SPICAV on Venus Express: Three spectrometers to study the global structure and composition of the Venus atmosphere

Spectroscopy for the investigation of the characteristics of the atmosphere of Venus (SPICAV) is a suite of three spectrometers in the UV and IR range with a total mass of 13.9 kg flying on the Venus Express (VEX) orbiter, dedicated to the study of the atmosphere of Venus from ground level to the outermost hydrogen corona at more than 40,000 km. It is derived from the SPICAM instrument already flying on board Mars Express (MEX) with great success, with the addition of a new IR high-resolution spectrometer, solar occultation IR (SOIR), working in the solar occultation mode. The instrument consists of three spectrometers and a simple data processing unit providing the interface of these channels with the spacecraft.A UV spectrometer (118–320 nm, resolution 1.5 nm) is identical to the MEX version. It is dedicated to nadir viewing, limb viewing and vertical profiling by stellar and solar occultation. In nadir orientation, SPICAV UV will analyse the albedo spectrum (solar light scattered back from the clouds) to retrieve SO₂, and the distribution of the UV-blue absorber (of still unknown origin) on the dayside with implications for cloud structure and atmospheric dynamics. On the nightside, γ and δ bands of NO will be studied, as well as emissions produced by electron precipitations. In the stellar occultation mode the UV sensor will measure the vertical profiles of CO₂, temperature, SO₂, SO, clouds and aerosols. The density/temperature profiles obtained with SPICAV will constrain and aid in the development of dynamical atmospheric models, from cloud top (∼60 km) to 160 km in the atmosphere. This is essential for future missions that would rely on aerocapture and aerobraking. UV observations of the upper atmosphere will allow studies of the ionosphere through the emissions of CO, CO⁺, and CO₂⁺, and its direct interaction with the solar wind. It will study the H corona, with its two different scale heights, and it will allow a better understanding of escape mechanisms and estimates of their magnitude, crucial for insight into the long-term evolution of the atmosphere.The SPICAV VIS-IR sensor (0.7–1.7 μm, resolution 0.5–1.2 nm) employs a pioneering technology: an acousto-optical tunable filter (AOTF). On the nightside, it will study the thermal emission peeping through the clouds, complementing the observations of both VIRTIS and Planetary Fourier Spectrometer (PFS) on VEX. In solar occultation mode this channel will study the vertical structure of H₂O, CO₂, and aerosols.The SOIR spectrometer is a new solar occultation IR spectrometer in the range λ=2.2–4.3 μm, with a spectral resolution λ/Δλ>15,000, the highest on board VEX. This new concept includes a combination of an echelle grating and an AOTF crystal to sort out one order at a time. The main objective is to measure HDO and H₂O in solar occultation, in order to characterize the escape of D atoms from the upper atmosphere and give more insight about the evolution of water on Venus. It will also study isotopes of CO₂ and minor species, and provides a sensitive search for new species in the upper atmosphere of Venus. It will attempt to measure also the nightside emission, which would allow a sensitive measurement of HDO in the lower atmosphere, to be compared to the ratio in the upper atmosphere, and possibly discover new minor atmospheric constituents.     

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.     

To the depths of Venus: Exploring the deep atmosphere and surface of our sister world with Venus Express

With its comprehensive suite of near-infrared instruments, Venus Express will perform the first detailed global exploration of the depths of the thick Venusian atmosphere. Through the near-daily acquisition of Visible and Infrared maps and spectra, three infrared-sensing instruments—the Planetary Fourier Spectrometer (PFS), the Venus Monitoring Camera (VMC), and the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS)—will comprehensively investigate the Thermal structure, meteorology, dynamics, chemistry, and stability of the deep Venus atmosphere. For the surface, these instruments will provide clues to the emissivity of surface materials and provide direct evidence of active volcanism. In so doing, ESA's Venus Express Mission directly addresses numerous high-priority Venus science objectives advanced by America's National Research Council (2003) decadal survey of planetary science.     

