Venus: A perspective at the beginning of planetary exploration

In situ measurements of the Venus atmosphere, made by the entry probes Venera 4, 5, 6, and 7, and data from the Mariner 5 flyby, have provided essentially new and reliable information and have powerfully contributed to our understanding of the nearest planet. The abundances of the principal atmospheric constituents and the temperature and pressure profiles down to the Venus surface were obtained for the first time. It was shown that the atmosphere is composed primarily of CO₂ and that N₂ (if any) and H₂O are relatively minor admixtures. In the region of the Venera 7 landing, the temperature and pressure at the Venus surface were established as equal to 747 ± 20°K and 90 ± 15 kgcm⁻². Space vehicles have also provided limited but quite important information on the physical properties of the Venus upper atmosphere and ionosphere, and on the interaction of the planet with the interplanetary environment. The main characteristics of the Venus atmosphere are discussed here with emphasis on the Venera results, including instrumentation, data processing, and altitude profiles.     

Venus Express—The first European mission to Venus

Venus Express is the first European mission to planet Venus. The mission aims at a comprehensive investigation of Venus atmosphere and plasma environment and will address some important aspects of the surface physics from orbit. In particular, Venus Express will focus on the structure, composition, and dynamics of the Venus atmosphere, escape processes and interaction of the atmosphere with the solar wind and so to provide answers to the many questions that still remain unanswered in these fields. Venus Express will enable a breakthrough in Venus science after a long period of silence since the period of intense exploration in the 1970s and the 1980s.The payload consists of seven instruments. Five of them were inherited from the Mars Express and Rosetta projects while two instruments were designed and built specifically for Venus Express. The suite of spectrometers and imaging instruments, together with the radio-science experiment, and the plasma package make up an optimised payload well capable of addressing the mission goals to sufficient depth. Several of the instruments will make specific use of the spectral windows at infrared wavelengths in order to study the atmosphere in three dimensions. The spacecraft is based on the Mars Express design with minor modifications mainly needed to cope with the thermal environment around Venus, and so a very cost-effective mission has been realised in an exceptionally short time.The spacecraft was launched on 9 November 2005 from Baikonur, Kazakhstan, by a Russian Soyuz-Fregat launcher and arrived at Venus on 11 April 2006. Venus Express will carry out observations of the planet from a highly elliptic polar orbit with a 24-h period. In 3 Earth years (4 Venus sidereal days) of operations, it will return about 2 Tbit of scientific data.Telecommunications with the Earth is performed by the new ESA ground station in Cebreros, Spain, while a nearly identical ground station in New Norcia, Australia, supports the radio-science investigations.     

Data on dynamics of the subcloud Venus atmosphere from Venera Spaceprobe measurements

This paper presents the principal results of wind velocity and turbulence measurements in the Venus atmosphere during the Venera flights.     

Venera 11 and Venera 12 observations of e.u.v. emissions from the upper atmosphere of Venus

The results obtained by two extreme ultra violet (e.u.v.) spectrophotometers flown near Venus on VENERA 11 and VENERA 12 in December 1978 are presented. Detectors were placed at discrete wavelength positions to measure e.u.v. emissions from the upper atmosphere of Venus while the spacecraft were drifting on their fly-by orbits. The emissions of HI 121.6 nm (Ly-α), HeI 58.4 nm, and OI 130.4 nm were measured with unprecedented sensitivity and spatial resolution. An OI signal of 500 Rayleigh (R) measured outside the disc suggested the existence of a large bulge of oxygen atoms. The e.u.v. emissions of two ionic species. OII 83.4 nm and HeII 30.4 nm, were measured for the first time in the atmosphere of Venus. The zero order detector of VENERA 12 indicated the presence of a very intense e.u.v. emission (28 kR) lying between the monitored wavelengths. This emission, which was only 3 kR for VENERA 11, is likely to be associated with the solar wind-ionosphere interaction.An attempt to measure ArI and NeI resonance emissions failed.The Lyman alpha (Ly-α) interplanetary background was 4 to 5 times larger than expected, suggestive of a very intense solar flux or an increase of the interplanetary density. The distribution of hydrogen indicates two populations with temperatures of 400 and 700 K.     

