|
|
| Morphology and dynamics of the upper cloud layer of Venus |
| 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. |
| Publication date: 29 Nov 2007 |
|
|
| South-polar features on Venus similar to those near the north pole |
| 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. |
| Publication date: 29 Nov 2007 |
|
|
| The loss of ions from Venus through the plasma wake |
| 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. |
| Publication date: 29 Nov 2007 |
|
|
| The structure of Venus' middle atmosphere and ionosphere |
| 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. |
| Publication date: 29 Nov 2007 |
|
|
| Venus as a more Earth-like planet |
| 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. |
| Publication date: 29 Nov 2007 |
|
|
| Venus Express |
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. |
| Publication date: 28 Nov 2007 |
|
|
| MAG: The Fluxgate Magnetometer of Venus Express |
| 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. |
| Publication date: 02 Nov 2007 |
|
|
| PFS: The Planetary Fourier Spectrometer |
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. |
| Publication date: 02 Nov 2007 |
|
|
| SPICAV: Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Venus |
| 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. |
| Publication date: 02 Nov 2007 |
|
|
| Spacecraft and Payload Data Handling |
| The success of a scientific mission is determined by the quality of the scientific
results. The prompt delivery of instrument and ancillary raw data to the instrument
teams and the delivery of reduced and calibrated data to the scientific community
are therefore key elements in the mission design. This paper describes the data flow
from the Venus Express spacecraft through the ground segment via the instrument
teams to the final scientific archive. Several software tools and standards are used
to support the data dissemination. The functionality of the individual tools is
explained, the interfaces to the individual groups are discussed and examples of the
graphical user interfaces are shown. |
| Publication date: 02 Nov 2007 |
|
|
| The Ground Segment and Mission Operations |
| Following launch and its 5-month interplanetary cruise, Venus Express was injected
into a 24 h orbit around Venus. It is now conducting science observations for two
Venusian days (486 Earth-days). This paper describes the ground system
infrastructure, the orbital requirements and mission control parameters, the flight
operations concept, principles and implementation for the various mission phases
and payload operations, and the mission products to be made available to the
scientific community. |
| Publication date: 02 Nov 2007 |
|
|
| The Venus Express Spacecraft System Design |
| 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. |
| Publication date: 02 Nov 2007 |
|
|
| The general objective of ASPERA-4: Analyser of Space Plasmas and Energetic |
| The general objective of ASPERA-4 (Analyser of Space Plasmas and Energetic
Atoms) is to study the solar wind-atmosphere interaction and characterise the
plasma and neutral gas environment in near-Venus space through energetic neutral
atom (ENA) imaging and local charged particle measurements. The studies address
the fundamental question: how strongly do the interplanetary plasma and
electromagnetic fields affect the atmosphere of Venus? ASPERA-4 comprises four
sensors: two ENA sensors, and electron and ion spectrometers. The Neutral Particle
Imager (NPI) measures the integral ENA flux (0.1-60 keV) with no mass or energy
resolution but relatively high angular resolution. The Neutral Particle Detector
(NPD) measures the ENA flux, resolving velocity (0.1-10 keV for hydrogen) and
mass (H and O) with a coarse angular resolution. The electron spectrometer (ELS)
is a standard top-hat electrostatic analyser (energy range 0.001-20 keV) in a very
compact design. These three sensors are on a scanning platform providing 4p
coverage. ASPERA-4 also contains an ion mass composition sensor, IMA (Ion Mass Analyser). Mechanically, IMA is a separate unit electrically connected to the
ASPERA-4 main unit. IMA provides ion measurements in the energy range
0.01-36 keV/q for the main ion components H+, He++, He+, O++, O+ and CO+2 ion
group with M/q > 40 amu/q. |
| Publication date: 02 Nov 2007 |
|
|
| VIRTIS: The Visible and Infrared Thermal Imaging Spectrometer |
| 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. |
| Publication date: 02 Nov 2007 |
|
|
| VMC: The Venus Monitoring Camera |
| 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. |
| Publication date: 02 Nov 2007 |
|
|
| Venus Atmospheric, Ionospheric, Surface and Interplanetary Radio- Wave Propagation Studies with the VeRA Radio Science Experiment |
| 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. |
| Publication date: 02 Nov 2007 |
|
|
| Venus Express Science Planning and Commanding |
| 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. |
| Publication date: 02 Nov 2007 |
|
|
| The Planet Venus and the Venus Express Mission |
Venus Express was launched in late October, 2005, and arrived at the planet in April 2006, where it is now in orbit and the return to Earth of new information about Venus' atmosphere, surface, and space environment has begun. The purpose of this special issue of Planetary and Space Science is to lay out the background to the mission, in terms of the planet and its mysteries as well as the spacecraft, its instruments, and the planned observations, in order to review the context in which the new results will be analysed and interpreted.
- The remainder of the abstract is truncated -
|
| Publication date: 15 Nov 2006 |
|
|
| Outstanding aeronomy problems at Venus |
| Of all the non-terrestrial ionospheres and thermospheres in our solar system those of Venus have been explored and studied the most. This is mainly because of the 14 year exploration of the well instrumented Pioneer Venus spacecraft and the theoretical studies prompted by the resulting observational information. However, there are still a number of important scientific questions that remain unanswered. These include: i) dynamics of the thermosphere; ii) the energy mechanisms/sources responsible for maintaining the elevated plasma temperatures in the ionosphere; iii) airglow/aurora intensities and their sources; and iv) hot atom populations. Venus Express is likely to help address some of the issues listed under i), iii) and iv) above.
|
| Publication date: 10 Nov 2006 |
|
|
| From Earth to Venus - Reaching our Sister Planet |
| In the early morning of 9 November 2005, Venus Express left Earth aboard a Soyuz launch vehicle and headed for Venus. After several months of interplanetary cruise, a perfect capture burn on 11 April 2006 placed the spacecraft in orbit around our neighbouring planet. Only 48 hours later, the first astonishing images of the south pole were received on Earth. A few weeks later, after orbital manoeuvres, Venus Express achieved its operational science orbit ready to begin several years of observations. |
| Publication date: 15 Aug 2006 |
|
|
