First observations with the ASPERA-4 experiment on Venus Express

Venus, the ’Morning Star’, is very similar to Earth in size, mass, density and volume, but evolved in a radically different way over the last four thousend million years. Its toxic and heavy atmosphere is made up almost entirely of carbon dioxide, clouds of sulphuric acid. Venus has a burning-hot surface temperature (≈ 750 K), and a high surface pressure (95 bar), compared to Earth (1 bar). Understanding why Venus became such an unfriendly and inhospitable planet is of crucial importance for comparative planetology and also useful for understanding the long-term climatic evolution processes on Earth.

The Venus Express mission aims at a global investigation of the atmosphere and the plasma environment of Venus and addresses several important aspects of the geology and surface physics. The basic idea of this mission is to observe from orbit the same target with different instruments at the same time. This provides a comprehensive, versatile and complete view of the different phenomena taking place on Venus. Venus Express is also a sister mission of Mars Express - a spacecraft which is in orbit around Mars with a similar set of instruments.

Venus Express is ESA’s first spacecraft to visit planet Venus. On 9 November 2005 Venus Express was successfully launched by a Soyuz-Fregat rocket, from the Baikonur Cosmodrome in Kazahkstan. After a five-month interplanetary journey to the inner solar system, the spacecraft arrived to destination on 11 April 2006 and was captured by the Venusian gravity. Venus Express is in a polar orbit which ranges between 66 000 (apocentre) and 250 kilometres (pericentre) altitude. The pericentre is located almost above the North pole (80± North latitude) and it takes 24 hours for the spacecraft to travel around the planet. The mapping mission will last for 2 Venusian days, about 500 Earth days.
The Venus Express payload consists of ASPERA-4 (Analyser of Space Plasma and Energetic Atoms), MAG (Magnetometer), PFS (Planetary Fourier Spectrometer), SPICAV/SOIR (Ultraviolet and Infrared Atmospheric Spectrometer), VeRa (Venus Radio Science Experiment), VIRTIS (Ultraviolet/visible/nearinfrared Mapping Spectrometer) and VMC (Venus Monitoring Camera).

Figure 1: ASPERA-4 data from 30 May 2006, between 01:00 UT and 03:30 UT. The upper panel shows the total counts of the Electron Spectrometer in the energy range of 20 - 1000 eV. The middle panel illustrates the total counts of the Ion Mass Analyzer, taking into account all ions, in the energy range of 10 - 10000 eV. The bottom panel shows the Venus Express altitude in Venusian radii as a function of time. All boundary crossings are marked by black vertical lines.

The ASPERA-4 experiment is an international collaboration of European and American scientists led by the Swedish Institute of Space Phyics in Kiruna. This instrument will investigate the interaction between the solar wind and the atmosphere of Venus by measuring outflowing particles from the planet’s atmosphere and the particles making up the solar wind. It will study how the molecules and ions escape the planet.
The ASPERA-4 instrument consists of 4 sensors:

the Neutral Particle Imager (NPI), a simple ENA (Energetic Neutral Atom) direction analyzer that surveys ENA fluxes with high angular resolution [Energy range: 0.1 - 60 keV]
the Neutral Particle Detector (NPD), performing ENA velocity and mass measurements [Energy range: 0.1 - 10 keV]
the Electron Spectrometer (ELS), performing electron energy measurements [Energy range: 0.01 - 20 keV]
the Ion Mass Analyzer (IMA), a mass resolving spectrograph that provides measurements of the main ion components (H, H2, He and O) [Energy range: 0.01 - 40 keV]

The ASPERA-4 design is a re-use of the ASPERA-3 design flown on Mars Express, adapted to suit the different thermal and radiation environments that will be encountered during the Venus Express mission. The MPS has developed and built in collaboration with IDA (TU Braunschweig) and IFSI (Rom) the electronics for the NPD sensor, and is taking part in the scientific data analysis.

The figure shows data acquired by ASPERA-4 on 30 May 2006. Energy spectrograms of the ELS and IMA sensors and the spacecraft altitude in Venusian radii (1 RV = 6051.5 km) as a function of time are displayed. The observations show the main features of the solar wind interactions with Venus. Due to the supersonic solar wind a bow shock is formed in front of the planet. The bow shock is a discontinuity in the flow marked by an abrupt increase in turbulence, and enhanced electron fluxes and plasma temperatures, together with a decrease in flow velocity. The region, bounded by the bow shock and the ’magnetic barrier’, containing the shocked, slowed down and heated solar wind, is called magnetosheath. In contrast to Earth Venus has no intrinsic magnetic field which can deflect solar wind ions. Thus solar wind ions and electrons, and the entrained interplanetary magnetic field (IMF), impact the ionosphere directly, driving currents that produce a magnetic barrier which stops the penetration of the solar wind. At the magnetic barrier the IMF is compressed and drapes around the ionosphere. As a consequence most of the shocked solar wind flows around the planet. During the eclipse (when the spacecraft is in the shadow of Venus) heavy ions of planetary origin, most probably oxygen, can be observed. On the outbound pass Venus Express crosses again all those regions but in reverse order.

The data shown in the figure are from the ’orbit commissioning phase’, which started on 22 April 2006, where the spacecraft instruments were being switched on and tested. Already in this early phase the instruments onboard Venus Express returned exciting data and we are now looking forward to the science operation phase which started on 4 June 2006.

Links

Venus Express at MPS

ASPERA-4 at MPS

Venus Express at IWF

Venus Express Homepage at ESA

Venus Express Special at ESA

Swedish Institute of Space Physics

ASPERA-4 site at IRF