The legacy of Venus Express: highlights from the first European planetary mission to Venus

The ESA/Venus Express mission spent more than 8 years in orbit around Venus to extensively study its atmosphere, ionosphere and plasma environment and unveil new aspects of its surface. Extensive reviews of the work of Venus Express are underway, to cover in-depth studies of the new face of Venus revealed by Venus Express and ground-based concurrent observations. This paper intends to give a summarized and wide overview of some of the outstanding results in all the science areas studied by the mission. This paper will first review the main aspects of the mission and its instrumental payload. Then, a selection of results will be reviewed from the outermost layers interacting with the Solar wind, down to the surface of Venus. As Venus Express is already considered by space agencies as a pathfinder for the future of Venus exploration, perspectives for future missions will be given, which will have to study Venus not only from orbital view, but also down to the surface to solve the many remaining mysteries of the sister planet of the Earth.     

Atmospheric tides on Venus: III. The planetary boundary layer

Diurnal solar heating of Venus' surface produces variable temperatures, winds, and pressure gradients within a shallow layer at the botton of the atmosphere. The corresponding asymmetric mass distribution experiences a tidal torque tending to maintain Venus' slow retrograde rotation. It is shown that including viscosity in the boundary layer does not materially affect the balance of torques. On the other hand, friction between the air and ground can reduce the predicted wind speeds from ∽5 to ∽1 m/sec in the lower atmosphere, more consistent with the observations from Venus landers and descent probes. Implications for aeolian activity on Venus' surface and for future missions are discussed.     

The frequency-area distribution of volcanic units on Venus: Implications for planetary resurfacing

The areas of volcanic units on Venus have been measured on the 1:5000000 geological maps published by NASA/USGS. These data were used to obtain a frequency-area distribution. The cumulative frequency-area distribution of 1544 specific occurrence of units cover six orders of magnitude from the largest unit (30 × 10⁶ km²) to the smallest (20 km²). The probability distribution function has been calculated. The medium and large volcanic units correlate well with a power-law (fractal) relation for the dependence of frequency on area with a slope of −1.83. There are fewer small units than the expected values provided by the power-law relation. Our measurements cover 21.02% of the planetary surface, 3.59% of the study area was found to be tessera terrain and is excluded from this study of volcanism. The measurements were restricted to areas where geological maps have been published. The analysis was performed on two independent areas of the planet, with a complete coverage of published maps. In both areas the largest volcanic unit covers a significant portion of the surface (58.75% and 63.64%, respectively). For the total measured volcanic units (excluding tessera), these two largest units (that could correspond to the same unit or not) cover the 61.18% and they are stratigraphically superimposed on older volcanic units which cover 3.37% of the area. The remaining area (35.45%) is occupied by younger volcanic units stratigraphically superimposed on the large volcanic unit(s). These results are based on the independent mapping of a large number of geologists with different ideas about the geodynamical evolution of Venus and different criteria for geological mapping. Despite this fact, the presence of these very large units is incompatible with the equilibrium resurfacing models, because their generation at different ages would destroy the crater randomness. Our frequency-area distribution of the mapped volcanic units supports a catastrophic resurfacing due to the emplacement of the largest unit(s) followed by a decay of volcanism. Our data for the frequency-area distribution of volcanic units provide new support for catastrophic resurfacing models. It is difficult to make our observations compatible with equilibrium, steady-state resurfacing models.     

The thermosphere of Venus and its exploration by a Venus Express Accelerometer Experiment

The forthcoming observations by Venus Express provide an ideal opportunity to comprehensively study the atmosphere of Venus for the first time since Pioneer Venus (1978–1992), and for the first time ever in detail at polar latitudes. This article reviews some of our current knowledge from space and ground-based observations about the upper atmosphere of Venus, such as its thermal structure, the global distribution of gases and dynamics. We discuss the processes most likely responsible for phenomena such as the cold nightside cryosphere, the cloud top superrotation and waves, and highlight outstanding scientific challenges for Venus Express measurements. In particular, we describe an experiment to measure atmospheric drag using the on-board accelerometers.     

