A set of key questions have been identified that formulate gaps in our current knowledge of Venus:

• What is the mechanism and what is the driving force of the super-rotation of the atmosphere?
• What are the basic processes in the general circulation of the atmosphere?
• What is the composition and chemistry of the lower atmosphere and the clouds?
• What is the past and present water balance in the atmosphere?
• What is the role of the radiative balance and greenhouse effect in the past present and future evolution of the planet?
• Is there currently volcanic and/or tectonic activity on the planet?

The answers to these questions, together with the comprehensive studies under the different themes described below, will lead to an improved understanding of perhaps the most fundamental question of all: Why has Venus evolved so differently compared to the Earth, in spite of the similarities in terms of size, basic composition and distance to the Sun?

The scientific objectives of the Venus Express mission have been concisely expressed within seven scientific themes. The aim is to carry out a comprehensive study of the atmosphere of Venus and to study the planet's plasma environment and its interaction with the solar wind in some detail. Dedicated surface studies are also performed. The seven scientific themes are:

1. Atmospheric dynamics
2. Atmospheric structure
3. Atmospheric composition and chemistry
4. Cloud layer and hazes
5. Radiative balance
6. Surface properties and geology
7. Plasma environment and escape processes

The first three themes are divided into sub-themes that refer to the upper, middle and lower parts of the atmosphere. The corresponding approximate limits for these regions are, above 110 km, between 110 and 60 km, and below 60 km. The scientific requirements within the sub-themes are broken down into units that can be directly addressed by individual measurements by the Venus Express scientific instruments.