Launches of Soviet spacecraft to Venus were started in 1961 (1,2). A full list of them is presented in Table 1. It includes all Soviet launches, successful and unsuccessful, declared officially and not declared. Until 1965, the leading industrial organization responsible for lunar and planetary spacecraft was OKB-1 (Osoboe Konstruktorskoe Byuro No. 1, in translation ”Special Design Bureau No. 1”) managed by Chief Designer S.P. Korolev. It developed designs, manufactured the spacecraft, made tests, and controlled spacecraft in flight. Simultaneously, it had a very extensive program of manned Earth orbiting missions. It became more and more difficult to be responsible for everything, and in 1965 the lunar and planetary projects were transferred form OKB-1 to another facility, the Design Bureau and Plant named after S.A. Lavochkin. The Design Bureau (”KB”) was managed by Chief Designer G.N. Babakin (1914-1971). Later (1974), this facility was renamed Nauchno-Proizvodstvennoe Obyedinenie imeni S.A. Lavochkina (NPOL), in translation Research-Industrial Association S.A.
Table 1. Soviet Missions to Venus
[Venera] 1VA No.1                         probe                               4 Feb 1961                             Molniya
Identical to Venera 1. Failed to depart from low Earth orbit due to fourth-stage failure (2). Registered as Sputnik 7 in the United States. Carried science instruments and a pennant in a ”landing apparatus,” which was an atmospheric entry probe expected to survive landing on the surface.
Venera 1                                       probe                         12 Feb 1961                              Molniya
Commuication failed in transit. Attitude control and radio system failures (2).
[Venera] 2MV-1 No.1                atm/surf probe           25 Aug 1962                                Molniya
A new design of Venera spacecraft. Carrier vehicle with a detachable entry probe. Probe expected to survive landing and carried a pennant. Failed to depart from low Earth orbit. Fourth-stage engine orientation system failed (2). Registered as Sputnik 23 in the United States.
[Venera] 2MV-1 No.2 atm/surf probe 8 Sept 1962 Molniya
Carried a pennant in an entry probe. Failed to depart from low Earth orbit. Fourth stage failed (2). Registered as Sputnik 24 in the United States.
[Venera] 2MV-2 No.1              flyby                            12 Sept 1962                                Molniya
Photo-flyby mission. Failed to depart from low Earth orbit. Third stage failed after 531 s (2). Registered as Sputnik 25 in the United States.
[Venera] 3MV-1A No.4A          test mission              19 Feb 1964                                  Molniya
Test launch of new spacecraft with launch vehicle. Launch failure. Third-stage engine failure (2).
[Venera] Cosmos 27            atm/surf probe            27 Mar 1964                                      Molniya
Failed to depart from low Earth orbit. Fourth-stage engine did not ignite due to power supply failure (2).
[Venera] Zond 1                 atm/surf probe           2 Apr 1964                                           Molniya
Spacecraft flew toward Venus, but communications failed in transit after 2 months. Leak in pressurized ”orbital” section caused loss of thermal control and failure of transmitters. Radio communications conducted through probe (2).
Venera 2                               flyby                          12 Nov 1965                                         Molniya
Flew by Venus at 23,950 km on 27 Feb 1966. Communications failed during Venus flyby due to failure of thermal control system. No data returned (2).
Venera 3                          atm/surf probe                16 Nov 1965                                        Molniya
Communications failed 17 days before arrival at Venus. Intended delivery of science instruments and a pennant with the USSR state emblem at arrival on 1 March 1966.
First spacecraft to impact another planet (2).
[Venera] Cosmos 96           flyby                             23 Nov 1965                                           Molniya
Failed to depart from low Earth orbit. Third stage terminated improperly, fourth stage did not ignite due to unstable flight, and spacecraft separated with large disturbances (2).
Venera 4                         atm/surf probe              12 Jun 1967                                                Molniya
Beginning with Venera 4, the full responsibility for planetary missions was transferred from OKB-1 to NPOL. First successful planetary atmospheric probe on 18 Oct 1967. Entered at 19°N38°E on the night side and transmitted for 94 minutes. Measured temperature, pressure, wind velocity, and CO2,N2, and H2O content over 25-55 km on night side of planet. Showed that the atmosphere is 90-95% CO2, and measured a temperature of 535°K before being crushed at 25 km. Detected no N2. Carrier vehicle included plasma and UV radiation experiments (3).
[Venera] Cosmos 167         atm/surf probe           17 Jun 1967                                                 Molniya
Same design and science as Venera 4. Failed to depart from low Earth orbit.
Venera 5                             atm/surf probe                5 Jan 1969                                                Molniya
Successful atmospheric probe; entered night side on 16 May 1969 at 3°S18°E. Probe measured temperature, pressure, wind velocity, and CO2,N2, and H2O content over 25-55 km on night side of planet. Transmitted for 53 minutes in the atmosphere. Lander crushed at 18 km. Flyby science same as Venera 4 (3).
