Venus Express is an orbiter for studies of the atmosphere, the plasma environment, and the surface of Venus. The payload of VEX comprises instruments basing on the development for missions like Rosetta and Mars Express. The mission is going to answer fundamental questions related to the global atmospheric circulation, the atmospheric chemical composition and its variations, the surface-atmospheric physical and chemical interactions including volcanism, the physics and chemistry of cloud layers, the thermal balance and role of trace gases in the greenhouse effect, the origin and evolution of the atmosphere, and the plasma environment and its interaction with the solar wind. This information is crucial in a comparative planetology context and notable for understanding the long-term climatic evolution processes on Earth. VIRTIS as a sensitive visible spectro-imager and IR spectrometer will contribute to this the mission with studies of the structure and composition of the lower atmosphere within the near IR-windows of Venus, the studies of middle and upper atmosphere and the surface /surface-atmosphere interactions. The mission will launched by a Soyuz/Fregat from Baikonur in November 2005. The launcher will place the spacecraft into a transfer orbit to Venus. After a cruise phase of about 153 days the spacecraft will be captured by Venus and five days later it will be transferred into its operational orbit.
With MEX, VEX and Bepi Colombo a comprehensive program for studies of each planet in the inner Solar system enables a global approach for studies of the evolution and origin of the terrestrial planets.
Main scientific objectives of VIRTIS
During the first attempts of imaging spectroscopy on Venus night side by NIMS/Galileo and VIMS/Cassini fly-bys it was shown how powerful this method allows to investigate clouds and atmospheric phenomena. According to the technical parameters of VIRTIS described in section 2 the main scientific goals of VIRTIS at Venus are the following:
- Study of the lower atmosphere composition below the clouds and its variations (CO, OCS, SO 2 , H 2 O) from night side observations
- Study the cloud structure, composition and scattering properties (day side observations)
- Cloud tracking in the UV (70 km, day side) and IR (50 km, night side)
- Determination of the temperature field and zonal wind in an altitude range from 60 – 100 km (night side)
- Lightning search (night side)
- Mesospheric sounding: understanding the transition region between troposphere and thermosphere
- Non- LTE O 2 emission (night/day side) at 1.27 µm (95-110 km)
- CO 2 fluorescence (day side): non –LTE emissions at 4.3 µm (>80 km)
- Limb observations (CO, CO 2 ), atmospheric vertical structure (> 60 km, day/night side)
- Search for variations related to surface/atmosphere interactions, dynamics, meteorology and volcanism
- Temperature mapping of the surface, search for hot spots related to volcanic activity
To achieve the scientific objectives VIRTIS must observe the night and day side and work with fill imaging and spectroscopy capabilities. The spatial resolution of VIRTIS is better than 13 km at apoapsis. This resolution fulfils the requirement for cloud structure observations. It is also consistent with surface studies in the IR, because the scattering in the Venus clouds blurs the thermal flux coming from the surface over a scale range comparable to the cloud height (30km).
The VIRTIS observations are separated into two categories:
- Spectral mode (h< 12000 km) will be used for joint VIRTIS/PFS observations. The coverage will reach about 15 % after 7 orbits. In particular the cloud variability and atmospheric composition will be tracked
- Spectral imaging mode (h> 12000 km) with cube spectro-image reconstruction
Scientific and HW contributions of the German OS team to the experiment
The DLR OS team is responsible for the adaptation of the VIRTIS Rosetta SW to the new mission profile at Venus, the support of adaptation and integration of the Main Electronics, the all over instrument integration and the operational tests. The team is managing the instrument operations which is derived from the Rosetta system.
The experiment is leaded by P. Drossart from LESIA, Paris, F. The German role is the national team leadership (G. Arnold, Deputy PI). The German Deputy PI ensures the provision of the science data to the German science community in cooperation with Institute of Planetary Research.
Within the VIRTIS VEX science team the German group will contribute to the data evaluation by studies of the radiative transfer of the lower and upper atmosphere of Venus and volcanic surface phenomena.