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Adv. Space Res. Vol. 29, No. 2, pp. 255-264,2002 Q 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved.

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THE LAVOISIER MISSION : A SYSTEM OF DESCENT PROBE AND BALLOON FLOTILLA FOR GEOCHEMICAL INVESTIGATION OF

THE DEEP ATMOSPHERE AND SURFACE OF VENUS

E. Chassefiere’, J.J. Berthelier2, J.-L. Bertauxj, E. Qubmerais3, J.-P. Pommereau3, P. Rannouj, F. Raulin4, P. Co114, D. Coscia4, A. Jambon5, P. Sarda6, J.C. Sabroux’, G. Vitter8, A. Le Pichonc, B. Landeaup, P. Lognonnel”, Y. Cohen’u, S. VergniolelO, G. Hulotlu, M. Mandea’O, J.-F. Pineaull, B. BCzard’2, H.U. Keller13, D. Titov13, D. Breuer14, K. Szego R. Rodrigo19, F.W. Taylor20, A.-M.

15, Cs. Ferencz16, M. Roos-Serote17, 0. Korablev18, V. Linkin18, Harri21

’ Laboratoire de Mteorologie Dynamique (LMD/IPSL), Universite Pierre et Marie Curie, Boite 99, 75252 Paris Cedex 05, France 2 Centre d’Etude des Environnements Terrestres et Planetaires (CETP/IPSL), France 3 Service d’Adronomie (SW IPSL), France 4 Laboratoire Interuniversitire des Sj&mes Atmosphtriques (LISA, Univ. Paris 11 & 7), France 5 Laboratoire MAGIE (Univ. Paris 6), France 6 Groupe de Geochimie des Gaz Rares (Univ. Paris 11), France 7 Institut de Protection et de Stirete’ Nucleaire (IPSN), France 8 Laboratoire d’Electrochimie et de Physicochimie des Materiaux et des Interfaces (LEPMI), France 9 Commissariat a I’Energie Atomique (CEA), France 10 Institut de Physique du Globe de Paris (IPGP), France 1 1 Socitte ALBEDO Technologies, St Sylvestre, France ‘2 Observatoire de Paris, section de Meudon (DESPA/OP), France 13 Max Planck Institut f?ir Aeronomy (MPAe), Germany 14 Institute fir Planetology (Univ. Muenster), Germany ‘5 Hungarian Academy of Science (KFKI), Hungary ‘6 Space Research Group, Department of Geophysics (Univ. Eotvos), Hungary 17 Observatorio Astronomico de Lisboa (Univ. ‘8 Space Research Institute (IKI), Russia

Lisbon), Portugal

‘9 Institute de Astrofisica de Andalucia (CSIC), Spain 20 University of Oxford, United Kingdom 21 Finnish Meteorological Institute (FM,), Finland

ABSTRACT

Lavoisier mission is a joint effort of eight European countries and a technological challenge aimed at investigating the lower atmosphere and the surface of Venus. The mission consists of a descent probe and three balloons to be deployed below the cloud deck. Its main scientific objectives may be summarized as following : (i) composition of the deep atmosphere (elemental/ isotopic), oxygen fugacity ;

: noble gas (elemental/isotopic), molecular species vertical/horizontal/temporal variability ; (ii) infrared spectroscopy

and radiometry (molecular composition, radiative transfer) ; (iii) dynamics of the atmosphere : p, T, acceleration measurements, balloon localization through VLBI, meteorological events signed by acoustic waves, atmospheric mixing as imprinted on radioactive tracers ; (iv) surface morphology and mineralogy through near infrared imaging on dayside, surface temperature through NIR imaging on nightside. Additional tentative objectives are search for (a) atmospheric electrical activity (optically, radioelectrically, acoustically), (b) crustal outgassing and/or volcanic activity : acoustic activity, horizontal/vertical distribution of radioactive tracers, (c) seismic activity : acoustic waves transmitted from crust to atmosphere, and (d) remanent and/or intrinsic magnetic field. Lavoisier was proposed to ESA in response to the F2/F3 mission Announcement of Opportunity at the beginning of 2000, but it was not selected for the assessment study. A wide international partnership was created for this occasion, including Finland (FMI), France (IPSL, MAGIE, Universite Orsay, IPSN, INPG, CEA, IPGP, Obs. Paris-Meudon), Germany (MPAe, Univ. Muenster), Hungary (KFIU, Univ. Eotvos), Portugal (OAL), Russia (IIU), Spain (IAA), United Kingdom (Univ. Oxford).

0 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved.

255

256 E. Chassefitre et al.

GENERAL SCIENTIFIC OBJECTIVES One of the most promising area in Solar System science is the comparative study of the three terrestrial

planets (Venus, Earth, Mars). Why did the three planets evolve in such different ways, from relatively comparable initial states? The small size of Mars, favouring atmospheric escape and allowing a relatively fast cooling of the interior, certainly played a role in making the present Mars so inhospitable. Venus has almost the same size and density as Earth, and was probably initially endowed with similar amounts of volatile material. The absence of water in significant amounts, possibly explained by intense hydrogen escape at early epochs, and of molecular oxygen in the present atmosphere, which requires extremely strong primitive escape and/or massive oxidation of surface material, remains poorly constrained and understood. Because of the massive carbon dioxide atmosphere, there is a large greenhouse effect at the surface of Venus, where the temperature is +70°C. The way such a strong greenhouse built up and stabilized is not well understood. How did the atmosphere evolve, under the combined effects of escape and interaction with solid planet? How did the history of tectonism and volcanism influence, or was influenced by, atmospheric greenhouse, mantle convection and core solidification rate? Did the last general resurfacing event, -0.5-l Gyr ago, occur in coincidence with complete freezing of the core, and vanishing of a primitive dynamo? Which processes are presently controlling the thermochemical equilibrium between the surface and the atmosphere, what is the nature of the feedback processes, what are the roles of carbon, sultir and chlorine cycles? Is there a present volcanic activity?