The planetary fourier spectrometer (PFS) onboard the European Venus Express mission

The planetary fourier spectrometer (PFS) for the Venus Express mission is an infrared spectrometer optimized for atmospheric studies. This instrument has a short wavelength (SW) channel that covers the spectral range from 1700 to 11400 cm⁻¹ (0.9–5.5 μm) and a long wavelength (LW) channel that covers 250–1700 cm⁻¹ (5.5–45 μm). Both channels have a uniform spectral resolution of 1.3 cm⁻¹. The instrument field of view FOV is about 1.6 ° (FWHM) for the short wavelength channel and 2.8 ° for the LW channel which corresponds to a spatial resolution of 7 and 12 km when Venus is observed from an altitude of 250 km. PFS can provide unique data necessary to improve our knowledge not only of the atmospheric properties but also surface properties (temperature) and the surface-atmosphere interaction (volcanic activity).PFS works primarily around the pericentre of the orbit, only occasionally observing Venus from larger distances. Each measurements takes 4.5 s, with a repetition time of 11.5 s. By working roughly 1.5 h around pericentre, a total of 460 measurements per orbit will be acquired plus 60 for calibrations. PFS is able to take measurements at all local times, enabling the retrieval of atmospheric vertical temperature profiles on both the day and the night side.The PFS measures a host of atmospheric and surface phenomena on Venus. These include the:(1) thermal surface flux at several wavelengths near 1 μm, with concurrent constraints on surface temperature and emissivity (indicative of composition); (2) the abundances of several highly-diagnostic trace molecular species; (3) atmospheric temperatures from 55 to 100 km altitude; (4) cloud opacities and cloud-tracked winds in the lower-level cloud layers near 50-km altitudes; (5) cloud top pressures of the uppermost haze/cloud region near 70–80 km altitude; and (6) oxygen airglow near the 100 km level. All of these will be observed repeatedly during the 500-day nominal mission of Venus Express to yield an increased understanding of meteorological, dynamical, photochemical, and thermo-chemical processes in the Venus atmosphere. Additionally, PFS will search for and characterize current volcanic activity through spatial and temporal anomalies in both the surface thermal flux and the abundances of volcanic trace species in the lower atmosphere.Measurement of the 15 μm CO₂ band is very important. Its profile gives, by means of a complex temperature profile retrieval technique, the vertical pressure-temperature relation, basis of the global atmospheric study.PFS is made of four modules called O, E, P and S being, respectively, the interferometer and proximity electronics, the digital control unit, the power supply and the pointing device.     

Venus exploration with the Venera 9 and Venera 10 spacecraft

In October 1975 the Venera 9 and 10 space vehicles reached Venus. Two landers separated from the spacecraft and soft-landed on the illuminated side of the planet while their remaining orbiters were inserted into highly elliptical orbits, with pericenters at about 7600 km. These flights became a very important step in the Soviet program of Venus exploration. For the first time two panoramas of the Venus surface were returned to the Earth. Both landers and orbiters were equipped with various scientific instruments for studying the structure and dynamics of the atmosphere, physical properties and structure of the clouds, light attenuation in the atmosphere and illumination properties of the surface at the landing sites, and the composition, structure, and interaction processes in the Venus upper atmosphere and environment. The experiments were of complex character due to the simultaneous measurements from landers and orbiters, while the orbiters delivered very important information provided by systematic observations of the planet with great time and space coverage. In this report the principal characteristics of the flights, construction of the spacecraft, instrumentation, and scheme of landing on the surface are described. The preliminary results of the measurements obtained and their tentative interpretation are discussed.     

The deep interior of Venus, Mars, and the Earth: A brief review and the need for planetary surface-based measurements

A comparison of the internal structure of Earth-like planets is unavoidable to understand the formation and evolution of the solar system, and the differences between Earth’s, Mars’, and Venus’ atmospheres, surfaces and tectonic behaviors. Recent studies point at the role of core structure and dynamics in the evolution of the atmosphere, mantle and crust. On Earth, the crust thickness and the radius and physical state of the cores are known for almost one century, since the advent of seismological observations, but the lack of long-term surface-based geodetic, electromagnetic and seismological observations on the other planets, results in very large uncertainties on the crust thickness, on the temperature and composition of their mantle, and on the size and physical state of their cores. According to the currently available geodetic data, Mars’ dimensionless mean-moment-of-inertia ratio is equal to 0.3653±0.0008. When combined with geochemical observations and with the inputs of laboratory experiments on planetary materials at high pressure and high temperature, this result constrains a narrow range of density values for Mars’ mantle and favors a light [6200-6765 kg m⁻³] sulfur-rich core, but it still allows for a 1600-1750 km range for the core radius, i.e. an uncertainty at least ten times larger than the precision obtained in 1913 by Gutenberg for the Earth’s core. Mars’ mantle density distribution may be explained by a large range of temperatures and mineralogical compositions, either olivine- or pyroxene-rich. The unknown mean thickness of Mars’ crust makes necessary a number of working assumptions for the interpretation of gravimetric and magnetic data. The situation is worse for Venus, and the most conservative model of its deep interior is a transposition of the Earth’s structure scaled to Venus’ radius and mass. The temperature conditions at the surface of this planet hardly make possible long-term ground-based measurements, but this is indeed feasible at the surface of Mars. Precise measurements of Mars’ crust thickness, core radius and structure, and the proof of the existence or absence of an inner core, would put tight constraints on mantle dynamics and thermal evolution, and on possible scenarios leading to the extinction of Mars’ magnetic field about 4.0 Ga ago. Long-lasting surface-based geodetic, seismological and magnetic observations would provide this information, as well as the distributions as a function of depth of the density, elastic and anelastic parameters, and electrical conductivity. Current studies on the structure of Earth’s deep interior demonstrate that the latter data set, when constrained by laboratory experiments, may be inverted in terms of temperature, chemical, and mineralogical compositions.     

