2.11 Venus Express. Introduction. Mission status

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2 2.11 Venus Express Venus Express was launched from Baikonur Cosmodrome on 9 November A Soyuz-Fregat rocket put the 1200 kg spacecraft almost perfectly on the ideal trajectory towards Earth s twin planet, where it arrived 11 April 2006 and entered routine operations in June Already during its very first orbit around Venus, the instruments on board captured stunning images of the southern hemisphere and of the south polar vortex. The mission, originally proposed to ESA in response to a March 2001 Call for Ideas for reuse of the Mars Express platform, is aimed at study of the atmosphere, plasma environment and surface of Venus. It was developed in a record time of less than 4 years. The demanding schedule set out at the very beginning of the project was strictly followed and Venus Express was ready for launch on the first day of the launch window. The spacecraft design is derived from Mars Express, reusing most subsystems with only minor modifications. Important differences are found in the thermal control system, which had to be redesigned in order to cope with the much higher heat input from the Sun at Venus and the high albedo of Venus itself. For the same reason, the solar panels were completely redesigned. The new design is based on high-temperature GaAs cells, and the total area could be reduced to about half that of Mars Express since the solar radiation is about twice as intense at Venus. When Venus Express arrived at Venus, an orbit-insertion manoeuvre placed the spacecraft in a highly-elliptical 9-day orbit around Venus. Following a series of apocentre-lowering manoeuvres, the final 24-h polar orbit, with a pericentre altitude of km and an apocentre altitude of km, was reached on 7 May The initial pericentre was located at around 80ºN latitude and is drifting slowly northwards. The scientific observations are shared between the pericentre region, where high-resolution studies of small-scale features are carried out, and near apocentre and intermediate regions, where global features and dynamical processes are studied. The acquired data is transmitted to Earth in each orbit during the 8 h following the pericentre pass. ESA s deep-space tracking station at Cebreros (E), of which Venus Express was the first major customer, is the nominal ground station for spacecraft control and data downlink. The other 35 m station, at New Norcia, Australia, is used during mission-critical operations and for radio-science support during dedicated campaigns at certain phases of the mission. The spacecraft successfully completed its first two Venusian sidereal days (486 Earth days) of science observations in October 2007 and has continued to work well during its first mission extension that will last until end-april Since routine science operations at Venus commenced, the mission has gone through a number of critical phases: Introduction Mission status Solar superior conjunction, leaving the spacecraft mostly inactive with only essential systems in operation and a low data rate transmitting only housekeeping parameters. The spacecraft is rotated 180 about the z-axis in order to avoid solar illumination of certain faces of the spacecraft. Quadrature entry (when the Sun-Venus-Earth angle starts to exceed 90 degrees). The spacecraft is rotated 180 deg about the y-axis to avoid illumination of certain faces of the spacecraft. It uses a secondary antenna for Earth communication (see Fig ). Solar inferior conjunction. Again the spacecraft is rotated 180 about the z-axis. For further information, see 77

3 Quadrature exit (as above, with the Sun-Venus-Earth angle dropping below 90 again.) The spacecraft is again rotated 180 about the y-axis. The main high ain antenna is used again. The status of the spacecraft is very good with nominal performance of all subsystems and instruments, with the following exceptions: The PFS instrument has not been operational since launch due to a malfunctioning mechanism. Regular attempts have been and are still being made in order to reactivate the system. An unexplained decrease in transmitted S-band signal level occurred at the end of This precludes a small sub-class of Radio Science observations. A sudden increase in both the VIRTIS-H and VIRTIS-M cryocooler motor currents, a few weeks apart, led to a temporary suspension of VIRTIS operations. Following a number of tests the instruments resumed operations, and have been functioning nominally since. During 2007, the Venus Express science ground segment and its staff were moved from ESTEC to ESAC, Spain. Scientific objectives The main goal of the mission is to conduct a comprehensive study of the atmosphere of Venus and to study in some detail the plasma environment and the interaction between the upper atmosphere and the solar wind. Several aspects of the surface and surface-atmosphere interactions are also studied. In order to organise properly the topics to be studied and to ensure that the full potential of the mission is exploited, seven Science Themes have been defined, each theme with its own detailed set of objectives: atmospheric dynamics; atmospheric structure; atmospheric composition and chemistry; cloud layers and hazes; radiative balance; surface properties and geology; and plasma environment and escape processes. Addressing these themes to a proper depth will enable solutions to many of the fundamental questions that are still open for Venus. These include: what is the mechanism of global atmospheric circulation; what are the mechanisms and the driving force behind the atmospheric super-rotation; what is the chemical composition and what are its spatial and temporal variations in the short- and long-term; what is the role of the cloud layers and the trace gases in the thermal balance of the planet; what is the importance of the greenhouse effect; how can the origin and the evolution of the atmosphere be described; what has been and what is the role of atmospheric escape for the present state of the atmosphere; what role does the solar wind play in the evolution of the atmosphere; and is there still active volcanism and seismic activity on Venus? Resolving these issues is of crucial importance for understanding 78