The Magellan mission, through radar imaging, topography and gravity measurements, provided an enormous amount of information, allowing substantial advances in Venus geodynamics. Our knowledge of Venus geochemistry is less advanced, despite Pioneer Venus and soviet missions, which provided a good preliminary insight into atmospheric composition (and surface composition), but technological advances should allow to go much further. We propose a geochemical mission consisting of a Venus descent probe/balloon flotilla system oriented toward : (i) measuring with a high accuracy the noble gas elemental and isotopic composition ; (“) h II c aracterizing the main chemical cycles (carbon, sulfur, chlorine) in the low atmosphere, and their interaction with surface minerals ; (iii) helping in improving radiative transfer models through the spectral measurement of infrared fluxes, and measuring the surface temperature ; (iv) constraining small-scale and meso-scale dynamics of the deep atmosphere (winds, atmospheric mixing, meteorological events like storms) ; (v) constraining the morphological and mineralogical characteristics of the surface through visible-near infrared spectra-imaging ; (vi) searching for still hypothetical phenomena like : lightnings, volcanic activity, crustal outgassing, seismic activity, remanent magnetization.

Noble Gases and Light Elements : Elemental and Isotopic Composition The primary atmospheres of the terrestrial planets may be either derived by outgassing from the solid

interior or accreted from the outside (e.g. addition of cometary material). Secondary processes such as escape, outgassing from the solid planet, radiogenic contribution, solar wind addition, sputtering, chemical conversion (carbonate precipitation, photosynthesis, water decomposition etc...) may have significantly altere their primary composition. Noble gases are ideal tracers to understand the formation and evolution f the ! atmosphere of terrestrial planets. Their chemical inertness and the presence of numerous isotopes among which radiogenic ones (e.g. 4He, 40Ar), nucleogenic ones (e.g. 21Ne) and fissiogenic ones (134-136Xe) permit to identify a number of possible sources to planetary atmospheres as well as the fingerprint of physical processes previously mentioned. Unlike chemical compounds, they can be found in the atmospheres of the terrestrial planets, in meteorites of various types and in the solar wind. Attempts at modelling the terrestrial atmospheric evolution have confirmed the high potentiality of these elements. Comparative planetology is clearly the next step to go.

The composition of Venus atmosphere according to the previous measurements by Pioneer and Venera missions is : CO2 (96.5 %), N2 (3.5%), He (12 ppm), Ar (70 ppm), Ne (7 ppm), Kr (0.7-0.05 ppm), Xe (?). The only element for which an isotopic composition is available is Ar but it is worth considering a confirmation because of possible interferences in the previous measurements (e.g. with HCI) especially if an accurate measurement of the 36Ar/38Ar is necessary. Therefore estimates of He, Ne, Kr and Xe isotopes would provide invaluable information necessary for comparative planetology and to constrain models of atmospheric formation and evolution (for both Venus and Earth).

- Helium : the abundance and isotopic composition of helium results from the competition between : (i) outgassing from the solid interior ; (ii) escape from the atmosphere ; (iii) input from solar wind. On Earth, outgassing from the solid interior consists of two well identified fluxes : one rich in 3He (primordial) from the mantle and a purely radiogenic one in relation with crustal outgassing. The same components will have to be considered on Venus but with a completely different relative importance: volcanism on Earth in relation with plate tectonics is a major phenomenon. Release from the crust is a rapid process because of erosion. On Venus the much higher temperature should promote escape from deeper crustal layers but how does erosion play a role? Input from the solar wind is limited on Earth because of the magnetic field. The absence of a significant field on Venus will have an influence on the abundance and isotopic composition of He in Venus atmosphere.

- Neon : in the terrestrial atmosphere, Ne isotopes are intermediate between a planetary component (chondritic) and a solar component. The question is whether this results from mixing of these two

The Lavoisier Mission 257

components (mantle Ne is solar) or more simply, from mass fractionation as a result of massive escape from the atmosphere in its early stages. Mesuring Ne isotopes in the atmosphere of Venus will greatly help constraining such problems.

- Argon : 40Ar is the radiogenic product of 4oK. On Earth, it is a major component of the atmos here (l%), testifying of the importance of outgassing. On Venus, 4nAr is less abundant than on Earth while 3 Ar is g

far more abundant, illustrating a drastic difference between the two planets. Where does 36Ar come from? Its ratio to Ne precludes a significant solar contribution. Its total abundance corresponds to what is recorded in the most gas rich chondrites, a very surprising result.

- Krypton and Xenon : Krypton isotopes may provide some information on the escape processes because of the possibly mass fractionated pattern easy to interprete in the absence of radiogenicinucleogenic contributions. Its ratio to Ar and Xe would permit to characterize its source in Venus atmosphere. Xenon carries more information than anyone of the other noble gases because of its numerous isotopes including radiogenic/tissiogenic isotopes from short and long lived parents. Is the terrestrial missing xenon a unique signature of our planet? Does Venus exhibit isotopic anomalies in 129Xe, 134-336Xe? Are Venus atmospheric Xe isotopes mass fractionated relative to the Earth/Solar references?