Tectonics of Venus: A review

The article presents a new tectonic scheme of Venus and gives the following interpretation of the planet's main structural units: (1) plains — areas of flood volcanism over stretched crust; (2) dome-like uplifts — areas of uplifting and volcanic activity above the mantle hot-spots; (3) coronae —former dome-like uplifts, partially subsided and diffused by gravity; (4) ridge belts — fold zones; (5) tesserae — fragments of ductile compression and shortening of crust; (6) supercoronae — coronae formed in the course of further evolution and relaxation of Beta-type uplifts. Ishtar Terra is considered to be a fragment of an ancient tessera paleocontinent, on the edge of which the Lakshmi supercorona is superimposed. Aphrodite Terra is considered as a belt of mantle hot-spot structures (dome-like uplifts, coronae, supercoronae, volcanoes, rifts).Three types of planetary belts have been distinguished on Venus: uplifted 'weakened' belts with an abundance of mantle hot-spot structures; a northern fan of ridge belts; and belts of low basalt plains. The center of the planetary system of uplifted weakened belts is situated in Atla Regio.The present tectonic structure of Venus is inferred to have formed during two stages of evolution characterized by different tectonic regimes. Stage I is a regime of soft ductile plates (formation of tessera uplifts and volcanic plains). Stage II is a formation of 'weakened' uplifted planetary belts, various tectonic regimes of mantle hot-spots, and plains-forming volcanism.'Geology and Tectonics of Venus', special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci. Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT, Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).     

Characterization of rock populations on planetary surfaces: Techniques and a preliminary analysis of Mars and Venus

Characteristics of rock populations on the surfaces of Mars and Venus can be derived from analyses of rock morphology and morphometry data. We present measurements of rock sizes and sphericities made from Viking lander images using an interactive digital image display system. The rocks considered are in the gravel size range (16–256 mm in diameter). Mean sphericities, form ratios, and roundness factors are found to be very similar for both Viking lander sites. Size distributions, however, demonstrate differences between the sites; there are significantly more cobble size fragments at VL-2 than at VL-1. A model calling for aphanitic basalts emplaced as ejecta or lava flows at the Viking sites is supported by the rock shape, size, and roundness data.Morphologic features pertaining to the modification history of a rock are considered for Mars and Venus. A multi-parameter clustering algorithm is utilized to objectively categorize martian and venusian rocks in terms of various criteria. Erosional markings such as flutes are demonstrated to be most important in separating VL-1 rock morphologic groups, while rock form (i.e., shape) represents the primary separator of subpopulations at VL-2 and the Venera landing sites. Fillets are common around VL-1 and Venera 10 fragments. Obstacle scours occur frequently only at VL-1. Cavities in rocks are ubiquitous at all lander sites except Venera 9. Eolian processes, possibly assisted by local solution weathering, are a strong candidate for the origin of cavities and flutes in martian rocks.     