Venera 6                            atm/surf probe              10 Jan 1969                                                   Molniya
Same design and science as Venera 5. Successful atmospheric probe; entered night side 17 May 1969 at 5°S23°E. Transmitted for 51 minutes in the atmosphere. Lander crushed at 18 km. Venera 5 and 6 found that the atmosphere is 93-97% CO2, 2-5% N2,and less than 4% O2 (3).
Venera 7                             atm/surf probe              17 Aug 1970                                                       Molniya
First successful planetary lander on 15 Dee 1970. Landed on night side at 5°S 351°E and transmitted for 23 minutes from the surface. Measured a temperature of 747 K on the surface. No pressure data transmitted due to a failure in the data acquisition system.
The Venera 7 probe was the first to survive atmospheric heat and pressure and reach the surface (3).
[Venera] Cosmos 359     atm/surf probe                       22 Aug 1970                                                       Molniya
Same design and science as Venera 7. Failed to depart from low Earth orbit.
Venera 8                       atm/surf probe                        27 Mar 1972                                                         Molniya
Landed 22 Jul 1972 on day side near terminator at 10°S 335°E. Returned atmospheric temperature, pressure, wind speed, composition, and light levels during descent.
Transmitted data for 50 min on the surface and reported a K-U-Th gamma-ray surface composition analysis. Measured a temperature of 743°K and a pressure of 93 bar at the surface (3).
[Venera] Cosmos 482                    atm/surf probe             31 Mar 1972                                                     Molniya
Same design and science as Venera 8. Failed to depart from low Earth orbit. Fourth stage misfired.
Venera 9 orbiter/lander 8 Jun 1975 Proton-D
New design of heavy Venera spacecraft using the Proton launcher. Dispatched successful lander and orbited Venus 22 Oct 1975. The first Venus orbiter, first picture from the surface of another planet, and first use of orbiter as relay for a planetary probe. Descent probe landed on day side at 32°N 291°E and communicated through the orbiter for 53 minutes. Lander measured atmospheric composition, structure, and photometry on descent and obtained B/W images and K-U-Th gamma-ray analysis on the surface. Orbiter returned imagery, IR-radiometry, spectrometry, photopolarimetry, radio occultation, and plasma data (4,5).
Venera 10                                   orbiter/lander                          14 Jun 1975                                              Proton-D
Same design and science as Venera 9. Dispatched successful lander on day side at 16°N 291°E and orbited Venus 25 Oct 1975. Venera 9 and 10 found the lower boundary of the clouds at 49 km and three distinct cloud layers at altitudes of 57-70 km, 52-57 km, and 49-52 km. Both orbiters ceased operations in March 1976 (4,5).
Venera 11                                flyby/lander                               9 Sep 1978                                                   Proton-D
Landed 25 Dec 1978 on the day side at 14°S 299°E. Measured atmospheric temperature, pressure, wind velocity, spectra of short wavelength radiation, chemical and isotope composition, aerosols, and thunderstorm activity. Surface imaging and XRF sapling systems failed. Contact lost after 95 minutes on the surface. Flyby spacecraft carried UV spectrometer, plasma instruments, and lander relay communications (6).
Venera 12                          flyby/lander                                      14 Sep 1978                                                  Proton-D
Same design and science as Venera 11. Landed 21 Dec 1978 on the day side at 7°S 294°E. Same science as Venera 11 and included cloud particle composition. Surface imaging and XRF sapling systems also failed. Continued transmitting data for 110 minutes until flyby spacecraft went below the horizon (6).
Venera 13                        flyby/lander                                      30 Oct 1981                                                    Proton-D
Landed 1 Mar 1982 on the day side at 7.5°S 303.0°E. Conducted atmospheric and cloud science and both BW and color imagery of the surface, as well as XRF analysis of the surface material. Contact with lander lost after 127 minutes (8).
Venera 14                      flyby/lander                                       4 Nov 1981                                                         Proton-D
Same design and science as Venera 13. Landed 5 Mar 1982 on the day side at 13.4°S 310.2°E. Contact with lander lost after 63 minutes (8).
Venera 15                       orbiter                                               2 Jun 1983                                                            Proton-D
Entered Venus orbit 10 Oct. Radar mapper covered the planet from 30°N to North Pole at
1-2 km resolution. The middle atmosphere and clouds were examined by IR
spectrometry (9,10).
Venera 16                       orbiter                                              7 Jun 1983                                                     Proton-D
Same design and science as Venera 15. Entered Venus orbit 14 Oct. Radar mapper with same coverage and resolution as Venera 15. IR instrument failed (9,10).
