Venus Express: Scientific Goals, Instrumentation, and Scenario of the Mission

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1 ISSN , Cosmic Research, 2006, Vol. 44, No. 4, pp Pleiades Publishing, Inc., Original Russian Tet D.V. Titov, H. Svedhem, D. McCoy, J.-P. Lebreton, S. Barabash, J.-L. Bertau, P. Drossart, V. Formisano, B. Haeusler, O.I. Korablev, W. Markiewicz, D. Neveance, M. Petzold, G. Piccioni, T.L. Zhang, F.W. Taylor, E. Lellouch, D. Koschny, O. Witasse, M. Warhaut, A. Acomazzo, J. Rodrigues-Cannabal, J. Fabrega, T. Schirmann, A. Clochet, M. Coradini, 2006, published in Kosmicheskie Issledovaniya, 2006, Vol. 44, No. 4, pp Venus Epress: Scientific Goals, Instrumentation, and Scenario of the Mission D. V. Titov a, h, H. Svedhem b, D. McCoy b, J.-P. Lebreton b, S. Barabash c, J.-L. Bertau d, P. Drossart e, V. Formisano f, B. Haeusler g, O. I. Korablev h, W. Markiewicz a, D. Neveance i, M. Petzold j, G. Piccioni k, T. L. Zhang l, F. W. Taylor m, E. Lellouch e, D. Koschny b, O. Witasse b, M. Warhaut n, A. Acomazzo n, J. Rodrigues-Cannabal n, J. Fabrega o, T. Schirmann o, A. Clochet o, and M. Coradini p a Ma-Planck-Institute for Solar System Research, Katlenburg-Lindau, Germany b ESA/ESTEC, Noordwijk, Netherlands c Institute of Space Physics, Kiruna, Sweden d Service d Aéronomie du CNRS/IPSL, BP.3, 91371, Verriéres-le-Buisson, France e LESIA, Observatoire de Paris, 5 Place Janssen, Meudon, France f Istituto di Fisica dello Spazio Interplanetario, via del Fosso del Cavaliere 100, Roma, Italy g Universitat der Bundeswehr, München, Germany h Space Research Institute, Russian Academy of Sciences, Profsoyuznaya ul. 84/32, Moscow , Russia i Belgian Institute for Space Aeronomy, 3 av. Circulaire, B-1189, Brussels, Belgium j Universität zu Köln, Institut fur Geophysik und Meteorologie, A.-Magnus-Platz, Köln, Germany k Istituto Astrofisica Spaziale (INAF-IAS), via del Fosso del Cavaliere 100, Roma, Italy l Institute of Space Research, Graz, Austria m Oford University, UK n ESA/ESOC, Darmstadt, Germany o EADS Astrium, Tolouse, France p ESA Headquarters, Paris, France Received December 5, 2005 Abstract The first European mission to Venus (Venus Epress) is described. It is based on a repeated use of the Mars Epress design with minor modifications dictated in the main by more severe thermal environment at Venus. The main scientific task of the mission is global eploration of the Venusian atmosphere, circumplanetary plasma, and the planet surface from an orbiting spacecraft. The Venus Epress payload includes seven instruments, five of which are inherited from the missions Mars Epress and Rosetta. Two instruments were specially designed for Venus Epress. The advantages of Venus Epress in comparison with previous missions are in using advanced instrumentation and methods of remote sounding, as well as a spacecraft with a broad spectrum of capabilities of orbital observations. PACS numbers: Ea DOI: /S INTRODUCTION From the very advent of space era Venus is an etremely attractive object for planetary investigations. Early eploration of the planet using the spacecraft of the Venera series, Pioneer Venus, and Vega ( ), together with ground-based observations allowed one to describe basic physical and chemical conditions in the atmosphere and on the surface of the planet. At the same time, many problems concerning physical processes that maintain so eotic conditions on the planet remained unsolved. Global mapping of the surface by Pioneer Venus, Venera-15, Venera-16, and Magellan, along with investigations by landing modules on the surface, yielded a lot of data for understanding geology and geophysics of the surface. Venus Epress should make similar systematic study of the atmosphere. This is especially necessary for atmosphere below the clouds, which remains virtually uneplored, ecluding some eperiments on descent probes and observations by Galileo and Cassini flyby spacecraft. Fundamental scientific problems in physics of Venus are related to global circulation of the atmosphere, its composition, chemical and physical interactions of the atmosphere with the surface (including possible volcanic activity), processes in the cloud layer, heat balance and the role of various gases in maintaining the green- 334

2 VENUS EXPRESS: SCIENTIFIC GOALS, INSTRUMENTATION 335 house effect, origin and evolution of the atmosphere, circumplanetary plasma, and the interaction of the solar wind with the planet. In addition, the problems of history of volcanism on Venus, global tectonics, and physical properties of the surface remain in many respects open [30]. In spring of 2001 the European Space Agency (ESA) suggested the scientific community to use repeatedly the design of the Mars Epress spacecraft in order to realize some new astronomical or planetary mission. The project to use a modified spacecraft with already available scientific payload in the Venus orbit [1] was chosen by ESA from nine proposed projects. Eisting systems of the spacecraft and scientific instruments developed for the Mars Epress and Rosetta missions, as well as already formed scientific and industrial teams, allowed ESA to prepare the spacecraft for launch in less than four years. Venus Epress will give an opportunity to make a breakthrough in eploration of Venus. The instruments designed for Mars Epress and Rosetta, supplemented with two new eperiments, were found to be quite suitable for these purposes. The new generation of eperiments will be able to penetrate into the undercloud atmosphere due to observations in the so-called transparency windows, in which the emission of the lower atmosphere or even of the surface can escape to space through the thick cloud layer. This will allow one to disclose many mysteries in physics of the planet, to study a mechanism maintaining hurricane winds and polar vortices in the atmosphere, and to find a relationship of etreme climatic conditions with volcanic and tectonic processes. BASIC SCIENTIFIC GOALS Venus Epress is aimed at a global investigation of the Venusian atmosphere, circumplanetary plasma, and some properties of the surface from the orbit. In this section we describe the main scientific tasks of the mission, commenting how observations by Venus Epress will help solving them. Structure of the Atmosphere Previous investigations of the lower atmosphere of Venus (0 60 km) were restricted to measurements by 16 descent probes, mainly at low