Spacecraft techniques for lunar research

The most significant findings about the Moon obtained by spacecraft so far, have resulted from measurements of gravity, electromagnetic properties, seismicity, mechanical properties, geologic features, composition, ages, and the lunar environment. A number of major lunar questions remain to be answered. Other properties, measurable with spacecraft, which may provide data critical to answering these questions include geometrical shape, motions, and heat flow. In this paper specific measurements that should provide critical data for each of these questions are identified, with some candidate techniques. Among the suggested techniques that have not yet been used are very long baseline interferometry (Earth-Moon baseline), gravity gradiometry, elemental analysis by neutron interactions, and remotely-controlled on-Moon microscopy.Different kinds of missions are suitable for the different measurements: lunar orbiters, soft-landers, long-range surface traverses, and sample return to Earth are all needed. The choice of manned vs remotely-controlled missions does not depend on scientific requirements but on other considerations. Both manned and remotely-controlled techniques have been used for orbiters, landers, and sample return, neither for a long-range traverse.Paper presented to the NATO Advanced Study Institute on Lunar Studies, Patras, Greece, September 1971.     

Network science, NetLander: a european mission to study the planet Mars

The planet Mars has many Earth like characteristics, but its evolution is different. An important future step in Mars’ geophysics is to deploy a network of stations at the surface of Mars inorder to study a wide range of properties of this planet, going from its deep interior structure to its atmosphere. Each ground station (small landers) will contain the same scientific instruments/experiments. The collected data will improve our knowledge of the Martian interior, surface and atmosphere, as well as its evolution. An important part of these objectives can only be achieved by a network of surface stations, as a network gives unique possibilities for performing studies of global scale phenomena and studies requiring simultaneous measurements from several sites.     

On the Thermal Protection Systems of Landers for Venus Exploration

The landers of the Soviet Venera series—from Venera-9 to Venera-14—designed at the Lavochkin Association are a man-made monument to spectacular achievements of Soviet space research. For more than 40 years, they have remained the uneclipsed Soviet results in space studies of the Solar System. Within the last almost half a century, the experiments carried out by the Venera-9 to Venera-14 probes for studying the surface of the planet have not been repeated by any space agency in the world, mainly due to quite substantial technical problems. Since that time, no Russian missions with landers have been sent to Venus either. On Venus, there is an anoxic carbon dioxide atmosphere, where the pressure is 9.2 MPa and the temperature is 735 K near the surface. A long-lived lander should experience these conditions for an appreciable length of time. What technical solutions could provide a longer operation time for a new probe investigating the surface of Venus, if its thermal scheme is constructed similar to that of the Venera series? Onboard new landers, there should be a sealed module, where the physical conditions required for operating scientific instruments are maintained for a long period. At the same time, new high-temperature electronic equipment that remains functional under the above-mentioned conditions have appeared. In this paper, we consider and discuss different variants of the system for a long-lived sealed lander, in particular, the absorption of the penetrating heat due to water evaporation and the thermal protection construction for the instruments with intermediate characteristics.     

The composition of the atmosphere of Venus below 100 km altitude: An overview

We review the progress in our understanding of the composition of the Venus atmosphere since the publication of the COSPAR Venus International Reference Atmosphere volume in 1985. Results presented there were derived from data compiled in 1982–1983. More recent progress has resulted in large part from Earth-based studies of the near-infrared radiation from the nightside of the planet. These observations allow us to probe the atmosphere between the cloud tops and the surface. Additional insight has been gained through: (i) the analysis of ultraviolet radiation by satellites and rockets; (ii) data collected by the Vega 1 and 2 landers; (iii) complementary analyses of Venera 15 and 16 data; (iv) ground-based and Magellan radio occultation measurements, and (v) re-analyses of some spacecraft measurements made before 1983, in particular the Pioneer Venus and Venera 11, 13 and 14 data. These new data, and re-interpretations of older data, provide a much better knowledge of the vertical profile of water vapor, and more information on sulfur species above and below the clouds, including firm detections of OCS and SO. In addition, some spatial and/or temporal variations have been observed for CO, H₂O, H₂SO₄, SO₂, and OCS. New values of the D/H ratio have also been obtained.     

On the optical studies of the atmospheric water vapour from the surface of Mars

Remote observations of the atmospheric water vapour from the Mars orbit were usually carried out to study its global distribution and variability. Measurements of the water vapour abundance onboard the landers have recently become an important complement to the orbital sounding. Narrow-band filter photometry and spectroscopy of the solar radiation from the surface of the planet proved to be a powerful tool in the study of atmospheric water. The Imager for Mars Pathfinder (IMP) was the first instrument to measure its amount from the surface. The Surface Stereo Imager (SSI) onboard the Mars Polar Lander (MPL) was to follow but the spacecraft was lost at landing. Nevertheless significant expertise in the optical measurements of atmospheric H₂O was gained during these missions. This paper summarizes this experience emphasizing the radiative transfer aspects of the problem. The results of this study could be of importance for future missions to Mars.     