Windblown sand on Venus: Preliminary results of laboratory simulations

Small particles and winds of sufficient strength to move them have been detected from Venera and Pioneer-Venus data and suggest the existence of aeolian processes on Venus. The Venus wind tunnel (VWT) was fabricated in order to investigate the behavior of windblown particles in a simulated Venusian environment. Preliminary results show that sand-size material is readily entrained at the wind speeds detected on Venus and that saltating grains achieve velocities closely matching those of the wind. Measurements of saltation threshold and particle flux for various particle sizes have been compared with theoretical models which were developed by extrapolation of findings from Martian and terrestial simulations. Results are in general agreement with theory, although certain discrepancies are apparent which may be attributed to experimental and/or theoretical-modeling procedures. Present findings enable a better understanding of Venusian surface processes and suggest that aeolian processes are important in the geological evolution of Venus.     

Chemical composition of Venus atmosphere and clouds: Some unsolved problems

In spite of many spacecrafts that visited Venus, chemical composition of the Venus atmosphere and clouds present many challenging problems in observation and theory. The following problems are briefly discussed below: (1) molecular oxygen above the clouds, (2) lightning, (3) the blue absorption in the clouds, (4) mode 3 particle controversy and the Vega X-ray fluorescent observations, (5) search for new chlorine and sulfur species, and (6) vertical and spatial variations of water vapor and CO.     

Equilibrium resurfacing of Venus: Results from new Monte Carlo modeling and implications for Venus surface histories

Venus’ impact crater population imposes two observational constraints that must be met by possible model surface histories: (1) near random spatial distribution of ～975 craters, and (2) few obviously modified impact craters. Catastrophic resurfacing obviously meets these constraints, but equilibrium resurfacing histories require a balance between crater distribution and modification to be viable. Equilibrium resurfacing scenarios with small incremental resurfacing areas meet constraint 1 but not 2, whereas those with large incremental resurfacing areas meet constraint 2 but not 1. Results of Monte Carlo modeling of equilibrium resurfacing (Strom et al., 1994) is widely cited as support for catastrophic resurfacing hypotheses and as evidence against hypotheses of equilibrium resurfacing. However, the Monte Carlo models did not consider intermediate-size incremental resurfacing areas, nor did they consider histories in which the era of impact crater formation outlasts an era of equilibrium resurfacing. We construct three suites of Monte Carlo experiments that examine incremental resurfacing areas not previously considered (5%, 1%, 0.7%, and 0.1%), and that vary the duration of resurfacing relative to impact crater formation time (1:1 [suite A], 5:6 [suite B], and 2:3 [suite C]). We test the model results against the two impact crater constraints.Several experiments met both constraints. The shorter the time period of equilibrium resurfacing, or the longer the time of crater formation following the cessation of equilibrium resurfacing, the larger the possible areas of incremental resurfacing that satisfy both constraints. Equilibrium resurfacing is statistically viable for suite A at 0.1%, suite B at 0.1%, and suite C for 1%, 0.7%, and 0.1% areas of incremental resurfacing.     

Geologic interpretation of the near-infrared images of the surface taken by the Venus Monitoring Camera,Venus Express

We analyze night-time near-infrared (NIR) thermal emission images of the Venus surface obtained with the 1-μm channel of the Venus Monitoring Camera onboard Venus Express. Comparison with the results of the Magellan radar survey and the model NIR images of the Beta-Phoebe region show that the night-time VMC images provide reliable information on spatial variations of the NIR surface emission. In this paper we consider if tessera terrain has the different NIR emissivity (and thus mineralogic composition) in comparison to the surrounding basaltic plains. This is done through the study of an area SW of Beta Regio where there is a massif of tessera terrain, Chimon-mana Tessera, surrounded by supposedly basaltic plains. Our analysis showed that 1-μm emissivity of tessera surface material is by 15–35% lower than that of relatively fresh supposedly basaltic lavas of plains and volcanic edifices. This is consistent with hypothesis that the tessera material is not basaltic, maybe felsic, that is in agreement with the results of analyses of VEX VIRTIS and Galileo NIMS data. If the felsic nature of venusian tesserae will be confirmed in further studies this may have important implications on geochemical environments in early history of Venus. We have found that the surface materials of plains in the study area are very variegated in their 1-μm emissivity, which probably reflects variability of degree of their chemical weathering. We have also found a possible decrease of the calculated emissivity at the top of Tuulikki Mons volcano which, if real, may be due to different (more felsic?) composition of volcanic products on the volcano summit.     