Concerning isotopes of light elements : H, C, N and 0, they exhibit highly variable isotopic compositions depending on the diverse chondrite classes and planetary objects. They result from : (i) nucleosynthetic processes ; (ii) heterogeneity in the Solar nebula ; (iii) fractionation processes upon accretion ; (iv) differentiation and evolution of planetary objects. On Venus, where the temperature is significantly higher than on the Earth, fractionation is expected to be less significant. From the isotopic composition one therefore expects mostly information dealing with comparative planetology. Under this aspect, oxygen isotopes (160, 170, 180) will be of particular significance as they permit establishing different fractionation lines for the diverse objects of the Solar system. In the future, analyses of solid samples will require an accurate knowledge of this atmospheric reference.

Main Chemical Cycles in the Low Atmosphere Several questions concerning the molecular composition of the Venus atmosphere remain unresolved.

The most important ones are : (i) Middle atmosphere: “What is 02 abundance near the cloud tops?” (Von Zahn et al., 1983) ; (ii) Lower atmosphere: “Better measurements of composition, including vertical and horizontal variation of such species as SO2, SO, H2S, Cl2, COS, and H20. In particular, detailed comparison of in-situ and remote observations of Hz0 in deep atmosphere” (Esposito et al., 1997) ; (iii) Surface-atmosphere interactions: “There is still a pressing need for a deep atmospheric spacecraft to measure the chemical composition of the lower 22 km of the atmosphere. The gases of most interest are H20, CO, S@, OCS, Sl_8, and H2S, and the method to measure them is probably infrared spectroscopy...” (Fegley et al., 1997).To reach a coherent picture of the chemical cycles operating throughout the whole Venus atmosphere, coupled in its lower layers with surface through thermochemical equilibrium processes between vapor and mineralogical phases, requires the simultaneous measurements of the concentration of molecular species, the oxygen fugacity, and, if possible, phenomena signing mixing processes, competing with thermochemical processes in the low atmosphere.

The question of the (never measured) value of the oxygen fugacity near the surface and in the first kilometers above it, is of crucial importance. Calculations of the oxygen mixing ratio, from the CO/CO2 ratio and the temperature, thanks to spectrometric observations of the upper Venusian atmosphere from the Earth, or on the basis of mass spectrometric/gas chromatographic analysis down to the surface of the planet (Pioneer-Venus and Venera 12), are somewhat unsatisfactory because of dubious chemical equilibria attainment throughout the atmospheric column and/or uncertainties in atmospheric homogeneity and compositional analysis. Given the 17+1 ppm carbon monoxide mixing ratio measured on board Venera 12 in the lower Venusian atmosphere (12 km), a 50% confidence interval on such a value extrapolated down to the planetary surface give rise to an order of magnitude incertitude on the calculated oxygen mixing ratio at 73 5 K, a variation of 10 K yielding a factor 3 error on the inferred value. The most comprehensive study of the redox state of the lower atmosphere of Venus still proposes an almost two orders of magnitude uncertainty of the surface oxygen fugacity, centered at 10-20.85 bars (Fegley et al., 1997).

Abundances of gases like SO2, COS, HCI are controlled by thermochemical reactions with minerals at the surface. The CO2 pressure is assumed to be maintained through equilibrium with surface calcium silicates. Although thermodynamical calculations are in rather good agreement with observation (temperature, CO2 pressure), the basic question of the stability of such a system is still unanswered. Concerning SO2, equilibrium values definitely differ from observed values, and existing measurements significantly differ from each other for unknown reasons. The finding by Bertaux et al (1996), from UV absorption measurements of SO2 from the Vega descent probes, that the SO2 mixing ratio regularly decreases with decreasing altitude from 100 pmv at 40 km altitude, just below the clouds, down to 25 ppmv at 10 km altitude, is particularly intriguing, It contradicts the current model of thermochemical equilibrium with calcite which results in an equilibrium value of SO2 mixing ratio at the surface of loo-130 ppmv (Fegley et al, 1997), in agreement with the different existing measurements below the clouds, but not with the low value measured by Vega in the low atmosphere. As mentioned by Bertaux et al (1996), such a vertical variation could be associated with horizontal variations,

258 E. Chassefikre et al.

through the existence of sinks and sources, and therefore of regions where the vertical gradient is reversed. Horizontal coverage, by using balloons, could provide an answer to this question.

Another point mentioned by Bertaux et al (1996) is the strong link between SO2 and CO concentrations, through both the gas/surface reaction previoulsy mentioned and the gas/gas CO+S02 reaction. A simultaneous measurement of CO, 02 fugacity and SO2 should allow to better characterize the Venus chemical system. Characterization of surface material, in particular the Fe-bearing minerals, would considerably help in understanding the general equilibrium of low atmosphere. The question of the intensity of the vertical atmospheric mixing by turbulence and large scale dynamical processes deserves to be mentioned. In the absence of any information, it is impossible to derive the vertical SO2 flux from the vertical gradient observed by Bertaux et al (1996). Another important implication is that vertical mixing times t,ix are expected to be of the same order as chemical times t&em associated with the main thermochemical equilibria, like the CO/SO2 equilibrium previously mentioned. If t,ix<t&m, a gas like CO is uniformly mixed with altitude, whereas if t,ix>t,hem, thermochemical equibrium is realized everywhere, and the CO mixing ratio increases with altitude. Measuring the vertical profiles of reactive species, and temperature, in the O-IO km altitude range should therefore allow to put constraints on vertical mixing, together with other means, like vertical and horizontal profiles of radioactive tracers (radon).

