In the broad context of planetary science, Mars represents an important transition between the outer volatile-rich, more oxidised regions of the accretion zone of the terrestrial bodies (asteroid belt) and the inner, more refractory and less oxidised regions from which the Earth, Venus and Mercury accreted.
This special position of Mars and its traditional character is also manifested by its size, the degree of internal activity, the age of its surface features, and the density of its atmosphere. These properties are intermediate between those of the large terrestrial planets (Earth, Venus) and the smaller planetary bodies (Mercury, the Moon, the asteroids). Exploration of Mars is crucial for a better understanding of the Earth from the perspective of comparative planetology.
The scientific objectives of the Mars Express mission represent an attempt to fulfill part of the lost scientific goals of the ill-fated Russian Mars-96 mission.