4 Figure Geometry of Venus orbit around the Sun (Venus itself is not shown). For thermal reasons the spacecraft has to be rotated 180 about the z-axis at the superior and inferior solar conjunctions and 180 about the y-axis at the entry and the exit of the quadrature phase. the long-term evolution of climatic processes on the sister planets Venus, Earth and Mars, and will significantly contribute to general comparative planetology. Great challenges were presented to the team in defining the set of instruments to be carried by Venus Express. Since the schedule was postulated a priori, the choice of instruments was naturally restricted to units not requiring significant new development; existing qualified designs from previous projects were clearly preferred. The obvious candidates were the instruments developed for Mars Express and Rosetta. After a detailed assessment, three Mars Express instruments were chosen together with two Rosetta instruments, enhanced with a new, miniaturised 4-band camera and a new Magnetometer (with heritage from the Rosetta lander). In addition, a very high-resolution IR solar occultation spectrometer was added to the SPICAM instrument from Mars Express to make SPICAV/SOIR for Venus Express. This new instrument, with a spectral resolution of more than , is able to identify a number of isotopes and is particularly important for studying the escape of hydrogen from the planet and so contributes to a better understanding of the evolution of water on Venus. The resulting instrument complement includes a combination of two spectrometers, an imaging spectrometer and a camera, covering the range from UV to thermal-ir, along with a plasma analyser and a magnetometer. These instruments together have the capability of sounding the entire atmosphere from the surface to above 200 km altitude. The Radio Science investigation will use the spacecraft communication system enhanced with an ultra-stable oscillator, to make high vertical resolution investigations by occultation and to carry out surface studies by bistatic radar techniques. The elements of the scientific payload are listed in Table As it turns out, despite the limitations in the freedom of choice, the payload is a first-class set of instruments well optimised for the mission, and all aspects of the scientific objectives are addressed to a proper depth. 79

5 Table The Venus Express scientific payload. Code Technique Principal Investigator ASPERA Plasma analyser. S. Barabash (IRF-Kiruna, S) Energetic neutral atom imager MAG Magnetometer T. Zhang (IFW, Graz, A) PFS High-resolution IR Fourier spectrometer (presently non-operational) V. Formisano (IFSI-INAF, Rome, I) SPICAV/ UV & IR atmospheric spectrometer for J.-L. Bertaux (SA/CNRS, Verrières-le-Buisson, F) SOIR solar/stellar occultations and nadir observations A.-C. Vandaele (BIRA-IASB, Brussels, B) VeRa Radio occultation instrument B. Häusler (Universität der Bundeswehr, München, D) VIRTIS UV-visible-IR imaging and high-resolution spectrometer P. Drossart (CNRS/LESIA & Observatoire de Paris, F) G. Piccioni (IASF-INAF, Rome, I) VMC Wide-angle Venus Monitoring Camera W. Markiewicz (MPS, Katlenburg-Lindau, D) Venus Express is the first spacecraft to fully exploit the near-ir spectral windows, discovered in the 1980s. These windows, at wavelengths between 1 μm and 5 μm, through which radiation from the lower atmosphere and even the surface escapes to space, allow mapping of the atmosphere in three dimensions. The mission is also addressing open questions on the plasma environment, focusing on non-thermal atmospheric escape, particularly of water. Scientific achievements All of the science objectives have been addressed during the two first years of operations, even if some specific topics need to be studied for a longer duration before definitive conclusions can be drawn. Great progress has been achieved in several fields, in particular concerning atmospheric dynamics. The VMC and VIRTIS ultraviolet bands have been used extensively to get a good picture of the dynamics of the altitude of the cloud tops, i.e. at about 70 km (see Fig ). At these wavelengths, around 370 nm, a still unknown gas or aerosol in the clouds shows a strong absorption, while in the visible range the planet is completely featureless. The UV band thus allows tracking of the cloud motion and enables the study of the dynamics both on a global and a local scale. Three distinctly different dynamical regimes have been identified. At low latitudes, where the solar heating causes packets of air to rise due to convection, a mottled structure appears. In the polar region, a vast permanent vortex surrounded by a cold collar is approximately centred on the pole. The eye of the vortex can take different shapes that change on a timescale of less than a day. The intermediate latitudes show a banded structure that resembles a laminar flow. This region seems to be dominated by high altitude hazes with smaller droplet sizes, residing slightly above the main cloud deck and therefore obscuring these. Mapping of the cloud altitudes has shown that the clouds in the vortex are located about 5 km lower than for the rest of the planet and so appear as a hole in the main cloud layer. The VIRTIS instrument has made extensive studies of the polar vortex region with several campaigns of 4-dimensional data (two spatial, one spectral and time) and have synthesised movies of the motion in this region (see Fig ). The rotation 80

