ESA Science & Technology - Publication Archive

We report measurements of night-side airglow emission in the atmosphere of Venus in the OH (2-0), OH (1-0), O2(a-X) (0-1), and O2(a-X) (0-0) bands. This is the first detection of the first three of these airglow emissions on another planet. These observations provide the most direct observational constraints to date on H, OH, and O3, key species in the chemistry of Venus' upper atmosphere. (This is an abbreviated abstract from the paper "First detection of hydroxyl in the atmosphere of Venus", by Piccioni et al, published in Astronomy & Astrophysics, A&A 483, L29-L33 (2008))

The European space Agency (ESA) undertook a bold experiment with the Mars Express mission: to develop rapidly a low cost platform for planetary exploration. The myriad scientific achievements of this mission prove the success of the experiment. ESA took a second bold step by adapting the Mars platform for the Venus Express mission, and doing so rapidly and most cost-effectively. While the differences in Venus and Mars necessitated several changes in instrumentation, there are many objectives that remain the same at the two planets. When we issued a call to the MEX and VEX communities for a volume of brief articles covering the latest results from these two missions, the response from those examining the interaction of the solar wind and energetic particles with the planets was most swift. The authors were asked to keep their presentations to four published pages. The guest editor in turn attempted to shepherd these papers through the reviewing process quickly. In those instances where the editor had a conflict of interest, R. J. Strangeway assumed the duties of the editor.

The articles that passed review before the press deadline are included therein. They include discussions of the various plasma boundaries at Venus and Mars, the nature of their plasma environments, the discovery of energetic neutral particles, the configuration of the magnetic field near the planets, space weather and the loss of atmosphere. Papers included contain both modeling and observational work and are written by some of the newest members of the community as well as many of the veteran research scientists. We especially thank the referees of these papers who responded promptly to help speed these early results to the readers of Planetary and Space Science.

The occurrence of lightning in a planetary atmosphere enables chemical processes to take place that would not occur under standard temperatures and pressures. Although much evidence has been reported for lightning on Venus, some searches have been negative and the existence of lightning has remained controversial. A definitive detection would be the confirmation of electromagnetic, whistler-mode waves propagating from the atmosphere to the ionosphere. Here we report observations of Venus' ionosphere that reveal strong, circularly polarized, electromagnetic waves with frequencies near 100 Hz. The waves appear as bursts of radiation lasting 0.25 to 0.5 s, and have the expected properties of whistler-mode signals generated by lightning discharges in Venus' clouds.

The atmosphere and ionosphere of Venus have been studied in the past by spacecraft with remote sensing or in situ techniques. These early missions, however, have left us with questions about, for example, the atmospheric structure in the transition region from the upper troposphere to the lower mesosphere (50-90 km) and the remarkably variable structure of the ionosphere. Observations become increasingly difficult within and below the global cloud deck (<50 km altitude), where strong absorption greatly limits the available investigative spectrum to a few infrared windows and the radio range. Here we report radio-sounding results from the first Venus Express Radio Science (VeRa) occultation season. We determine the fine structure in temperatures at upper cloud-deck altitudes, detect a distinct day-night temperature difference in the southern middle atmosphere, and track day-to-day changes in Venus' ionosphere.

Venus has no significant internal magnetic field, which allows the solar wind to interact directly with its atmosphere. A field is induced in this interaction, which partially shields the atmosphere, but we have no knowledge of how effective that shield is at solar minimum. (Our current knowledge of the solar wind interaction with Venus is derived from measurements at solar maximum.) The bow shock is close to the planet, meaning that it is possible that some solar wind could be absorbed by the atmosphere and contribute to the evolution of the atmosphere. Here we report magnetic field measurements from the Venus Express spacecraft in the plasma environment surrounding Venus. The bow shock under low solar activity conditions seems to be in the position that would be expected from a complete deflection by a magnetized ionosphere. Therefore little solar wind enters the Venus ionosphere even at solar minimum.