Vega 1                         flyby/lander/balloon                          15 Dec 1984                                                        Proton-D
Venus flyby 11 June 1985 using a gravity assist maneuver to redirect the spacecraft to Halley’s comet. Deployed an entry vehicle with a balloon and lander on the night side of the planet at 8.1°N 176.7°E. Conducted atmospheric science on descent. Balloon released on descent and floated for 48 hours measuring downdrafts of 1 m/s and average horizontal wind of 69 m/s. Drifted approx 10,000 km at about 54 km altitude. Lander soil analysis was made by the gamma spectrometer; the drilling device to provide a sample for the X-ray fluorescence spectrometer started to work in the atmosphere and thus failed to get a sample. Spacecraft bus flew by Venus and continued on to flyby Halley’s Comet at 8890 km distance on 6 Mar 1986 (11,12).
Vega 2                  flyby/lander/balloon                                21 Dec 1984                                                         Proton-D
Same design and science as Vega-l. Deployed balloon and lander in the night-side Venus atmosphere at 7.2°S 179.4°E on 15 Jun 1985 flyby with similar results. Same measurements as Vega 1. Sample acquisition with XRF and gamma-ray analyses on the surface were both successful. Spacecraft bus continued on to flyby Halley’s Comet at 8030 km distance on 9 Mar 1986 (11,12).
Lavochkin; the brief name “Lavochkin Association” is also used sometimes unofficially. For simplicity, we will use the designation “NPOL” for all periods covered below.
Almost all Soviet missions to Venus were named ”Venera” because Venera is the name of this planet in Russian. The only exception was the last one, it was “Vega,” not “Venera” (see below). OKB-1 made eleven attempts to launch Venera spacecraft from February 1961 to November 1965, but all of them were unsuccessful; either the launcher failed, or something failed in the spacecraft systems on the way to the planet. Venera 1 got on a trajectory to the planet, but communication was lost in the middle of the journey. Venera 3 reached the planet, but communications failed 17 days earlier. Some brief information about other failed missions is given in Table 1. In reality, they were not vain efforts. Very new techniques were created, and time was necessary for step by step developments, tests, and improvements. Meanwhile, the U.S. Mariner 1 went off course during launch in July 1962. A month later Mariner 2 was launched successfully and after a 3.5-month flight, flew by Venus and scanned the planet with infrared and microwave radiometers. That was the first successful planetary mission.
The first success in space studies of Venus was achieved by the Soviets in 1967. Venera 4 reached the planet and fulfilled its goals. It was separated from the bus and entered the atmosphere, slowing as it descended. The parachute opened, and measurements of the atmospheric properties were taken down to an altitude 25 km above the surface. The probe was destroyed there due to high density and/or high temperature. Venera 4 was the first successful planetary atmospheric probe. By this flight scheme, the bus from which the probe is separated enters the atmosphere and burns up. Separation was made several days before arrival. Data from such probes were transmitted to Earth directly by a low-gain antenna with a slow rate of 1 bit/s.
Accidentally or not, this success coincided with the above mentioned transfer of work from OKB-1 to NPOL. The technical documentation for Venera 4 had been prepared mainly in OKB-1, although NPOL designers did some updating. Of course, the Venera 4 design embodied the entire experience of OKB-1 in the development of planetary spacecraft. However, it was reinforced by their own NPOL know-how in design, manufacturing, and tests of rocket/aviation technology. After Venera 4, all Soviet planetary spacecraft were built by NPOL with its own team in cooperation with many other industrial firms.
The U.S. flyby probe, Mariner 5, arrived at Venus 1 day after Venera 4 and conducted some remote studies of the atmosphere. Then for about 10 years, NASA did nothing to explore Venus besides the Mariner 10 flyby on its way to Mercury. In contrast, the Soviet Union at that time was sending missions to Venus regularly, in the beginning using each astronomical window (about a 1.5-year gap) and later each second window (about 3 years). Other spacecraft that had increasingly better system environment protection followed Venera 4. Venera 7 (1970) made the first soft landing and transmitted signals directly from the surface. However, something failed in the data acquisition system, and only temperature data were transmitted, although they were much more detailed than those taken on earlier missions.
All probes from Venera 4 to Venera 7 landed on the night side of the planet due to navigational requirements. However, a day-side landing was very desirable for understanding the Venusian climate. This would provide the possibility of seeing if solar light reaches the surface of the planet or not. For this reason, Venera 8 (1972) was intentionally landed on the illuminated part of the planet, although very near the terminator. Measurements showed that some solar energy flux actually reaches the surface, as required by the greenhouse effect concept.