Initial Venus Express magnetic field observations of the Venus bow shock location at solar minimum

In this study, magnetic field measurements obtained by the Venus Express spacecraft are used to determine the bow shock position at solar minimum. The best fit of bow shock location from solar zenith angle 20-120° gives a terminator bow shock location of 2.14 R_V (1 R_V=6052 km) which is 1600 km closer to Venus than the 2.40 R_V determined during solar maximum conditions, a clear indication of the solar cycle variation of the Venus bow shock location. The best fit to the subsolar bow shock is 1.32 R_V, with the bow shock completely detached. Finally, a global bow shock model at solar minimum is constructed based on our best-fit empirical bow shock in the sunlit hemisphere and an asymptotic limit of the distant bow shock which is a Mach cone under typical Mach number of 5.5 at solar minimum. We also describe our approach to making the measurements and processing the data in a challenging magnetic cleanliness environment. An initial evaluation of the accuracy of measurements shows that the data are of a quality comparable to magnetic field measurements made onboard magnetically clean spacecraft.     

Preliminary studies concerning subsurface probes for the exploration of icy planetary bodies

It is well-known that the permanent terrestrial ice sheets (glaciers and polar caps) contain a lot of information about the recent geological history and in particular about climatic changes. Extrapolating this fact to other ice sheets in the solar system (e.g. the Mars polar regions, the icy moons of the outer planets, etc.), we may expect a similar wealth of information. To obtain this information it is possible to drill holes or melt the ice by a heated probe, which in this way is able to penetrate the surface and investigate the deeper layers in situ. In the latter case the driving agent is the heating power and the weight of the probe. In this paper we consider the application of such “melting probes” for exploring the structure of ice sheets in extraterrestrial environments. We describe several laboratory experiments with simple melting probes performed under cryo-vacuum conditions and compare the results with tests in a terrestrial environment. The experiments revealed that under space conditions the downward motion of a heated probe in an ice sheet is characterized by intermittent periods of sublimation and melting of the surrounding ice, sometimes interrupted by periods where a part of the probe's outer surface is frozen to the surrounding ice. This leads to a temporary blocking of the probe's downward motion. A similar situation can occur when the trailing tether is frozen in behind the probe. During the periods of ice sublimation the penetration process is significantly more power consuming, due to the large difference between the latent heat of sublimation and the latent heat of melting for water ice.     

Zonal winds at high latitudes on Venus: An improved application of cyclostrophic balance to Venus Express observations

Recent retrievals of zonal thermal winds obtained in a cyclostrophic regime on Venus are generally consistent with cloud tracking measurements at mid-latitudes, but become unphysical in polar regions where the values obtained above the clouds are often less than or close to zero. Using a global atmospheric model, we show that the main source of errors that appear in the polar regions when retrieving the zonal thermal winds is most likely due to uncertainties in the zonal wind intensity in the choice of the lower boundary condition.Here we suggest a new and robust method to better estimate the lower boundary condition for high latitudes, thereby improving the retrieved zonal thermal winds throughout the high latitudes middle atmosphere. This new method is applied to temperature fields derived from Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) data on board the Venus Express spacecraft. We obtain a zonal thermal wind field that is in better agreement with other, more direct methods based on either retrieving the zonal winds from cloud tracking or from direct measurements of the meridional slope of pressure surfaces.     

Earth and Venus: A comparative study

The observed density of Venus is about 2% smaller than would be expected if Venus were a twin planet of the Earth, possessing an identical internal composition and structure. In principle, this could be explained by a process of physical segregation of metal particles from silicate particles in the solar nebula prior to accretion, so that Venus accreted from relatively metal-depleted material. However, this model encounters severe difficulties in explaining the nature of the physical segregation process and also the detailed chemical composition of the Earth's mantle. Two alternative hypotheses are examined, both of which attempt to explain the density difference in terms of chemical fractionation processes. Both of these hypotheses assume that the relative abundances of the major elements Fe, Si, Mg, Al, and Ca are similar in both planets. According to the first hypothesis, a larger proportion of the total iron in Venus is present as iron oxide in the mantle, so that the core-to-mantle ratio is smaller than in the Earth. This model implies that Venus is more oxidized than the Earth, with its lower intrinsic density (i.e., corrected to equivalent pressures and temperatures) due to the larger amount of oxygen present. The difference between oxidation states is attributed to differing degrees of accretional heating arising from the relatively smaller mass of Venus. On the other hand, the second hypothesis maintains that Venus is more reduced than the Earth, with its mantle essentially devoid of oxidized iron. The difference intrinsic densities is attributed to the Earth accreting at a lower temperature than Venus as a result of the Earth's greater distance from the center of the nebula. As a result, large amounts of sulfur accreted on the Earth but not on Venus. The sulfur, which entered the core, is believed to have increased the mean density of the Earth because of its relatively high atomic weight. The hypothesis also implies that most of the Earth's potassium, because of its chalcophile properties, entered the core.These hypotheses are evaluated in the light of existing data. The second hypothesis leads to an intrinsic density for Venus which is only 0.4% smaller than that of the Earth. This difference is much smaller than is believed to exist. A wide range of chemical evidence is found to be unfavorable to this second hypothesis, but to be consistent with the interpretation that Venus is more oxidized than the Earth, as required by the first hypothesis.     