latitudes [2, 3]. They demonstrated the structure of the atmosphere below 30 km to be almost independent of latitude and time of the day (Fig. 1). However, there are virtually no accurate measurements of temperature and its gradient in the lower atmosphere. The structure of the middle atmosphere ( km) was studied by the infrared radiometer onboard Pioneer Venus and by the Fourier spectrometer onboard the Venera-15 spacecraft [4]. At these altitudes, temperature strongly depends on latitude and local time, which is determined by radiation and dynamic processes poorly studied at the moment too. H, km atmosphere 3 1 clouds 2 H 2 SO 4 +? CO 2, N 2, H 2 O, SO 2, COS, CO, HCl T, K The temperature of the thermosphere ( km) was derived from observations of night ultraviolet glow by Pioneer Venus and from density measurements [5, 6]. These observations indicated relatively low ( 300 K) dayside temperatures and abrupt drop of temperature down to 110 K while passing through the terminator on the night side. Such a behavior turned out to be quite unepected, and it has no analogs in the Solar System. Our knowledge about the thermosphere of Venus remains incomplete due to several reasons. First, until the present time, satellite observations were made with a limited spatial resolution. Second, nightglow observations allow one to study a layer with a thickness of km, i.e., they are made with poor resolution in vertical direction. And third, most observations were made at equatorial regions. Thus, the global distribution of temperature and density is not known so far in detail. Venus Epress will study the structure of the atmosphere using spectroscopic observations in a broad spectral range, from ultraviolet to thermal infrared emissions, and by the radio occultation method. The upper atmosphere ( km) will be studied with a high vertical resolution during solar and stellar occultations [7]. Spectroscopy of CO 2 in the 4.3 µm band and in 15 µm band will provide for remote sounding of temperature from 60 to 100 km with a vertical resolution of about 2 km. These observations will cover the planet completely with resolution of a few tens of kilometers [8, 9]. The radio occultation eperiment of the atmosphere will allow one to penetrate deeper (80 40 km) and to get temperature profiles with an altitude resolution of several hundred meters [10]. Thermal mapping of the regions with a large altitude drop in the transparency windows on the night side will I II III Fig. 1. Structure of the Venusian atmosphere [3, 5]. Temperature profiles below 100 km: low latitudes (1), 60 (2), and 75 (3). Profiles above 100 km are given by dotted (day) and dashed (night) lines. Rectangles in the right-hand side of the figure present the altitude ranges to be studied in the eperiments SPICAV (I), PFS and VIRTIS (II), VERA (III), and in thermal sounding on the night side (IV).

3 336 TITOV et al. H, km SO 2 H 2 O SO 2 H 2 O H 2 O CO CO H 2 O 50 H 2 O SO 2 HCl HCl CO H 2 O H 2 O Contents, ppm allow one to determine the temperature profile in the lower altitude scale [8, 9, 11]. In [12], a hypothesis that at every altitude the surface temperature is equal to temperature of the atmosphere was used in order to reconstruct the temperature gradient near the surface from ground-based measurements. This is important for determining the atmospheric stability parameter. CO Fig. 2. Contents of minor gas constituents of the atmosphere of Venus. Rectangles show the data of previous studies according to reviews [14, 31]. Vertical bars with arrows demonstrate the limits of detection (or presumed errors of measurements) for Venus Epress eperiments. Composition and Chemistry of the Atmosphere Carbon dioide and nitrogen are the basic components of the Venus atmosphere. Sulfur-bearing gases, carbon oide, water vapor, and other minor components are present in the atmosphere with abundances of from several pro mille (ppm) to a few thousand pro mille [13, 14, 31]. Figure 2 shows miing ratios for basic minor components of the atmosphere determined in the preceding missions. Though their quantities are small, minor components of the atmosphere participate in comple chemical cycles. There is a photochemical factory in the upper troposphere (60 70 km), where the reactions between CO 2, SO 2, H 2 O, and chlorine-bearing gases result in formation of sulfuric acid aerosols. Chemistry of the lower atmosphere is determined by thermal decomposition of sulfuric acid and by thermochemical reactions between sulfur- and carbon-bearing gases and water. Apparently, minerals of the surface also play an important role in supporting a certain level of abundance for some atmospheric gases [15]. Early studies of Venus resulted in a general idea about the atmosphere composition. However, our understanding of chemical cycles on Venus is far from being complete. For eample, the content of basic sulfur-bearing gas SO 2 whose abundance in the atmosphere is a few hundred ppm vastly eceeds balance when carbonates are on the surface. Moreover, for 14 years of observations Pioneer Venus measured a strong drop of this gas abundance near the cloud tops, which can indicate to volcanic activity. Earlier observations also found considerably inhomogeneous distribution of water vapor near the cloud tops. To study variations of the abundance of minor gas components with latitude and local solar time is the important problem in studies of composition and chemistry of the Venusian atmosphere. This is especially true for the lower atmosphere which, with the eception of scarce observations by landing modules and remote sounding during flybys of Galileo and Cassini missions, remains almost uneplored. Determination of the atmosphere composition is also of importance for understanding the heat balance in the atmosphere and the greenhouse effect. The set of optical instruments of Venus Epress designed for remote sensing in a wide spectral range (from ultraviolet to thermal infrared emissions) is perfectly suitable for solving this problem. Table 1 and Fig. 2 present the ranges of altitudes and the levels of abundances for various gases that will be measured by the satellite s instruments. The abundance of such gases as SO 2, SO, H 2 O, HCl, and CO and their variations will be permanently determined near the cloud tops, which will allow one to understand the process of photochemical production of the sulfuric acid aerosol. The discovery of spectral