Infoscience

Journal article

A simple chondritic model of Mars

SNC meteorites (Shergottites, Nakhlites and Chassigny) define a fractionation line in a delta(17)O/delta(18)O diagram as expected for rocks differentiated from a formerly homogenized parent body, which is intermediate between H ordinary chondrites and EH enstatite chondrites. The planet Mars is located between the Earth and the asteroid belts, the potential source of ordinary chondrites. Since oxygen is a major component of the terrestrial planets, our model assumes that Mars composition is a mixture of two chondritic sources whose proportions are calculated by mass balance based on oxygen isotopes only. Two possible model compositions can be derived: if the average isotopic composition of SNC is relaxed along its fractionation Line (Model 1) and if the end members are average H and EH chondrites, one obtains a 30:70 H:EH mixture. if the average isotopic composition of SNC is a robust feature, then an extreme composition of H chondrites must be selected which yields the proportion, 55:45 for the H:EH components. This composition carries the same oxygen isotopic composition as the iron inclusions in the IIE of the conjectured end member of the ordinary chondrite group. The proportions obtained this way enable to calculate two model compositions for all the refractory elements and oxygen. Model 1 can be discarded as it does not permit to fit reasonably the physical properties of the planet. Mass and composition of the core (Model 2) is easily derived (23% of Mars mass, containing 16% S); the remainder forming the bulk mantle composition. Comparison with recent estimates based on the composition of SNC meteorites reveals only minor differences, essentially for Si, Mg and Fe; this is because of our choice of non-CI chondritic composition, unlike previous models. Discussion of the assumptions made in previous models confirms that the new composition is in agreement with the SNC compositions. The model also permits to calculate adequate physical properties of the planet like its zero pressure mantle density, density profile with depth and dimensionless moment of inertia. The superiority of the present model resides in the minimal number of necessary hypotheses and the possibility to test it, using physical and chemical data about the planet: as far as we know of, all constraints can be satisfied within the errors of the measurements or uncertainty of the model. The Fe abundance and the low Mg/Si ratio imply that pyroxenes and garnet at depth will play a major role in the differentiation of the planet, a feature which differs markedly from the terrestrial mantle. (C) 1999 Elsevier Science B.V. All rights reserved.

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