Planetary Science

Telling Time on Mars

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Science  22 Feb 2008:
Vol. 319, Issue 5866, pp. 1011
DOI: 10.1126/science.319.5866.1011b

Our primary information on the surface age and geologic history of Mars comes from crater densities on different parts of the planet and from dating of Martian meteorites (though their source region on Mars is not known with certainty). Crater ages imply that most of the crust is over 2 billion years old. In contrast, dating of one distinct group of Martian meteorites, by a variety of isotopic systems, has generally indicated young ages, from a few hundred thousand to about 1 billion years. These numbers imply that Mars had enough recent magmatic activity to be sampled by a significant fraction of the available Martian meteorites. A problem, however, is that all of these rocks show evidence of alteration and impact events (required for but not limited to their excavation from Mars), as well as hints of the very early geochemical history of Mars, so interpreting their ages is not generally straightforward. Bouvier et al. present additional analyses of two of the purportedly young meteorites using several isotopic systems and evaluate the ages of all of the Martian meteorites. In general, the Sm-Nd and Lu-Hf ages derived from minerals were young, but the Pb-Pb whole-rock ages were much older (∼4 billion years). In contrast to some other recent analyses, the authors interpret this difference, and ultimately the other young ages, as indicating later alteration that has re-initialized mineral-based isochrons. — BH

Earth Planet. Sci. Lett. 266, 105 (2008).

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