transparency windows [16] through which the thermal emission of the lower atmosphere can escape into space has given us a good opportunity to investigate the lower atmosphere remotely from the orbit. The spectral analysis of its composition in the transparency windows is one of the main tasks of Venus Epress. More specifically, its scientific tasks include monitoring of abundance of such gases as H 2 O, SO 2, COS, CO, H 2 O, HCl, and HF together with their spatial and temporal variations. These observations will substantially enrich our knowledge about chemistry and dynamics of the Venusian atmosphere. Composition of the upper atmosphere will be investigated during solar and stellar eclipses [7]. The profile of CO will be measured from the cloud tops up to ~120 km (the range in which photochemical production of carbon oide takes place). The vertical profiles of SO 2, H 2 O and HDO, HCl and HF will be determined up to altitudes of 80 km above the clouds (Fig. 2). Cloud Layer Venus is fully covered by the cloud layer occupying the altitude range from 50 to 70 km. The optical thickness of clouds changes from 20 to 40, depending on the wavelength and location [13, 17]. In the visible range Venus looks like a uniform disk. In ultraviolet range

4 VENUS EXPRESS: SCIENTIFIC GOALS, INSTRUMENTATION 337 Table 1 Gas Wavelength, µm H, km VIRTIS PFS SPICAV SOIR VERA VMC H 2 O (365 cm 1 ) HDO , , , 70 23, , CO COS SO (1150 cm 1 ) (1360 cm 1 ) (520 cm 1 ) 70 HCl H 2 SO contrast details are seen on the cloud tops. They are caused by the presence of specie whose origin is still unknown. To reveal the nature of ultraviolet absorber which absorbs about a half of the radiation flu received by Venus from the Sun is one of the main tasks in future investigations of the planet. The second important problem is related to determination of composition of the large aerosol particles discovered by the major probe of Pioneer Venus. The instruments of remote sounding onboard Venus Epress will analyze the structure, composition, and dynamics of the cloud layer. Cameras will study the spatial and vertical distribution of the unknown ultraviolet absorber [9, 11]. Eclipse and limb observations [7], together with nadir sounding in the thermal range [8], will allow one to investigate the structure of the upper part of clouds and overcloud haze. Observations in the transparency windows on the night side will make it Bow shock Magnetosheath Solar wind Ionopause Waves on ionopause Mantle Ionosphere Ion flow to nightside Ion sludge Clouds and streamers Ion outflow Filaments Hole B Ion tail Venus Epress orbit Day Night Hole Central tail beam B Tail beam Magnetotail Fig. 3. The structure of the Venusian ionosphere induced by interaction of the solar wind with the planet according to [29]. The radial scale is increased twice above the planet surface. The black line shows the Venus Epress orbit.

5 338 TITOV et al. Resolving power, λ/ λ SPICAV SOIR SPICAV IR SPICAV UV VIRTIS M VMC VIRTIS N PFS Wavelength, µm M SPICAV VIRTIS N UV IR 1 PFS Fig. 4. Spectral characteristics of optical instruments onboard Venus Epress. VMC possible to determine variations of the total optical thickness of the cloudy layer [9, 11]. Spectroscopy of the atmosphere composition near the cloud tops will allow one to construct a model of the formation and evolution of sulfuric acid clouds. Finally, the study of correlations between the spatial distribution of the unknown ultraviolet absorber and atmospheric gases will be of utmost importance for revealing the mystery of this substance. Dynamics of the Atmosphere One can distinguish two modes of global circulation in the Venusian atmosphere: the retrograde zonal superrotation in the troposphere and mesosphere [18] and a flow from the subsolar region to antisolar one [19]. Monitoring the UV details of clouds and trajectories of descent probes and balloons in the atmosphere gives evidence that the planet s troposphere is in a permanent retrograde zonal rotation. In this rotation the wind velocity changes from ~100 m/s near the cloud tops down to zero near the surface. A slower meridional motion from the equator to poles is imposed on this circulation, and it culminates in huge polar dipole vortices. It seems that the field of winds in the mesosphere can be well approimated by a balance of the pressure gradient and the centrifugal force (cyclostrophic balance), which results in an almost regular zonal flow. The velocity of the thermal wind calculated in this approimation using measurements of the temperature field in the mesosphere decreases in the overcloud atmosphere being inferior to the thermosphere circulation from dayside to the night side. No attempts to model the zonal circulation of the Venusian atmosphere have been successful so far, which indicates that the mechanism supporting it is not completely understood. In particular, there are no answers to the following questions: Fig. 5. Comparative fields of view of the optical instruments onboard Venus Epress on the disk of Venus from a distance of km. 1) is meridional circulation represented by a single global Hadley cell or there are several such cells? 2) how polar dipole vortices close two main modes of circulation and what are their main properties and behavior? 3) how the zonal super-rotation in the troposphere/mesosphere goes over into circulation from dayside to the night side in the thermosphere? In order to solve the above problems, one needs detailed observations of the wind velocity in the atmosphere of Venus. Venus Epress will quantitatively analyze the wind field at altitudes of about 70 km by monitoring UV details on the cloud tops and by observing the motion of contrasts on infrared images of the night side [9, 11]. Remote sounding of temperature will allow one to determine the field of thermal winds in the mesosphere [8] and to check the boundaries of applicability of the cyclostrophic approimation. Observations of the night glow in the bands of O 2, NO, O, and H will help to understand the global circulation of the thermosphere at altitudes of km. Eventually, we epect that Venus Epress will get the three-dimensional picture of the atmosphere dynamics. In addition, cameras onboard the satellite will be able to study the phenomena on different scales, from planetary polar vortices and waves to such local phenomena as convection with a spatial resolution of order of several hundred meters.