The first set of missions to Venus, up to Venera 8, was conducted with the Molniya launchers (the R7 rocket plus fourth-stage L). These missions and their results are described in Ref. 3. Since 1975, NPOL used the more powerful rocket Proton (plus fourth-stage D). A new generation of Venera spacecraft was born due to this change, larger than the previous, with the conversion ofthe bus to the orbiter or flyby module. In both cases, the bus was used for the lander data relay. In Venera 9 and 10 missions, the first Venus orbiters were created, and new landers descended with greater success than previous ones. These landers (Fig. 1) transmitted to Earth the first panoramas of the Venusian surface. After this, there were six more successful Soviet missions (Venera 11, 12; Venera 13, 14; Vega 1, 2) with landers (but without orbiters) and two missions with orbiters (but without landers), Venera-15 and 16 with the synthetic aperture radar (SAR). The results of these latest missions are described in Refs. 4-13.
All Soviet planetary missions (except the last, Mars 96) were duplicated: two identical spacecraft were always launched. In Soviet conditions, it cost only 15-20% more than a single mission, but such duplication provided a significant increase in the overall mission reliability.
Soviet Venera landers were unique. The new generation landers survived on the surface not less than two hours providing panoramic imaging (B/W and color) and measurements of the surface material composition by X-ray fluorescence (XRF) spectrometry. The XRF experiment was extremely difficult because of the necessity of bringing a sample into the interior part of the lander. In all cases, communication between the lander and the bus were interrupted only by geometry, not due to limited lander lifetime.
Venera 9 and 10 probe, general view. An artist's concept: the probe after landing (22 and 25 October 1975). Full mass about 1560 kg. Diameter 2.4 m. They transmitted scientific information (including panoramas) from the surface of Venus for 53 and 65 min, respectively.
Figure 1. Venera 9 and 10 probe, general view. An artist’s concept: the probe after landing (22 and 25 October 1975). Full mass about 1560 kg. Diameter 2.4 m. They transmitted scientific information (including panoramas) from the surface of Venus for 53 and 65 min, respectively.
The latest Soviet missions to Venus, Vega 1 and Vega 2 (1984) were very ambitious.
Together with ”classical” landers, balloons were targeted at the Venusian atmosphere, and after that the buses were redirected to Halley’s comet, becoming cometary probes. The Vega mission was prepared and conducted with broad international cooperation. Scientific and technical groups from eight countries participated in developing the science payload. Twenty radio astronomical observatories observed drifts of balloons in the atmosphere using VLBI and Doppler methods. This network was organized by Centre National d’Etudes Spaciales (CNES) which was especially interested in the balloon part of the Vega missions.
After Vega 1 and 2, no more Soviet missions were sent to Venus. Possibly it was not the right decision. In reality, exploration of Venus was something like a Soviet ”ecological niche” within the world of space science. For economic reasons, the Soviet Union could not compete with the United States in every field of space research. M.V. Keldysh mentioned that in such conditions a reasonable strategy must be to concentrate efforts in some narrow selected directions. Venusian exploration was just such a direction in space science.
Several topics were dedicated to the analysis of numerous results of Soviet and U.S. missions to Venus (14-20). The Soviet scientific input will be outlined very briefly below.
Results of Studies: The Atmosphere and Its Interaction with the Solar Wind
Vertical Structure of the Atmosphere from the Surface to 100 km. A lot of in situ measurements were made by Venera probes using temperature and pressure sensors at altitudes from about 60 km to the surface. It was shown that the temperature and pressure profiles are nearly the same on the day and night sides of Venus at these altitudes. Venera and Pioneer results obtained before 1979 were combined in the COSPAR Venus Reference Atmosphere, VIRA (16). At zero reference level (corresponding to a radial distance 6052 km from the center of mass of the planet), the temperature is 735°K, and the pressure is 92 bar according to VIRA. The temperature and pressure decline with altitude to values of about 260°K and 0.2 bar at 60 km. An important input was provided by Vega 2, the latest of all Venus probes (see Table 1). For the first time, very precise measurements of T, P profiles were made down to the surface (12). The temperature profile is presented in Fig. 2 These T, P measurements were better than those on previous Soviet Venera landers. Temperature sensors on the U.S. Pioneer probes had nearly the same precision as those on Vega but failed below 11 km. The analysis of the Vega 2 temperature profile at low altitudes shows the presence of convection in lower layers of the atmosphere. This region of convective instability probably extends up to altitudes of 25-30 km. The atmosphere is stable between 30 and 50 km, but above 50 km (and up to 55 km) becomes unstable again. Consequently, the atmosphere of Venus (in contrast to the terrestrial one) has two convective zones separated by a wide stable interval with a subadiabatic lapse rate. The Vega 1 and Vega 2 balloons (13) drifted at altitudes of 52-53 km just inside the upper convective zone and confirmed the presence of pronounced turbulence there.