Radiation Budget of Venus Atmosphere The understanding of the mechanisms of Venus atmosphere heat balance and persistence of its high

surface temperature are very important for the studies of the climate evolution on terrestrial planets. Lower atmosphere is hidden by the dense cloud layer and difficult to sound, but it is assumed to be important for the maintenance of the prominent four-day superrotation. Knowledge of the thermal balance and role water vapor plays in it is also important for understanding the chemical evolution of the surface of planet.

At present, the greenhouse effect is generally accepted as the only explanation of the thermal structure of mid-to-lower Venus atmosphere. There are, however, some evidences that wave dynamics may yield an important contribution to the thermal balance below the cloud deck. These evidences are : (i) major part of solar radiation is being absorbed inside cloud deck ; (ii) temperature profile is latitudinally invariant, nearly linear, essentially subadiabatic and only slightly alters its shape in the cloudy region ; (iii) the balance between visible and IR radiation in the lower atmosphere is so far not well established. The answers to these questions require the knowledge of upward and downward fluxes in the lower atmosphere in the wide range from NIR to thermal IR. In present, there are no experimental evidences about the shape of the IR spectrum of Venus under the clouds. The efforts to model theoretically the radiation transfer (Marov et al., 1985 ; Pollack et al., 1993) do not produce convincing results due to major difficulties in computing spectra at high pressure and temperature. Conventional Voigt profile of spectral lines is not appropriate for the high-pressure collisional broadening resulting in essentially sub-lorenzian form factor. Another source of uncertainty is the major contribution of hot bands and water vapor, with contents suggested to be highly variable. These factors affect drastically the spectrum shape and, consequently, determine the radiation transfer balance in transmission windows.

Lightning and Radio-Electric Emissions Conditions prevailing on the surface and in the lower atmosphere of most of the planets in the Solar

System are likely to give rise to their electrification with, as a possible consequence, electrical break-up and discharges. Besides its own interest as a fundamental process in planetary environments, atmospheric electricity may also have a significant role in several domains of atmospheric physics such as dynamics of dust particles and clouds, meteorology and gas chemistry particularly because electrical discharges may produce minor species more efficiently than photochemical processes. Furthermore, it is generally admitted that, at least on the Earth, the appearance of prebiotic molecules and the first steps in the life development were linked with a strong electrical activity in the primitive atmosphere.

The existence of lightning on Venus has been the subject of considerable debates since American and Russian missions were flown to the planet (e.g. Russell, 1991). Lightnings require processes that are able to generate and separate electrical charges of opposite polarity. In reference to the case of the Earth, charge separation generally occurs in regions of intense vertical convection with embedded aerosols with a wide size spectrum. Although information available on the lower atmosphere of Venus did not bring evidence of such conditions, the observations were certainly much too limited to preclude the possible existence of electrical charging mechanisms in particular in the cloud layer and more specifically in the middle of this layer, around 50 km. Levin et al. (1983) have indeed demonstrated that large electric fields can build up if one takes into account the existence of particles of larger sizes and a more intense mass loading than was observed by the descent probes. If lightning are to occur, they should arise from it&a-cloud discharges since, due to the high pressure at ground level, the breakdown voltage is certainly too large for electrical discharges to occur below a few kilometer altitude.

The search for lightning was first performed by analyzing optical data of Venera 9 (Krasnopolsky, 1980) and from telescope observations from Earth (Grebowsky et al., 1997). A number of spurious events were detected with rather different temporal signatures but which, nevertheless, were identified by the authors as

The Lavoisier Mission 259

as indicating the existence of lightning. Electrical strokes can be detected not only through optical emission but also by means of the radio-electric noise that is generated by the corresponding electrical current, as observed from descent probes, mainly Venera 9 and Venera IO (Ksanfomality, 1980). Plasma wave measurements on-board Pioneer Venus Orbiter were used to try to retrieve the signature of transient events bearing some resemblance with low-frequency wave emission that follow terrestrial lightning. Burst observed in the 1OOHz channel in the night sector are consistent with escaping ionospheric whistler waves (Grebowsky et al., 1997) ; owing to the possible propagation of electromagnetic waves in the sub-ionospheric wave guide over large distances, they are indicative of regions where ionospheric conditions favor the escaping of whistler waves rather than of optimal conditions of generation. Magnetic fluctuations observed on the Venera descent probes have been interpreted as evidence of persistent dayside lightning frontal storms.

Interior, Surface, Surface/Atmosphere Interactions Independent estimates of the present volcanic activity on Venus based on geophysical, geological, and

geochemical data generally suggest maximum extrusion rates of approximately 0.4 km3 yr’. Considering that extrusions are assumed to account for only 5-10% of the total crust production, the upper limit of the crustal growth rate including intrusions may be about 4 km3 yr’. This value is much smaller than the terrestrial crust production rate of approximately 20-30 km3 yr l. On Earth, crust production occurs predominantly at plate boundaries, which provide effective pathways for ascending magma to produce new crust at divergent and convergent margins. On Venus, however, there is no plate tectonics at present. Instead, the convecting mantle is covered by an outer “one-plate” layer, or rather, a stagnant lid, through which magma transport is much more difficult. Such an absence of plate tectonics is reflected on Earth only in the interior of tectonic plates, and in fact, the terrestrial intraplate crust production rate is similar to the assumed maximum crust production rate of Venus. However, no final evidence exists that there is present-day crust production on Venus, at all.

