14. In these respects Venus acts just like a magnetized planet.
Electrons and ions originating at and behind the bow shock and accelerated back toward the Sun and found on field lines connected to the bow shock. These back streaming particles excite waves. The electrons in particular excite waves at VLF frequencies. These waves are particularly intense along the field line that is tangent to the bow shock as is illustrated in: Figure C (53k).
However, there are also important differences. The first indication of these differences is the long term variability of the location of the bow shock. Figure 15 illustrates this behavior. When the solar EUV output is high at solar maximum, the terminator bow shock is about 2.45 Venus radii (RV) above the terminator. At solar minimum it is about 2.15 RV [Zhang et al., 1990]. The subsolar bow shock undergoes a similar variation. It appears that the solar cycle variations in the Venus upper atmosphere and ionosphere affect the shock location perhaps in analogy with how the variation in a comet's neutral atmosphere affects its solar wind interaction.
In a manner similar to comets, the solar wind picks up ions from Venus' oxygen exosphere and accelerates them in the antisolar direction as illustrated in Figure 16 [Luhmann and Kozyra, 1991]. We expect that this flux would be modulated by the solar cycle because both the Venus exosphere and the ionizing solar EUV flux should be solar cycle dependent. Indeed, such a dependence is found observationally [Moore et al., 1990]. Also like a comet, Venus has a magnetotail [Russell et al., 1981] which is induced by the solar wind interaction. The solar wind magnetic field is draped around Venus in the interaction. Magnetosheath flux tubes passing closest to the planet are slowed in the interaction and are believed to be mass loaded by the interaction with the ionosphere. Figure 17 illustrates the tail formation process.
The success of this mission was due to many factors, one of the most important being the dedication of the many individuals associated with this project: from the initial planning of the mission, to the construction of the spacecraft, to its operations and scientific analysis phase. The selection of a coordinated payload was important for developing scientific understanding rather than just amassing data. The selection of interdisciplinary scientists to participate on an equal footing with the experiment teams, and later of guest investigators with limited terms was also critical. A spirit of cooperation amongst many of these teams and investigators was also crucial.
Finally certain technical capabilities were critical to the success of the mission. First, the ability to adjust periapsis altitude to the lowest possible altitude consistent with spacecraft safety provided 3 seasons of in-situ data in the upper ionosphere. A small data storage unit provided coverage when the spacecraft was out of view of the Earth or tracking stations were not available. Variable data rates allowed the bit error rate to be controlled as Venus varied in distance from the Earth. Multiple data formats were also used to tailor available data rates to the measurement objectives at any one time. Finally the long life of the mission enabled many secular studies to be pursued like the decay of the SO2 haze and some solar cycle effects.
Nevertheless, much remains to be done. The pickup ions have not been properly investigated. We do not know the chemical composition energy spectrum or accurate loss rate of these ions. The rate of occurrence of lightning remains poorly constrained. Optical measurements of lightning are sorely needed. The ionosphere has been probed at solar maximum but in-situ data at solar minimum are not available. Furthermore, global ionospheric probing such as available on Earth from top-side sounders would be highly desirable. The plasma wave measurements on Pioneer Venus were extremely crude. The antennas were short and insensitive and the frequency resolution limited. A modern plasma wave instrument could do much to characterize the plasma physical processes occurring around Venus. In-situ measurements of the Venus neutral exosphere would also be highly desireable. Finally, the aurora remain unexplained and would be an important target of any future exploration.
Those desiring more details than possible herein are referred to the special issue of the Journal of Geophysical Research in December 1980 and the book, Venus [Hunten et al., 1983]. More recent review articles have appeared on the solar wind interaction [Luhmann, 1986; 1990]; on the magnetosheath and magnetotail [Phillips and McComas, 1991]; on the structure of the ionosphere [Brace and Kliore, 1991]; on the dynamics of the ionosphere [Miller and Whitten, 1991]; on the magnetization of the ionosphere [Luhmann and Cravens, 1991]; on the plasma wave environment of Venus [Strangeway, 1991]; on the structure and luminosity of the thermosphere [Fox and Bougher, 1991] and on Venus lightning [Russell, 1991].