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.
This book results from a workshop at the International Space Science Institute (ISSI) in Bern, Switzerland, where geochemists, geophysicists, and photogeologists have combined their expertise to constrain the timescales and geological processes in the evolution of Mars. In order to achieve this goal, the ages of cratered lunar surfaces, which are dated precisely by the radiometric ages of returned samples, have been taken as a reference for the inner solar system chronology. The derived ages of cratered geological units on Mars indicate ongoing geological activity from about 4.5 Gyr ago till modern geologic time. Ancient surfaces involve primordial crustal material with strong magnetization that has remained from Mars' core formation within the first 20 Myr of the solar system, whereas other surfaces were created during major geological processing at about 3-4.5 Gyr ago, probably associated with a denser atmosphere and more fluvial environment, and also to exposures of volcanism.The youngest surfaces indicate volcanism, weathering, gullying, exhumation, and modest water release all operating within the last few 100 Myr, with the youngest detected flows at less than 10 Myr ago.
Neither Earth nor Moon offers such a complete geological record. This picture is consistent with radiometric age data of Martian meteorites which indicate that Mars has not only ancient crustal material, as represented in the ALH84001 meteorite, but at least some geologically young igneous rocks with ages of a few 100 Myr. Remote sensing of the Martian surface identifies two broad groups of igneous rock units, basaltic and andesitic, as is confirmed by in-situ chemical analyses at the Pathfinder landing site.Based on these results, the book contains an update on the overall stratigraphic system and geologic processes of the Martian surface, and a recent review on the newest models of the Martian interior structure and on the knowledge about the history of the Martian atmosphere and hydrosphere.