Geology of the Venera and Vega Landing-Site Regions

Using Magellan radar images of the Venera and Vega landing-site regions, we carried out a photogeologic analysis and mapping of these regions, the soil composition of which was earlier analyzed by the spacecraft of these series. All these landing-site regions are shown to possess a similar set of geologic situations, which are typical of the planet as a whole, and a similar sequence of geologic evolution. Judging by the results of mapping, a geochemically advanced material analyzed at the Venera-8 landing site appears to be a complex of shield plains (Psh). Tholeiitic compositions obtained by Venera-9/10 and Vega-1/2 probably represent the unit of plains with wrinkle ridges (Pwr), which is most common on Venus. The material of tholeiitic composition analyzed by Venera-14 is likely to be the unit of relatively young lobate plains (Pl).     

The content of uranium,thorium, and potassium in the rocks of Venus as measured by Venera 8

A gamma ray spectrometer recording on the surface of Venus from Venera 8 reveals a content of radioactive potassium, uranium, and thorium very similar to acid magmatic rocks on the Earth. Venus is evidently a differentiated planet.     

Preliminary results on the Venus atmosphere from the Venera 8 descent module

The Venera 8 descent module measured pressure, temperature, winds and illumination as a function of altitude in its landing on July 22, 1972, just beyond the terminator in the illuminated hemisphere of Venus. The surface temperature and pressure is 741 ± 7°K and 93 ± 1.5kgcm^?2, consistent with early Venera observations and showing either no diurnal variation or insignificant diurnal variation in temperature and pressure in the vicinity of the morning terminator. The atmosphere is adiabatic down to the surface. The horizontal wind speed is low near the surface, about 35m/sec between 20 and 40km altitude, and increasing rapidly above 48km altitude to 100–140m/sec, consistent with the 4-day retrograde rotation of the ultraviolet clouds. The illumination at the center of the day hemisphere of Venus is calculated to be about 1% of the solar flux at the top of the atmosphere, consistent with greenhouse models and high enough to permit photography of the Venus surface by future missions. The attenuation below 35km altitude is explained by Rayleigh scattering with no atmospheric aerosols; above 35km there must be substantial extinction of incident light.     

On the wind-velocity measurements from Venera spacecraft data

Some features of the wind-velocity determination based on the results of Doppler shift measurements from the Venera probes during their descent in the Venus atmosphere are discussed. The validity of assumptions used in the reduction and analysis of these data are treated in connection with the preceding paper by Ainsworth and Herman. We conclude that the Venera velocity profiles are a valid representation of Venus atmospheric conditions.     

Venus lightning: Comparison with terrestrial lightning

Terrestrial lightning is generated by the separation of electric charge residing on water-ice particles in clouds, a few kilometers above the electrically conducting surface of the Earth. It is detected optically, electromagnetically, and aurally. The majority of discharges occur within or between clouds with about one third discharging to the surface of the Earth. Upward-propagating lightning also occurs with effects extending into the ionosphere. On Venus, the clouds are close to 50 km above the surface of the planet, where the temperatures and pressures are near those of Earth’s surface. In contrast the atmospheric pressure near the surface of Venus is nearly 100 times that of Earth. Thus, while intra- and inter-cloud lightning is expected to occur in a manner similar to that on Earth, we do not expect discharges from the clouds to the surface to occur. Upward-going lightning may be more frequent at Venus because the ionosphere is closer to the clouds. As at Earth, Venus lightning has been detected optically and electromagnetically from a variety of platforms. We find that some of the observed properties of lightning are different at the two planets. Many of the differences in the electromagnetic waves detected by spacecraft can be attributed to effects during ionospheric propagation to the spacecraft. We review the differences in the ionospheres of Earth and Venus and how they affect observations. We use both the Pioneer Venus electric antenna observations as well as the Venus Express magnetic measurements.     

Structure of the Venus atmosphere

The structure of the Venus atmosphere is discussed. The data obtained in the 1980s by the last Soviet missions to Venus: orbiters Venera 15, 16 and the entry probes and balloons of Vega 1 and 2 are compared with the Venus International Reference Atmosphere (VIRA) model. VIRA is based on the data of the extensive space investigations of Venus in the 1960s and 1970s. The results of the IR Fourier Spectrometry experiment on Venera 15 are reviewed in detail. This instrument is considered as a precursor of the long wavelength channel of the Planetary Fourier Spectrometer on Venus Express.     