Previous models of the thermal evolution of Venus including mantle differentiation have shown that the high temperatures and the strong convection vigour in the early evolution result in a rapid crustal growth. Associated with the crustal growth is a strong early mantle degassing. Volatiles like H20, CO2, and noble gases are released into the atmosphere. The evolution of the atmosphere is, therefore, strongly related to the crust production and the associated mantle degassing but also to the capture of in-falling comet-like material enriched in volatile components. In the subsequent evolution, the atmosphere has been fed by continuing crustal growth and mantle degassing. These models assume a continuously evolving planet. On Venus, however, the cratering record of the surface indicates a global or near-global resurfacing event about 750 to 500 Ma ago. The volume that erupted is estimated to exceed the largest igneous provinces on Earth by as much as an order of magnitude. Whether this volcanic activity ceased abruptly or gradually is not clear, yet. Also the explanation of this resurfacing event has been debated diversely. Possible explanations for this catastrophic event include episodic mantle overturning or a transition from a plate tectonic-like convective regime to the present stagnant-lid convective regime. The consequences of the resurfacing for the evolution of the atmosphere and the surface temperature are still uncertain, as well. Models propose an increase of the surface temperature due to a strong release of greenhouse gases. Alternatively, it has been suggested that the surface temperature did not change significantly due to a chemical-albedo feedback.

Future models of Venus’ mantle convection, thermal evolution, and atmospheric evolution shall provide an explanation for the resurfacing event. For better-constrained models, more precise information on the present state of the atmosphere and the interior structure of the planet including its temperature distribution is needed. To obtain this data base, the knowledge of a possible existence and the amount of present-day crust production is most relevant. This information can be obtained through the atmospheric data provided by the Lavoisier mission (molecular composition, distribution of radioactive tracers, volcanic gas, aerosol plumes). With the help of the above described interrelations between the various factors influencing the crust production or its rate, more precise present-day values for the mantle temperature, the convection velocity, and the lithosphere thickness can be determined. These parameters can be used, in turn, as boundary conditions for the coupled thermal and atmospheric evolution models in order to lead to a comprehensive understanding of the planet Venus and its evolution.

If there has been an irreversible decrease of tectonic/volcanic activity in the last few hundred million years, following episodic stages of strong heat release from interior, and therefore core solidification, and if the vanishing of the internal dynamo did not occur prior to the last episode of general resurfacing, frozen lava floods could keep the trace of the vanishing dynamo. The surface temperature of Venus (47OOC) is below the Curie point of some pure magnetic minerals contained in basalts, like magnetite (58O’C). Nevertheless, X-ray fluorescence measurements at the surface of Venus (Venera 13, 14, Vega 2) show that the Venusian rocks contain titanium, in amounts similar to terrestrial basalts, suggesting that the main magnetic phase could be titanomagnetite, with a Curie temperature which may be as low as 150-300°C. But note that iron oxides, during formation, are progressively enriched in titanium, with a subsequent evolution (lowering) of the Curie temperature. Moreover, the Curie temperature depends on the physical state of the magnetic material (granulometry, degree of impurity), which may change with time and location. For these reasons, the Curie

260 E. Chassefike et al.

temperature is in fact a Curie temperature interval, whose lower and upper limits are difficult to predict in the case of Venus

Searching for a remanent magnetic field is therefore interesting, but must be considered as tentative. A positive detection of a remanent magnetic field would give information on the thermal history, and past core dynamo. As in the case of lightnings, horizontal coverage is expected to bring substantial additional science. Note that the presence of a weak residual intrinsic magnetic field cannot be excluded. Considering magnetic fields external to the planet we note the presence of fields of the order of 100 nT in the magnetosheath between the ionosphere and uninterrupted solar wind. This region extends from about 300 km to 2000 km altitude on the day side, and it is likely that there is an induced magnetotail downstream of the planet in the solar wind.

Main Scientific Objectives We propose to reach the previous objectives through the use of a system including a descent probe and an

ensemble of 3 balloons, deployed at various locations on Venus (North/South hemispheres, Day/Night sides), floating during one day at 10 km altitude and descending down to the surface in a final stage. Through this system, we plan to address the following general scientific questions : - Composition of the deep atmosphere : noble gas (elemental/isotopic), molecular species (elemental/ isotopic), oxygen fugacity ; vertical/horizontal/temporal variability. - Infrared spectral fluxes (molecular composition, radiative transfer). - Dynamics of the atmosphere : p, T, acceleration measurements, balloon localization through VLBI, meteorological events signed by acoustic waves, atmospheric mixing as imprinted on radioactive tracers (like radon). - Surface morphology and mineralogy through near infrared imaging on day-side (in well chosen spectral intervals in the 0.6- 1.3 pm range), surface temperature through NIR imaging on nightside. - Search for atmospheric electrical activity (optically, radioelectrically, acoustically). - Search for crustal outgassing and/or volcanic activity : acoustic activity, horizontal/vertical distribution of radioactive tracers. - Search for seismic activity : acoustic waves transmitted from crust to atmosphere. - Search for remanent and/or intrinsic magnetic tield.