The vertical profiles of the atmosphere between 60 and 90 km were measured by several means, including accelerometers on descent probes and infrared spectrometry of the thermal radiation emitted to space (Venera 15). The analysis of orbiter (Venera 9, 10, 15, 16) radio occultation data gave a set of profiles between 40 and 90 km. Observations with an IR Fourier spectrometer FS-1/4 on board the Venera 15 orbiter (9) provided about 2000 spectra in the range from 6 to 35 mm with a spectral resolution of 5-7 cm ~1 and spatial resolution of about 200 km. These spectra contain rich information about the atmospheric temperatures, aerosol, and some minor constituents (H2O and SO2). The altitude temperature profiles were found from the spectral shape of the strong CO2 band centered at 15 mm. The temperature at the altitude of 90 km is about 170°K. The shape of the altitude profile depends on latitude. In equatorial and midlatitudes (<45°), there is a monotonic decrease with altitude. In latitudes >45°, temperature inversions can be seen ordinarily with a minimum near 60-65 km and a maximum near 70-75 km. These structures are generated by the dynamic transfer of heat. Solar time and local variations were also observed. Winds. There are very strong zonal winds in the atmosphere of Venus. The principal mode of global circulation in the Venusian atmosphere is retrograde superrotation; the entire atmosphere below 85 km moves in the same direction as the solid planet itself, but at much greater speed. Maybe only the lowest layers move in the opposite direction, but there are no measurements of wind direction in that part of the atmosphere. The first estimates of wind speeds below 60 km were obtained by Doppler tracking of Venera 4. Similar measurements were made on all later probes. On Vega 1 and 2, observations of balloon motions were added: about 70 m/s at a nearly fixed altitude of 54 km. Wind speed decreases below this altitude down to several m/s at 10 km. Wind velocities above 40 km were derived also from the thermal profiles obtained by Venera 9, 10, 15, 16 radio occultations using cyclostrophic balance equations. Venera 15 thermal profiles obtained by IR spectrometry were used even more effectively for wind retrievals. They show wind speeds up to 130 m/s at altitudes of 70-75 km with two pronounced daily maxima, as expected because of thermal tides. Close to the surface (at an altitude of 1.3 m) on Venera 9 and 10 landers, a wind speed of about 1 m/s was measured by anemometers. It is not possible to say anything about the direction of this surface wind.
Vega 2 probe, the measured vertical profile of atmospheric temperature.
Figure 2. Vega 2 probe, the measured vertical profile of atmospheric temperature.
Chemical and Isotopic Composition of Atmospheric Gases.
The first direct measurements of atmospheric chemical composition were made with simple chemical sensors on earlier Venera missions. They showed that CO2 and N2 are the main constituents (97% and 3%, taking into account also some latest qualifications). Much more sophisticated instruments have worked on later probes (from Venera 11 to 14), namely, mass spectrometers and gas chromato-graphs. The results of mass spectrometry confirmed a strong anomaly of the argon isotope ratio (36Ar/40Ar) discovered several months earlier by the U.S. Pioneer large probe. This ratio on Venus is about 1 instead of 0.003 as in the terrestrial atmosphere. It was an important observation because of the different nature of these isotopes; one ofthem (36Ar) is so-called primary, the second (40Ar) is radiogenic. It was found that all primary (nonradiogenic) isotopes of noble gases are much more abundant on Venus than on Earth. Ratios of nonradiogenic isotopes of noble gases in some cases also showed some small differences from terrestrial ratios. It was found (on Venera 13 and 14) that the Ne20/Ne22 ratio is 12.2. This is larger than that on Earth (10.1) and less than that in the solar wind (13.7). This means that solar wind implantation may influence the evolution of the atmosphere of Venus.
Gas chromatographs measured the abundance of several gases. Among them are SO2 and CO (130 and 28ppm, respectively, at 42 km in altitude, ”Venera 12”). Later, the UV spectrometer on Vega probes did find that the SO2 mixing ratio is 20-25 ppm at 12 km, 38 ppm at 22 km, and 125-140 ppm at 42 km (the last in near agreement with Venera 12). SO2 abundance in the upper clouds (~70km) is much less and varies very strongly with latitude: 0.03 ppm at the low and middle latitudes and 0.100 to 1ppm in the North Polar region according to IR spectra obtained by the Venera 15 Fourier spectrometer. There are significant place-to-place and time variations of SO2 abundance in the upper clouds.