6 VENUS EXPRESS: SCIENTIFIC GOALS, INSTRUMENTATION 339 Energy Balance and the Greenhouse Effect High temperatures of the Venusian surface are a consequence of a strong greenhouse effect caused by the eistence in the atmosphere of such gases as CO 2, H 2 O, SO 2, and of sulfuric acid clouds [20, 21]. Less than 10% of the solar flu incident onto the upper boundary of the atmosphere penetrates down to the surface. However, strong absorption by greenhouse gases prevents the surface from being cooled by way of emission in the thermal range. This results in a difference of almost 450 K between temperatures of the surface and of the cloud tops, which is an absolute record among the terrestrial planets. Venus Epress will regularly measure in a wide range of wavelengths the flues of emission coming from the planet both on the day and night sides. This will allow one to characterize quantitatively the radiation heat balance and to reveal the role of various gases in the planetary heat balance and in the greenhouse effect. Near-Planet Plasma and Erosion of the Atmosphere Investigation of the processes of losses of matter by the planet is directly related to origin and evolution of the Venusian atmosphere. Venus is similar to the Earth in size, density, and position in the Solar System. It is quite probable that both the planets were supplied in their formation with equal amounts of volatiles. The main problem is how the Venusian atmosphere evolved under the action of processes of erosion in the upper atmosphere and interacting with the hot surface. At the present time Venus is obviously depleted of water, which is probably eplained by the intense loss of hydrogen at earlier stages of evolution. Similarly, the absence of considerable amount of free oygen suggests its intensive bounding by minerals of the surface. Understanding of these processes is based on measurements of abundances of noble gases and their isotopes, and it is rather approimate at the moment. The history of water on the planet is recorded in the ratio D/H. Deuterium was found on Venus in the amounts eceeding terrestrial values by a factor of 150 [22]. Such enrichment by deuterium can be eplained by higher rate of escape of light hydrogen atoms from the upper atmosphere. The escape rate of gases is determined, first, by the abundance of its molecules in the upper atmosphere and, second, by specific features of interaction of the solar wind with the atmosphere. The quantitative analysis of the rate of erosion of the atmosphere on its upper boundary requires knowing the content of atoms in the upper atmosphere. Preceding missions to Venus have no instruments capable to solve these problems. Venus has no internal magnetic field, which leads to substantial difference from the Earth as far as the loss of matter by the planet is concerned. The upper atmosphere of Venus is not protected by the magnetic field against the incident solar wind. As a result, the larger part of the eosphere of Venus is permanently inside the solar plasma flow, where the processes of ionization and charge echange lead to efficient removal of atmospheric gases by the solar plasma stream. In the process of interaction of the solar wind with the ionosphere, comple structures are formed on the night side through which a considerable amount of matter leaves the atmosphere (Fig. 3). These processes determine the loss of heavy molecules (for eample, of oygen) for which the thermal mechanism is inefficient. Venus Epress will investigate the circumplanetary plasma, its interaction with the solar wind, and escape processes by measuring the flues of energetic neutral atoms, ions, and electrons together with regular monitoring of the magnetic field [23, 24]. These contact measurements will be supplemented with remote sounding of the structure and composition of overcloud atmosphere up to altitudes of about 200 km [7], as well as by probing the ionosphere structure in a radio occultation eperiment [10]. Investigation of the Surface Radar mapping of the surface from the Magellan satellite showed that the surface of Venus is one of the youngest in the Solar System. Volcanism and tectonic processes have strongly changed the planet s surface [25], having formed strongly strained plateaus (tessera) and etended plains. Many scientists adhere to the hypothesis of global and catastrophic renovation of the crust: a unique mechanism for the terrestrial planets [26]. Up to now the important problems of eistence of volcanic activity on the planet in the modern epoch and of the role of the surface in chemistry of the atmosphere remain unsolved. In spite of the fact that there are no special instruments onboard the Venus Epress spacecraft for investigating the surface, the satellite will supplement previous studies in several ways [28]. Radar sounding will be eecuted by emitting the radio signals to a chosen region on the surface and by detecting the reflected signal by ground-based antennas. These eperiments will be concentrated on studying the highlands Aphrodite Terra, Beta Regio, Atla, and Mawell Montes [10]. Magellan recorded anomalously strong reflection in these regions. A gravitational eperiment will investigate the mass distribution in the region of Atalanta Planitia. Optical instruments will make thermal mapping of the surface in transparency windows near 1 µm on the night side, which will allow one to make up an emissivity map and, probably, to find some traces of volcanic activity. Spectroscopic measurements of the composition of the lower atmosphere will give indirect evidence about the character of interaction between the atmosphere and the surface. It is quite probable that Venus is a seismically active planet. As is well known, seismic waves propagating in the solid body of the planet can be transmitted and

