Mars is about one-eighth the mass of the Earth and it may provide an analogue of what the Earth was like when it was at such an early stage of accretion. The fur ther growth of the Earth was sustained by major collisions with planetesimals and planets such as that which resulted in the formation ofthe Earth's moon (Hartmann and Davis, 1975; Cameron and Ward, 1976; Wetherill, 1986; Cameron and Benz, 1991). This late accretionary history, which lasted more than 50 Myr in the case of the Earth (Halliday, 2000a, b), appears to have been shorter and less catastrophic in the case of Mars (Harper et ai. , 1995; Lee and Halliday, 1997). In this article we review the basic differences between the bulk composition of Mars and the Earth and the manner in which this plays into our understanding of the timing and mechanisms of accretion and core formation. We highlight some of the evidence for early cessation of major collisional growth on Mars. Finally, we reevaluate the isotopic evidence that Mars differentiated quickly. Fundamental differences between the composition of Mars and that of other terrestrial planets are apparent from the planet's slightly lower density and from the compositions of Martian meteorites. The low density is partially explicable if there is a greater proportion of more volatile elements.

Inhalt
Introduction: A New Chapter in Mars Research.- Introduction: A New Chapter in Mars Research.- Chronology of Mars and of the Inner Solar System.- Stratigraphy and Isotope Ages of Lunar Geologic Units: Chronological Standard for the Inner Solar System.- Cratering Records in the Inner Solar System in Relation to the Lunar Reference System.- Mars/Moon Cratering Rate Ratio Estimates.- Ages and Geologic Histories of Martian Meteorites.- Cratering Chronology and the Evolution of Mars.- Evolution of the Interior and Surface of Mars.- The Accretion, Composition and Early Differentiation of Mars.- Geophysical Constraints on the Evolution of Mars.- Geological Processes and Evolution.- The Martian Surface Composition.- Chemical Composition of Rocks and Soils at the Pathfinder Site.- History and Fate of the Martian Atmosphere and Hydrosphere.- Geomorphologic Evidence for Liquid Water.- Alteration Assemblages in Martian Meteorites: Implications for Near-Surface Processes.- Aeolian Processes and Their Effects on Understanding the Chronology of Mars.- Martian Seeps and Their Relation to Youthful Geothermal Activity.- The Atmosphere of Mars as Constrained by Remote Sensing.- Martian Volatiles: Isotopic Composition, Origin, and Evolution.- Epilogue.- Summary: New Views and New Directions in Mars Research.