ESA's Mars Express Mission – Europe on Its Way to Mars

Introduction 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 Earth, Venus and Mercury accreted. Its size, the degree of internal activity, the age of its surface features, and the density of its atmosphere are manifestations of Mars’ special position and its transitional character. Its parameters are intermediate between those of the large terrestrial planets (Earth, Venus) and the smaller planetary bodies (Mercury, Moon, and asteroids). Although geologically less evolved, the Martian surface is more Earth-like than that of any other terrestrial planet. Consequently, the exploration of Mars is crucial for a better understanding of the Earth from the comparative-planetology perspective.