7 340 TITOV et al. amplified by the atmosphere. Near the epicenter, the disturbances reaching the upper atmosphere lead to modulation of the temperature field, and they can be observed in the form of characteristic emissions in the 4.3 µm CO 2 band [9]. SCIENTIFIC PAYLOAD Seven instruments constitute the scientific payload of the Venus Epress spacecraft. Five of them have been taken from the Mars Epress and Rosetta missions with minor modifications: ASPERA is an analyzer of space plasma and energetic neutral atoms; Planetary Fourier Spectrometer (PFS) is a high-resolution infrared spectrometer; SPICAV is a spectrometer for observing solar and stellar occultations; VERA is a radio eperiment; and VIRTIS is a sensitive spectral camera and high-resolution spectrometer for visible and infrared ranges. These eperiments are supplemented by two instruments specially designed for Venus Epress: wideangle camera (VMC) and a magnetometer. In this chapter we briefly describe scientific tasks of the instruments and their characteristics. ASPERA: Analyzer of Cosmic Plasma and Energetic Neutral Atoms The instrument ASPERA-4 is a copy of the Mars Epress instrument with the same name [23]. It includes four sensors; two for detecting energetic neutral atoms (ENA), one for electron spectrometer, and one for ion spectrometer. The detector of neutral atoms measures the flu of ENA in the range kev without analysis in masses and energies, but with a fairly high angular resolution The detector of neutral particles will measure the ENA flu in the range kev with resolution in energy and with capability to discern masses from H to O, but with a coarse angular resolution of The electron spectrometer represents a compact electrostatic analyzer for measurements of the flu of electrons from 1 to 20 kev with a resolution of 8%. These three detectors are mounted on a rotating platform with a viewing angle of 360. The instrument also includes an ion mass analyzer for measuring the basic ion components (H + +, H 2, He +, O + + +, O 2, and CO 2, as well as ion group with M/q > 40 a.m.u./q) in the energy range from 0.1 to 10 kev/c. The solar wind interacts directly with the ionosphere of Venus, since the planet has no magnetic field (Fig. 3). The main task of the ASPERA-4 eperiment is to investigate this interaction and to probe the nearplanet space. The method to be used consists in mapping the flues of energetic neutral atoms and charged particles. The ASPERA eperiment will fulfill the following tasks: (1) to investigate the interaction of the solar wind with the atmosphere of Venus; (2) to study quantitatively the influence of plasma processes on the atmosphere; (3) to get the global distributions of plasma and neutral gas; (4) to study quantitatively the composition and flow of the matter escaping the planet; (5) to investigate the structure of plasma environment; and (6) to measure parameters of the undisturbed solar wind. These observations will allow us to solve the fundamental problem of interaction between the solar wind and the planet without magnetic field and of the role of this interaction in the atmosphere evolution. The similarity and distinction of these processes for terrestrial planets should be also understood. ASPERA will make observations along the entire orbit, thus probing all zones of circumplanetary plasma, from disturbed plasma near the planet to undisturbed solar wind in the apocenter. PFS: Infrared Fourier Spectrometer The planetary Fourier spectrometer (PFS) [8] is optimized for eploration of the atmosphere. The instrument is inherited from the Mars Epress mission with minor modifications. Two channels of PFS cover a wide range of wave lengths from 0.9 to 45 µm. The boundary between them is close to 5 µm, which approimately corresponds to dividing the Venus spectrum in the ranges of reflected solar radiation and proper thermal emission. The spectral resolution of PFS is about 1 cm 1. Its field of view is 1.6 and 2.8 for shortwave and longwave channels, respectively, which gives spatial resolutions of 7 and 12 km in the pericenter. PFS has scanning mechanism that allow the instrument to make calibration measurements and to change the direction of the field of view when observing the planet. The main goal of the PFS eperiment is to study the mesosphere and upper part of the cloud layer of Venus. Spectral measurements in the channel of thermal emission and, especially, in the CO 2 band around 15 µm will be used for remote sensing of the vertical temperature and aerosol structure at altitudes of 60 to 100 km. Such meridional measurements are especially suitable for mapping the thermal wind in the mesosphere. Observations of the escaping thermal and reflected solar flues in a broad spectral interval will allow the radiation balance of the planet to be studied in detail. High spectral resolution makes PFS an ideal instrument for studying the atmosphere composition. Observations in the regions of absorption bands of H 2 O, SO 2, CO, and other atmospheric gases will allow one to determine their abundance and altitude distributions in the overcloud atmosphere. The upper boundary of the cloud layer will be located using ëo 2 absorption bands. Observations in the transparency windows on the night side will provide for a possibility to study the composition of the subcloud atmosphere. However, averaging of hundreds of spectra will be required for this, in order to get sufficient signal-to-noise ratio. Sufficiently broad field of view of the instrument restricts PFS observations basically to distances lower

8 VENUS EXPRESS: SCIENTIFIC GOALS, INSTRUMENTATION 341 than km, which will allow one to cover in the nadir mode the region from latitude 50 south up to the North Pole. Scanning capability of the instrument will allow one to have a look somewhat further to the south. It is planned to get spectra in every orbit. SPICAV: Spectrometer for Observation of Solar and Stellar Occultations and for Nadir Observations SPICAV is a comple of three spectrometers designed to study the atmosphere of Venus by the method of solar and stellar occultations and using limb and nadir geometry [7]. SPICAV-UV is a high-sensitivity instrument operating in the range nm with a spectral resolution of 1.5 nm and using an intensified CCD detector. SPICAV-IR has the working interval from 0.7 to 1.7 µm and resolving power R = The second IR-channel (SOIR) is designed to study the upper atmosphere in the range µm with etremely high spectral resolution R ~ This channel consists of a dispersion spectrometer operating in high orders of diffraction and of an acoustooptical filter isolating one order of diffraction. The HgCdTe matri (Sofradir) cooled down to 90 K serves as a detector. The UV and IR channels of SPICAV repeat the SPICAM eperiment onboard Mars Epress with the only difference that the spectral range of the IR channel was etended to 0.7 µm. The SOIR unit was developed and designed specially for Venus Epress. The main goal of the SPICAV eperiment onboard Venus Epress is to study the structure and composition of the planet s upper atmosphere including the mesosphere and lower thermosphere. The eperiment using the method of solar and stellar occultations will be used at Venus for the first time. Observations of CO 2 absorption bands in the ultraviolet range will allow one to sound the vertical profiles of density and temperature in the altitude range from 70 to 180 km. The