Dynamics of the Venus ionosphere: A two-dimensional model study

A two-dimensional model of the ionosphere of Venus which simulates ionospheric dynamics by self-consistently solving the plasma equations of motion, including the inertial term, in finite difference form has been constructed. The model, which is applied over the solar zenith angle range extending from 60 to 140° and the altitude range 100 to 480 km, simulates the measured horizontal velocity field quite satisfactorily. The ion density field is somewhat overestimated on the dayside because of the choice model neutral atmosphere and underestimated on the nightside because of setting the ionopause height at too low an altitude. It is concluded that solar photoionization on the dayside and ion recombination on the nightside are the processes mainly responsible for accelerating the plasma to the observed velocities. The plasma flow appears to be sufficient to maintain the nightside ionosphere at or near the observed median level of ion densities.     

Names on the maps of Venus: A pre-MAGELLAN review

The topographic features on Venus are named with female names: craters and paterae have names of famous women as well as female first names, non-crater features have names of mythological characters. The first 80 names appeared on the maps after the Pioneer Venus mission; about 300 more names after the Venera 15 and 16 missions. 376 features of 17 types are now named. All the names are listed with their coordinates and attribute data. A large number of new names will be necessary after the MAGELLAN mission when detailed maps will be produced for nearly the whole surface of Venus.'Geology and Tectonics of Venus', special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci., Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT, Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).     

A two-dimensional model of the ionosphere of Venus: Thermal structure

A two-dimensional spectral model of energetics in the ionosphere of Venus has been constructed. The effects of horizontal bulk transport of heat and the heat flux saturation have been taken into account. The model is capable of explaining the observed high ion temperatures for solar zenith angles greater than 140°. An external heat input to ions of 1–2 × 10⁻⁴ergs cm⁻²sec⁻¹ almost uniformly distributed over the entire planet gives good agreement with the average ion temperature data from the PVO retarding potential analyzer. The effects of varying the magnitude of the horizontal plasma velocity, including the vertical component of bulk velocity, changing the altitude dependence of the velocity profile, and making the ionopause height a function of solar zenith angle have been discussed.     

Study of a thermal drill head for the exploration of subsurface planetary ice layers

The recently discovered water vapor plumes on Saturn's moon Enceladus, the polar caps of planet Mars and the possible ice volcanism on the Jovian satellites call for suitable techniques to explore deep ice layers of the solar system bodies. This paper presents a novel approach to deliver scientific probes into deeper layers of planetary ice. Several existing locomotion concepts and techniques for such probes are presented. After studying the mathematical framework of the melting locomotion process, melting tests with different head forms were done to evaluate the influence of the head's geometry on the melting process. This work led to a novel concept of a thermal drill head, using heat and mechanical drill in combination to penetrate the ice. We compare the performance of such a hybrid concept versus the melting penetration alone by a mathematical model and tests in ice with a prototype of the melting drill head.     

Structural architecture and a new tectonic perspective of Ovda Regio, Venus

The structural architecture of the Ovda Regio, Venus, derived from regional and detailed structural mapping of several key segments, reveals a new tectonic perspective, for the first time that varies from most of the existing tectonic models. The interpreted structural features include folds of different styles and scales, mega shear zones, ribbon structures, and other kinematic indicators. While concentric folding is predominant in the western Ovda, the eastern Ovda is characterized by shear folding. Two mega shear zones are recognised: dextral NW-SE trending GMSZ and a complimentary sinistral NE-SW trending KKSZ. Two tectonic stages are identified in a coherent and continuous strain history involving initial N-S compression that gave rise to regional east-west folding providing the fundamental tectonic architecture of a mountain fold-thrust belt. Ribbon structures in a broad radial pattern were developed contemporaneously with this folding. The second tectonic stage saw the development of a conjugate pair of mega shear zones and a range of kinematic indicators, consistent with continued N-S compression and the pre-existing fold pattern was modified resulting in the development of new fold structures, particularly in the eastern Ovda. The structural characteristics of Ovda Regio are analogous to those of Himalaya-Tibet collision front and also the Precambrian mobile belts of southern India.