Formation and Crustal Evolution of Mars: A Solar System Perspective

Stuart Ross Taylor

Department of Geology, Australian National University

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Abstract: Mars (r = 3390 km) is a small rocky planet 11% of Earth mass. How did it form? The inner solar system with its small rocky planets is distinct from the region of the giant planets that dominate the outer reaches of the system. A basic reason for this division was the depletion of ice and volatile elements and loss of gaseous elements (H, He) in the inner nebula, associated with violent solar activity in the earliest stages of nebular evolution. Jupiter and Saturn formed within 1-5 m.y. while the gas was still present. Within the inner nebula, only bodies large enough to survive the early intense heating episodes from the early Sun were left.

The terrestrial planets accreted 50-100 m.y. later from these left-over dry rocky planetesimals after the gaseous components of the nebula had been dissipated. Mars has a higher content of volatile elements (e.g. K) and a lower uncompressed density than the Earth.

Computer simulations of the accretion process in the inner solar system show that about 100 moon-size bodies, 10 Mercury-size and 3-5 Mars-size bodies would have formed the final population of planetesimals existing just before the final sweep-up into Venus and the Earth. Mars and Mercury, only about 1/20 Earth mass, are survivors of this population. The formation of the Earth’s Moon was the result of a collision with a body the size of Mars.

During accretion, Mars was probably melted, forming a Fe/FeS core. From the isotopic data, it appears to have differentiated at an early stage, forming a crust, now about 50 km thick. The global dust storms provide average samples of the martian crust. The chemistry of Viking and Pathfinder soils, SNC meteorites derived from Mars and the form of the giant martian volcanoes all lead to the conclusion that the magmatic history of Mars is dominated by basalts. The martian soils have high S and Cl contents, derived from volcanic activity in the absence of an oceanic sink.

Mars is a one-plate planet and recent plume activity has been restricted to the Tharsis Plateau and Olympus Mons. Despite its small size, Mars has the highest volcano, the largest canyon, the flattest surface and the greatest range in relief (30 km) of the terrestrial planets.

PROFILE: Professor Stuart Ross Taylor has degrees in chemistry and geology and completed his PhD in geochemistry at Indiana University. He lectured at Oxford University and as Emeritus Professor and Visiting Fellow at the Dept of Geology at ANU he is currently working with colleagues at NASA and ESA on the New Views of the Moon Project. He has worked on the composition and evolution of the Moon, the continental crust, tektites and impact glasses and many other topics involving trace element geochemistry. He was involved in the study of lunar samples from the first sample return in 1969, when he was a member of the Preliminary Examination Team at NASA Johnson Space Centre and carried out the first analysis of the first lunar sample returned to Earth. As a NASA Principal Investigator, he worked on models for lunar composition, evolution and origin. Professor Ross Taylor has published 230 papers in scientific journals and nine books and Asteroid 5670 is named Rosstaylor in his honour. He is also a Fellow of the Australian Academy of Science.