H2O abundance in the lower atmosphere was estimated many times using different kinds of instruments including chemical sensors, gas chromatographs, and optical spectrophotometers (Fig. 3). There was a big discrepancy among the results obtained in these measurements, from 30 to 1000 ppm, approximately. It seems now that only optical spectrophotometers provided a realistic estimate. According to optical measurements, the H2O column abundance is about 1 g/cm2, and the mixing ratio is 30-40 ppm between the surface and the main cloud deck. Low resolution spectrophotometers were installed on Venera 11, 13, and 14 to measure the spectrum of the solar radiation diffusing through the atmosphere in the range of 0.4-1.2 mm. These spectra contain several pronounced H2O and CO2 absorption bands (Fig. 3). In spite of the relatively low abundance of H2O, this gas provides a substantive input to the overall infrared opacity for the upwelling thermal (1>2 mm) radiation in the atmosphere of Venus. H2O abundance within the clouds is even lower. About 5-10 ppm (with some latitudinal, place-to-place, and daily variations) was found in the upper clouds from analysis of spectra obtained with Venera 15 IR spectrometry.
Clouds and Hazes. Clouds cover the whole planet Venus, making its surface invisible from outside. It was known from ground-based observations that their upper boundary is at an altitude of about 65-70 km above the surface. Venera 9 and 10 probes (1975) discovered their lower boundary at an altitude of about 49-50 km. It was found independently by two experiments, nephelometer and photometer. They showed also that the main cloud deck (located between these two boundaries) consists of three layers of different optical density: the lower (the most dense), middle (most transparent), and upper clouds. The atmosphere above and below the main cloud deck is also not free from aerosols: there is an upper haze above the upper clouds and a lower haze below the lower clouds. In the main cloud deck, the typical sizes of particles are of the order of 1-2 mm according to measurements with the nephelometer. The size spectrum is wide and varies with altitude. The average optical thickness of the main cloud deck is about 30. Direct solar radiation does not penetrate through the clouds, but there is a high enough flux of scattered solar radiation so that cloud particles consist of almost transparent material. The vertical structure of the upper clouds was studied in detail by IR spectra obtained by Venera 15. They showed that the aerosol scale height in the low and middle latitudes is large, about 5-6 km, but is much smaller (down to 1-2 km) in high latitudes. An important fact derived from these spectra is that their continuum (outside of CO2 and H2O bands) is in fair agreement with micron size particles that consist of sulfuric acid (at a concentration of about 75%). However this does not mean that H2SO4 is the only chemical component of the particulate matter in clouds at all altitudes. Middle and lower clouds may include particles of other chemicals. The strongest evidence of this was obtained by one of the experiments of the Vega probes; cloud particles were collected, and their elemental composition was estimated by a simplified X-ray fluorescent method. Sulfur was identified everywhere in the main cloud deck, but chlorine and phosphorus were also found in the middle and especially in the lower clouds. Moreover, phosphorus, not sulfur dominated here.
Venera 11: Examples of spectra of solar light penetrating the lower atmosphere of Venus due to atmospheric scattering. Numbers near curves are altitudes above the surface (in km). The overall shape of spectra and intensities are defined by extinction in clouds and atmospheric gas. CO2 and H2O absorption bands are easily visible.
Figure 3. Venera 11: Examples of spectra of solar light penetrating the lower atmosphere of Venus due to atmospheric scattering. Numbers near curves are altitudes above the surface (in km). The overall shape of spectra and intensities are defined by extinction in clouds and atmospheric gas. CO2 and H2O absorption bands are easily visible.
Solar and Thermal Fluxes. The photometer installed on Venera 8 discovered that part of the solar light penetrating through the atmosphere reaches the surface after multiple scattering in the clouds and gas. More sophisticated (mul-tiband) photometers on Venera 9, 10, 11, 12 and spectrophotometers (with continuous spectral scanning) on Venera 11, 13, 14 provided much more information about light fluxes from different directions and at all altitudes from about 60 km down to the surface. They showed, in particular, that the illumination of the surface (spectrally integrated) is about 3-5% of the solar illumination at the upper boundary of the clouds. The surface albedo is low (about 0.1), so that the incoming solar flux is absorbed by the surface and warms it. This minor energy flux (together with the high opacity of the atmosphere to infrared thermal radiation) may be enough to support the high temperature of the surface and the lower atmosphere due to the greenhouse effect.
The thermal radiation within the atmosphere was not measured by Soviet probes, but there were measurements by orbiters of outgoing thermal radiation from the planet. The first time, it was measured by Venera-9 and 10 by a very simple single filter radiometer, and later on Venera 15 by the IR Fourier spectrometer. The first of these sets of measurements was made only at low latitudes, but the second covered a wide latitude range from the equatorial to polar regions. This second confirmed the strong peculiarity of the thermal radiation field of Venus. The radiance is higher in polar regions than in low latitudes, as was discovered some years earlier by the Pioneer orbiter.