Spectral characteristics of rocks: Effects of composition and texture and implications for the interpretation of planet surface compositions

In spectroscopic remote sensing for the exploration of the surface compositions of Earth and terrestrial planets, reflectance spectra with very low spectral contrast and even devoid of diagnostic absorption bands can be observed, which make the interpretation of the component minerals ambiguous. Using selected examples of terrestrial rock samples from intrusive and effusive geologic systems, we discuss compositional and textural properties related to these particular spectral shapes. We show that: (1) this spectral behaviour is common for coarse grains of multimineral rocks, where the optical coupling is expected to occur between welded mineral particles; (2) it is emphasised by the presence of opaque minerals with various compositions, such as ulvospinel, magnetite and chromite in effusive rock groundmass and in intrusive rocks; (3) it is controlled by the number of silicate phases within which the FeO is distributed, irrespective of the total iron content in the rock: a rock composition with a high number of iron-bearing minerals producing this kind of low contrast, almost featureless spectra is indicated here as “critical mode”; (4) it is also strongly intensified by aqueous alteration of silicates.These observations suggest unpredictable combinations of several different petrographic variables affecting the spectra of some compact rocks, and stimulate both targeted studies to quantitatively relate spectral and petrographic parameters, and the development of appropriate methods of spectral decomposition. Our ongoing work is at present focused on the spectroscopic effects of the FeO concentration in transparent neutral plagioclase, the different compositions of the opaque neutral minerals, and the iron bearing amorphous phases.We also discuss the analogy between the rocks used in the analysis reported here and the crustal rock compositions observed on Mars and inferred for Mercury as well as the compatibility of the factors responsible for the low spectral contrast of terrestrial rock samples with the factors expected for the two planets. We observe that a coarse-grained surface and a composition approaching a critical mode could explain the featureless Acidalia spectra on Mars, and suggest that the still open questions about Mercury’s surface regolith characteristics and composition do not exclude a priori the contribution of some of the factors examined in this paper to the peculiar surface properties of this planet.     

Tectonics and Evolution of Coronae on Venus: Preliminary Results of Tectonophysical Modeling

The formation of annular features on Venus, the so-called coronae, is modeled. It is common practice to associate their formation with the uplift and relaxation of hot mantle diapirs. We managed to partially reproduce the topography and structural pattern of the initial stage of corona evolution, a radially fractured dome, by lifting and lowering a piston under a layer of sand with consistent oil or moist clay. We failed to model a dense radial fracturing, which is typical of the earliest stage of corona evolution. We were able to reproduce the necessary attribute of coronae, concentric structures, which are commonly assumed to be formed at the stage of dome relaxation. Their formation mechanism in our experiments can serve only as a partial analog of the processes that produce corona rims. There is an obvious need to use more accurate models. Nevertheless, our modeling shows that the brittle deformations manifest themselves more clearly than do the plastic ones in the formation of dome-shaped uplift during the generation of natural coronae. The modeling also shows that the pattern of deformation within the dome-shaped uplift depends to some extent on the relationship between the layer thickness and the cross-sectional piston sizes. The latter can be a model for the relationship between the lithosphere thickness and the cross-sectional sizes of the mantle diapir that form a corona.     

Venus: Cloud optical depth and surface albedo from Venera 8

We have used the measurements of the solar flux obtained by the Venera 8 spacecraft inside the atmosphere of Venus and the values of the Venus spherical albedo to deduce the characteristics of the clouds and of the ground. The method used is the exponential kernel approximation and the results have been tested by exact computations with the spherical harmonics method.A cloud layer with an optical thickness $\text{τ}{}_1\text{? 144}$, an albedo for single scattering $\text{ω}{}_0\text{= 0.9998}$ in the rear infrared, above a Rayleigh layer between 0 and 32 km and a ground of reflectivity ? = 0.4, gives a good agreement with the experimental results. A model with two cloud layers is also discussed.     