6 Figure An image of the southern hemisphere of Venus taken through a UV filter by VMC on 26 July 2007 showing the cloud tops at about 70 km altitude. The South Pole is at the terminator at the lower end of the image. The equatorial region and low latitudes (in the upper part of the image) show clear signs of convection cells due to solar heating. A bright cloud with few features covering a large portion of the polar region is expanding northwards towards lower latitudes, where it will dissolve within a few days. (ESA/VMC/MPS) period of the vortex is 2.5 days and is slightly variable. The rotation period of the mid and low latitudes is about four days. This high rate is remarkable since the solid planet rotates at a period of as much as 243 days and is therefore referred to as a superrotation. The energy to drive this superrotation must be coming from the solar heating but the process has until now not been fully understood. The new data from Venus Express allow more detailed models to be constructed and the modellers seem now to be on the right track to explain this problem. The upper atmosphere dynamics have been studied by following airglow generated by descending oxygen atoms recombining into O 2 at the night side of the planet. Figure shows how the upper atmosphere circulation takes place, with a global solar to anti-solar motion. This is different to the situation in the middle atmosphere where a Hadley-type circulation dominates. This is characterised by an updraft around the equator, due to solar heating, and a poleward transport to mid latitudes of about 1 m s 1 at the upper cloud level. At mid latitudes the air sinks and returns to the equatorial regions somewhere at a lower altitude. This return circulation has not yet been observed and is one of the remaining major uncertainties of the model. The atmospheric thermal structure has been addressed mainly by the SpicaV and Vera investigations through stellar/solar and terrestrial occultation respectively. SpicaV covers the altitude range km and VeRa covers the range km. A strong inversion layer has been found by SpicaV on the night side at an altitude of km. This is believed to be due to compressional heating by the downdraft of the air which is the same effect that causes the oxygen airglow described above. The VeRa data show a second but less pronounced inversion associated with the main cloud layer between 62 and 75 km altitude. The implication of this inversion still needs to be further studied. 81

7 Figure The detailed fine structure of the southern polar vortex, imaged here at 5.1 μm wavelength by the VIRTIS instrument on 11 August 2007, is studied in order to find an explanation of the origin and the driving forces of the whole vortex system. This highly variable and intriguing structure rotates at a rate of once per 2.5 days. This false colour image shows the calibrated radiance, corresponding to the temperature at a level of about 65 km altitude, with yellow/white being the highest temperature and dark/red being the lowest temperature. (ESA/VIRTIS/IASF-INAF/ Obs. de Paris) Figure A simplified sketch showing the upper atmosphere dynamics. The solar UV radiation dissociates CO 2 molecules and the products travel from the solar to the anti-solar side of the planet where the air cools off and descends. The oxygen airglow, which is the observable parameter, is a result of the recombination of oxygen atoms that takes place once the pressure has become sufficiently high. This occurs at about 96 km altitude on the anti-solar side of the planet. (R. Hueso) Several minor species have been identified by spectral analysis and profiles have been made throughout the atmosphere by the Virtis and SpicaV instruments. These include H 2 O, HDO, CO, SO 2, HCl, HF and COS. More species are expected to be identified after further studies of the extensive dataset. The ratio of HDO to H 2 O is of particular interest since it gives a clue to the history of water on the planet. Thermal escape of hydrogen is more efficient than for deuterium and indeed there is an enhancement of deuterium found. In addition the ratio varies with altitude. The importance of this is presently being studied. The Aspera instrument detects both hydrogen and oxygen in the exosphere at a ratio of 2:1, indicating that water is still being lost from the planet. A small amount of helium is also found to escape. The magnetometer is the only instrument onboard that operates continuously. It has made a large number of in situ characterisations of the different plasma regions and their borders like bow shock and induced magnetopause. These data were taken at around solar minimum condition and complement the earlier data from Pioneer Venus taken at solar maximum. The magnetometer has also detected whistler waves near Venus. This is interpreted as proof of frequent lightning in the atmosphere. The intensity may even exceed that of the Earth. This has important consequences for the chemical composition of the atmosphere, since lightning can be an important source of energy for synthesis of complex molecules that cannot be formed otherwise. Surface imaging is possible thanks to the infrared spectral windows, of which the window at 1.0 μm is the deepest and is where most of the radiation actually comes from the surface. However a large fraction that comes from the lower atmosphere 82

8 Figure Using the 1.0 micron spectral window has allowed the VMC team to produce this detailed thermal map of the surface. Dark/blue is the lowest temperature, corresponding to high altitude/mountains and orange/red is the highest temperature, corresponding to low altitude. The thermal gradient, or lapse-rate, at low altitudes is approximately 10K/km. Maps like this will be used to search for hotspots indicating volcanism and other geologic activity and to identify areas of anomalous thermal emissivity. (ESA/VMC/MPS/DLR) needs to be removed from these images. This is done by estimating the atmospheric contribution by using data from the sides of the centre wavelength and from other less deep windows. A resulting image from VMC data is shown in Fig Such images will be used for searching for geologic activity and volcanic and lava field hot spots. This activity is very work intensive and has just started. The first results from Venus Express were published in nine papers in a special section of the journal Nature on 29 November The next major publication will be two dedicated issues of the Journal of Geophysical Research, to appear in the second half of 2008, which together will contain about 60 papers based on Venus Express results. The first years of operation have provided a wealth of data that are contributing significantly to the knowledge of Venus. However, many questions do remain. The spacecraft is still in very good condition and is capable of providing many more answers. The case for a mission extension beyond April 2009 is therefore presently being defined. 83