Airglow. Night-side emission in the range 4000-7000 A was discovered by high sensitivity spectrometers of the Venera 9 and 10 orbiters. The observed spectrum consisted of seven peaks that were later identified as a superposition of several bands of molecular oxygen, O2. These bands can be emitted only if O2 is a small constituent. For this reason, they dominate on Venus, but are absent in the terrestrial night emission spectrum. Possibly, the ”ashen light” of Venus that is observed sometimes from Earth may be explained by this emission. It is generated at an altitude of 100 km. Far UV emission in the lines of H, He, O, and O+ was measured by filter photometers of Venera 9, 10 and spectrometers of Venera 11, 12 flyby modules. Abundances of these atomic constituents and the upper atmosphere temperature were estimated by analysis of these data. Electrical Activity in the Lower Atmosphere. The experiment ”Groza’ (”Thunderstorm”) was conducted on Venera 11, 12, 13, and 14 for observations of low frequency (LF) electromagnetic emission in the range 10-80 kHz. Sporadic (impulsive) radiation was actually observed whose general characteristics were similar to the LF emission of terrestrial thunderstorms. It was supposed originally that this emission is generated in the clouds. Another hypothesis assumes that a possible source may be located much lower and even connected with some volcanic activity. An optical flash on the night side of Venus (lightning) was observed once by the spectrometer of Venera 9.
Ionosphere. Multiple radio occultation of orbiters Venera 9 and 10 on two frequencies (wavelengths 32 and 8 cm) provided the first evidence of strong variability of the electron density (ne) profile depending on solar zenith angle and conditions of interaction with the solar wind. During daytime, the main maximum (with ne about 4 x 105cm ~ 3) was located at an altitude of about 150 km, and the ionosphere goes up to 300-800 km (ionopause level). The night ionosphere had its main peak (about 104cm ~3) at 130-140 km, and there were no electrons above 170-200 km. Sometimes, two peaks were visible in the night profiles. Additional measurements were made by radio occultation of Venera 15 and 16.
Solar Wind Interaction with the Atmosphere. This was investigated by two plasma spectrometers and a magnetometer onboard the Venera 9 and 10 orbiters. They show the constant presence of a bow shock, which heats and compresses the solar wind flow. Venus has no intrinsic magnetic field, but a so-called magnetosheath is generated on the internal side of the bow shock. The ionized flow of the magnetosheath can interact directly with the neutral atmosphere due to charge exchange and photoionization. This adds mass to the solar wind because the upper atmosphere consists mainly of oxygen. The Ve-nusian ionosphere is not protected from interaction with this flow due to an absence of an intrinsic magnetic filed. The planetary ion flow on the night side forms a planetary plasma tail. The night ionosphere is supported by the plasma flow from the day side and also by electron precipitations. Findings of Venera 9 and 10 in studies of the solar wind interaction with the ionosphere of Venus were developed later by experiments on board Pioneer Orbiter.

Results of Studies: The Surface

Surface Morphology and Geology. These have been studied by Soviet missions to Venus on two different scales. In 1983-1984, Venera 15 and 16 orbiters provided side-looking radar images of the northern 21% of the planet surface at 1-2 km spatial resolution. An example is presented in Fig. 4. These were the first images on which landforms larger than a few kilometers across were seen, and thus local and regional geological structures could be easily identified. Simultaneously, the topography of that part of the planet was measured with 5 x 50 km footprints and a 30-m rms error of the relative altitudes. Even earlier (in 1975 and 1982), Venera 9, 10, 13, and 14 landers sent to Earth TV panoramas (see Fig. 5) of the landing sites showing centimeter-scale details of the surface in vicinities closest to the landers and of progressively larger land-forms at a distance.
Analysis of the Venera 15-16 data allowed us to understand the key features of Venusian geology, later confirmed by the U.S. Magellan mission results. It showed that the part of Venus studied was dominated by vast lava plains. Besides plains, more than 30 large (>100km in diameter), almost 1000 intermediate (20-100 km), and more than 20,000 small (<20km) volcanic constructs have been identified. Within the plains, “islands” and “continents” of high-standing blocks of specific terrain named “tessera” were seen. The tessera are morphologically rough due to multiple tectonic deformations predating, in most cases, the emplacement of volcanic plains. Volcanic-tectonic features, named “coronae,” circular to ovoidal and hundreds of kilometers across, were also observed within the plains. They are specific to Venus and are not observed on other planetary bodies. About 150 impact craters have been identified in the Venera 15-16 images. Their area density showed that the observed ensemble of volcanic and tectonic features and terrains had a mean surface age of about 0.5 to 1 billion years. Excellent preservation of the observed features for this long time provided evidence that surface processes such as wind erosion and deposition had very low mean rates on Venus. The observed area distribution of the features and terrains, including close to random spatial distribution of impact craters, led us to conclude that the plate-tectonic style of planet geodynamics typical of Earth was not the case in the morphologically identifiable (< 1 billion years) part of the geologic history of Venus, at least for the part of the planet studied.
Venera 15: An example of the radar images showing the western part of the volcanic plateau Lakshmi and tectonic mountains Akna Montes.