Venus, unlike Earth, is an extremely dry planet although both began with similar masses, distances from the Sun, and presumably water inventories. The high deuterium-to-hydrogen ratio in the venusian atmosphere relative to Earth's also indicates that the atmosphere has undergone significantly different evolution over the age of the Solar System. Present-day thermal escape is low for all atmospheric species. However, hydrogen can escape by means of collisions with hot atoms from ionospheric photochemistry, and although the bulk of O and O₂ are gravitationally bound, heavy ions have been observed to escape through interaction with the solar wind. Nevertheless, their relative rates of escape, spatial distribution, and composition could not be determined from these previous measurements. Here we report Venus Express measurements showing that the dominant escaping ions are O⁺, He⁺ and H⁺. The escaping ions leave Venus through the plasma sheet (a central portion of the plasma wake) and in a boundary layer of the induced magnetosphere. The escape rate ratios are Q(H⁺)/Q(O⁺) = 1.9; Q(He⁺)/Q(O⁺) = 0.07. The first of these implies that the escape of H⁺ and O⁺, together with the estimated escape of neutral hydrogen and oxygen, currently takes place near the stoichometric ratio corresponding to water.

Venus has thick clouds of H₂SO₄ aerosol particles extending from altitudes of 40 to 60 km. The 60-100 km region (the mesosphere) is a transition region between the 4 day retrograde superrotation at the top of the thick clouds and the solar-antisolar circulation in the thermosphere (above 100 km), which has upwelling over the subsolar point and transport to the nightside. The mesosphere has a light haze of variable optical thickness, with CO, SO₂, HCl, HF, H₂O and HDO as the most important minor gaseous constituents, but the vertical distribution of the haze and molecules is poorly known because previous descent probes began their measurements at or below 60 km. Here we report the detection of an extensive layer of warm air at altitudes 90-120 km on the night side that we interpret as the result of adiabatic heating during air subsidence. Such a strong temperature inversion was not expected, because the night side of Venus was otherwise so cold that it was named the 'cryosphere' above 100 km. We also measured the mesospheric distributions of HF, HCl, H₂O and HDO. HCl is less abundant than reported 40 years ago. HDO/H₂O is enhanced by a factor of 2.5 with respect to the lower atmosphere, and there is a general depletion of H₂O around 80-90 km for which we have no explanation.

The upper atmosphere of a planet is a transition region in which energy is transferred between the deeper atmosphere and outer space. Molecular emissions from the upper atmosphere (90-120 km altitude) of Venus can be used to investigate the energetics and to trace the circulation of this hitherto little-studied region. Previous spacecraft and ground-based observations of infrared emission from CO₂, O₂ and NO have established that photochemical and dynamic activity controls the structure of the upper atmosphere of Venus. These data, however, have left unresolved the precise altitude of the emission owing to a lack of data and of an adequate observing geometry. Here we report measurements of day-side CO₂ non-local thermodynamic equilibrium emission at 4.3 m, extending from 90 to 120 km altitude, and of night-side O₂ emission extending from 95 to 100 km. The CO₂ emission peak occurs at 115 km and varies with solar zenith angle over a range of 10 km. This confirms previous modelling, and permits the beginning of a systematic study of the variability of the emission. The O₂ peak emission happens at 96 km +- 1 km, which is consistent with three-body recombination of oxygen atoms transported from the day side by a global thermospheric sub-solar to anti-solar circulation, as previously predicted.

Venus has no seasons, slow rotation and a very massive atmosphere, which is mainly carbon dioxide with clouds primarily of sulphuric acid droplets. Infrared observations by previous missions to Venus revealed a bright 'dipole' feature surrounded by a cold 'collar' at its north pole. The polar dipole is a 'double-eye' feature at the centre of a vast vortex that rotates around the pole, and is possibly associated with rapid downwelling. The polar cold collar is a wide, shallow river of cold air that circulates around the polar vortex. One outstanding question has been whether the global circulation was symmetric, such that a dipole feature existed at the south pole. Here we report observations of Venus' south-polar region, where we have seen clouds with morphology much like those around the north pole, but rotating somewhat faster than the northern dipole. The vortex may extend down to the lower cloud layers that lie at about 50 km height and perhaps deeper. The spectroscopic properties of the clouds around the south pole are compatible with a sulphuric acid composition.

Venus is completely covered by a thick cloud layer, of which the upper part is composed of sulphuric acid and some unknown aerosols. The cloud tops are in fast retrograde rotation (super-rotation), but the factors responsible for this super-rotation are unknown. Here we report observations of Venus with the Venus Monitoring Camera on board the Venus Express spacecraft. We investigate both global and small-scale properties of the clouds, their temporal and latitudinal variations, and derive wind velocities. The southern polar region is highly variable and can change dramatically on timescales as short as one day, perhaps arising from the injection of SO₂ into the mesosphere. The convective cells in the vicinity of the subsolar point are much smaller than previously inferred, which we interpret as indicating that they are confined to the upper cloud layer, contrary to previous conclusions but consistent with more recent study.

Venus is Earth's near twin in mass and radius, and our nearest planetary neighbour, yet conditions there are very different in many respects. Its atmosphere, mostly composed of carbon dioxide, has a surface temperature and pressure far higher than those of Earth. Only traces of water are found, although it is likely that there was much more present in the past, possibly forming Earth-like oceans. Here we discuss how the first year of observations by Venus Express brings into focus the evolutionary paths by which the climates of two similar planets diverged from common beginnings to such extremes. These include a CO₂-driven greenhouse effect, erosion of the atmosphere by solar particles and radiation, surface-atmosphere interactions, and atmospheric circulation regimes defined by differing planetary rotation rates.

Venus Express is the first dedicated mission to Venus in over a decade. It was built by the European Space Agency, launched from Baikonur on a Soyuz-Fregat launcher on 9 November 2005. It arrived at Venus on 11 April 2006 and is in a polar orbit, with a period of ~24 hours. Since arrival its suite of instruments has been collecting data on the atmosphere and magnetosphere. In this focus, eight Letters describe the results obtained so far, while a Progress paper by Svedhem et al. gives an overview of the mission.

Studying the dynamics of the Venus atmosphere, one of the main goals of the Venus Express mission, requires global imaging of the planet. The Venus Monitoring Camera (VMC) meets this goal by having the relatively wide field-of-view of 17.5º. VMC is recording images using four narrowband filters, from UV to near-IR, all sharing one CCD. The spatial resolution is 0.2-45 km per pixel, depending on the distance from the planet. The planet's full disc is captured near the apocentre of the orbit. VMC is complementing the mission's other instruments by tracking cloud motions at ~70 km (cloud tops) and at ~50 km (main cloud layer) altitudes, mapping oxygen night-glow and its variability, mapping the nightside thermal emission from the surface, and studying the lapse rate and water content in the lower 6-10 km. In addition, VMC is providing imaging context for the whole mission, and its movies of the atmosphere are of significant interest for science and the public outreach programme.

The VIRTIS imaging spectrometer built for ESA's Rosetta cometary mission is a versatile instrument that is also well-suited to Venus observations. The discovery of the near-IR windows in the atmosphere of Venus from ground-based observations in the 1980s showed that the surface of the planet can be studied via IR observations over the nightside. Imaging spectroscopy in the visible and near-IR can study the atmosphere from the uppermost layers down to the deepest levels. With its unique combination of mapping capabilities at low spectral resolution (VIRTIS-M) and high spectral resolution slit spectroscopy (VIRTIS-H), the instrument is ideal for making extensive IR and visible spectral images of the planet.

The Venus Express Radio-Science Experiment (VeRa) is using radio signals at X- and S-band (3.5 cm and 13 cm wavelengths, respectively) to probe the Venus surface, neutral atmosphere, ionosphere and gravity field, and the interplanetary medium. An ultrastable oscillator (USO) is providing a high-quality onboard reference frequency source; instrumentation on Earth is sampling amplitude, phase, propagation time and polarisation of the received signals. Simultaneous coherent measurements at the two wavelengths allow separation of dispersive media effects from classical Doppler shift. The execution of a radio-science experiment involves the precise interaction of many complex spaceborne and ground-based systems. The quality of the measurements depend critically not only on the noise performance of the USO, the quality of the radio link and the performance of the ground station, but also on the precision of the timing, ephemeris data, orbit prediction and the attitude-control manoeuvres that are needed to perform the experiments and to extract the data.