The composition and vertical structure of the lower cloud deck on Venus

The opportunity to determine the planetwide temperature and cloud structure of Venus using radio occultation techniques arose with Pioneer Venus. Amplitude and Doppler data provided by the radio occultation experiment offered a unique and powerful means of examining the atmospheric properties in the lower cloud region.Absorption due to gaseous components of the atmosphere was subtracted from the measured absorption coefficient profiles before they were used to compute cloud mass contents. This absorption was found to represent a small part of the total absorption, depending on the latitude. In the main cloud deck, gaseous absorption contributes 10 to 20%, however, at the bottom of the detected absorption layer the sulfuric acid vapor contributes up to 100% due to increased vapor pressures. The clouds are the primary contributing absorbers in the 1- to 3-bar level of the Venus atmosphere. Below about 3 bars, depending on the latitude, absorption due to sulfuric acid vapor dominates.If a cloud particle model consisting of a solid nonabsorbing dielectric sphere with a concentric liquid sulfuric acid coating is invoked, the absorptivity of the particles increases from that of a pure sulfuric acid liquid sphere, and the mass content derived from the absorption coefficient profiles decreases. As the ratio of the core radius to the total radius (q) increases, absorption increases by more than a factor of 10 for high values of q. In the case of pure sulfuric acid droplets, the conductivity is sufficiently high that some of the field is excluded from the interior of the droplet thereby reducing the absorption. When a dielectric core of nonabsorbing material is introduced, the surface charge density is reduced and the absorption increases.The mass contents for all orbits in the equatorial region of Venus were calculated using values of q from 0 to 1. The resulting profiles match the probe mass content profiles at similar locations when a q of 0.97 is chosen.The wavelength dependence of the absorption for the spherical shell model varies with q from 1/λ² for pure liquid to λ^0.2 for a large core. A q of from 0.96 to 0.98 results in a wavelength dependence of 1/λ^1.0 to 1/λ^1.4 which matches the radio occultation absorption wavelength dependence and the microwave opacity wavelength dependence.Mass content profiles using a q of 0.97 were determined for occultations in the polar, collar, midlatitudinal, and equatorial regions assuming q remains constant over the planet. The results show considerable variability in both the level and the magnitude of the lower cloud deck. The cloud layer is lowest in altitude in the polar region. This might be expected as the temperature profile is cooler in the polar region than over the rest of the planet. The mass content is greatest in the polar and collar regions; however, many of the collar profiles were cut off due to fluctuations resulting from increased turbulence in the collar region. The mass contents are least dense in the midlatitude regions. There is a sharp lower boundary at about 1.5 bars in the equatorial and midlatitude regions and at about 2.5 bars in the polar region. Measurements made by the Particle Size Spectrometer and nephelometers also showed sharp lower cloud boundaries at this level.     

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.     

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.     

Tectonic Structure,Classification, and Evolution of Arachnoids on Venus: Preliminary Results

An analysis has been done of the topography and geologic structure of arachnoids—specific radial/concentric volcannic-tectonic structures on the surface of Venus. A representative sample (53 arachnoids) from 265 structures of this type, which are listed in the catalog of volcanic structures of the surface of Venus (Crumpler and Aubele, 2000), has been studied. The overwhelming majority of arachnoids are shown to be depressions that are commonly outlined by concentric extensional structures. Following Head et al. (1992) and Aittola and Kostama (2001), the assumption is confirmed and substantiated that arachnoids are formed by gravitational relaxation of small magmatic diapirs. Several types of arachnoids are identified on the basis of an analysis of structural patterns characteristic of such structures. It is also shown that the formation of different types of arachnoids depends on the depth of the magmatic diapir under the surface, on the thickness and reologic properties of the structures superposed on the evolving magmatic diapir, and on the character of regional stress fields that arise in the process of formation of such structures. The conclusion is drawn that most of the arachnoids were formed due to the gravitational relaxation of magmatic diapirs within the brittle part of the lithosphere, and some of them appeared as a result of the gravitational relaxation of radially fractured centers—novae. It is also shown that arachnoids are long-lived and multistep structures. At least some of them began to evolve before the formation of regional plains with wrinkle ridges, and their development ended after this event.     