UV channel will also investigate the altitude distribution of sulfurbearing gases SO 2 and SO above clouds. One of the main tasks of SOIR is to study HDO in the upper atmosphere using spectra in the band 3.7 µm. This channel will also measure contents of SO 2, COS, CO, HCl, HF, and other minor constituents in the upper atmosphere (Table 1). Due to its high spectral resolution and the use of geometry of solar occultations, SOIR is ideally suitable for searching for new molecules, especially hydrocarbonates (CH 4, C 2 H 2 ), nitrogen oides (NO, N 2 O, ), and chlorine-bearing components (CH 3 Cl, ClO 2 ) playing important part in chemistry of the atmosphere. Eclipse observations will also be used to study the structure and optical properties of the overcloud haze. These observations will improve considerably our understanding of the composition and chemistry of the Venusian atmosphere and will make a contribution to studies of the atmosphere evolution and of the peculiarities of water loss by the planet. SPICAV will also operate in the nadir mode. Observations on the night side will be concentrated on transparency windows, which will allow one to probe the composition of the lower atmosphere, the optical thickness of the cloudy layer, and the surface temperature. Mapping of night glow in the bands of NO and O 2 will be used for establishing the circulation pattern of the thermosphere [19]. The processes of loosing atoms of H and O in the upper atmosphere will be studied by observing the emissions of these gases. Solar occultations occur in certain seasons with duration of about one month. In this time SPICAV observations will have high priority. The observations will be basically made near the pericenter. Beyond these seasons, SPICAV will participate in observations equally with other eperiments. VERA Radio Eperiment The radio eperiment onboard Venus Epress uses a radio system of the spacecraft operating in X and S frequency ranges (wavelengths of 3.5 and 13 cm) in order to sound the neutral atmosphere and ionosphere and to find properties of the surface and gravitational field, as well as of the interplanetary space [10]. An ultra-stable oscillator serves as a high-quality reference source of onboard signal for the spacecraft s transmitter. The ground-based instruments analyze the amplitude, phase, propagation time, and polarization of a signal received. Simultaneous measurements at two wave lengths allow one to separate effects due to propagation in the interplanetary space and the Doppler effect. The ultra-stable oscillator onboard Venus Epress is a direct descendent of the similar unit onboard the Rosetta spacecraft. The main goal of the radio eperiment is to study the vertical structure of the neutral atmosphere and ionosphere of Venus by the radio occultation method. The sounding of the neutral atmosphere will allow one to reconstruct the profiles of density and temperature at altitudes of 40 to 80 km with a vertical resolution of a few hundred meters, which eceeds the capabilities of temperature sounding in the thermal range substantially. Such observations will allow one to find the structure of waves in the mesosphere and troposphere, as well as to determine the abundance of sulfur acid vapors above the clouds. The structure of the ionosphere or the vertical profile of electron density will be determined in the overcloud atmosphere up to altitudes of about 600 km. Studies of the surface in the radio eperiment will include bi-static radar sounding of the surface and some investigations of the gravity field. The former will be concentrated in mountain regions where the radar eperiment on the Magellan spacecraft has found anomalously high reflection coefficient. These observations will allow the surface roughness and dielectric coefficient to be determined on spatial scales from cen-

9 342 TITOV et al. timeters to meters. A search for gravity anomalies will allow one to better understand the structure of the crust and lithosphere of Venus. And, finally, the radio eperiment will study dynamic processes in the solar corona using disturbances of the signal on its way from Venus to the Earth. The radio eperiment will be made basically near the pericenter. It will require a specific geometry and a maneuver of the spacecraft for high-precision antenna pointing. Radio occultation investigations of the atmosphere will be possible at certain periods when VERA will have a high priority. For one and a half years of observations in orbit the radio occultation will cover virtually all latitudes and longitudes, and certain geological structures on the surface will be sounded [27]. VIRTIS: Mapping Spectrometer for Visible and Infrared Ranges The VIRTIS instrument includes two channels: (i) a mapping spectrometer (VIRTIS-M) operating in the range µm with a moderate spectral resolution (R ~ 200) and (ii) a high-resolution (R ~ 1200) spectrometer VIRTIS-H for a range of 2 5 µm [9]. The angular resolution of both channels is 0.25 mrad, which will allow one to map the planet with a spatial resolution of about 20 km from the apocenter. The VIRTIS instrument onboard Venus Epress is a copy of the instrument installed on the Rosetta cometary probe. The VIRTIS eperiment has a broad spectrum of scientific tasks. First of all, the high sensitivity of VIR- TIS makes it an ideal instrument for sounding the atmosphere composition on the night side by way of measuring weak emissions in the transparency windows. VIRTIS will sound the abundance of such gases as H 2 O, CO, SO 2, and COS in the lower atmosphere (Table 1) and the cloud opacity. If variations of the atmosphere compositions are discovered, this can be an indirect evidence of the character of atmospheric dynamics, interaction of the atmosphere with the surface, or volcanic activity. On the dayside VIRTIS will measure the composition of the atmosphere and aerosol near the upper boundary of the cloudy layer. The second task of VIRTIS is to study the atmosphere dynamics. The eperiment will measure the wind velocities at altitudes of 70 and 50 km using the observed displacement of cloudy layer details in the ultraviolet and infrared ranges. In addition, mapping of O 2 emissions at 1.27 µm will be used. The spatial distribution and variation of this emissions give evidence of circulation in the lower thermosphere ( km). In the final analysis, VIRTIS will accomplish threedimensional sounding (tomography) of the general circulation of the atmosphere. The third task of VIRTIS is to sound the temperature and aerosol structure of the mesosphere in the range km. To this end, measurements of thermal emission