Magnetic states of the ionosphere of Venus observed by Venus Express

Strong ultraviolet radiation from the Sun ionizes the upper atmosphere of Venus, creating a dense ionosphere on the dayside of the planet. In contrast to Earth, the ionosphere of Venus is not protected against the solar wind by a magnetic field. However, the interaction between charged ionospheric particles and the solar wind dynamic and magnetic pressure creates a pseudo-magnetosphere which deflects the solar wind flow around the planet (Schunk and Nagy, 1980). The combination of changing solar radiation and solar wind intensities leads to a highly variable structure and plasma composition of the ionosphere. The instrumentation of the Venus Express spacecraft allows to measure the magnetic field (MAG experiment) as well as the electron energy spectrum and the ion composition (ASPERA-4 experiment) of the upper ionosphere and ionopause. In contrast to the earlier Pioneer Venus Orbiter (PVO) measurements which were conducted during solar maximum, the solar activity was very low in the period 2006-2009. A comparison with PVO allows for an investigation of ionospheric properties under different solar wind and EUV radiation conditions. Observations of MAG and ASPERA have been analyzed to determine the positions of the photoelectron boundary (PEB) and the “magnetopause” and their dependence on the solar zenith angle (SZA). The PEB was determined using the ELS observations of ionospheric photoelectrons, which can be identified by their specific energy range. It is of particular interest to explore the different magnetic states of the ionosphere, since these influence the local plasma conductivity, currents and probably the escape of electrons and ions. The penetration of magnetic fields into the ionosphere depends on the external conditions as well as on the ionospheric properties. By analyzing a large number of orbits, using a combination of two different methods, we define criteria to distinguish between the so-called magnetized and unmagnetized ionospheric states. Furthermore, we confirm that the average magnetic field inside the ionosphere shows a linear dependence on the magnetic field in the region directly above the PEB.     

Deep radio occultations and “evolute flashes”; their characteristics and utility for planetary studies

Radio occultation studies of the structure of planetary atmospheres have generally involved relatively shallow penetration of the spacecraft behind the limb of the planet in the plane of the sky. Current radio link sensitivities allow detection of the radio signals at all occultation depths, whenever the planet-spacecraft distance is sufficiently large for the refraction to occur at atmospheric heights where microwave absorption is not too large. Voyager 1 at Jupiter and Voyager 2 at Saturn will pass almost directly behind the planets as viewed from the Earth. Thus they will pass through the caustics that corresponds to the focal line of a spherical planet, expanded by oblateness into a surface approximating a four-cusp cylinder. In the plane of the sky, the projection of this surface approximates the evolute of the planet's limb. As the spacecraft passes behind the planet with its antenna tracking the occulting limb, the strength of the radio signals received on Earth will at first decrease due to defocusing in the atmosphere, but then increase as the evolute is approached, because of the focusing caused by limb curvature. Inside the evolute there are four simultaneous signal paths over four limb positions. If we neglect absorption, focused signals for an instant could become orders of magnitude stronger than for the unocculted spacecraft. Measurements of the frequency and intensity of deep occultation signals, and of the timing and character of these “evolute flashes”, could provide information on atmospheric absorption, turbulence, and structure, and on details of the shape of the atmosphere at the focusing limbs as affected, for example, by planetary gravitational moments, rotation, and zonal winds. Such observations will be attempted with Voyager and potentially could be very fruitful in the Pioneer Venus and Galileo (Jupiter) orbiting missions.     

Aeolian regime of the surface of Venus

Recent probes of the planet Venus reveal a probable surface temperature exceeding 700K and a pressure exceeding 100 atm. A very dusty lower atmosphere may exist which is composed of micron-sized particles kept airborne by mild turbulence and a gentle circulation of deep adiabatic currents. A study of surface conditions responsible for generation and persistence of surface dust clouds is of fundamental importance in the radiative and dynamic properties of the atmosphere. Also spurious radar echoes may be caused by suspended particulate matter, thus explaining the high relief reported by radar altimeters.Equations describing transportation and deposition of dust and sand have been solved for the surface conditions of Venus. It is concluded that the minimum wind velocity for initiating grain movement is about one order of magnitude smaller than on Earth. In addition, this minimum wind velocity occurs for smaller particles on Venus than on Earth. Once the particles are raised, they can be maintained aloft for longer periods of time and over a larger size range on Venus.Surface structures such as ripples evolved from aeolian deposition are likely to be of smaller vertical dimensions but larger horizontally when compared with equivalent structures on Earth.