SPICAV (SPectroscopy for the Investigation of the Characteristics of the Atmosphere of Venus) is a suite of three UV-IR spectrometers dedicated to the study of the atmosphere of Venus, from ground level to the outermost hydrogen corona at more than 40 000 km altitude. It is derived from the SPICAM instrument already flying on Mars Express with great success, with the addition of the new Solar Occultation IR (SOIR) high-resolution spectrometer working in the solar occultation mode. In nadir orientation, SPICAV UV (110-310 nm) will analyse the albedo spectrum to retrieve SO₂ and the distribution of the UV-blue absorber (of unknown origin) on the dayside with implications for cloud structure, and atmospheric dynamics. On the nightside, the g and d bands of NO will be studied, as well as emissions produced by electron precipitations. In the stellar occultation mode, the UV sensor will measure the vertical profiles of CO₂, temperature, SO₂, SO, clouds and aerosols. The density/temperature profiles obtained with SPICAV will constrain and aid in the development of dynamical atmospheric models, from cloud top (~60 km) to 160 km in the atmosphere. UV observations of the upper atmosphere will allow studies of the ionosphere through the emissions of CO, CO⁺ and O₂ ⁺, and its direct interaction with the solar wind. It will study the H corona, with its two different scale heights, and it will allow a better understanding of escape mechanisms and estimates of their magnitude, crucial for insight into the long-term evolution of the atmosphere.

Although the Venus Express and Mars Express spacecraft are very similar, key modifications were made to meet the requirements of a Venus mission. This paper provides an overview of the main mission drivers that led to the design changes, and describes the main spacecraft functions.

The Planetary Fourier Spectrometer (PFS) is an infrared spectrometer optimised for atmospheric studies, with a short-wavelength (SW) channel covering the spectral range 1800-11400 cm^-1 (0.9-5.5 mm) and a long-wavelength (LW) channel covering 250-1800 cm^-1 (5.5-45 mm). Both channels have a uniform spectral resolution of 1.3 cm^-1. It is the first Fourier spectrometer at Venus covering the 1-5 mm range. The SW field of view is about 1.6º FWHM, and 2.8º FWHM for the LW, which corresponds to spatial resolutions of 7 km and 12 km, respectively, when Venus is observed from a height of 250 km. PFS can provide unique data for improving our knowledge not only of the atmosphere properties but also the surface properties (temperature) and surface-atmosphere interaction (volcanic activity).

The SW channel uses a PbSe detector cooled to 200-220K, while the LW channel is based on a pyroelectric (LiTaO3) detector working at room temperature. The intensity of the interferogram is measured at every 112 nm displacement of the mirrors (corresponding to 450 nm optical path difference), by using a laser diode monochromatic light interferogram (a sine wave), whose zero crossings control the double pendulum motion. PFS works primarily around the pericentre of the orbit, only occasionally observing Venus from large distances. Each measurement takes 4 s, with a repetition time of 11.5 s. By working for about 1.5 h around pericentre, a total of 460 measurements per orbit can be acquired, plus 60 for calibrations. PFS can take measurements at all local times, facilitating the retrieval of surface temperatures and atmospheric vertical temperature profiles on both the day and night sides.

The MAG (Magnetometer) instrument of Venus Express is investigating the plasma environment of Venus. Although Venus has no intrinsic magnetic moment, its magnetic field plays an important role in the interaction of the solar wind with the planet. The hardware, with heritage from the ROMAP Rosetta Lander magnetometer, consists of two sensors, an electronics box and a carbon fibre boom. One sensor is located on the tip of the boom, while the other is mounted on the spacecraft body; this configuration allows the magnetic effects of spacecraft origin to be separated from the ambient space magnetic field.

The Venus Express Science Operations Centre (VSOC) has the task of defining and performing, under the direct responsibility of the Project Scientist, the science operations for the mission. VSOC ensures that all the science objectives can be fulfilled within the operational constraints. This paper focuses on the planning and commanding activities, and provides an overview of the VSOC activities, outlines its architecture down to the hardware level, and summarises the science planning process. The data-handling and archiving are dealt with in a companion paper.