of the atmosphere in the range 4 5 µm will be used. The fundamental absorption band of CO 2 is within this range. These observations will supplement the thermal sounding of the PFS eperiment, providing for high-resolution observations in the southern hemisphere. The limb geometry will make it possible for VIRTIS to study the vertical structure of the overcloud aerosol haze with an altitude resolution from hundreds of meters to 2 km. In addition, VIRTIS will make thermal mapping of the surface in the transparency windows near 1 µm, which should supplement the radar studies of Magellan and make it possible to discover hot spots associated with volcanic activity. The spatial resolution of these observations is limited by scattering in the cloud layer, and it will not be better than 50 km. Night observations will be also used to search for lightning events and to observe thermosphere emissions of CO 2, which, theoretically, can give indirect evidence in favor of seismic activity on the planet. The research program of VIRTIS requires that the eperiment would be performed both in the pericenter and far from the planet. The most important task of mapping is to observe the motion of cloud details and to create a spectral mosaic of the southern hemisphere from the apocenter. At distances closer than km to the surface the mapping capability of VIRTIS will be limited by fast motion of the spacecraft, which will prevent one from making full reconstruction of images. Here, the images will consist of separate points distributed along the orbit. Another limitation will be represented by a large amount of data that can be obtained in the eperiment. This problem will be solved by properly choosing scientific priority and by using the data compression. VMC Camera for Monitoring Observations VMC is a wide-angle digital camera for observations of the atmosphere and surface of Venus through four narrow-band filters. The instrument is a single unit including optical part, a CCD detector ( piels), and electronics [11]. The camera has 17.5 (0.3 rad) field of view and angular resolution of 0.75 mrad/piel, which corresponds to spatial resolutions of 0.2 km and 50 km in the pericenter and apocenter, respectively. The detector and electronics provide for a dynamical range of 6000 DN, and eposure time can be chosen from 0.5 ms to 30 s. Magnetometer The magnetometer of Venus Epress is designed for measuring the magnitude and direction of the magnetic field near the planet [24]. Since Venus has no proper magnetic field, these measurements will characterize the field frozen into plasma. The instrument includes two detectors and electronics. One detector is installed on the upper panel of the spacecraft, and another one is mounted on the end of a meter-long boom. Such a con-

10 VENUS EXPRESS: SCIENTIFIC GOALS, INSTRUMENTATION 343 figuration allows one to separate a signal of the eternal magnetic field from the background of noise produced by the spacecraft systems. The magnetometer has a wide dynamical range from ±32.8 to ± nt with resolution, respectively, from 1 to 128 pct. The magnetometer of Venus Epress descends from the ROMAP instrument of the landing module of the Rosetta lander. It will be a first miniature magnetometer with two detectors installed onboard a scientific satellite. Since Venus has no proper magnetic field, the larger part of the upper atmosphere of the planet is permanently in the solar plasma flow. The incident solar wind interacts directly with the ionosphere of Venus, where the comple structures of plasma clouds, tail rays, lines, and ionospheric holes are formed. The planet intensively looses atmospheric gases in these regions (Fig. 3). The orbit of Venus Epress will intersect the regions of interaction of the solar wind with the ionosphere. The magnetometer will make measurements with high temporal resolution in the magnetosheath, on the magnetic barrier, in the ionosphere, and in the magnetotail. It will eplore the boundaries between different plasma regions. These observations will produce important data necessary for interpretation of the ASPERA result. It should be noted that Venus Epress will make observations in the solar minimum period, thus supplementing the measurements made by the Pioneer Venus spacecraft. The eperiment will also search for thunderstorms by detecting electromagnetic waves generated during atmospheric electric discharges. The magnetometer will be in operation along the entire orbit. At distances more than km from the planet the magnetic field will be measured with a frequency of 1 Hz. For two hours in the pericenter region the frequency of measurements will be increased up to 32 Hz, and immediately in the pericenter the instrument will make measurements with a maimum frequency of 128 Hz (searching for thunderstorm discharges). Magnetometer ASPERA VIRTIS PFS VERA SPICAV SOIR VMC Fig. 6. The Venus Epress spacecraft and arrangement of its payload. General Performance of Scientific Payload Venus Epress will deliver to the planet a powerful comple of spectral and mapping eperimental equipment. Its capabilities include both high-resolution spectrometers and a wide-angle filter camera. Figure 4 shows spectral characteristics of the instruments of Venus Epress. The optical eperiments cover the spectral range from ultraviolet (0.1 µm) to thermal infrared (50 µm) emission with a resolving power of 100 to This instrumentation will provide for a possibility to study (in great detail and with high precision) the structure, composition, energy balance, and other properties of the atmosphere and the surface. Such a set of optical instruments will operate in the Venus orbit for the first time. Figure 5 presents a comparison of the fields of view of the optical instruments. They vary from a wide-angle camera that grasps the entire Venus disk in the apocenter down to the instruments whose field of view is a few degrees. VIRTIS has the best angular resolution (0.25 mrad), which allows it to reach spatial resolutions of 100 m and 16 km in the pericenter and apocenter, respectively. Different fields of view of the Venus Epress instruments allow one to combine the observations with high spatial resolution and observations giving the general contet. One should note two other important advantages of Venus Epress over preceding observations. First, VIR- TIS and VMC give a possibility to get almost instantaneous pictures. By comparison, it was required about 4 hours for Pioneer Venus to make one picture, since spatial scanning was provided by rotation of the spacecraft itself. Second, Venus Epress has a three-aial system of stabilization, which provides for considerable opportunity of pointing the instruments. This will make it possible for Venus Epress to fully cover all latitudes and longitudes of the planet with much better spatial and temporal resolution than in all preceding missions. The use of various observational methods and a broad spectral range will provide for complete and reliable coverage of the entire field of research programs in the altitude range from the surface to the thermosphere. In many cases the measurements of different instruments will supplement each other. The principles of mutual complementarity and team work are actively used when planning observations onboard the Venus Epress. SPACECRAFT Venus Epress re-uses the design of the Mars Epress spacecraft, as well as some available onboard