Observed composition of the ionosphere of Venus: Implications for the ionization peak and the maintenance of the nightside ionosphere

Across the nightside of Venus, daily measurements from the PV Orbiter Ion Mass Spectrometer often indicate an ionosphere of relatively abundant concentration, with a composition characteristic of the dayside ionosphere. Such conditions are interspersed by other days on which the ionosphere appears to largely “disappear” down to about 200 km, with ion concentrations at lower heights also much reduced. These characteristics, coupled with observations of strong day to night flows of O⁺ in the upper ionosphere, support arguments that ion transport from the dayside is important for the maintenance of the nightside ionosphere. Also, U.S. and Soviet observations of nightside energetic electron fluxes have prompted consideration of impact ionization as an additional nightside ion source. The details of the ion and neutral composition at low altitudes on the nightside provide an important input for further analysis of the maintenance process. In the range 140–160 km, strong concentrations of O₂⁺ and NO⁺ indicate that the ionization peak is at times composed of at least two prominent ion species. Nightside concentrations of O₂⁺ and NO⁺ as large as 10⁵ and 10⁴/cm³, respectively, appear to require sources in addition to that provided by transport. The most probable sources are considered briefly, and no satisfactory explanation is yet found for the observed NO⁺ concentrations. Further analysis beyond the scope of this paper is required to resolve this issue.     

Diffuse scattering of radar on the surface of Venus: Origin and implications for the distribution of soils

Previous analysis of PV altimeter data has shown that ~25% of the surface of Venus is characterized by low values of reflectivity, interpreted as being due to the presence of porous materials such as soils. However, examination of a corrected reflectivity data set in combination with PV altimeter data suggests that no more than 5% of the surface of Venus is covered by soils more than several to tens of cm in depth. Most regions of apparent low reflectivity are instead interpreted to be due to the presence of small (5–50 cm) roughness elements on the surface that cause diffuse scattering at the 17 cm PV wavelength. Regions of low apparent reflectivity are of interest because of a correlation with tessera, a complex tectonic unit mapped from Venera 15/16 SAR data. Regions of tessera are characterized by a complex system of intersecting ridges and valleys thought to be of tectonic origin. Examination of possible models for the form of diffuse scatterers in the tessera suggests that they are rock fragments and originate from a mass-wasting process that is linked to the rugged nature of the terrain. Further, these diffuse scatterers are associated with other tectonic landforms, suggesting that they originate as part of tectonic deformation of the surface. Viewed from a geologic standpoint, the PV data sets are important tools for understanding tectonic, volcanic, and degradational processes on Venus, as well as for future interpretation of data from the Magellan mission.     

Constraints on the composition of the Venus atmosphere from microwave measurements near 1.35 cm wavelength

The determination of the brightness temperature of Venus near 1.35 cm wavelength is reviewed. The observed brightness temperature is compared with models for the microwave emission based on the physical and chemical structure of the atmosphere as obtained from spacecraft. Upper limits are set on the concentrations of microwave-absorbing minor constituents. In particular, upper limits are determined for SO₂ (180 ppm) and H₂O (0.3%) for a mixing-ratio profile that is uniformly mixed up to the cloud bottom at 50 km and is rapidly depleted ($\text{scale height}\text{? 1}\text{km}\text{)}$ at higher altitudes. The total optical depth of the cloud region at or above 50 km is <0.17 at 1.35 cm wavelength. The SO₂ upper limit is only in marginal agreement with the spacecraft results, and it may be that the latter have been overestimated, or that the distribution of SO₂ is more complex than given by the uniform mixing model.