TECHNICAL AND OPERATIONAL ASPECTS

Payload Description

Descent Probe (see table 1) - Noble gas/stable isotope mass spectrometer (elemental/composition). The may spectrometer (MS) is

composed of a preparation line and a mass spectrometer. A sample of a fraction of cm will be taken for noble gas analysis. The exact amount will depend on the exact volume of the mass spectrometer and its sensitivity. In order to obtain high precision determination in the isotopic compositions, it is necessary to first purify the gases and to separate the gases from one another. This can be achieved by reacting the gases on hot Ti sponge. The time necessary for purification (a few minutes) will have to be determined as the time allowed for analysis will be rather short. For a high precision one must take enough time (counting statistics), as the less abundant isotopes are about 105 (helium) to 300 times less abundant than the major isotope. For a complete analysis of all isotopes, a minimum of 23 peaks (plus background) have to be analyzed, plus subsidiary peaks to resolve possible interferences. This problem can be partly resolved if argon can be separated from the other less abundant gases. We therefore propose to introduce gases into the mass spectrometer through a membrane. This will permit to analyse first He and Ne. A second membrane will permit to introduce Ar, and finaly Kr and Xe will be analyzed. The choice of the membranes requires some preliminary work. In order to obtain a high precision on isotopic ratios, several cycles (5-10) of data acquisition will be programmed : for counting times of 5 seconds on each peak a total time of 55 minutes is necessary for the analysis of all isotopes of the five noble gases including purification. A schematic diagram of the preparation line is displayed in Fig. 1.

-Gas chromatograph (GC), electrochemical oxygen sensor (OES), optical gas analyser (OLGA) (molecular/isotopic composition). Detailed descriptions of these instruments may be found in the proposal (E. Chassefiere and D. Coscia, A mission to Venus : Lavoisier, Answer to ESA call for mission proposals for two fleximissions (F2 and F3), submission date : 27/01/2000). The dual column GC consists of a MolSieve-type column for the separation of the noble gases (Ne, Ar, Kr, Xe), N2, 02, CO and CH4, and a Silica-Plot or Plot U polymer column for SO2, CO2, COS, H2S, H20, NH3 and C2 hydrocarbons. OES works as a fuel cell. OLGA may use the TDLAS active method (Tunable Laser Diode), or UV absorption and/or emission (fluorescence measurement).

- Infrared spectrometer (atmospheric composition, radiative transfer), Visible/Near Infrared spectro- imager (atmospheric composition, surface mineralogy/morphology on dayside, surface temperature on night side). The VISS (Venus Imaging Solid-state Spectrometer) uses the AOTF technology (Acousto-optic Tunable System). It may be used for reflectance spectroscopy (0.5-1.2 urn range), in order to determine the

The Lavoisier Mission 261

mineralogy of surface material, as well as probing of atmospheric gases in the thermal infrared (1.7-4 urn and beyond). Several windows exist, where the transparency lenght (z=l) is small (a few hundred meters to a few kilometers). It is divided into a VIS-NIR and a IR instrument. An option could be to use a grating spectrometer (Visible Near-Infrared Spectra-imager VNIR).

Membrane

LAVOISIER : preparation line

Fig. I. Schematic diagram of the preparation line.

- Radioactive tracer detector (signature of crust outgassing/volcanism, atmospheric mixing). The APlD instrument (Alpha Particle Ionisation Detector) is aimed at measuring the airborne naturally occurring radionuclides of the uranium and thorium series, in order to constrain outgassing and atmospheric vertical mixing.

- Atmospheric sensor package APTIV (p, T sensors, accelerometer).

Table 1. Power, Mass and Telemetry Budget of the Descent Probe

Instrument

MS

Power Mass Volume (average) (kg) (Liter) 5.6 5 I

Data Rate (kbitsls) 0.1

Gc OES ; A.;2 OLGA/TDLAS 1.3 014 OLGA/U’ 5 1

VISSNIS-NIR 2.5 VISSAR 6 ! 1 APID 0.1-l 2:3 APTIV 0.5 0.7 TOTAL 31.9 13.0 Alternative to VISS/VlS-NIR : VNIS 7 3 * A data compression rate of 5 is assumed.

1.6 0.01 0.2 1

!

;5 9:31

0.15* 0.001

A::

*

;.;:

0:1 0.36* 8.1

4.5 3

Balloon Probes (see table 2) - Gas chromatograph (GC), electrochemical oxygen sensor (OES), radioactive tracer detector (APID). - Infrared radiometer (radiative transfer, composition). The VERBE experiment (Venus Energy and

Radiation Budget Experiment) consists of a small radiometer operating in four spectral ranges : 1.7 urn, 2.4 urn, 3.7 urn, and an integral channel : 0.8-4 urn, looking upward and downward.

- Radio science (RS) : localization of balloons by VLBI (atmospheric dynamics). - Radioelectric receiver (RWAEI) and optical photometer (LOD) (for lightning detection). - Acoustic and magnetic sensor AMS (volcanoes, lightnings, meteorological events ; quakes ;

remanent/intrinsic magnetic field). - Near infrared camera NIRI for surface characterization (morphology/mineralogy on dayside,

temperature on nightside). - Electromagnetic sounding of inner planet by magnetometry (AMS). - Relaxation probe for conductivity measurements (RP).

262 E. Chassefike et al.

Table 2. Power, Mass and Telemetry Budget of the Balloon Probes

Instrument Power Mass Volume Data Rate (average) (kg) (Liter) (kbits/s)

Gc 2.7 1.5 1.6 0.03 OES AMS NIRI LOD APID VERBE RWAEl RP APTIV

0 0.5 co.1 2 0.1-l 1.2 1.7

:::

0.02 0.01

;.:2 co.3

0:07 0.15 0.25 2.3 3 co.5 0.1

~*~-1.1 co.3

0:7 0.4- 0.5 1.2

0.00 0.1 0.4* co.0 0.1 0.2* 0.4* 0.03 0.36

TOTAL 10.2 8.1 * A data compression rate of 5 is assumed.