Figure 4. Venera 15: An example of the radar images showing the western part of the volcanic plateau Lakshmi and tectonic mountains Akna Montes.
TV panoramas taken by Venera 9, 10, 13, and 14 spacecraft that landed thousands kilometers apart from each other showed strikingly similar small-scale morphology: bedrock consisting of sequences of close to horizontal, numerous, centimeter-thick beds, forming local highs, and structureless soil in local depressions. At the Venera 9 site, which is on a steep (~30°) slope, the bedrock is seen in the form of platy rock fragments that are part of the on-slope talus. A clod of soil several centimeters across was thrown onto the supporting ring at the landing of Venera 13. Several sequential TV pictures showed clearly that the clod was gradually removed by wind during the approximately one-hour observation time.
Venera 13: TV panorama showing outcrops of bedded material and soil.
Figure 5. Venera 13: TV panorama showing outcrops of bedded material and soil.
Surface Albedo and Color.
Photometric analysis of the Venera 9, 10, 13, and 14 TV panoramas showed that the albedo of the soil is about 0.03-0.04, and the albedo of the bedrock varies from 0.05 to 0.12. Surface color was not an easy thing to determine even when Venera 13 and 14 sent color TV panoramas back to Earth because the sunlight reaching the planet surface is not neutrally white but prominently orange. The most reliable data on surface color were received by photometers of the Venera 9 and 10 probes. It was concluded that only basalts are similar to the surface of Venus in regard to spectral characteristics.
Chemical Composition of the Surface Material. This was determined by two methods, gamma-ray spectrometry and X-ray fluorescence spectrometry. The first was used by Venera 8, 9, 10, and Vega 1 and 2. The gamma-spectrometer was inside the pressure- and temperature-protected lander compartment and measured spectra of gamma irradiation of potassium, uranium, and thorium of the surface material that penetrated through the compartment walls. The measurements showed that the contents of these three radioactive elements in the surface materials of the Venera 9, 10 and Vega 1 and 2 sites were the same as in terrestrial tholeiite basalts. In the case of Venera 8, the contents of these elements found were significantly higher, reaching the level typical for such terrestrial rocks as alkaline basalt, monzonite, or syenite. A second technique was used by Venera 13,14, and Vega 2. The landers had a drilling device and a system of delivering the acquired surface material sample to the inside of the protected lander compartment, where the measurements were done. Contents of Si, Ti, Al, Fe, Mn, Mg, Ca, K, S, and Cl were determined. In the surface materials of the Venera 14 and Vega 2 sites, the contents of the listed elements found were the same as in terrestrial tholeiite basalts. In the case of Venera 13, the content found corresponds to alkaline (leucite) basalt.
Physicomechanical Characteristics and Electric Resistivity of the Surface Material. The measurements started with attempts to measure the density of the surface material with the help of a gamma-ray densitometer at the Venera 9 and 10 landing sites. In the case of Venera 9, the attempt was unsuccessful; the densitometer sensor unit stood upon two rock fragments standing apart with a gap in between so that the sensor was not in the required contact with the solid surface. In the case of Venera 10, the sensor unit stood upon a bed rock outcrop, although the unit inclination to the outcrop surface was not exactly determined, which left the possibility of an equivocal estimate. The preferred estimate of the bulk density of the rock is 2.8 7 0.1 g cm ~3, but the alternative estimate of ~1.5 x gcm~ 3 is also possible. Then, estimates of bulk densities and bearing capacities of the surface material were done at the Venera 13 and 14 sites from dynamic loading data (when the lander impacted the surface) and using the trellis girder. These results showed that the bedrock material at these sites had physicomechanical properties similar to those of packed sintered sand, namely, a density 1.4-1.5 g cm ~ 3 and bearing capacity of 4-10 kg cm ~ 2. The soil is a very weak and porous material with a density from 1.15-1.2gcm~3 and a bearing strength ~2kgcm~2. The electric resistivity of the surface material was unexpectedly low: 73 to 89 ohm m.

Concluding Comments

A lot of valuable information about Venus was obtained by the Soviet (and also U.S.) missions to this planet. Most of the data (although still not all) has been carefully analyzed. A set of new goals and new possibilities was identified by this activity. Proposals for new missions to Venus are offered from time to time by scientists from different countries; however, space agencies have not paid serious attention to this planet in recent years. Mars and small bodies dominate their programs. We think that it is not a well-balanced approach. For a better understanding of the past and future of our Earth, we need careful and systematic studies of both of its neighbor planets, Mars and Venus, not just one. The valuable technical experience obtained earlier is still available. At the same time, new approaches are visible, like use of near-infrared day emissions for deep sounding of the atmosphere from orbit, high temperature electronics for landers and balloons, etc. Possibly it is time now for a return to the exploration of Venus on a new level.

Next post:

Previous post: