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International audience; This paper deals with the stability of clahtrate hydrates at low temperature and low pressure in the conditions prevailing in the atmosphere and at the surface of Mars. We fit the classical van der Waals model to compare deviation from experimental results published in the literature. It appears to be acceptable and allows to simulating a Martian gas, CO2 dominated (95.3%) plus nitrogen (2.7%) and argon (2%). The hydrate is a CO2-based hydrate which is unstable during Mars' summer and stable during Mars' winter. The proportion of methane in the hydrate is estimated and is found to be from one tenth to one third of the composition of the gas phase. The proportion depends on the crystallographic structure which is assumed to be formed. In fact, both the structure I and II appear to be stable in the conditions of Mars' winter. The consequences of these results on our understanding of the atmospheric cycle of Martian methane are drawn and analyzed. We propose upper limits on (i) the seasonal variation of methane due to a hypothesized alternate formation of CO2/N2/Ar/CH4 hydrates on the seasonal polar caps, and (ii) on the lifetime of atmospheric methane with respect to an hypothesized continuous trapping under hydrate form on the south polar cap. We show that these mechanisms have only small effects, and cannot play a significant role in the dynamics of methane in present Mars' atmosphere. Hypothesized clathrate hydrates trapped in the permanent south polar cap could include methane in relative proportions between 0.1 and 0.4 times the average global atmospheric ratio. Searching for the spectral signatures of clathrate hydrates on the caps and, if detectable, of inhomogeneities of the CH4 mixing ratio in possible local atmospheric plumes forming during the sublimation of polar hydrate pockets in spectroscopic data from existing (Mars Express, MRO) and future (TGO-Exomars) missions is an interesting challenge of Mars science and astrobiology.

A link is established between the vertical structure of the eddy diffusion coefficient and the shape of the ozone profile in the middle atmosphere of Mars. By using a 2-D model of the effect of internal gravity waves on the general circulation pattern (Theodore et al., Icarus105, 512, 1993a), the eddy diffusion coefficient K is calculated as a function of altitude in solstitial conditions for various latitudes covering polar, mid-latitude and equatorial regions. The vertical profile of K is mainly characterized by a steep increase with altitude above a typlcal level of ≈40 km, in the region of the atmosphere where breaking waves are expected to release their energy. By using a 1-D steady state photochemical model (Blamont and Chassefiere, Icarus104, 324, 1993), which is proven to provide a realistic diurnal profile of ozone, it is shown that the enhanced vertical transport in the breaking region results in the formation of a thin ozone layer whose contrast is well correlated with the altitude of the breaking level. This correlation is quantitatively analyzed and discussed in view of future observations of ozone by solar and stellar occultation from the Mars 94 space-craft (SPICAM experiment).

MarsNEXT is a mission in Phase A study by the European Space Agency in the context of preparations for a future mission for Mars Sample Return (MSR). The MarsNEXT mission demonstrates key technologies for European involvement in MSR, which will be an international effort, and also addresses unanswered questions on the interior, atmosphere and history of Mars. These science objectives are realized through a network of landers, which work in synergy with an orbiting spacecraft, a configuration which is required in order to achieve a global understanding of Mars.

International audience; It has been suggested that the present release rate of methane to the Martian atmosphere could be the result of serpentinization in the deep subsurface, followed by the conversion of H2 to CH4 in a CO2-rich fluid. Making this assumption, we show that the cryosphere could act as a buffer storing under the form of micron-size methane clathrate particles the methane delivered from below by hydrothermal fluids and progressively releasing it to the atmosphere at the top. From an extrapolation of the present CH4 release rate back to the past, we calculate that up to several hundred millibars (~200-2000 mbar) of CO2, resulting from the oxidation of the released CH4, in addition to the volcanic supply (~400 mbar), should have accumulated in the atmosphere in the absence of a CO2 sink. We reassess the capability of escape to have removed CO2 from the atmosphere by C escape, and show that it is not significant. We suggest that atmospheric carbon is recycled to the crust through an active subsurface hydrological system, and precipitates as carbonates within the crust. During episodic periods of magmatic activity, these carbonates are decomposed to CO2 dissolved in running water, and CO2 can react with H2 formed by serpentinization to build CH4. CH4 is then buffered in the subsurface cryosphere, above the water table, and finally released to the atmosphere, before being recycled to the subsurface hydrological system, and converted back to carbonates. We propose a typical evolution curve of the CO2 pressure since the late Noachian based on our hypothesis. Contrary to the steady state carbon cycle at work on Earth, a progressive damping of the carbon cycle occurs on Mars due to the absence of plate tectonics and the progressive cooling of the planet.

Solar occultations performed with an ultraviolet spectrometer on board the Soviet spacecraft Phobos 2 (Blamont et al. 1989, Planet and Space Sci. 39, 175-187) provided data on the vertical profile of the ozone density in the martian atmosphere above ≈35 km altitude in the equatorial region (0°-20° N latitude) near the northern spring equinox (Ls = 0°-20°). Measurements were made close to the evening terminator. Due to instrumental problems, only six occultations may be analyzed in the near UV wing of the Hartley continuum (≈270-283 nm). Ozone is detected at one time with a high degree of confidence below ≈55 km and above ≈37 km altitude. Its number density shows a maximum of ≈108 cm-3 around 42-45 km. Its scale height above 45 km is 6 ± 2 km. Ozone, present in two other cases with a similar vertical profile, is absent, not in excess of ≈2 × 107 cm-3, for the three other observations. An analysis based on a one-dimensional steady-state photochemical model indicates that, when ozone is present, the eddy diffusion coefficient K is ≈2 × 106 cm2 sec-1 around 40-45 km altitude within a factor of ≈2 on either side. Below 50 km. the water vapor mixing ratio is lower than ≈2 × 10-5. Such a low ratio is consistent with a temperature decreasing with increasing altitude, with a typical temperature of ≈ 150 K at ≈50 km altitude. Above 50 km, the H2O mixing ratio is definitely lower than 10-5, strongly suggesting saturation at high altitude. The agreement between the modeled and observed profiles of ozone is significantly improved by assuming a sharp increase of K over a typical scale of one atmospheric scale height around 40-45 km altitude, from ≈5 × 105 cm2 sec-1 below 40 km up to ≈107 cm2 sec-1 above 45 km. A possible explanation is turbulence induced by breaking gravity waves. The absence of ozone found in at least one-half of cases may be simply reproduced by using the thermal structure and humidity conditions derived from the analysis of the clouds (Chassefiere et al., 1992, Icarus 97, 46-69), i.e., a temperature and a H2O mixing ratio of respectively ≈165 K and ≈10-4 at 50 km altitude.

It has been suggested that the haze aerosols in Titan's atmosphere might present an irregular structure, rather similar to the morphology of aggregates experimentally synthesized by Bar-Nun et al. ( J. geophys. Res. 93 , 8383, 1988). The theoretical approach of West ( Appl. Opt. 30 ,5316, 1991) and West and Smith ( Icarus 90 , 330, 1991). which uses a fractal concept to numerically generate aggregates, allowed us to support this idea and to provide constraints on their size and shape by comparing the observed and modelled polarization properties of such particles. The building mechanism of these aerosols, when analysed using microphysical modelling (Cabane et al., Icarus 96 , 176. 1992). leads naturally to aggregates. They are formed of spherical compact monomers, which build up in the region of photochemical synthesis, and whose radius depends mainly on the atmospheric pressure at the formation level. The subsequent growth of aggregates in the settling phase is treated here by introducing the fractal dimension as a parameter of the model ( D f ≈ 2 in the case of cluster-cluster aggregation). Using this fractal model, a vertical distribution of size and number density of the aggregates is obtained down to ≈ 80 km for different production altitudes. The previous estimate of the formation altitude of photochemical aerosols (≈ 350–400 km) is confirmed when comparing the number of monomers per aggregate deduced from the present study with the value proposed by West and Smith. The vertical profile of the effective radius of aggregates is calculated as a function of the visible optical depth derived from Voyager imaging. A good fit with the radius derived from Voyager forward-scattering measurements is obtained (≈ 0.3–0.5 μm), still using a low formation altitude. Finally, it must be emphasized that, for the first time, observational and theoretical results about the size and the structure of particles are reconciled.

From a comparison between the different observations of Martian methane existing today, including the new TES methane maps (Fonti and Marzo, 2010), we show that all sets of data are globally consistent with each other, and that a well definite seasonal cycle of methane has been at work for at least 10 yr. With a simple model of the balance between the loss fluxes of H and O, using up-to-date values of the escape fluxes, we show that the long-standing enigma of the imbalance between H and O escape fluxes may be solved by assuming that the missing sink of oxygen is the oxidation of methane. If no H2 is released together with CH4, a good agreement is found between the present CH4 flux and the value imposed by the balance between H and O escape fluxes, an average over the last approximate to 103 yr. If H2 is released together with CH4, as expected if CH4 originates in serpentinization, the average level of CH4 during the last 103 yr should have been at least ten times lower than the present one. The lack of present H2 release could suggest a long-term storage of methane in the subsurface under the form of clathrates, whereas H2 has been lost to the atmosphere shortly after being produced. We suggest that the thin layer of CO2 ice covering the permanent southern polar cap could result from the release of methane since the end of the last obliquity transition (time scale: 1 Myr), at an average rate of 0.1 Mt yr-1), consistent with the values derived from: (i) the present observations of methane (time scale: 10 yr), (ii) the estimate from the observed imbalance between the H and O escape fluxes (time scale: 1 kyr). If so, the present release of methane from subsurface clathrates would have acted at a similar rate since at least 3 Myr

International audience; Probing of Hermean exosphere by ultraviolet spectroscopy (PHEBUS) is a double spectrometer for the Extreme Ultraviolet range (55-155 nm) and the Far Ultraviolet range (145-315 nm) devoted to the characterization of Mercury's exosphere composition and dynamics, and surface-exosphere connections. This French-led instrument is implemented in a cooperative scheme involving Japan (detectors), Russia (scanner) and Italy (ground calibration). PHEBUS will address the following main scientific objectives relative to Mercury's exosphere: determination of the composition and the vertical structure of the exosphere; characterization of the exospheric dynamics: day to night circulation, transport between active and inactive regions; study of surface release processes; identification and characterization of the sources of exospheric constituents; detection and characterization of ionized species and their relation with the neutral atmosphere; space and time monitoring of exosphere/magnetosphere exchange and transport processes; study and quantification of escape, global scale source/sink balance and geochemical cycles synergistically with other experiments of BepiColombo (Mercury Sodium Atmospheric Spectral Imager (MSASI), Mercury Plasma Particle Experiment (MPPE) on Mercury Magnetospheric Orbiter (MMO); Mercury imaging X-ray spectrometer (MIXS), Search for exosphere refilling and emitted neutral abundance (SERENA) on Mercury Planetary Orbiter (MPO)). Two gratings and two detectors are used according to a specific, compact design. The spectrum detection is based on the photon counting method and is realized using micro-channel plate (MCP) detectors with Resistive Anode Encoder (RAE). Typical photocathodes are CsI or KBr for the extreme ultra-violet (EUV) range, CsTe for the far ultra-violet (FUV) range. Extra visible lines are monitored using a photo-multiplier (PM) that is also used in photon counting mode. In order to prevent sensitivity losses which are critical in UV ranges, a minimum of reflections is achieved inside the instrument using only an off-axis parabola and a set of holographic gratings. A one degree-of-freedom scanning system allows to probe, at the highest possible signal-to-noise ratio, selected regions and altitude ranges of interest. Different modes of observation will be used sequentially (vertical scans, along-orbit scans, grazing observations at twilight, etc.). During the mission, the instrument will be regularly calibrated on well chosen stars, in such a way to quantitatively estimate the overall degradation of the sensitivity of the instrument.

International audience; The evolution of Earth-like habitable planets is a complex process that depends on the geodynamical and geophysical environments. In particular, it is necessary that plate tectonics remain active over billions of years. These geophysically active environments are strongly coupled to a planet's host star parameters, such as mass, luminosity and activity, orbit location of the habitable zone, and the planet's initial water inventory. Depending on the host star's radiation and particle flux evolution, the composition in the thermosphere, and the availability of an active magnetic dynamo, the atmospheres of Earth-like planets within their habitable zones are differently affected due to thermal and nonthermal escape processes. For some planets, strong atmospheric escape could even effect the stability of the atmosphere

Phobos 2 spectrometer measurements of solar occultations close to the evening terminator have furnished data on the vertical structure of the Martian aerosols near the northern spring equinox. Since the thermal structure derived from saturated IR profiles of water vapor does not allow the reaching of the CO2 frost point at cloud altitudes, said clouds' particles may be formed by H2O ice. Dust was also monitored at two wavelengths; it is assumed that the dust particles are levitated by eddy mixing. A parallel is drawn between these thin clouds and the polar mesospheric clouds observed on earth.

Recent developments in our understanding of the morphology (shape and size) of haze aerosols in Titan's atmosphere, aggregate particles and their associated optical properties (West 1990, West and Smith 1991), have been considered in light of a microphysical modeling of aerosols. Using an Eulerian model rather similar to the one of Toon et al. (1980), it is shown that the growth of particles may be divided in two stages. The first one corresponds to the initial growth near the formation altitude by accretion of very small elementary particles (synthetized oligomers). Due to the large number of accreted small particles, this stage leads to the formation of nearly spherical particles (called “monomers” by West and Smith). In the second stage, these particles (“monomers”) settle in the atmosphere and stick together forming aggregates, of the same kind as those obtained experimentally by Bar-Nun et al. (1988). The classical assumption of spherical particles, which is made in all existing models, is shown to be valid during the first growth stage. The mean ratio between the masses of two particles colliding together is quantified as a function of altitude using a specific index (mass sharing index, MSI). From this criterion, the two previously defined regions (the formation region inside which monomers are generated, the settling region where aggregates build up) may be clearly separated and the boundary between them precisely defined and located. Identifying the monomer radius with the mass-averaged radius at the altitude of the boundary, it is shown that this radius is extremely sensitive to the altitude where aerosols are created. The reason is that the residence time of particles near the formation altitude, which determines to a large extent the size of monomers, is an increasing function of the pressure. The strong dependence of the monomer radius on the pressure (thus the altitude) is calculated using different sets of parameters (eddy diffusion coefficient, electrical charge, mass production rate, …) in order to cover the widest range of possibilities. Using parameters favored by West in his analysis of the polarizing properties of Titan's haze, the formation altitude of aerosols is found to lie in the range from 350 to 400 km.

International audience; A time dependent model of hydrogen hydrodynamic escape powered by solar EUV flux and solar wind and, accounting for oxygen frictional escape, has been implemented in primitive Venus conditions The model is. constrained by the present Ne-20/Ne-22 and Ar-36/Ar-38 ratios in Venus atmosphere. It suggests that the net total amount of water delivered to the planet during accretion (approximate to 10-100 Myr) is not in excess of the content of approximate to 5 Terrestrial Oceans (5 TO). In our preferred scenario, 60% of the oxygen (3 TO) is left behind the hydrogen during the first 100 Myr. From a comparison with Earth's case, we suggest that hydrodynamic escape has dried up Venus atmosphere early in its history (approximate to 70 Myr), triggering the crystallization of the magma ocean, and leaving no available water in the atmosphere to condense out and form an Earth-size water ocean On. the contrary, Earth, possibly endowed with more water, and subject to a weaker hydrodynamic escape, would have remained wet after the crystallization of its magma ocean. We suggest that the oxygen left behind the escaping hydrogen during the main hydrodynamic phase on Venus has been dissolved in the magma ocean, and lost through oxidation. In the proposed scenario, the dense Venus CO2 atmosphere doesn't result from an initial episode of runaway (or moist) greenhouse, but has been formed during the crystallization of the magma ocean, by progressive exsolution of carbon dioxide, at a time when the atmospheric partial pressure of water was of a few hundred bar. In the subsequent period, from approximate to 100 to approximate to 500 Myr, the hydrogen of the water delivered by comets may have been removed by continuing thermal escape, resulting at 500 Myr in a water global equivalent layer (GEL) of a few meters depth (or less), probably under the form of water vapor in the atmosphere, and a molecular oxygen atmosphere of approximate to 10 bar or so. At later times, pick-up ion escape may have removed most of the remaining water, and led to the present D/H atmospheric enhancement factor of 150. The approximate to 10 bar of oxygen may have been absorbed by crustal oxidation

International audience; In this paper, we summarize our present understanding of Mars' atmosphere, magnetic field, and surface and address past evolution of these features. Key scientific questions concerning Mars' surface, atmosphere, and magnetic field, along with the planet's interaction with solar wind, are discussed. We also define what key parameters and measurements should be performed and the main characteristics of a martian mission that would help to provide answers to these questions.Such a mission—Mars Environment and Magnetic Orbiter (MEMO)—was proposed as an answer to the Cosmic Vision Call of Opportunity as an M-class mission (corresponding to a total European Space Agency cost of less than 300 M€). MEMO was designed to study the strong interconnection between the planetary interior, atmosphere, and solar conditions, which is essential to our understanding of planetary evolution, the appearance of life, and its sustainability.The MEMO main platform combined remote sensing and in situ measurements of the atmosphere and the magnetic field during regular incursions into the martian upper atmosphere. The micro-satellite was designed to perform simultaneous in situ solar wind measurements. MEMO was defined to conduct:• Four-dimensional mapping of the martian atmosphere from the surface up to 120 km by measuring wind, temperature, water, and composition, all of which would provide a complete view of the martian climate and photochemical system;• Mapping of the low-altitude magnetic field with unprecedented geographical, altitude, local time, and seasonal resolutions;• A characterization of the simultaneous responses of the atmosphere, magnetic field, and near-Mars space to solar variability by means of in situ atmospheric and solar wind measurements.

Recent observations and missions to Mars have provided us with new insight into the past habitability of Mars and its history. At the same time they have raised many questions on the planet evolution. We show that even with the few data available we can propose a scenario for the evolution of the Martian atmosphere in the last three billion years. Our model is obtained with a back integration of the Martian atmosphere, and takes into account the effects of volcanic degassing, which constitutes an input of volatiles, and atmospheric escape into space. We focus on CO2, the predominant Martian atmospheric gas. Volcanic CO2 degassing rates are obtained for different models of numerical model crust production rates [Breuer, D., Spohn, T. 2003. Early plate tectonics versus single-plate tectonics on Mars: Evidence from magnetic field history and crust evolution. J. Geophys. Res. - Planets, 108, E7, 5072, Breuer, D., Spohn, T., 2006. Viscosity of the Martian mantle and its initial temperature: Constraints from crust formation history and the evolution of the magnetic field. Planet. Space Sci. 54 (2006) 153–169; Manga, M., Wenzel, M., Zaranek, S.E., 2006. Mantle Plumes and Long-lived Volcanism on Mars as Result of a Layered Mantle. American Geophysical Union Fall Meeting 2006, Abstract #P31C-0149.] and constrained on observation. By estimating the volatile contents of the lavas, the amount of volatiles released in the atmosphere is estimated for different scenarios. Both non-thermal processes (related to the solar activity) and thermal processes are studied and non-thermal processes are incorporated in our modelling of the escape [Chassefiere, E., Leblanc, F., Langlais, B., 2006, The combined effects of escape and magnetic field history at Mars. Planet. Space Sci. Volume 55, Issue 3, Pages 343–357.]. We used measurements from ASPERA and Mars Express and these models to estimate the amount of lost atmosphere. An evolution of the CO2 pressure consistent with its present state is then obtained. A crustal production rate of at least 0.01 km3/year is needed for the atmosphere to be at steady state. Moreover, we show that for most of the scenarios a rapid loss of the primary (and primordial) atmosphere due to atmospheric escape is required in the first 2 Gyr in order to obtain the present-day atmosphere. When CO2 concentration in the mantle is high enough (i.e. more than 800 ppm), our results imply that present-day atmosphere would have a volcanic origin and would have been created between 1 Gyr and 2 Gyr ago even for models with low volcanic activity. If the volcanic activity and the degassing are intense enough, then the atmosphere can even be entirely secondary and as young as 1 Gyr. However, with low activity and low CO2 concentration (less than 600 ppm), the present-day atmosphere is likely to be for the major part primordial.

International audience; Mars Environment and Magnetic Orbiter was proposed as an answer to the Cosmic Vision Call of Opportunity as a M-class mission. The MEMO mission is designed to study the strong interconnections between the planetary interior, atmosphere and solar conditions essential to understand planetary evolution, the appearance of life and its sustainability. MEMO provides a high-resolution, complete, mapping of the magnetic field (below an altitude of about 250 km), with an yet unachieved full global coverage. This is combined with an in situ characterization of the high atmosphere and remote sensing of the middle and lower atmospheres, with an unmatched accuracy. These measurements are completed by an improved detection of the gravity field signatures associated with carbon dioxide cycle and to the tidal deformation. In addition the solar wind, solar EUV/UV and energetic particle fluxes are simultaneously and continuously monitored. The challenging scientific objectives of the MEMO mission proposal are fulfilled with the appropriate scientific instruments and orbit strategy. MEMO is composed of a main platform, placed on a elliptical (130 × 1,000 km), non polar (77° inclination) orbit, and of an independent, higher apoapsis (10,000 km) and low periapsis (300 km) micro-satellite. These orbital parameters are designed so that the scientific return of MEMO is maximized, in terms of measurement altitude, local time, season and geographical coverage. MEMO carry several suites of instruments, made of an ‘exospheric-upper atmosphere’ package, a ‘magnetic field’ package, and a ‘low-middle atmosphere’ package. Nominal mission duration is one Martian year.

A spectrometer designed at the Space Research Institute, Moscow, for solar occultation measurements of the Martian atmosphere from the Phobos spacecraft has the spectral range 2706–2740 and 5293–5372 cm −1 in the first and second diffraction orders, respectively, with resolving power λ / Δλ = 1200. Data on the water vapor distribution in the Martian atmosphere were derived from nine occultation profiles at low latitudes and give a mean mixing ratio of 150 ppm below 12 km decreasing to 3 ppm at 40 km. The total amount of the water vapor is 10 μm of precipitable water. Mean scale height of water vapor is 3.8 km between 12 and 45 km. Small-scale variations of the water vapor density, though close to the experimental errors, may be caused by tidal waves with an amplitude of 2 K near 20 km and 5 K at 40 km with a further increase to 15 K at 100 km. The volume extinction coefficients and their spectral slopes at 1.9 and 3.7 μm have been measured to study the properties of aerosols in the Martian atmosphere.

As a part of the payload of the first European Polar Platform, the GOMOS instrument has been proposed by a group of 25 scientists from six countries. It consists of a telescope feeding two spectrographs, mounted on a dedicated steerable platform. The transmittance of the atmosphere between 250 and 675 nm is measured by comparing the spectrum of a star outside the atmosphere, and through it. The ozone tangential column is determined from its UV and Chappuis band absorption. This self-calibrated method is particularly well suited for the study of ozone long term trend. The altitude of each single measurement is precisely known (± 50 m), independently of altitude uncertainties. About 25 stellar occultations per orbit, and 350 per day, spread over all latitudes can be performed from 90 km down to 15–20 km of altitude. NO 2 , NO 3 , H 2 O, T(z) and aerosols are also simultaneously determined, important parameters associated to the ozone equilibrium. The ability to measure ozone long-term trends is calculated.

International audience; The origin and evolution of Venus', Earth's, Mars' and Titan's atmospheres are discussed from the time when the active young Sun arrived at the Zero-Age-Main-Sequence. We show that the high EUV flux of the young Sun, depending on the thermospheric composition, the amount of IR-coolers and the mass and size of the planet, could have been responsible that hydrostatic equilibrium was not always maintained and hydrodynamic flow and expansion of the upper atmosphere resulting in adiabatic cooling of the exobase temperature could develop. Furthermore, thermal and various nonthermal atmospheric escape processes influenced the evolution and isotope fractionation of the atmospheres and water inventories of the terrestrial planets and Saturn's large satellite Titan efficiently.

The origin and evolution of Venus’, Earth’s, Mars’ and Titan’s atmospheres are discussed from the time when the active young Sun arrived at the Zero-Age-Main-Sequence. We show that the high EUV flux of the young Sun, depending on the thermospheric composition, the amount of IR-coolers and the mass and size of the planet, could have been responsible that hydrostatic equilibrium was not always maintained and hydrodynamic flow and expansion of the upper atmosphere resulting in adiabatic cooling of the exobase temperature could develop. Furthermore, thermal and various nonthermal atmospheric escape processes influenced the evolution and isotope fractionation of the atmospheres and water inventories of the terrestrial planets and Saturn’s large satellite Titan efficiently.

Science goals to understand the origin, history and environment of Venus have been driving international space exploration missions for over 40 years. Today, Venus is still identified as a high priority science target in NASA's Solar System Exploration Roadmap, and clearly fits scientific objectives of ESA's Cosmic Vision Program in addition to the ongoing Venus Express mission, while JAXA is planning to launch its own Venus Climate Orbiter. Technology readiness has often been the pivotal factor in mission prioritization. Missions in all classes-small, medium or large-could be designed as orbiters with remote sensing capabilities, however, the desire for scientific advancements beyond our current knowledge point to in-situ exploration of Venus at the surface and lower atmosphere, involving probes, landers, and aerial platforms. High altitude balloons could circumnavigate Venus repeatedly; deep probes could operate for extended periods utilizing thermal protection technologies, pressure vessel designs and advancements in high temperature electronics. In situ missions lasting for over an Earth day could employ a specially designed dynamic Stirling Radioisotope Generator (SRG) power system, that could provide both electric power and active thermal control to the spacecraft. An air mobility platform, possibly employing metallic bellows, could allow for all axis control, long traversing and surface access at multiple desired locations, thus providing an advantage over static lander or rover based architectures. Sample return missions are also featured in all planetary roadmaps. The Venus exploration plans over the next three decades are anticipated to greatly contribute to our understanding of this planet, which subsequently would advance our overall knowledge about Solar System history and habitability.

Mariner 10, the only spacecraft that ever passed close to Mercury, revealed several unexpected characteristics: an intrinsic magnetosphere, the highest mean density of any Solar System terrestrial planet and a very thin non-collisional atmosphere. Mercury's atmosphere is very poorly explored since only three atomic elements, H, He and O, were observed during the three flybys of Mariner 10. The measurements done by radio and solar occultations provided upper limits on the neutral and ion densities. These measurements pointed out the close connection between species in Mercury's exosphere and its surface, which is also the case for the Moon. Mariner 10 observations also characterized the vertical distributions and the day to night contrasts of Mercury's exosphere for its lightest components H and He (Broadfoot, A.L., et al., 1976. Mariner 10: Mercury atmosphere. Geophys. Res. Lett. 3, 577–580). More than a decade later, the first observation from a ground-based observatory of Mercury's sodium (Na) exospheric component was reported (Potter, A.E., Morgan, T.H., 1985. Discovery of sodium in the atmosphere of Mercury. Science 229, 651–653). Since then, potassium and more recently calcium have been identified in Mercury's exosphere. The bright Na resonant scattering emission has been often observed since 1985. This large set of observations is now the best source of information on Mercury's exospheric mechanisms of ejection, dynamics, sources and sinks. In particular, several of these observations provided evidence of prompt and delayed effects, both localized and global, for the very inhomogeneous Mercury's Na exosphere. These inhomogenities have been interpreted as the trace of Mercury's magnetosphere–solar wind interaction and have highlighted some of the main sources of exospheric material. Some of these features have been also interpreted as the trace of a global dayside to night side circulation of Mercury's exosphere and therefore have highlighted also the relation between exospheric production and upper surface composition. Hopefully, new sets of in situ measurements will be obtained within the next decade thanks to Messenger and Bepi-Colombo missions. Until then, ground-based observations and modelling will remain the only approaches to resolve questions on Mercury's exosphere. Mercury's exospheric composition and structure as they are presently known are described in this paper. The principal models for the main short and long times terms variations and local and global variations of Mercury's exosphere are described. The mechanisms of production and their characteristics are also given. Mercury's exosphere can also be seen as part of the coupled magnetosphere–upper surface–exosphere system and several of the links between these elements are essential to the interpretation of most of the ground-based observations. The relation between Mercury's planet composition and its exospheric composition is also considered, as is the global recycling, sources and sinks of Mercury's exosphere.

Within the framework of Pasteur-Exomars, we have proposed to measure in situ water vapor (H2O, HDO, H218O, H217O) and carbon dioxide (CO2, 13C16O2, 16O12C18O, 16O12C17O) isotopes and also CO, CH4 and N2O by absorption spectroscopy using near infrared laser diodes. The Service d'Aéronomie has a relevant experience in trace-gas detection with laser diodes. We have developed, with the support of the CNES and the CNRS, the SDLA diode laser spectrometer to measure in situ H2O (at 1.39 µm), CH4 (1.65 µm) and CO2 (at 1.60 µm) in the middle atmosphere from stratospheric balloons. The concentrations are obtained with a precision error of a few percent and with a high temporal resolution that ranges from 10 ms to 1 s. The developed laser probing technique should be also highly efficient to study the Martian atmosphere as there are much higher amounts of water vapor and carbon dioxide in the Martian atmosphere than in the lower stratosphere (H2O: 200 ppmv at 6 hPa on Mars, 5 ppmv at 10 hPa in the low stratosphere (LS); CO2 95% on Mars, 360 ppmv in the LS). Hence, we propose to adapt the laser probing technique to the Martian atmosphere. The main objectives are to determine water vapor and carbon dioxide fluxes and to study boundary layer properties. The sensor will provide in situ daily, diurnally resolved measurements of near-surface H2O and CO2 concentrations over seasonal time scales. The additional isotopic measurements will provide quantitative constraints on the evolution of atmospheric composition and on the history of water on Mars.

We have developed a 1D thermal model of Mercury's regolith, in order to simulate the heat diffusion in the upper subsurface (first 10 m). We assume in our model that the thermophysical properties of the Hermean regolith are similar to those of the lunar regolith. We apply our thermal model to the Caloris basin which slopes induce distortions of the surface temperature compared to results obtained for a perfect spherical planet. This thermal model is then coupled with a 3D Monte Carlo model of Mercury's sodium exosphere [Leblanc, F., Johnson, R.E., 2003. Icarus 164, 261–281; Leblanc, F., Delcourt, D., Johnson, R.E., 2003b. J. Geophys. Res. 108 (E12), doi:10.1029/2003JE002151/.5136], in order to describe the signatures of Caloris basin on Mercury's sodium exosphere in term of temporal and spatial variabilities. In particular, we find a motion of the maxima of sodium density in the exosphere towards the Northern hemisphere similar to the one observed by Potter et al. [Potter, A.E., Morgan, T.H., Killen, R.M., 1999. Planet. Space Sci., 47, 1441–1449] but did not reproduce the observed change of the emission brightness. The main conclusion of this study is that the Caloris basin–exosphere relations might be observable from the Earth which we hope will motivate new observations of Mercury's exosphere.

[1] Since the Surveyor and Apollo missions and up to the recent Lunar Prospector mission, 222Rn and 210Po have been key isotopes for understanding gas release events and their spatial and temporal variations on the Moon. Comparatively, these isotopes have drawn much less attention on Mars, if any, despite the wealth of information it could bring on the uppermost meters of the regolith, the exchange of volatiles at the surface, and the atmospheric aerosol cycle. Here we present a statistical analysis of the high-energy end of alpha spectra obtained by the alpha particle X-ray spectrometer onboard Mars Exploration Rover Opportunity and report evidence of 210Po, a decay product of 222Rn, attached to atmospheric dust. The 210Po surface activity on rocks and soils at the landing site is lower than 3.1 × 10−4 Bq cm−2, but analysis of spectra obtained on the dust capture magnet reveals a 210Po activity of (4.6 ± 2.4) × 10−3 Bq cm−2 (±2σ). This difference is due to the very low dust cover index at the landing site. Owing to frequent dust devils, regional and global dust storms that mobilize substantial amounts of dust and homogenize the dust surface layer, we infer that the global average 222Rn exhalation rate is significantly greater on Mars than on the Moon. This comparison supports the hypothesis that on Mars, radon emanation could be comparatively enhanced by the presence of water in the surficial soil. Analysis of atmospheric spectra yields a radon activity upper limit of 16 ± 5 Bq m−3 during nighttime at the landing site.

Future missions to Mars will offer the opportunity to continue the search for organic molecules accessible from the surface, and to better quantify the cycling of volatile elements through geochemical pathways. This presentation describes an analytical instrument suite that is designed to measure elemental, isotopic, and potential organic signatures contained in the atmosphere and near surface reservoirs on Mars. The Mars analytical chemistry experiment (MACE) combines two unique mass-spectrometer-based instruments to accomplish these measurements. The first instrument combines a sample handling system with a reusable pyrolysis oven for processing solid materials. Evolved volatile gases from the pyrolyzer are either oxidized for elemental analysis, or sent through a preconcentrator into a 2D gas chromatograph for separation of organics. The processed gas stream is then sent to a high resolution dynamic time-of-flight mass spectrometer for detection. The second instrument is designed primarily for direct atmospheric measurements, using a combination of catalyst beds, getters, and cryogenic traps to separate and concentrate species of interest, such as noble gases. Concentrated gases are subsequently detected with a second, dedicated static mass spectrometer to avoid possible contamination from the pyrolysis of organics in the first process. A breadboard version of each of these instruments has been demonstrated in the laboratory. In this presentation, we discuss the design, applicability, and capabilities of the MACE suite in more detail

International audience; This tutorial deals with the question of atmospheric escape on Mars. After a brief introduction describing the general context of Mars escape studies, we will present in Section 2 a simplified theory of thermal escape, of both Jeans and hydrodynamic types. The phenomenon of hydrodynamic escape, still hypothetical and not proved to have ever existed on terrestrial planets, will be treated with the help of two well known examples: (i) the isotopic fractionation of xenon in Mars and Earth atmospheres, (ii) the paradox of missing oxygen in Venus atmosphere. In Section 3, a simplified approach of non-thermal escape will be developed, treating in a specific way the different kinds of escape (photochemical escape, ion sputtering, ion escape and ionospheric outflow). As a matter of illustration, some calculations of the relative contributions of these mechanisms, and of their time evolutions, will be given, and the magnitude of the total amount of atmosphere lost by non-thermal escape will be estimated. Section 4 will present the state of knowledge concerning the constraints derived from Mars isotopic geochemistry in terms of past escape and evolution. Finally, a few conclusions, which are more interrogations, will be proposed.

International audience; In view to prepare Mars human exploration, it is necessary to promote and lead, at the international level, a highly interdisciplinary program, involving specialists of geochemistry, geophysics, atmospheric science, space weather, and biology. The goal of this program will be to elaborate concepts of individual instruments, then of integrated instrumental packages, able to collect exhaustive data sets of environmental parameters from future landers and rovers of Mars, and to favour the conditions of their implementation. Such a program is one of the most urgent need for preparing human exploration, in order to develop mitigation strategies aimed at ensuring the safety of human explorers, and minimizing risk for surface operations. A few main areas of investigation may be listed: particle and radiation environment, chemical composition of atmosphere, meteorology, chemical composition of dust, surface and subsurface material, water in the subsurface, physical properties of the soil, search for an hypothesized microbial activity, characterization of radio-electric properties of the Martian ionosphere. Scientists at the origin of the present paper, already involved at a high degree of responsibility in several Mars missions, and actively preparing in situ instrumentation for future landed platforms (Netlander - now cancelled, MSL-09), express their readiness to participate in both ESA/AURORA and NASA programs of Mars human exploration. They think that the formation of a Mars Environment working group at ESA, in the course of the AURORA definition phase, could act positively in favour of the program, by increasing its scientific cross-section and making it still more focused on human exploration. © 2004 Published by Elsevier Ltd on behalf of COSPAR.

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 : noble gas (elemental/isotopic), molecular species (elemental/isotopic), oxygen fugacity; 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 (KFKI, Univ. Eotvos), Portugal (OAL), Russia (IKI), Spain (IAA), United Kingdom (Univ. Oxford). (C) 2002 COSPAR. Published by Elsevier Science Ltd. All rights reserved.

International audience; The effect of a typical solar energetic particle event on the Martian atmosphere is described. Sputtering, Lyman α emission, atmospheric heating, and the radiation flux at the surface are calculated using a model of the solar wind interaction with the Martian atmosphere. The effect of the crustal magnetic field is also investigated. We provide estimates of the associated escape flux of neutral atoms and molecules. We compare the atmospheric energy deposition due to the incident solar energetic particles with the energy deposition due to other sources such as solar EUV/IR, solar wind protons and Martian energetic neutral atoms derived from pickup ions. We show that atmospheric sputtering efficiency is small compared with that for pickup ions for the present epoch but that the energy deposition at the homopause is important compared with the EUV/IR energy deposition. While some areas of the atmosphere are shielded from the incident flux by the crustal magnetic fields, the energetic particle flux reaching the surface is virtually unattenuated. This study of “space weather” at Mars raises a number of questions that need to be addressed with new observations and models.

International audience; We present the principle of the EXOCAM chamber, devoted to the study of physical-chemical interactions between the atmosphere and the surface and subsurface in Mars conditions. The purpose of this experiment is to reach a better knowledge of the physical and chemical processes that altered the atmosphere-soil coupled system. We describe the scientific goals of EXOCAM, the multiple fields that will benefit from this experiment and the instrumentation that is devoted to the analysis of the results. We also give a description of the chamber and its main devices.

International audience; DYNAMO is a small Mars orbiter planned to be launched in 2005 or 2007, in the frame of the NASA/ CNES Mars exploration program. It is aimed at improving gravity and magnetic field resolution, in order to better understand the magnetic, geologic and thermal history of Mars, and at characterizing current atmospheric escape, which is still poorly constrained. These objectives are achieved by using a low periapsis orbit, similar to the one used by the Mars Global Surveyor spacecraft during its aerobraking phases. The proposed periapsis altitude for DYNAMO of 120–130 km, coupled with the global distribution of periapses to be obtained during one Martian year of operation, through about 5000 low passes, will produce a magnetic/gravity field data set with approximately five times the spatial resolution of MGS. Additional data on the internal structure will be obtained by mapping the electric conductivity. Low periapsis provides a unique opportunity to investigate the chemical and dynamical properties of the deep ionosphere, thermosphere, and the interaction between the atmosphere and the solar wind, therefore atmospheric escape, which may have played a crucial role in removing atmosphere and water from the planet.

Biemann 1 calls into question our preliminary interpretations of our experimental results2. A comparison of laboratory and flight measurements should settle the uncertainties he raises. In addition to evaluating instrumental characteristics such as the 'piston effect', a technique we used for injecting oven-gas content for gas chromatography–mass spectrometry (GC–MS), such studies should enable a wide range of possible compositions for Titan's aerosols to be investigated. Our aerosol collection and pyrolysis (ACP) measurements in Titan's atmosphere can then be revisited.

It is suggested that aerosol particles forming the detached and main haze layers of Titan's atmosphere do not originate in the same atmospheric levels. Particles present above approximately 350 km could be formed of polyacetylenes synthetized in the 500-800 km altitude range through successive insertion reactions involving the C2H radical under the action of solar ultraviolet photons (Yung et al., Astrophys. J. Suppl. 55, 465, 1984). They might contain C-N oligomers in comparable amounts, as well as C-H-N oligomers synthetized at high altitude (900-1000 km) by the action of suprathermal Saturn plasma electrons. Physically, they are expected to consist of fluffy aggregates of density approximately 0.01-0.1 g cm-3. Their mass production rate is small (10(-15)-10(-14) kg m-2 s-1), that is typically 10% or less of the main haze production rate. Due to their low fall velocity, they are very sensitive to large scale horizontal motions and one substantial part of them may be swept away by meridional circulation at the detached haze level. The altitude range where these aerosols are created is well above the range proposed by Cabane et al. (Planet. Space Sci. 41, 257, 1993) for aerosols of the main haze layer, on the basis of a new fractal microphysical modeling of Titan's aggregates, that is approximately 350-400 km. A natural outcome of this apparent discrepancy is to suppose that there is a second formation region, below approximately 400 km altitude, giving rise to the main haze layer. The aim of the present paper is to review the different possible formation mechanisms of this main haze layer and assess their ability to account for the observed characteristics of the haze. Several conditions are established. The first one, called "condition A", concerns the formation altitude range imposed by fractal modeling. Possible chemical and energy sources are examined. Two additional constraints, relative to the minimum gas mass ("condition B") and input energy ("condition C") required for efficient conversion of gas into aerosols, are defined. By comparing the production rates of the haze, as derived from microphysical models, and of gaseous chemical species, as derived from photochemical models, five possible source constituents are identified: N2, CH4, C2H2, C2H6 and HCN. Polymerization of C2H2 into (C2H2)n through action of solar ultraviolet photons is shown to be rather improbable (condition A is hardly satisfied). From both our current knowledge of the gaseous phase photochemistry, through modeling and laboratory experiments, and existing models of the interaction between Saturn magnetosphere and Titan atmosphere, the formation of C-H-N polymers through action of Saturn magnetospheric energetic particles (E approximately 100 keV), is proposed as the basic polymerization mechanism in the lower formation region (conditions A,B and C are jointly satisfied).

Titan, the main satellite of Saturn, has been observed by remote sensing for many years, both from interplanetary probes (Pioneer and Voyager's flybys) and from the Earth. Its N2 atmosphere, containing a small fraction of CH4 (approximately 2%), with T approximately 90 K and P approximately 1.5 bar at the ground level, is irradiated by solar UV photons and deeply bombarded by energetic particles, i.e. Saturn mangetospheric electrons and protons, interplanetary electrons and cosmic rays. The resulting energy deposition, which takes place mainly below 1000 km, initiates chemical reactions which yield gaseous hydrocarbons and nitriles and, through polymerisation processes, solid aerosol particles which grow by coagulation and settle down to the ground. At the present time, photochemical models strongly require the results of specific laboratory studies. Chemical rate constants are not well known at low temperatures, charged-particle-induced reactions are difficult to model and laboratory simulations of atmospheric processes are therefore of great interest. Moreover, the synthesis of organic compounds which have not been detected to date provides valuable information for future observations. The origin and chemical composition of aerosols depend on the nature of chemical and energy sources. Their production from gaseous species may be monitored in laboratory chambers and their optical or microphysical properties compared to those deduced from the observations of Titan's atmosphere. The development of simulation chambers of Titan's extreme conditions is necessary for a better understanding of past and future observations. Space probes will sound Titan's atmosphere by remote sensing and in situ analysis in the near future (Cassini-Huygens mission). It appears necessary, as a preliminary step to test on-board experiments in such chambers, and as a final step, when new space data have been acquired, to use them for more general scientific purposes.

International audience; The possible existence of stable warming at middle altitudes in the martian atmosphere was first examined by J. R. Barnes (J. Geophys. Res. 95, 1401, 1990) in one-dimensional modeling of the effect of breaking gravity waves on the general circulation. This work was motivated by infrared observations of D. Deming, M. J. Mumma, F. Espenak, T. Kostius, and D. Zipoy (Icarus 66, 366, 1986) which suggested temperatures far from radiative equilibrium above polar regions at solstice. New observational evidence was obtained from heterodyne measurements of the J = 2-1 12CO and 13 CO transitions made during the 1988 opposition. At this date, corresponding to southern summer solstice, these lines were mapped as a function of the position on the martian disk with the IRAM 30-m telescope. Wind velocities near 50 km altitude and vertical thermal profiles were derived from the Doppler shift and the line profiles, respectively, at various locations on Mars. The northern (winter) hemisphere profiles indicate the presence of a warm layer in the 40-60-km altitude range, with a relative warming of ≈5 K compared to smoothly decreasing temperature profiles such as those found in the southern hemisphere. This may be interpreted in terms of a temperature inversion in the northern middle atmosphere. Such a phenomenon may be similar to the stable inversion layers observed in the terrestrial mesosphere, which are believed to be a response to the gravity wave activity. We show that a zonally averaged model of the martian middle atmosphere, using the parametrization of gravity waves effects propounded by R. S. Lindzen (J. Geophys. Res. 86, 9707, 1981). is able to produce both a plausible zonal wind field consistent with the measurements of E. Lellouch, J. J. Goldstein, S. W. Bougher, G. Paubert and J. Rosenqvist (Astrophys, J. 383, 401, 1991) and a thermal field in good agreement with the observations. The 2-D modeling further indicates that weak equatorial thermal inversions may occur, due to the penetration of the summer hemisphere retrograde jet into the winter hemisphere.

Global Martian atmospheric results derived from the infrared imaging spectrometer ISM flown aboard the Phobos 2 Soviet spacecraft are presented. Over low altitude regions the expected CO mixing ratio of (8 +/- 3) x 10(-4) is measured. Variations of the 2.35-micrometers feature are inconsistent with this value over the Great Martian Volcanoes. If the 2.35-micrometers band is entirely attributable to carbon monoxide, the CO mixing ratio is typically depleted by a factor of 3 over these high altitude areas. Orography should play a major role in the existence of this CO "hole." If, however, these spectral variations at 2.35 micrometers are due to the surface composition, the fraction of the surface covered by the responsible mineral must smoothly decrease as the surface elevation decreases. This phenomenon implies a strong interaction between the surface and the atmosphere for the Great Martian Volcanoes. Diurnal behavior and latitudinal variations of water vapor are globally consistent with Viking measurements. During the Phobos observations, the water vapor amounts over the bright equatorial regions range around 11 pr-micrometers during the day. These amounts are slightly larger than those inferred from 1976 to 1979. The lack of global dust storms during 1988-1989 could explain the enhancement of H2O in the atmosphere.

Lyman-alpha background maps collected by the Ultraviolet Spectrometer (UVS) instrument on board Voyager I and 2 during the whole cruise are compared with current models of the interaction between the Sun and an homogeneous interstellar flow (without any perturbation nor filtration at the heliopause). These data provide precise measurements of the solar wind ionization rate, through the location of the maximum emission region (MER) upwind from the sun. The inferred rate implies a higher interstellar density as compared with previous results. Systematic departures clearly appear, indicating an excess of emission from the downwind ionization cavity as compared with the model. Possible explanations (radiative transfer, heliospheric interface modifications.) are discussed.

A fraction of Voyager 1 and 2 UVS measurements of the interplanetary Lyman-alpha background during the 1977-1983 period (radial distance 1-18 AU) is presented and compared with results from current models of the interaction between the sun and the neutral interstellar gas. It is shown that there are some systematic departures from the predicted pattern: 1) the position and brightness of the upwind maximum emission region on one part, and the downwind/upwind intensity ratio on the other part are contradictory and require two different values of the solar ionization: the downwind emission is most likely larger than predicted. 2) there is an excess of emission from the anti-solar directions as compared with regions close to the sun when moving outwards. Possible explanations include incorrect modeling (multiple scattering, non stationary processes), and inhomogeneous wind flow as a result of the heliospheric interface perturbations.

During the first year of the cruise of Voyager 1 and 2 spacecrafts, when they were still near the Sun, many measurements of the interplanetary hydrogen glow, due to resonant scattering of solar photons, show a clear Ly-β signature at 102.6 nm. The comparison between Ly-α and Ly-β glows exhibits significant discrepancies which can not be explained by radiation transfer in the Ly-α interplanetary line. They might be due, at least partially, to the temporal variability of the solar flux, larger at Ly-α center than at Ly-β center. A study of OSO-8 high resolution Ly-α and Ly-β solar spectra shows indeed that the temporal/spatial variability of the line center solar Ly-α intensity is about three times the one of the line center solar Ly-β intensity.

Four experiments flown on board Phobos 2 provided information on the characteristics of the dust particles suspended in the Martian atmosphere: Auguste (UV-visible-IR spectrometer working in solar occultation geometry), ISM (IR spectrometer measuring the light of the Sun reflected by the planet), Termoskan (scanning radiometer mapping the planetary thermal radiation), KRFM (UV-visible multiphotometer providing limb-to-limb profiles). These experiments, which sounded equatorial regions (20°S–20°N) near the northern spring equinox (LS = 0–20°), are shown to yield a reasonably consistent picture of the dust distribution over the whole altitude range from the ground level, or just above, outside the boundary layer, up to ≈25 km. The vertical profiles of particle volume mixing ratio and effective (projected area-weighted) radius deduced from Auguste measurements, performed in the 15–25 km altitude range, are extrapolated down to the ground by using a simple, physical parameterization of the altitude dependence of dust mixing ratio and radius. This parameterization, which must be understood as describing the vertical distribution of dust in the zonal average and on the mesoscale in latitude, assumes that gravitational settling and eddy diffusion are the only two processes driving vertical dust transport. The vertically averaged effective radius and optical depth of dust particles, as well as vertical profiles of related quantities, are obtained. Optical depth at 1.9-μm wavelength is found to be 0.2 on average, with a typical variation of ±0.1 with time and space. This result is similar to that obtained from ISM spectra analysis. It is also consistent with the Termoskan and KRFM measurements, which yield near-infrared optical depths of 0.12–0.26 and 0.12–0.24, respectively. The particle number density near the surface, as derived from extrapolation of solar occultation profiles, is in the range 1–3 cm-3, in good agreement with Termoskan results (1–2 cm-3). The scale height of the dust volume mixing ratio just above the surface is ≈8–9 km on average, that is, of the same order as the background atmospheric scale height. The vertically averaged effective radius of dust particles is found to lie in the range 1.7±0.2 μm, possibly ≈2 μm in the case of a large effective variance of 0.4. The most likely ISM value is 1.2 μm, with a rather large uncertainty of ±0.4 μm, mainly due to the fact that the spectral dependence of the Minnaert coefficient is not well known. Because ISM data used in the present work were obtained on the Tharsis plateau, at a mean altitude of ≈7 km, the ISM radius must be compared to the Auguste vertically averaged radius for z > 7 km, that is, ≈1.5±0.2 μm. Auguste and ISM radii are therefore consistent at the 1-σ level. Three typical vertical profiles of the dust particle radius and number density, obtained by averaging all solar occultation profiles, including their extrapolated parts below ≈15 km, are proposed as reference models, for three selected values of the effective variance of the particle size distribution (0.10, 0.25, and 0.40).

A value of the eddy diffusion coefficient K of ≈1.5×106 cm2 s−1 in the middle atmosphere of Mars was obtained from Phobos 2 solar occultation measurements of dust, ozone, and clouds at low latitude. The aim of the present study is to complete this picture by using a steady state photochemical one-dimensional model. The main regulation mechanism of O2 is the reaction of O with itself, whose rate depends on the value of K in the middle atmosphere. By comparing calculated and observed values of the O2 abundance, an upper limit of ≈2×107 cm−2 s−1 on K is inferred. By including an additional constraint provided by H2 balance, a lower limit of ≈4×106 cm−2 s−1 may be placed. It results from the present analysis that the most realistic value of K to be used in works resorting to one-dimensional modeling (long-term evolution, escape, surface/atmosphere exchanges) is ≈107 cm2 s−1 rather than ≈106 cm2 s−1. The difference between theoretical and observational values might be due to the regular occurrence of global dust storms, whose effect should be to increase the yearly average value of K. The present study suggests less than 3 precipitable micrometer (pr-μm) of the yearly averaged water vapor column, unless H2O is confined in a layer near the ground. Although the first possibility seems more probable, the second hypothesis cannot be ruled out. It could reflect a continuous supply of H2O from the regolith to the atmosphere on a seasonal scale. The loss to production ratio of CO, which has a lifetime of ≈5 years, is shown to depart from unity by no more than ≈10% over a wide range of atmospheric conditions. The stability of the Martian atmosphere is therefore realized in the classical frame of homogeneous chemistry.

International audience; Elastic collisions between neutral helium atoms from interstellar origin and protons from both the interstellar plasma in the heliospheric interface and the solar wind inside the heliosphere modify parameters of the helium gas. Upstream the heliopause, the helium atoms release momentum to the plasma, which results in a decrease of their bulk velocity, and remove energy from the plasma, which results in an increase of their temperature. Inside the heliosphere, the major part of the energy communicated by solar wind protons to helium atoms is transferred through scarce frontal collisions, which results in the creation of a hot tenuous He component in the downstream (with respect to the sun) region. The modification of the spatial He distribution function near the sun due to slightly deflected atoms is negligible in first approximation. Lastly, we emphasize that the collisional effect is probably insufficient to explain the high temperature of helium derived from photometric measurements of the interplanetary medium at 58.4 nm (16 000 K). By taking into account two other effects which are the light transfer in the He resonance line inside the focusing cone and a possible latitudinal anisotropy of the solar ionizing EUV flux, leading both to a lowering of the contrast of the cone, we show that the temperature discrepancy between H (8 000 K : spectral measurements : line width, Doppler shift) and He (16 000 K : photometric measurements) can be reasonably explained by the combined actions of these three effects : elastic collisions, light transfer at 58.4 nm inside the focusing cone, solar EUV flux latitudinal anisotropy.

Lyman α and 58.4 nm HeI radiations resonantly scattered were observed with EUV spectrophotometers flown on Venera 11 and Venera 12. The altitude distribution of hydrogen was derived by limb observations from 250 km (exobase level) to 50,000 km. In the inner exosphere (up to ⋍ 2,000 km of altitude) the distribution can be described by a classical exospheric distribution with TC = 275 ± 25 K and n = 4−2+3 × 104 atom. cm−3 at 250 km. The integrated number density from 250 to 110 km (the level of CO2 absorption) is 2.1 × 1012 atom. cm−2, a factor of 3 to 6 lower than that predicted by aeronomical models. This number density decreases from the morning side to the afternoon side, or alternately from equatorial to polar regions. Above 2,000 km a “hot” hydrogen population dominates, which can be simulated by T = 103 K and n = 103 atom. cm−3 at the exobase level. The optical thickness of helium above 141 km (the level of CO2 absorption for 58.4 nm radiation) was determined to be τo = 3, corresponding to a density at 150 km of 1.6 × 106 cm−3. This is about 3 times less than what was obtained with the Bus Neutral Mass Spectrometer of Pioneer Venus, and about twice less than ONMS measurements, but is in agreement with earlier EUV measurement by Mariner 10 (2 ± 1 × 106 cm−3).

The high atmosphere of Mars (z > 30 km) has been observed from Phobos 2, using the solar occultation technique. Due to a major error in the pointing system, resulting from a wrong orientation of the spacecraft in the pointing software, the lower atmosphere is never observed in ultraviolet (220–330 nm) and high resolution near-infrared (760 nm, 936 nm) channels. Pointing data sometimes provide useful information on the visible opacity below 30 km. showing that the scale height of dust decreases from ≅ 8 km below 20 km to ≅ 4 km above. Among 32 occultations, four show the presence of a water ice cloud high in the atmosphere (z = 50 km), with a maximum tangential optical thickness varying from ≅ 0.05 to ≅ 2 and a vertical extent in the range from 5 to 15 km. Indications on the particles size (r ≫ 0.01 μm) are obtained using spectral information. From a simple cloud model, where eddy diffusion and sedimentation processes are taken into account, an upper limit of 107 cm2 s−1 is derived for the eddy diffusion coefficient. Similarly, the absence of ozone in detectable amounts above 30 km is interpreted as the signature of a rather weak value of K. Using a simple stationary photochemical model, a value of K as low as 106 cm2 s−1 seems to be required.

The HeI 58.4 nm resonance emission from the dayside upper atmosphere of Venus was recorded by two extreme ultraviolet (e.u.v.) spectrophotometers flown near Venus on Venera 11 and Venera 12 in December 1978. Scans across the illuminated disc are compared with models for which the radiative transfer equation in a spherically symmetric atmosphere is solved. The optical thickness of helium above the CO2 absorption level (∼ 140 km) is determined to be τ=3.5 ±1.5, independently of any calibration factor. This corresponds to a number density at 150 km of 2.2±1×106 atom. cm−3 compatible with original PV ONMS (Orbital Neutral Mass Spectrometer of Pioneer Venus) and Mariner10 e.u.v. measurements, but a factor of 2 below the Bus Neutral Mass Spectrometer Measurements. There are also indications that the density increases at high latitudes above 60°, which might be related to the complex thermal structure (cold polar collar) observed around 70° of latitude and ∼70 km of altitude. The escape rate from the top of the atmosphere is then estimated ∼5× 05 cm−2 s−1, and implications for the outgassing history of Venus are briefly discussed.

The results of four space experiments looking at resonance scattering of H and He atoms in the solar system are reported. The characteristics of the Local Interstellar Medium are determined to be T = 8000 ± 500 K, and the wind velocity is 20 ± 1 km.s-1. Larger values are found for Helium as a result of an artefact of the modeling. The degree of ionization is about 30% in the LISM. Such conditions were predicted to prevail by Mc Kee and Ostriker (1977), at the interface between a dense and cold cloud and a hot and tenuous ISM. In addition, the solar wind mass flux was measured at all latitudes, showing a 50% decrease when going from the ecliptic to the pole in the 1976–1977 period, linked to the presence of a polar coronal hole. Predictions are made concerning some results which will be collected with ULYSSES.