7.4 1.7

Launch and Operational Sequence The spacecraft at launch is composed of : (i) The bus, equipped with the telecommunication system,

which may be similar to the platform developed for the Mars-Express mission (mass : 440 kg) ; (ii) The descent probe, which is an heritage of the Huygens probe (mass : 190 kg) ; (iii) 3 Balloon Probes (mass : 140 kg each), which can benefit of the heritage of the Mars balloon ; (iv) Hydrazine for maneuvers (mass : -50 kg). The total mass of the spacecraft at launch is -1100 kg. The spacecraft is put on an Earth-Venus transfer orbit by a Soyuz-Fregat launcher. The arrival velocity at Venus, 3 months later, is about 4 km/s. At the pericenter, the velocity of the fly-by bus increases up to nearly 11.5 km/s. The date of arrival at Venus of the bus is denoted by to. The sequence of operations is as following :

I- tl = to-20 days : release of the the 3 Balloon Probes (BPS). By adequatly spinning the bus, the release velocities of the BPS, that is their velocities relative to the bus, are adjusted in such a way to distribute them on a great circle of typically 1500-2000 km radius at Venus level (initial required relative velocity of 1 m/s), centered on a point, to be defined, near Venus disk center. BPS are expected to sample latitudes and local times (e. g. (i) one in dayside southern hemisphere, (ii) one in dayside northern hemisphere, (iii) one in nightside). The mass of the Bus + Descent Probe (DP) system is now 680 kg.

ZZ- tl + 1 hr : chemical deceleration of the Bus+DP system by 200 m/s. By assuming a specific impulsion Isp of 310 s, the required mass of hydrazine for deceleration is about 45 kg. The mass of the Bus+DP system drops down to 635 kg.

ZZZ- t2 = to-19 days : release of the DP, placed on an adequate trajectory toward the target point on Venus. The mass of the bus system is now 445 kg.

IV- t2 + 1 hr : chemical deceleration of the Bus system by 30 m/s and reorientation of the bus on a fly-by trajectory. Required mass of hydrazine : 5 kg. The mass of the Bus system is now about 440 kg, that is the weight of the Mars Express platform.

V- Arrival at Venus in the following order : BPS at t0 - 24 hr, DP at to-2 hr, Bus at to. The parameters at arrival are : (i) Venus-Earth distance : 0.52 AU ; (ii) Sun-Earth/Earth-Venus angle : 43” ; (iii) Bus- Venus/Earth-Venus angle : -30”. The fly-by Bus and the Earth are facing nearly the same sector of the Venus surface, allowing using both Bus/Probes and Earth/Probes radio links (for localization purpose in the last case, by using VLBI technique) ; (iv) The asymptotic trajectory of the Bus, not strictly parallel to the ecliptic plane, points towards north ecliptic, with an angle of -30’ with respect to the ecliptic plane. The subBus point on Venus is therefore located near 30’ south latitude. The night-side sector represents about 2/3 of the full disk, and is located to the east. The day side-sector is therefore in the morning (due to the retrograde rotation of Venus).

VI- t0 - 24 hr : deployment of balloons at 10 km altitude and beginning of balloon observation phase. The Bus/BP distance is about 350,000 km. During the 24 hours of their operational phase, the balloons, carried by the wind (a few meters per second, up to 20 m/s), cover a distance smaller than 1500 km. Because the rotation of Venus is very slow (period : 117 days), the bus may be considered as facing the same sector of the Venus disk during the final one-day approach. If BPS are deployed at a distance of 1500-2000 km from the center of the disk, they are in constant radio visibility from the bus. BP-Earth and BP-Bus data transmission, localization of balloons by VLBI.

VII- to - 2 hr : (i) Progressive deflation of balloons and descent down to the surface, reached at to, 24 hours after deployment (duration of the descent phase : 2 hours). (ii) Injection of the DP in the Venus atmosphere for a 1 hr to 2 hrs (2 hrs assumed in the present scenario) descent phase. The descent phases of the DP (from 100 km to 0 km) and the 3 BPS (from 10 km to 0 km) are therefore synchronized, the 4 probes reach the surface at the same time. At to-2 hr, the Bus/probes distance is about 40,000 km, and the velocity of the bus is about 7 km/s. High data transmission rate between the probes and the Bus.

The Lavoisier Mission 263

VIII- t0 : (Hard) landing of probes and end of operations. The Bus/probes distance is of the order of 5000 km. The velocity of the bus is about 10 km/s.

IX- tX0 : The bus is occulted by the planet. Data are transmitted to Earth in a subsequent phase, after deoccultation of the bus.

System Concept and Thermal Control The bus is designed to take the Descent and Balloon Probes to Venus and also to carry a data relay system

for communicating with Earth. An off-the-shelf technology is planned to be used wherever possible. In particular, the bus concept and structure should be based on the Mars Express spacecraft. In this hypothesis, the total mass of the bus, including propellant requires for manoeuvre and braking phases, is about 485 kg. Up to 3 Balloon Probes can be accomodated on the bus and then released to Venus. Each probe is made of a gondola protected by a thermal shield during the entry phase. The gondola has a hot and a cold compartment. The hot compartment is the outer balcony of the gondola and is dedicated to the sub-systems which can withstand the atmospheric environment (in particular the thermal conditions). These sub-systems are mainly the power s/s and the Helium pressurized vessels. The cold compartment is similar to a thermal cocoon. This tighten spherical structure, containing mainly the scientific payload, ensures a thermal control of its internal volume within an acceptable range (up to 40°C). The thermal shield is designed to absorb the thermal flux due to the entry of the probe from the upper Venusian atmosphere down to the altitude measurement. The shield concept could be based on the technology retained for the Huygens probe front shield sub-system. Its weight can be evaluated to 15 kg.