11 344 TITOV et al. systems [32]. Small modifications of the satellite are caused basically by four times higher flu of the solar radiation at Venus and by the necessity to accommodate some new instruments on the spacecraft. Such an approach allowed ESA to minimize both risks and costs of the project and, as a result, to prepare the spacecraft for launch in November Venus Epress is a spacecraft of cubic form with a dimension of about 1.7 m (Fig. 6). Onboard systems and eperimental equipment are arranged on walls and inner boards of the satellite that are mounted around the fuel tank. Venus Epress has a three-ais system of attitude control. It includes star trackers, gyroscopes, and reaction wheels. Communication with the spacecraft will be maintained with the help of one low gain and two high gain antennas: the main antenna with a diameter of 1.3 m and the auiliary 30-cm antenna for communication in the periods of Venus lower conjunction when the distance to the Earth is small. The maimum rate of data transmission from the Venus orbit will be equal to 228 kbit per second. The electrical system will provide for a high degree of autonomy of the satellite and for sufficient amount of power at all stages of the flight including solar eclipses in orbit lasting about 1 h. The spacecraft has symmetrically located panels of gallium-arsenide solar batteries with a total area of 5.7 m 2 which provide for a power of 1400 W. Lithium storage batteries with a total capacity of 24 A h serve as onboard accumulators of electric power. The onboard system of data acquisition has two channels: high- and low-rate channels. The first of them assumes direct recording of data into the onboard storage device with a capacity of 12 Gbit. This channel will be used by the VIRTIS and VMC instruments which produce a large amount of data. The second channel includes a special interrogation unit. Collection of data from other instruments and their recording to the storage device will be performed through this channel. The propulsion system includes a cruise propulsion unit with a thrust of 415 N (in order to reach the Venus orbit and to form a working orbit) and eight low-thrust engines (10 N) of the system of attitude control and orbital maneuvers. The total onboard supply of two-component propellant is 570 kg, which is sufficient for injection into orbit and for operation in this orbit for 3 years. MISSION SCENARIO AND WORKING ORBIT Venus Epress was launched on November 9, 2005 from the Baikonur cosmodrome in Kazakhstan using Russian rocket launcher Soyuz with a Fregat booster. In the first month after the launch the systems of spacecraft and the eperimental equipment were tested. This operation included tests of separate instruments, taking images of the Earth and the Moon, and observations of point-like sources (such as stars and Venus) and the Sun. These tests demonstrated that all systems of the satellite and eperimental equipment operated nominally. The focusing of optical instruments was also checked, the precise location of optical aes in space was determined, and the thermal regime was investigated. In the beginning of the flight the magnetometer boom was opened, and this instrument has started regular observations. The first measurements of the particle flues of interplanetary plasma by ASPERA were also successful. At the transfer stage a regular communication with Venus Epress was kept, and some additional tests of the instrumentation were made. On April 11, 2006 the Venus Epress spacecraft was inserted into an orbit of a planetary artificial satellite. The first ten-day capture orbit was strongly elongated with a maimum distance from the planet km. Already in this orbit, along with necessary dynamical operations, the first observation runs of Venus were made. Such a hurry is justified by the fact that the capture orbit provided for unique opportunity of observations that would not take place again. For eample, VIRTIS had an opportunity to see full disk of Venus from a distance of km, which was important for studying the general circulation of the atmosphere in the southern hemisphere and, especially, for searching for the dipole vorte in the south polar region. The conditions of observing heliospheric energetic neutrals by ASPERA were also unique. SPICAV made a search for a hydrogen corona around Venus. Several subsequent orbits were used to form the working orbit with a revolution period of 24 h and a distance in the apocenter about km. After that several weeks are spent for trying out various modes of orbital observations which require precise coordination of operations eecuted by the spacecraft, scientific payload, and ground-based control center. The regular scientific observations will continue until October 2007 (nominal mission). To this time two Venusian days will elapse so that the observations will cover the daily planetary cycle twice. The spacecraft lifetime is designed for two additional Venusian daily cycles, i.e., for 500 days more of orbital observations (etended mission). The polar working orbit is chosen for Venus Epress with a pericenter at 78 N (Fig. 7). The orbit is strongly elongated with the distance in the pericenter/apocenter 250/66000 km and with a revolution period of 24 h. Under the action of gravitational attraction of the Sun the pericenter height will increase during the nominal mission with a rate of 1 2 km per day. In order to keep the pericenter height in the range km, regular orbit corrections will be made. In addition, the pericenter will drift slowly poleward. No compensation of this drift is planned. The resources of the spacecraft are planned for three years of orbital observations. Further strategy can include three possible variants. The first is to continue operations for supporting the pericenter at low altitude