At an altitude of about 20 km, slightly higher than the measurement altitude, the thermal shield is released, the balloon envelope is deployed from the gondola and begins to be inflated with Helium. The balloon envelope is designed to receive a total Helium mass of 6 kg, corresponding to an inflated volume of about 2 m3, in order to stabilize the 60 kg gondola at the measurement altitude of 10 km. The Helium gas is contained in pressurized vessels made of metallic insert (Titanium thin wall sphere) reinforced by an external carbon composite therm0 structure. Two vessels of 40 cm diameter, pressurized at 60 MPa, are required per balloon probe. The estimated mass of a single vessel is 20 kg. After complete inflation of the balloon envelope, the vessels are released from the gondola in order to remove their 40 kg mass and to reduce the total mass to 60 kg. After altitude stabilisation of the balloon probes, the operational phase can start. The objective is to have 20 hours of scientific operations at 10 km altitude, and then to allow the descent of the gondola down to the surface by venting the Helium content of the envelope. This phase will take about 2 hours, the estimated science operation phase of the balloon probes is then close to 24 hours.

The descent probe is designed to perform scientific measurements during its 1 hr (or 2 hrs) descent time through the Venusian atmosphere from 80 km altitude to the ground. The concept is similar to the balloons probes, but with increased system dimensions (hemisphere of 160 cm diameter instead of 80 cm diameter sphere) and weight (250 kg instead of 115 kg) in order to accomodate the specific payload. The structure is an hemispherical thermal cocoon and an outer balcony protected by a thermal shield. The shield is used both to absorb the thermal flux due to the atmospheric descent and to allow aerobraking of the probe. A parachute could be eventually added to reach the best compromise between scientific return optimisation and system constraints such as system thermal behavior and power ressources. Some of the instruments require windows to look towards the ground. A first option could be to integrate these optical windows to the shield. A second option could be to integrate the windows on the bottom part of the probe and to release the shield during the descent. The outer balcony “hot compartment” is dedicated to sub-systems which can be in isothermal conditions with the probe environment (mainly telecom parts such as antenna, batteries, ..). The thermal insulation of the cocoon is designed to limit the internal temperature to acceptable value (about 40°C) during the 1 hr to 2 hrs lifetime. The internal cold compartment receive the complete payload and the other sub- systems (electronics, ..).

rl

isolant eau

isolant pee

charge utile

Fig. 2. Schematic view of the cocoon.

264 E. Chassefikre et al.

Both the balloons and descent probes have a complex and variable thermal environment during their operational phase. All the parts which can withstand this environment are not thermally protected in order to reduce the size of the temperature controlled area. This is the case in particular for the hardware components of the telecom s/s and for the batteries (described below). The temperature controled area is designed to keep all the thermally sensitive equipments below 40”C.This task is more critical on the balloon probes, because of their 24 hours lifetime, than in the descent probe. Taking into account the limited power ressources, a concept of a passive thermal cocoon made of concentric layers of PCM (Phase Change Materials) have then been validated for the balloon probes.

The thermal dynamic simplified model is based on a spherical gondola. A preliminary thermal study has been led at LETIEF. The concept of the gondola consists of a spherical cocoon with a series of concentric layers having the following characteristics (from the outer edge to the inner) : insulating layer, liquid water layer, insulating layer, paraffin layer, inner corn artment with the payload. The thermal conductivity of the insulator has been assumed to be 0.04 W m-l K- . For the paraffin, a calorific capacity of 1000 J kg-1 K-l is P assumed, for a volumic mass of 800 kg mm3. Melting temperature is assumed to be 40°C and the latent heat of fusion is 260,000 J kg-l. About 4000 configurations have been analyzed. The better compromise consists of a 80 cm diameter cocoon, with successive layers, from the outer to the inner, of thicknesses : 8 cm (insulator), 2 cm (water), 8 cm (insulator), 4 cm (paraffin). The volume of the central compartment is 24 I and the time of survival about 32 hours. This time has been obtained with conservative assumptions, in such a way to account for the necessary weakening of performances due to thermal “leaks” (entry gas and optical lines, structure). The weight of the cocoon, including insulator, water and paraffin, and payload (20 kg) is 60 kg.

The use of high temperature batteries aboard the balloon and descent probes is an important key to solve the thermal control requirement by reducing the mass located in the cold compartment and the volume of this compartment. It means that the power sub-system should work at about 700 K. Similar lithium and sodium batteries are under development. These batteries allow a stable energy storage at ambient temperature, it means that no significant energy loss is expected after integration of the power s/s and during the cruise to Venus. They can then be operated in the range 600-800 K. The lithium-sulphur batteries are commonly operated in the range 620-670 K. Practical energy densities in the range loo-150 Wh/kg are reported on the optimized couple Li/Fe$ batteries. The sodium-sulphur @la&) batteries have a similar operating temperature range and practical energy densities of 100 Whkg are reported. A technological trade-off is required between these technologies but a mean energy density of 100 Wh/kg, compatible with the Venus environmental constraints, can be retained for the power s/s mass and performance estimation.

ACKNOWLEDGMENTS We are grateful to G. Lefebvre, E. Palomo , 0. Riou, E. Surugue and J.F. Durastanti, from LETIEF, for

thermal studies, and A. Ribes, from CNES, for telecommunication studies, which were led in the frame of the preparation of the Lavoisier proposal, submitted on 27/01/2000 in answer to ESA call for mission proposals for two fleximissions (F2 and F3).

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